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A decade of harbour porpoise occurrence in German waters—Analyses of aerial surveys, incidental sightings and strandings
Journal of Sea Research 56 (2006) 65 – 80 www.elsevier.com/locate/seares
A decade of harbour porpoise occurrence in German waters— Analyses of aerial surveys, incidental sightings and strandings Ursula Siebert a,⁎, Anita Gilles a , Klaus Lucke a , Martje Ludwig a , Harald Benke b , Karl-Hermann Kock c , Meike Scheidat a a
Research and Technology Centre, Christian-Albrechts-University of Kiel, Hafentörn 1, 25761 Büsum, Germany b German Oceanographic Museum, Katharinenberg 14-20, 18439 Stralsund, Germany c Federal Research Centre for Fisheries, Palmaille 9, 22767 Hamburg, Germany Received 22 March 2005; accepted 26 January 2006 Available online 14 March 2006
Abstract Data on the occurrence of harbour porpoises (Phocoena phocoena) in German waters from 1988 to 2002 were collected from dedicated aerial surveys, incidental sightings and strandings. Aerial surveys conducted in 1995 and 1996 revealed a mean abundance of 4288 (in 1995) and 7356 harbour porpoises (in 1996) in the German North Sea study area. Mean abundances of harbour porpoises in the German Baltic Sea, divided into two subunits (blocks B and C), were estimated at 980 and 1830 (in 1995 and 1996 resp.) and at 601 (in 1995; there were no sightings in block C during the 1996 survey). From 1988 to 2002, 791 incidental sightings of harbour porpoise pods were reported in German and partly Danish coastal waters of the North and Baltic Seas. In the period 1990 to 2001, 996 harbour porpoises were found stranded along the German North Sea coast and 17 animals were identified as by-catch. In the same period 229 harbour porpoises were found stranded along the German Baltic Sea coast and 105 animals were incidentally taken in fisheries. The proportion of by-caught harbour porpoises was significantly larger in the Baltic Sea. Different monitoring methods are helpful for different aims and management issues: aerial surveys cover large areas in a short time and provide information on density, abundance, distributional patterns and seasonality. Incidental sighting and stranding networks provide indications of general distribution, seasonal variation in abundance, age distribution, by-catch and of areas which are important in the harbour porpoise’s life cycle. Comparison of data from the North and Baltic Seas revealed a higher abundance of harbour porpoises in the North Sea than in the Baltic Sea. Altogether the data sets demonstrated a strong seasonality of harbour porpoise occurrence off the German coast with highest numbers during the summer months. Important habitats for harbour porpoises were detected west of the islands of Sylt and Amrum in the North Sea and around the Schlei estuary, in waters west of Fehmarn and the Fischland-Darss area in the Baltic Sea. © 2006 Elsevier B.V. All rights reserved. Keywords: Phocoena phocoena; Aerial surveys; Incidental sightings; Strandings; By-catch; North Sea; Baltic Sea
1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (U. Siebert). 1385-1101/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2006.01.003
Aerial and shipboard surveys, incidental sightings and strandings all show that the harbour porpoise (Phocoena phocoena) is by far the most common cetacean in the German parts of the North and Baltic Seas
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(Benke et al., 1998; Hammond et al., 2002; Scheidat et al., 2004). Yet, the status of populations in these waters remained unclear. There is reason for concern as several national and international research projects indicate that harbour porpoise numbers in the central Baltic Sea have decreased dramatically over the last 50 years (Berggren and Arrhenius, 1995; Schulze, 1996; Benke et al., 1998; Hammond et al., 2002). The major cause of unnatural harbour porpoise death in the North and Baltic Seas is thought to be incidental mortality in fishing gear, primarily in bottom-set gillnets (Kock and Benke, 1996; Vinther, 1999; Vinther and Larsen, 2004). Other anthropogenic activities, such as chemical and noise pollution (e.g. caused by the construction of offshore windmill farms or the use of military low frequency sonar) may affect harbour porpoise adversely as well (Jepson et al., 1999, 2003; Siebert et al., 1999). To protect the harbour porpoise as required by several international agreements (ASCOBANS, OSPARCOM, HELCOM) and to identify appropriate management measures for the harbour porpoise stocks in the North and Baltic Seas, the occurrence, abundance and distribution of these animals need to be known. The ASCOBANS parties pursue these objectives with a range of approaches (see ASCOBANS (2002) national reports: http://www.ascobans.org). Stranding networks around the North and Baltic Seas currently exist in Belgium, the Netherlands, Germany, the United Kingdom, Poland and Sweden. Incidental sighting schemes were established in the Netherlands, the United Kingdom, Germany, Denmark, Poland and Sweden. Dedicated aerial and shipboard surveys only for cetaceans or surveys in combination with bird or oil pollution surveys, or surveys in connection with the planning of offshore windfarms have been conducted by the Netherlands, the United Kingdom, Germany, Denmark, Sweden and Poland. Overall Germany has invested most in harbour porpoise surveys over the last years, resulting in a leading position. The first systematic NW European survey for small cetaceans (SCANS) was conducted in 1994 (Hammond et al., 2002), and a repeat took place in July 2005. In Germany research projects were funded by federal ministries and the states of Schleswig-Holstein and Mecklenburg-Vorpommern with the intent to gather knowledge on the status of harbour porpoises in German near-shore waters and to comply with legal requirements. In addition to the assessment of health status (Siebert et al., 1999, 2001; Wünschmann et al., 2001; Das et al., 2004), genetic structure (Tiedemann et al., 1996; Huggenberger et al., 2002) and reproduction status
(Benke et al., 1998), projects were designed to obtain data on the occurrence and abundance of harbour porpoises in waters off Schleswig-Holstein and Mecklenburg-Vorpommern. This paper summarises data from stranding networks, incidental sightings and dedicated aerial surveys gathered in the period 1988 to 2002. The major aims were to compare and discuss the different methods and to recommend the appropriate choice for future monitoring programmes. 2. Methods 2.1. Baltic/North Sea aerial surveys in 1995 and 1996 Aerial surveys were conducted on pre-designed zigzag track lines (Fig. 1) in the western Baltic Sea and in the eastern North Sea in 1995 (July, October) and in 1996 (July). Standard line-transect methodology was used to estimate abundance (Buckland et al., 2001) of porpoise pods in three survey blocks (A, B and C; Fig. 1). We anticipated that sighting rates would be too low to allow effective strip width (esw) to be estimated using the tandem survey technique (Hiby and Lovell, 1998). As one of the two observers of the 1995 and 1996 surveys had also participated in SCANS 1994 and survey methods were the same, the effective strip widths estimated during the SCANS survey were also applied to the present results to estimate abundance. The data were stratified by the subjective sighting condition assessments ‘good’ and ‘moderate’ and 130m esw for good conditions and 80m esw for moderate conditions were used. The surveys were conducted with a high-winged aeroplane equipped with bubble windows in order to allow unobstructed observation of the water surface on both sides, from the abeam line forward to the track line. The method for designing the cruise tracks as well as the data collection method and the calculation of porpoise pod abundance and 95% confidence intervals (CI) were described in detail in Hammond et al. (2002). The survey blocks cover the coastal waters of Germany (except Lower Saxony). Using pseudo-random starting positions, several replicate zigzag tracks were constructed in each block. The selection of a single track would thus provide non-zero coverage probability for any point in the block. A mean abundance estimate with confidence limits was derived by surveying replicate tracks. 2.2. Incidental sightings In the late 1980s, the Research and Technology Centre West Coast (FTZ) in Büsum (Germany) developed a handout for the identification of whale
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Fig. 1. Aerial survey areas. Block A is situated in the North Sea, blocks B and C in the Baltic Sea. The lines indicate the zigzag transect lines, of which there were six in block A, four in block B and six in block C. Map projection: Mercator.
sightings from platforms of opportunity. This handout included information on the species (particularly a description of the harbour porpoise) as well as a questionnaire used to record the opportunistic sightings in more detail. The questionnaire was filled out by members of the public who made an incidental observation of harbour porpoises or other cetacean species and sent back to the FTZ. The data recorded included date and time, position of observation, pod size and number of calves (half the size of the adults), identification and behaviour of the whales, weather conditions as well as wind and sea state. If a description of location was given this was converted into a position. It was not possible to obtain any effort quantification. Incidental sightings were mostly made by sailors, motorboat drivers, fishermen, passengers on board ferries or people on the shore. In addition, a large amount of data were provided by official institutions such as the Customs Service, Coast Guard and Federal Border Guard recording opportunistic sightings during their surveillance cruises. All questionnaires of incidental sightings were entered into a database and subsequently analysed. Only sightings in the three survey blocks were included in this study for comparison with the aerial surveys data (see Fig. 1).
This data set yielded information on the occurrence and distribution of harbour porpoises in parts of the North and Baltic Seas, group size and their frequency, the relative number of mother-calf pairs and the seasonal distribution of the sightings. Sightings were plotted using ArcGIS Desktop 8.2. 2.3. Strandings and by-catches A stranding network, including a year-round observer scheme, was established on the German coast of Schleswig-Holstein and Mecklenburg-Vorpommern with members of several nature conservation organisations. Since 1990, patrolling efforts for carcasses by national park rangers, seal hunters and environmental organizations along the various beaches followed a standard procedure so that effort did not vary between months and years. Detailed information on the stranding network is provided by Benke et al. (1998). Animals were collected and necropsied by the FTZ in Schleswig-Holstein and by the German Oceanographic Museum in Mecklenburg-Vorpommern between 1990 and 2001. This systematic investigation of carcasses allowed the collection of data from stranded and by-caught animals and included gender and age class assessment. Wherever possible (North
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by glare was 26.8° and 23.0° in 1995, 14.6° and 20.1° in 1996 and 25.5° and 27.3° during the SCANS survey. Thus, in the 1995 and 1996 surveys, the subjective assessments ‘good’ and ‘moderate’ appeared to depend more heavily on sea state than during SCANS 1994.
Sea: 86%; Baltic Sea: 84% of all cases) strandings were assigned to a predefined sector of the German coastline. Porpoises were classified as calves, subadults or adults, based preferably on growth layer counts in teeth (Lockyer, 1995) or, in cases where this was not possible, on the length of the animal (see Siebert et al., 2001):
3.1.1. Effort and sightings by block Total time of survey effort flown in ‘good’ and ‘moderate’ conditions was 34.3 h (19.7 + 14.6h) in 1995 and 1996. Detailed survey effort per block as well as the percentage of survey time under ‘good’, ‘moderate’ and ‘poor’ conditions are shown in Table 1. Abundance was only calculated for survey effort under ‘good’ or ‘moderate’ conditions. Most sightings of harbour porpoises were recorded in North Sea block A (Fig. 2; 39 sightings in 1995 and 70 sightings in 1996). In the Baltic Sea, more harbour porpoises were sighted in block B (Fig. 3; 10 sightings in 1995 and 15 sightings in 1996) than in block C (Fig. 3; 6 sightings in 1995 and 0 sightings in 1996). Further details are provided in Tables 2 and 3.
(1) Calves: harbour porpoises between 0 and 1 y old, or ≤ 100 cm long; (2) Subadults: harbour porpoises between 1 and 4 y old, or between 101 and 125cm long; (3) Adults: harbour porpoises more than 4 y old, or longer than 125cm. Harbour porpoises defined as ‘by-catch’ were either taken in a net or were found stranded (later referred to as ‘stranded by-catch’) with clearly identifiable net marks or other lesions as described by Kuiken et al. (1994). However, as a larger number of strandings were in a state of advanced decomposition it was impossible to identify netmarks or other lesions. Therefore, a higher number of by-caught animals may have been among the decomposed animals.
3.1.2. Estimation of porpoise pod abundance A detailed description of the method used was provided by Hammond et al. (2002). Tables 2 and 3 show pod abundance estimated for the blocks from each replicate track and the mean values of these estimates weighted by the relative coverage in 1995 and 1996. Coverage was expressed relative to the area searched by completing the track in good conditions (esw = 130m). This is the baseline of calculating the weighted mean, in which the tracks that had a higher coverage percentage are given more weight in the mean animal abundance estimate, than the tracks with a lower coverage percentage. Each track is considered a sample and from each track overall abundance could be calculated. However, only by including several tracks/samples is it
3. Results 3.1. Baltic/North Sea aerial surveys in 1995 and 1996 The average sea state, according to the Beaufort scale, in ‘good’ and ‘moderate’ conditions was 1.07 and 2.03 in the 1995 survey, 0.85 and 1.96 in the 1996 survey, and 1.12 and 1.76 during the SCANS survey. Coding the assessment of turbidity as ‘clear’ = 1, ‘moderate’ = 2 and ‘turbid’ = 3, the average turbidity values were 1.30 and 1.42 in the 1995 survey, 1.51 and 1.58 in the 1996 survey and 1.74 and 2.07 during the SCANS survey. The extent of the sector affected
Table 1 Aerial survey period, hours surveyed, percentage of survey time flown in ‘good’, ‘moderate’ and ‘poor’ conditions, number of harbour porpoise pod sightings and number of mother-calf pairs Block
A B C
Survey period
Hours total
‘good’ (%)
‘moderate’ (%)
‘poor’ (%)
July 1995 July 1996 October 1995 July 1996 October 1995 July 1996
5.35 5.80 9.73 7.20 4.65 1.60
43 63 40 80 26 100
30 37 44 20 62 0
27 0 16 0 12 0
Pod sightings (n)
Mothercalf pairs (n)
33 70 10 15 6 0
4 13 1 6 0 0
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Fig. 2. Harbour porpoise sightings in block A during the aerial surveys in 1995 (n = 33) and 1996 (n = 70). The symbol size indicates group size of pod sightings. Map projection: Mercator.
Fig. 3. Harbour porpoise sightings in blocks B and C during the aerial surveys in 1995 (B, n = 10; C, n = 6) and 1996 (B, n = 15; C, n = 0). The symbol size indicates group size of pod sightings. Map projection: Mercator.
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Table 2 Aerial survey 1995 Block
Track no.
Pod sightings (n)
Relative coverage (%)
Pod abundance estimate for block
Weighted mean pod abundance estimate (95% CI)
Mean pod size estimate (CV)
Mean animal abundance (95% CI)
A
3 5 6 1 2 3 4 1 2 3 4 5
10 0 23 0 3 2 5 2 0 0 2 2
43 31 86 25 71 42 38 11 54 38 14 73
3639 0 4934 0 571 654 1871 2405 0 0 2225 556
3634 (1300–6000)
1.18 (.06)
4288 (1500–7800)
817 (300–2400)
1.20 (.11)
980 (360–2880)
514 (200–2300)
1.17 (.14)
601 (233–2684)
B
C
Pod abundance estimated for each block, mean of estimates weighted by the relative coverage based on 1995 surveys and derived mean animal abundance. CI = 95% Confidence interval; CV = Coefficient of variation.
possible to calculate a 95% confidence interval (CI). The larger the difference between tracks in terms of abundance, the larger the resulting confidence interval for the overall abundance. The last two columns show mean pod size and estimated mean animal abundance (Tables 2 and 3). The mean animal abundance in block A (North Sea) varied from 4288 (95% CI: 1500–7800) in 1995 to 7356 (95% CI: 5040–12 600) harbour porpoises in 1996. In the Baltic Sea, highest mean abundance was estimated for block B resulting in 980 (95% CI: 360– 2880) animals in the survey year1995 and 1830 (95% CI: 960–3 840) animals in 1996. The second survey block in the Baltic Sea (block C) yielded 601 (95% CI: 233–2 684) animals in 1995. There were no sightings in block C during the 1996 survey.
3.2. Incidental sightings 3.2.1. Platforms of opportunity From 1988 to 2002, 791 incidental sightings of harbour porpoise pods were recorded at the FTZ: 279 sightings were made in the Baltic Sea and 512 in the North Sea (Table 4). Sightings were mostly reported from coastal waters off Schleswig–Holstein (North and Baltic Sea) as well as along the coast of Mecklenburg–Vorpommern and around the Danish islands in the Baltic Sea. 3.2.2. Seasonal changes In the winter months (December–February, 1988– 2002), there were 50 incidental sightings in the North Sea, with a total of 88 harbour porpoises seen. Fig. 4 provides
Table 3 Aerial survey 1996 Block
Track no.
Pod sightings (n)
Relative coverage (%)
Pod abundance estimate for block
Weighted mean pod abundance estimate (95% CI)
Mean pod size estimate (CV)
Mean animal abundance (95% CI)
A
1 2 3 4 5 6 1 2 3 1 2 6
22 9 9 12 5 13 6 4 5 0 0 0
73 15 29 48 22 43 82 75 48 22 37 37
5417 13 353 5739 4807 4708 534 1126 844 1636 0 0 0
5838 (4000–10 000)
1.26 (.14)
7356 (5040–12 600)
1144 (600–2400)
1.60 (.10)
1830 (960–3840)
B
C
0 (0–0)
− (–)
− (–)
Pod abundance estimated for each block, mean of estimates weighted by the relative coverage based on 1996 surveys and derived mean animal abundance. CI = 95% Confidence interval; CV = Coefficient of variation.
U. Siebert et al. / Journal of Sea Research 56 (2006) 65–80 Table 4 Incidental sightings of harbour porpoises in the German North and Baltic Seas (1988 to 2002)
Number of sightings Number of porpoises Average group size (±SD) Maximum group size Number of calves Percentage of calves
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Table 5 Frequency of different group sizes sighted incidentally in the North and Baltic Seas (1988–2002)
North Sea
Baltic Sea
Group size
North Sea
Baltic Sea
512 1113 2.2 ± 3.2 40 59 5.3
279 689 2.5 ± 1.7 10 13 1.9
1 2 3–4 5–8 >8
235 (45.9%) 169 (33.0%) 87 (17.0%) 14 (2.7%) 7 (1.4%)
85 (30.5%) 100 (35.8%) 62 (12.1%) 27 (5.3%) 5 (1.0%)
information on the relative distribution of incidental sightings throughout the year. In July, the sightings (n=98; 26%) of harbour porpoises reached their maximum. 60% of the sightings, representing 53% of all counted porpoises, were noted between May and September. The distribution of observations in the Baltic Sea revealed a slightly different picture with a very low number of sightings in winter (Fig. 4). Only four sightings with a total of seven animals were reported. 85% of the sightings were noted from May to September. These sightings corresponded to 82% of the porpoises. In July and August, the number of sightings (n=73; n=56) and observed porpoises (n=114; n=130) reached its maximum, which is similar to the situation in the North Sea. 3.2.3. Group size and composition Platforms of opportunity accounted for 1802 harbour porpoises seen in the North Sea (n = 1113) and Baltic Sea (n = 689). Average group size was 2.5 (SD = 1.7) porpoises per sighting in the Baltic Sea and 2.2 (SD = 3.2) in the North Sea (Table 4).
The largest harbour porpoise group observed in the North Sea consisted of 40 animals on two occasions. Five groups of ten animals were seen in the Baltic Sea. In the whole period 59 calves were reported from the North Sea and 13 from the Baltic Sea (Table 4). Of the total number of 791 sightings of harbour porpoise groups in the North and Baltic Seas, 320 sightings consisted of a single animal, and in 269 cases two porpoises were seen. Groups of three to four animals (29.1%) were sighted comparatively often as well (Table 5). A difference in observed group size was detected between the North and Baltic Seas: most sightings (45.9%) in the North Sea consisted of a single animal, whereas most sightings in the Baltic Sea comprised two animals (35.8%). Most calves were reported in July (North Sea only), August and September (Fig. 5). Calves were registered in the North Sea more than twice as often as in the Baltic Sea in July and August. The number of calves started to decrease in August in the North Sea, while it peaked in the Baltic Sea during the same period (Fig. 5).
Fig. 4. Seasonal distribution of incidental sightings (%) of harbour porpoises in the German part of the Baltic and North Sea (1988–2002).
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Fig. 5. Seasonal occurrence of harbour porpoise calves observed from platforms of opportunity in the German part of the Baltic and North Sea (1988–2002).
3.2.4. Distribution The positions of incidental sightings in the period 1988 to 2002 are presented in Fig. 6. These encompass all sightings made from ships including a
position reference (North Sea: 443 sightings, Baltic Sea: 197 sightings). In the North Sea the majority of sightings were made around the islands of Sylt and Amrum (Fig. 6).
Fig. 6. Incidental sightings of harbour porpoises in the German North Sea (block A, n = 443) and Baltic Sea Sea (block B, n = 188; block C, n = 9), 1988 to 2002. Map projection: Mercator.
U. Siebert et al. / Journal of Sea Research 56 (2006) 65–80 Table 6 Strandings of harbour porpoise in the German North Sea and Baltic Sea (1990–2001) North Sea Baltic Sea Total Number of strandings and by-catches 1013 By-catches (%) 1.7 Calves (%) 43.1 Subadults (%) 22.2 Adults (%) 34.7 Males (%) 49.4 Females (%) 50.6
334 32.3 34.0 33.7 32.3 50.4 49.6
1347 9.3 40.7 25.2 34.1 49.6 50.4
In the Baltic block B, most porpoises were observed in the Little Belt and around the island of Fyn. In the German part of the Baltic Sea harbour porpoises were mostly observed between Flensburg Bight and Eckernförde as well as west and northwest of the island of Fehmarn (Fig. 6). In the eastern part of block B only sporadic sightings were registered and only nine sightings were reported from block C. 3.3. Strandings and by-catches 3.3.1. North Sea 996 harbour porpoises were found stranded along the coast of Schleswig–Holstein and 17 animals were
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identified as by-catch (1990–2001; Table 6). 457 were females and 446 were males. The rest (n = 110) remained unidentifiable with respect to sex. The proportions of females and males did not vary between years (χ2homogeneity test; 12 y; N = 903, χ 2 = 7.8, df = 11, P = 0.73). The total number of strandings was clearly larger in the North Sea (Binomial test of 1,013:334: z = 18.74, p < 0.0001). Fig. 7 shows that most of the stranded animals (n=517) were found on the North Frisian island of Sylt. The overall number of reported strandings decreased from north to south, as did the proportion of calves (Spearman's rank correlation coefficient, rS = 0.71, N = 11 islands, P = 0.015). Around the island of Sylt 237 (47.4%) of the stranded carcasses were calves, 98 (19.6%) were subadults and 165 (33.0%) were adults. The island south of Sylt, Amrum, showed a higher proportion of adults (46.6%; n = 34), while subadults made up 13.7% (n = 10) and calves accounted for 39.7% (n = 29) of all strandings (Fig. 7). Seventeen porpoises were identified as by-catches (six males and eleven females). They were all reported from 1991 to 1994 and in 1996 (Fig. 8). Six of these were calves, seven were subadults and four were adults. Fifteen animals were found dead on the beach as stranded by-catch either showing characteristic netmarks or
Fig. 7. Strandings of harbour porpoises on the German coast of the North Sea (1990–2001). The number next to the symbols shows the total number of stranded harbour porpoises. The symbol indicates the relative proportion of the three different age classes. S = Sylt, F = Föhr, A = Amrum, B = Büsum, H = Helgoland. Map projection: Mercator.
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Fig. 8. Temporal changes in the number of by-catches and strandings of harbour porpoises in the German part of the Baltic (BS) and North Sea (NS).
entangled in part of a net. Two harbour porpoises were found dead in a bottom-set gillnet near the island of Helgoland (January 1994). After 1996 no by-catches were identified in the German Bight (Fig. 8). The mean reported annual by-catch of harbour porpoises between 1990 and 2001 was 1.4 animals (SD = 2.1). Strandings were found year round. Along the North Sea coast, most animals were found in the summer months while the smallest number of strandings was discovered from November to March (Fig. 9). There was a clear periodicity in the number of animals found, indicating systematic seasonal variation (clear and significant peaks of linear auto-correlation-function of monthly data at twelve months, Pearson's correlation
coefficient rP = 0.50, N = 132mo, P < 0.001). Stranded calves were mainly found in June and July (Fig. 10). 3.3.2. Baltic Sea The annual number of strandings was much smaller in the Baltic than in the North Sea. From 1990 to 2001, a total of 229 harbour porpoises (81 females, 80 males) were found stranded along the Baltic shores of Schleswig–Holstein and Mecklenburg–Vorpommern and 105 animals (50 females, 53 males) were by-caught. The rest (n = 70) remained unidentifiable with respect to sex. Again, the proportions of females and males did not vary between years (χ2-homogeneity test; 12y; N = 264, χ2 = 11.36, df = 11, P = 0.41).
Fig. 9. Monthly distribution of strandings (1990–2001) in the German part of the Baltic and North Sea, shown as percentage (%) relative to the total number of strandings in the two areas.
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Fig. 10. Monthly distribution of calves on strandings (1990–2001) in the German part of the Baltic and North Sea, shown as relative amounts (%).
Most strandings occurred along the Fischland–Darss peninsula in Mecklenburg–Vorpommern (n = 40), near Flensburg (n = 37) and on the western shores of Fehmarn
in Schleswig–Holstein (n = 31). Relatively few calves, but a high proportion of subadults stranded along the Baltic coastline as compared to the North Sea (Fig. 11,
Fig. 11. Strandings of harbour porpoises on the German coast of the Baltic Sea (1990–2001). The number next to the symbols shows the total number of stranded harbour porpoises. The symbol indicates the relative proportion of the three different age classes. F = Flensburg, S = Schlei estuary, E = Eckernförde, K = Kiel, FH = Fehmarn, D = Fischland-Darss. Map projection: Mercator.
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Table 6). Proportions of age classes clearly differed between years in the North Sea (N = 712 individuals, χ2 = 63.77, df = 22, P < 0.0001) and tended to do so in the Baltic Sea (N = 299 individuals, χ2 = 32.16, df = 22, P = 0.075). The highest number and proportion of calves (n = 20, 54.1%) occurred near the outflow of the Schlei Fjord. Only a small number of calves stranded east of the island of Fehmarn. In Mecklenburg–Vorpommern, the strandings showed a fairly equal proportion of calves, subadults and adults. The proportion of by-caught harbour porpoises was clearly larger in the Baltic Sea than in the North Sea (χ2 = 280.4, df = 1, P < 0.0001). The by-caught porpoises, either reported by fishermen or coast guards (n = 55), were mostly found in bottom-set gillnets, on the beach with characteristic netmarks or entangled in part of a net (n = 50). Most animals taken incidentally were subadults (n = 64), but 27 adults und 12 calves were reported as by-catch as well. The mean annual bycatch rate of harbour porpoises between 1990–2001 based on the animals which were delivered or identified as by-catches was 8.8 animals (SD = 5.1). The by-catch of porpoises occurred in every year of the study period. The highest number of by-catches in relation to stranded animals occurred in 1990 and 1991. Subsequently, by-catch never exceeded the number of strandings (Fig. 8). Strandings were reported in every month. Comparison of the seasonal distribution of strandings (Figs. 9 and 10) indicated that the birth season in the Baltic Sea was about one month later than in the North Sea (maximum of cross-correlation function of monthly data at a lag of one month, all data: rP = 0.46, N = 134mo, P < 0.001; calves only: rP = 0.39, N = 134 mo, P < 0.001). 4. Discussion 4.1. Harbour porpoise in the German North and Baltic Seas The analyses of aerial surveys, incidental sightings and strandings all showed that the density of harbour porpoises in the Baltic was lower than in the North Sea study block. The number of mother-calf pairs was lower as well with a percentage of calves of 1.9 in the Baltic Sea compared to 5.3 in the North Sea. Furthermore, in the Baltic, the density of harbour porpoises decreased from west towards the east, confirming results from earlier surveys (Heide-Jørgensen et al., 1992, 1993; Benke et al., 1998; Hammond et al., 2002; Berggren et al., 2003; Scheidat et al., 2004). Information on incidental sightings from recreational boats (e.g. sailing
boats) was biased towards the west, where considerably more sailing takes place than in the east. The decline in abundance of harbour porpoises towards the east was, however, not obvious from strandings. Results of the stranding scheme indicated that similar numbers of porpoises were found along the coasts of Mecklenburg–Vorpommern and of Schleswig–Holstein. A high proportion of the strandings were calves. It is unclear where these porpoises came from. The direction and velocity of currents in the western Baltic suggest that some of these animals originated further north (e.g. Danish waters). Several hot spots of strandings were found in the Baltic Sea. These included the Schlei estuary, the area west of the island of Fehmarn in Schleswig–Holstein, which acted as a barrier to water pushed eastward by the prevailing westerly winds, and the area of the Fischland–Darss peninsula in Mecklenburg–Vorpommern. The highest number of stranded calves was found in the Schlei area, followed by the Fischland–Darss and Fehmarn areas. Most strandings of harbour porpoises along the shores of the North Sea occurred from June to August. In this period the proportion of calves in strandings was highest as well. In the same period the highest number of incidental sightings occurred. The high number of incidental sightings in this area as well as strandings around the islands of Sylt and Amrum confirmed that this area is often and regularly frequented and thus might be an important habitat for breeding harbour porpoises (Sonntag et al., 1999). This observation was reinforced by the high proportion of mother-calf pairs in the summer months, confirmed by the results of aerial surveys and incidental sightings. Recently conducted aerial surveys are complementing this picture with high densities of harbour porpoises observed close to the Danish border and in the area of Amrum Outerbank (Scheidat et al., 2004). In the 11-y study period, significantly more strandings of harbour porpoises occurred along the shores of the North Sea than the Baltic Sea. In the North Sea area most animals stranded on the northerly beaches of Schleswig–Holstein, especially on islands that may have functioned as a barrier to carcasses drifting northward with the residual coastal current (Otto et al., 1990). Only 10% of the strandings in the North Sea occurred south of 54°20′N. By-catch clearly dominated in the Baltic Sea. The number of porpoises incidentally taken in fisheries was higher in the western part of Kiel Bight than in its eastern part. However, fishermen in the western part were much more cooperative to report by-catches
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(Pfander, pers. comm., 2004). As reported earlier (Kinze, 1994; Benke et al., 1998) more subadults than adults were taken incidentally in fishing gear, for unknown reasons. Among strandings, adults and calves dominated in the North Sea, whereas mostly calves and subadults were found stranded in the Baltic Sea. Reliable information on by-catch in fishery can only be obtained if independent observer schemes are established (Northridge and Hammond, 1999). The by-catch of harbour porpoises in gillnet fisheries in the North Sea at large before 2000 was substantial. Catches in the range of several thousand harbour porpoises were reported in the Danish gillnet fishery (Vinther, 1999; Vinther and Larsen, 2004). The British gillnet fisheries took an estimated 400–800 porpoises per year (ASCOBANS, 2002). Catches were comparatively low with 30–50 porpoises annually in the small German gillnet fleet (Kock and Flores, 2003). The by-catch has probably declined since 2000 when the activities in the Danish and British gillnet fleets declined (Northridge and Hammond, 1999; Vinther and Larsen, 2004). The present level of by-catch is unknown. It appears that by-catch was the primary cause of death in the Baltic Sea while infectious diseases and perinatal death predominated in the North Sea (Siebert et al., 2001; Wünschmann et al., 2001). The identification of by-catches among strandings is difficult as some net types do not cause netmarks. Additionally, characteristics such as netmarks, haemorrhages (Kuiken et al., 1994) etc. disappear with progressing decomposition of the carcass. Taking into account the high by-catch rates from the North Sea at large (Northridge and Hammond, 1999; Vinther and Larsen, 2004) it is very likely that the number of by-catches is underestimated at necropsies because of the state of preservation of a large number of carcasses. The monthly distribution of strandings and incidental sightings in the Baltic suggested a strong seasonality with the highest occurrence of porpoises in summer. The same pattern is apparent in different areas of the North Sea (Benke et al., 1998; Lockyer, 2003; Hasselmeier et al., 2004). In the Baltic, the number of strandings and the occurrence of calves or juveniles peak about one month later than in the North Sea (Hasselmeier et al., 2004). Harbour porpoises of the North and Baltic Seas form distinct subpopulations with little genetic exchange (Tiedemann et al., 1996; Andersen et al., 1997; Huggenberger et al., 2002). Harbour porpoises in the Baltic Sea in turn belong to two subpopulations: the Baltic Proper (or central Baltic Sea) subpopulation, with the Darss sill as the western limit, and the Western Baltic
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subpopulation including the area of the Kattegat, Belt Seas, Øresund, Kiel Bight, Lübeck Bight and the Fehmarn Belt (Tiedemann et al., 1996; Andersen et al., 1997; Larsen et al., 2000; Huggenberger et al., 2002). Given the limited exchange between the subpopulations, the by-catch numbers reported for the Baltic Sea are at an unacceptably high level, threatening the survival of these populations (Berggren et al., 2002). First steps to reduce by-catch, such as improved reporting schemes and the use of pingers, have been taken at a European level. Success of these measures remains to be seen. 4.2. Comparison of methods 4.2.1. Aerial surveys in 1995 and 1996 Aeroplanes have often been used to conduct linetransect surveys of cetaceans. The method has its merits in areas of comparatively high density: it is effective in obtaining information on the distribution and abundance of harbour porpoises. The advantage of aerial surveys over ship-based surveys is that large areas can be covered in less time and that it is easier to react quickly to appropriate weather conditions. One disadvantage is that it is not possible to perform acoustic surveys or to collect biotic and abiotic data at the same time (e.g. salinity, sea surface temperature etc.). Additionally, the method has its shortcomings in regions where the abundance of animals is low, such as the central Baltic Sea. Other methods such as ship-based acoustic surveys with towed hydrophones or the deployment of a net of passive acoustic monitoring devices, so called ‘PODs’ (porpoise detectors) may be more informative in such areas. 4.2.2. Incidental sightings The incidental sightings recorded since 1988 are a valuable source of additional information to obtain a seasonal and temporal overview of the distribution of harbour porpoises along the German coast of the North and the Baltic Seas. However, a number of shortcomings need to be taken into consideration: sightings are qualitative measurements collected in an unsystematic way; operational constraints among boat operations vary (e.g. ferries, recreational boats); species, mother-calf pairs and group size may be misidentified. Mean group size may be overestimated because larger groups are easier to detect than single porpoises; the heterogeneity of these data precludes any rigid statistical analysis. The distribution of sighting effort is not equal for all areas and all seasons as areas of increased boat traffic exist in both seas (e.g. in the Wadden Sea),
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especially of recreational boats. Correcting for sighting effort is impossible as data on the frequency and distribution of recreational boats are missing. Nevertheless, patterns from incidental sightings are similar to those obtained from aerial surveys and the stranding network. 4.2.3. Strandings and by-catches Strandings data contain useful information on spatial and seasonal distributions of harbour porpoises (Polacheck et al., 1995). Furthermore, they allow the collection of additional information required for management and protection issues. This includes data on genetic and age structure, assessment of health and reproductive status, information on nutrition and chemical burden and mortality rates. Furthermore, the occurrence of other cetacean species may be noticed best from stranding records. Patrolling efforts for carcasses along various beaches have been following a standard procedure since 1990 and consequently have not varied between months and years. This is very important to ensure that yearly changes or trends are not artificially produced by varying efforts. Analysis of stranding records yield limited information on trends of abundance. For instance, the origin of the carcasses is unclear because strandings depend on various oceanographic features (e.g. currents and wind, which may vary between years). When westerly winds prevail, more dead animals are washed ashore on the German North Sea coast. Also, carcasses of harbour porpoises tend to sink to the bottom, and resurfacing time varies from a few days to several months depending on water temperature and currents (Moreno et al., 1992). Analysis of stranding information mirrored the analyses of the aerial surveys in that harbour porpoises are considerably more abundant around the North Frisian Islands than further south. In summary, the most appropriate methods for investigating abundance and distribution of cetaceans are dedicated aerial and ship surveys. Standard linetransect methods are used to estimate the number of animals in a given area based on the distance sampling data thus collected. Preferably, the survey should be designed for harbour porpoises only and not be combined with other surveys, at least for areas of high density. Good survey conditions are rare for the German Exclusive Economic Zones in the Baltic and North Sea. Aerial surveys can be conducted in a flexible way and even small time windows of low wind speeds can be used for survey flights. Therefore, aerial surveys were found to be the best method for survey work in the
coastal areas of the German waters. However, planes are limited in their range of action and thus can cover offshore areas only with difficulty. For such areas, e.g. the Dogger Bank, ship surveys may be more suited to conduct abundance estimates. To detect seasonal distribution, surveys should be conducted at least four times per year. To accurately interpret the results of local surveys it is necessary to conduct large-scale surveys that include the entire population of harbour porpoises. The SCANS survey conducted in 1994 was such a survey and a repeat took place in 2005. The use of the line-transect method is problematic in the Baltic Proper as harbour porpoise density is very low. New survey techniques have been developed for those areas covered under the current MINOS+ project (Marine warm-blooded animals in the North and Baltic Seas: Foundations for assessment of offshore wind farms; funded as part of the German government's research focus on renewable energies; http://www.minos-info.org) and tested in SCANSII in 2005. The methods developed and tested in SCANSII included the use of passive acoustics and vessels of opportunity, and recommended a suite of monitoring protocols modified according to species and area. Strandings data yield useful information on spatial and seasonal distribution and the occurrence of rare species. In addition, strandings networks provide a unique opportunity for data collection on the biology and health status parameters from tissue sampling. Other data such as genetic structure, reproductive status, food intake or anthropogenic impact are important for the management of harbour porpoise populations which, in combination with information on distribution and abundance, allow protection of the animals. Incidental sightings are valuable in areas where no or very limited specific surveys are conducted, such as the Mediterranean or Black Sea. In German waters they are a valuable additional source of information but of limited scientific potential. However, engaging interested individuals in this cost-effective reporting scheme helps foster public awareness for marine mammals and may result in stronger support for scientific study and political action to protect the sea. Acknowledgements We wish to thank Lex Hiby and Phil Lovell, Conservation Research Ltd. in Cambridge, for their help with the aerial survey data. Our thanks go to the pilots of the survey planes, especially Leif Petersen from the Danish Air Survey, and the observers. Thanks to Nils Koesters for rescuing some of the data and thanks
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to all contributors of the stranding network and the incidental sightings reporting scheme in Schleswig– Holstein. We thank Gerhard Schulze, German Oceanographic Museum, for collecting data on strandings of harbour porpoises in Mecklenburg–Vorpommern, as well as the Society for the Conservation of Marine Mammals (GSM) for collecting sighting data in the Baltic Sea. The comments of Krishna Das, Helena Herr, Ursula Verfuß, Franciscus Colijn and three anonymous reviewers are greatly appreciated. The various studies were funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, the German Federal Ministry for Education and Research, and the Ministry for Environment, Nature Conservation and Agriculture of Schleswig–Holstein. References Andersen, L.W., Holm, L.-E., Siegismund, H.R., Clausen, B., Kinze, C. C., Loeschcke, V., 1997. A combined DNA-micro-satellite and isozyme analysis of the population structure of the harbour porpoise in Danish waters and West Greenland. Heredity 78, 270–276. ASCOBANS, 2002. Incidental take of small cetaceans. Report of the Ninth Meeting of the Advisory Committee to ASCOBANS, Bonn 2002, unpublished. http://www.ascobans.org/files/finalres6.pdf. Benke, H., Siebert, U., Lick, R., Bandomir, B., Weiss, R., 1998. The current status of harbour porpoises (Phocoena phocoena) in German waters. Arch. Fish. Mar. Res. 46, 97–123. Berggren, P., Arrhenius, F., 1995. Densities and seasonal distribution of harbour porpoises (Phocoena phocoena) in the Swedish Skagerrak, Kattegat and Baltic Seas. In: Bjørge, A., Donovan, G. P. (Eds.), Biology of the Phocoenids. Rep. Int. Whal. Comm., Spec. Issue, vol. 16, pp. 109–121. Berggren, P., Wade, P.R., Carlström, J., Read, A.J., 2002. Potential limits to anthropogenic mortality for harbour porpoises in the Baltic region. Biol. Cons. 103, 313–322. Berggren, P., Hiby, L., Lovell, P., Scheidat, M., 2003. Abundance of harbour porpoises in the Baltic Sea from aerial surveys conducted in summer 2002. Working paper presented at the ASCOBANS meeting, Bonn, Germany, April 2003. 17 pp. Buckland, S.T., Anderson, D.R., Burnham, K.P., Laake, J.L., Borchers, D.L., Thomas, L., 2001. Introduction to Distance Sampling. Estimating Abundance of Biological Populations. University Press, Oxford. Das, K., Siebert, U., Fontaine, M., Jauniaux, T., Holsbeek, L., Bouquegneau, J.M., 2004. Ecological and pathological factors related to trace metal concentrations in harbour porpoises Phocoena phocoena from the North Sea and adjacent areas. Mar. Ecol. Prog. Ser. 281, 283–295. Hammond, P.S., Berggren, P., Benke, H., Borchers, D.L., Collet, A., Heide-Jørgensen, M.P., Heimlich, S., Hiby, A.R., Leopold, M.F., Øien, N., 2002. Abundance of harbour porpoise and other cetaceans in the North Sea and adjacent waters. J. Appl. Ecol. 39, 361–376. Hasselmeier, I., Abt, K.F., Adelung, D., Siebert, U., 2004. Stranding patterns of harbour porpoises (Phocoena phocoena) in the German North and Baltic Seas; when does the birth period occur? J. Cet. Res. Mang. 6, 259–263.
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Heide-Jørgensen, M.P., Mosbech, A., Teilmann, J., Benke, H., Schultz, W., 1992. Harbour porpoise (Phocoena phocoena) densities obtained from aerial surveys north of Fyn and in the Bay of Kiel. Ophelia 35, 133–146. Heide-Jørgensen, M.P., Mosbech, A., Teilmann, J., Benke, H., Wulf, J., 1993. Abundance and distribution of harbour porpoises Phocoena phocoena in selected areas of the western Baltic and the North Sea. Helgol. Meeresunters. 47, 335–346. Hiby, A.R., Lovell, P., 1998. Using aircraft in tandem formation to estimate abundance of harbour porpoise. Biometrics 54, 1280–1289. Huggenberger, S., Benke, H., Kinze, C.C., 2002. Geographical variations in harbour porpoise (Phocoena phocoena) skulls; support for a separate non-migratory population in the Baltic proper. Ophelia 56, 1–12. Jepson, P.D., Baker, J.R., Allchin, C.R., Law, R.J., Kuiken, T., Baker, J.R., Rogan, E., Kirkwood, J.K., 1999. Investigating potential associations between chronic exposure to polychlorinated biphenyls and infectious disease mortality in harbour porpoises from England and Wales. Sci. Total Environ. 243/244, 339–348. Jepson, P.D., Arbelo, M., Deaville, R., Patterson, I.A.P., Castro, P., Baker, J.R., Degollada, E., Ross, H.M., Herráez, P., Pocknell, A.M., Rodríguez, F., Howie, F.E., Espinosa, A., Reid, R.J., Jaber, J.R., Martin, V., Cunningham, A.A., Fernández, A., 2003. Gas-bubble lesions in stranded cetaceans. Nature 425, 575–576. Kinze, C.C., 1994. Incidental catches of harbour porpoises (Phocoena phocoena) in Danish waters 1986–89. Rep. Int. Whal. Comm., Spec. Issue 15, 189–202. Kock, K.H., Benke, H., 1996. On the by-catch of harbour porpoise (Phocoena phocoena) in German fisheries in the Baltic and the North Sea. Arch. Fish. Mar. Res. 44, 95–114. Kock, K.-H., Flores, H., 2003. Fang und Beifang der deutschen Stellnetzfischerei in der Nordsee. Projektbericht, Ministerium für Verbraucherschutz, Ernährung und Landwirtschaft 1–52 (available through K.-H. Kock), unpublished. Kuiken, T., Simpson, V.R., Allchin, C.R., Bennet, P.M., Codd, G.A., Harris, E.A., Howes, G.J., Kennedy, S., Kirkwood, J.K., Law, R.J., Merrett, N.R., Phillips, S., 1994. Mass mortality of common dolphins (Delphinus delphis) in south west England due to incidental capture in fishing gear. Vet. Rec. 134, 81–89. Larsen, F., Teilmann, J., Desportes, G., 2000. Movements of harbour porpoises (Phocoena phocoena) in Danish and adjacent waters. In: J. Teilmann (Eds.), The behaviour and sensory abilities of harbour porpoises (Phocoena phocoena) in relation to bycatch in gillnet fishery. PhD thesis, Univ. Southern Denmark, Odense, unpublished. Lockyer, C., 1995. A review of factors involved in zonation in odontocete teeth, and an investigation of the likely impact of environmental factors and major life events on harbour porpoise tooth structure. In: Bjørge, A., Donovan, G.P. (Eds.), Biology of the Phocoenids. Rep. Int. Whal. Comm. Spec., Issue, vol. 16, pp. 511–527. Lockyer, C., 2003. Harbour porpoises (Phocoena phocoena) in the North Atlantic: Biological parameters. In: Haug, T., Desportes, G., Vikingsson, G.A., Witting, L. (Eds.), Harbour Porpoises in the North Atlantic. NAMMCO Sci. Publ., vol. 5, pp. 71–90. Moreno, P., Benke, H., Lutter, S., 1992. Behaviour of harbour porpoise (Phocoena phocoena) carcasses in the German Bight: surfacing rate, decomposition and drift routes. In: Bohlken, H., Benke, H. (Eds.), Untersuchungen über Bestand, Gesundheitszustand und Wanderungen der Kleinwalpopulationen (Cetacea) in deutschen Gewässern. Interim Rep., Institut für Haustierkunde, Universität
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Kiel, Germany, unpublished, available through FTZ, Büsum, Germany. Northridge, S.P., Hammond, P.S, 1999. Estimation of porpoise mortality in UK gill and tangle net fisheries in the North Sea and west of Scotland. Paper for the International Whaling Commission, Scientific Committee SC/51/SM42. Otto, L., Zimmermann, J.T.F., Furnes, G.K., Mork, M., Saetre, R., Becker, G., 1990. Review of the physical oceanography of the North Sea. Neth. J. Sea Res. 26, 161–238. Polacheck, T., Wenzel, F.W., Early, G., 1995. What do stranding data say about harbor porpoises (Phocoena phocoena)? In: Bjørge, A., Donovan, G.P. (Eds.), Biology of the Phocoenids. Rep. Int. Whal. Comm., Spec. Issue, vol. 16, pp. 169–179. Scheidat, M., Kock, K.-H., Siebert, U., 2004. Summer distribution of harbour porpoise (Phocoena phocoena) in the German North and Baltic Sea. J. Cet. Res. Mang. 6, 251–257. Schulze, G., 1996. Die Schweinswale. Neue Brehm Bücherei, Magdeburg. Siebert, U., Joiris, C., Holsbeek, L., Benke, H., Failing, K., Frese, K., Petzinger, E., 1999. Potential relation between mercury concentrations and necropsy findings in cetaceans from German waters of the North and Baltic Seas. Mar. Pollut. Bull. 38, 285–295.
Siebert, U., Wünschmann, A., Weiss, R., Frank, H., Benke, H., Frese, K., 2001. Post-mortem findings in harbour porpoises (Phocoena phocoena) from the German North and Baltic Seas. J. Comp. Path. 124, 102–114. Sonntag, R.P., Benke, H., Hiby, A.R., Lick, R., Adelung, D., 1999. Identification of the first harbour porpoise (Phocoena phocoena) calving ground in the North Sea. J. Sea. Res. 41, 225–232. Tiedemann, R., Harder, H., Gmeiner, C., Haase, E., 1996. Mitochondrial DNA sequence patterns of harbour porpoises (Phocoena phocoena) from the North and Baltic Sea. Z. Säugtierkunde 61, 104–111. Vinther, M., 1999. Bycatches of harbour porpoises (Phocoena phocoena L.) in Danish set-net fisheries. J. Cet. Res. Manag. 1, 123–135. Vinther, M., Larsen, F., 2004. Updated estimates of harbour porpoise (Phocoena phocoena) by-catch in the Danish North Sea bottom-set gillnet fishery. J. Cet. Res. Manag. 6, 19–24. Wünschmann, A., Siebert, U., Frese, K., Weiss, R., Lockyer, C., Heide-Jørgensen, M.P., Müller, G., Baumgärtner, W., 2001. Evidence of infectious diseases in harbour porpoises (Phocoena phocoena) hunted in the waters of Greenland and by-caught in the German North Sea and Baltic Sea. Vet. Rec. 148, 715–720.
A decade of harbour porpoise occurrence in German waters— Analyses of aerial surveys, incidental sightings and strandings Ursula Siebert a,⁎, Anita Gilles a , Klaus Lucke a , Martje Ludwig a , Harald Benke b , Karl-Hermann Kock c , Meike Scheidat a a
Research and Technology Centre, Christian-Albrechts-University of Kiel, Hafentörn 1, 25761 Büsum, Germany b German Oceanographic Museum, Katharinenberg 14-20, 18439 Stralsund, Germany c Federal Research Centre for Fisheries, Palmaille 9, 22767 Hamburg, Germany Received 22 March 2005; accepted 26 January 2006 Available online 14 March 2006
Abstract Data on the occurrence of harbour porpoises (Phocoena phocoena) in German waters from 1988 to 2002 were collected from dedicated aerial surveys, incidental sightings and strandings. Aerial surveys conducted in 1995 and 1996 revealed a mean abundance of 4288 (in 1995) and 7356 harbour porpoises (in 1996) in the German North Sea study area. Mean abundances of harbour porpoises in the German Baltic Sea, divided into two subunits (blocks B and C), were estimated at 980 and 1830 (in 1995 and 1996 resp.) and at 601 (in 1995; there were no sightings in block C during the 1996 survey). From 1988 to 2002, 791 incidental sightings of harbour porpoise pods were reported in German and partly Danish coastal waters of the North and Baltic Seas. In the period 1990 to 2001, 996 harbour porpoises were found stranded along the German North Sea coast and 17 animals were identified as by-catch. In the same period 229 harbour porpoises were found stranded along the German Baltic Sea coast and 105 animals were incidentally taken in fisheries. The proportion of by-caught harbour porpoises was significantly larger in the Baltic Sea. Different monitoring methods are helpful for different aims and management issues: aerial surveys cover large areas in a short time and provide information on density, abundance, distributional patterns and seasonality. Incidental sighting and stranding networks provide indications of general distribution, seasonal variation in abundance, age distribution, by-catch and of areas which are important in the harbour porpoise’s life cycle. Comparison of data from the North and Baltic Seas revealed a higher abundance of harbour porpoises in the North Sea than in the Baltic Sea. Altogether the data sets demonstrated a strong seasonality of harbour porpoise occurrence off the German coast with highest numbers during the summer months. Important habitats for harbour porpoises were detected west of the islands of Sylt and Amrum in the North Sea and around the Schlei estuary, in waters west of Fehmarn and the Fischland-Darss area in the Baltic Sea. © 2006 Elsevier B.V. All rights reserved. Keywords: Phocoena phocoena; Aerial surveys; Incidental sightings; Strandings; By-catch; North Sea; Baltic Sea
1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (U. Siebert). 1385-1101/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2006.01.003
Aerial and shipboard surveys, incidental sightings and strandings all show that the harbour porpoise (Phocoena phocoena) is by far the most common cetacean in the German parts of the North and Baltic Seas
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(Benke et al., 1998; Hammond et al., 2002; Scheidat et al., 2004). Yet, the status of populations in these waters remained unclear. There is reason for concern as several national and international research projects indicate that harbour porpoise numbers in the central Baltic Sea have decreased dramatically over the last 50 years (Berggren and Arrhenius, 1995; Schulze, 1996; Benke et al., 1998; Hammond et al., 2002). The major cause of unnatural harbour porpoise death in the North and Baltic Seas is thought to be incidental mortality in fishing gear, primarily in bottom-set gillnets (Kock and Benke, 1996; Vinther, 1999; Vinther and Larsen, 2004). Other anthropogenic activities, such as chemical and noise pollution (e.g. caused by the construction of offshore windmill farms or the use of military low frequency sonar) may affect harbour porpoise adversely as well (Jepson et al., 1999, 2003; Siebert et al., 1999). To protect the harbour porpoise as required by several international agreements (ASCOBANS, OSPARCOM, HELCOM) and to identify appropriate management measures for the harbour porpoise stocks in the North and Baltic Seas, the occurrence, abundance and distribution of these animals need to be known. The ASCOBANS parties pursue these objectives with a range of approaches (see ASCOBANS (2002) national reports: http://www.ascobans.org). Stranding networks around the North and Baltic Seas currently exist in Belgium, the Netherlands, Germany, the United Kingdom, Poland and Sweden. Incidental sighting schemes were established in the Netherlands, the United Kingdom, Germany, Denmark, Poland and Sweden. Dedicated aerial and shipboard surveys only for cetaceans or surveys in combination with bird or oil pollution surveys, or surveys in connection with the planning of offshore windfarms have been conducted by the Netherlands, the United Kingdom, Germany, Denmark, Sweden and Poland. Overall Germany has invested most in harbour porpoise surveys over the last years, resulting in a leading position. The first systematic NW European survey for small cetaceans (SCANS) was conducted in 1994 (Hammond et al., 2002), and a repeat took place in July 2005. In Germany research projects were funded by federal ministries and the states of Schleswig-Holstein and Mecklenburg-Vorpommern with the intent to gather knowledge on the status of harbour porpoises in German near-shore waters and to comply with legal requirements. In addition to the assessment of health status (Siebert et al., 1999, 2001; Wünschmann et al., 2001; Das et al., 2004), genetic structure (Tiedemann et al., 1996; Huggenberger et al., 2002) and reproduction status
(Benke et al., 1998), projects were designed to obtain data on the occurrence and abundance of harbour porpoises in waters off Schleswig-Holstein and Mecklenburg-Vorpommern. This paper summarises data from stranding networks, incidental sightings and dedicated aerial surveys gathered in the period 1988 to 2002. The major aims were to compare and discuss the different methods and to recommend the appropriate choice for future monitoring programmes. 2. Methods 2.1. Baltic/North Sea aerial surveys in 1995 and 1996 Aerial surveys were conducted on pre-designed zigzag track lines (Fig. 1) in the western Baltic Sea and in the eastern North Sea in 1995 (July, October) and in 1996 (July). Standard line-transect methodology was used to estimate abundance (Buckland et al., 2001) of porpoise pods in three survey blocks (A, B and C; Fig. 1). We anticipated that sighting rates would be too low to allow effective strip width (esw) to be estimated using the tandem survey technique (Hiby and Lovell, 1998). As one of the two observers of the 1995 and 1996 surveys had also participated in SCANS 1994 and survey methods were the same, the effective strip widths estimated during the SCANS survey were also applied to the present results to estimate abundance. The data were stratified by the subjective sighting condition assessments ‘good’ and ‘moderate’ and 130m esw for good conditions and 80m esw for moderate conditions were used. The surveys were conducted with a high-winged aeroplane equipped with bubble windows in order to allow unobstructed observation of the water surface on both sides, from the abeam line forward to the track line. The method for designing the cruise tracks as well as the data collection method and the calculation of porpoise pod abundance and 95% confidence intervals (CI) were described in detail in Hammond et al. (2002). The survey blocks cover the coastal waters of Germany (except Lower Saxony). Using pseudo-random starting positions, several replicate zigzag tracks were constructed in each block. The selection of a single track would thus provide non-zero coverage probability for any point in the block. A mean abundance estimate with confidence limits was derived by surveying replicate tracks. 2.2. Incidental sightings In the late 1980s, the Research and Technology Centre West Coast (FTZ) in Büsum (Germany) developed a handout for the identification of whale
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Fig. 1. Aerial survey areas. Block A is situated in the North Sea, blocks B and C in the Baltic Sea. The lines indicate the zigzag transect lines, of which there were six in block A, four in block B and six in block C. Map projection: Mercator.
sightings from platforms of opportunity. This handout included information on the species (particularly a description of the harbour porpoise) as well as a questionnaire used to record the opportunistic sightings in more detail. The questionnaire was filled out by members of the public who made an incidental observation of harbour porpoises or other cetacean species and sent back to the FTZ. The data recorded included date and time, position of observation, pod size and number of calves (half the size of the adults), identification and behaviour of the whales, weather conditions as well as wind and sea state. If a description of location was given this was converted into a position. It was not possible to obtain any effort quantification. Incidental sightings were mostly made by sailors, motorboat drivers, fishermen, passengers on board ferries or people on the shore. In addition, a large amount of data were provided by official institutions such as the Customs Service, Coast Guard and Federal Border Guard recording opportunistic sightings during their surveillance cruises. All questionnaires of incidental sightings were entered into a database and subsequently analysed. Only sightings in the three survey blocks were included in this study for comparison with the aerial surveys data (see Fig. 1).
This data set yielded information on the occurrence and distribution of harbour porpoises in parts of the North and Baltic Seas, group size and their frequency, the relative number of mother-calf pairs and the seasonal distribution of the sightings. Sightings were plotted using ArcGIS Desktop 8.2. 2.3. Strandings and by-catches A stranding network, including a year-round observer scheme, was established on the German coast of Schleswig-Holstein and Mecklenburg-Vorpommern with members of several nature conservation organisations. Since 1990, patrolling efforts for carcasses by national park rangers, seal hunters and environmental organizations along the various beaches followed a standard procedure so that effort did not vary between months and years. Detailed information on the stranding network is provided by Benke et al. (1998). Animals were collected and necropsied by the FTZ in Schleswig-Holstein and by the German Oceanographic Museum in Mecklenburg-Vorpommern between 1990 and 2001. This systematic investigation of carcasses allowed the collection of data from stranded and by-caught animals and included gender and age class assessment. Wherever possible (North
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by glare was 26.8° and 23.0° in 1995, 14.6° and 20.1° in 1996 and 25.5° and 27.3° during the SCANS survey. Thus, in the 1995 and 1996 surveys, the subjective assessments ‘good’ and ‘moderate’ appeared to depend more heavily on sea state than during SCANS 1994.
Sea: 86%; Baltic Sea: 84% of all cases) strandings were assigned to a predefined sector of the German coastline. Porpoises were classified as calves, subadults or adults, based preferably on growth layer counts in teeth (Lockyer, 1995) or, in cases where this was not possible, on the length of the animal (see Siebert et al., 2001):
3.1.1. Effort and sightings by block Total time of survey effort flown in ‘good’ and ‘moderate’ conditions was 34.3 h (19.7 + 14.6h) in 1995 and 1996. Detailed survey effort per block as well as the percentage of survey time under ‘good’, ‘moderate’ and ‘poor’ conditions are shown in Table 1. Abundance was only calculated for survey effort under ‘good’ or ‘moderate’ conditions. Most sightings of harbour porpoises were recorded in North Sea block A (Fig. 2; 39 sightings in 1995 and 70 sightings in 1996). In the Baltic Sea, more harbour porpoises were sighted in block B (Fig. 3; 10 sightings in 1995 and 15 sightings in 1996) than in block C (Fig. 3; 6 sightings in 1995 and 0 sightings in 1996). Further details are provided in Tables 2 and 3.
(1) Calves: harbour porpoises between 0 and 1 y old, or ≤ 100 cm long; (2) Subadults: harbour porpoises between 1 and 4 y old, or between 101 and 125cm long; (3) Adults: harbour porpoises more than 4 y old, or longer than 125cm. Harbour porpoises defined as ‘by-catch’ were either taken in a net or were found stranded (later referred to as ‘stranded by-catch’) with clearly identifiable net marks or other lesions as described by Kuiken et al. (1994). However, as a larger number of strandings were in a state of advanced decomposition it was impossible to identify netmarks or other lesions. Therefore, a higher number of by-caught animals may have been among the decomposed animals.
3.1.2. Estimation of porpoise pod abundance A detailed description of the method used was provided by Hammond et al. (2002). Tables 2 and 3 show pod abundance estimated for the blocks from each replicate track and the mean values of these estimates weighted by the relative coverage in 1995 and 1996. Coverage was expressed relative to the area searched by completing the track in good conditions (esw = 130m). This is the baseline of calculating the weighted mean, in which the tracks that had a higher coverage percentage are given more weight in the mean animal abundance estimate, than the tracks with a lower coverage percentage. Each track is considered a sample and from each track overall abundance could be calculated. However, only by including several tracks/samples is it
3. Results 3.1. Baltic/North Sea aerial surveys in 1995 and 1996 The average sea state, according to the Beaufort scale, in ‘good’ and ‘moderate’ conditions was 1.07 and 2.03 in the 1995 survey, 0.85 and 1.96 in the 1996 survey, and 1.12 and 1.76 during the SCANS survey. Coding the assessment of turbidity as ‘clear’ = 1, ‘moderate’ = 2 and ‘turbid’ = 3, the average turbidity values were 1.30 and 1.42 in the 1995 survey, 1.51 and 1.58 in the 1996 survey and 1.74 and 2.07 during the SCANS survey. The extent of the sector affected
Table 1 Aerial survey period, hours surveyed, percentage of survey time flown in ‘good’, ‘moderate’ and ‘poor’ conditions, number of harbour porpoise pod sightings and number of mother-calf pairs Block
A B C
Survey period
Hours total
‘good’ (%)
‘moderate’ (%)
‘poor’ (%)
July 1995 July 1996 October 1995 July 1996 October 1995 July 1996
5.35 5.80 9.73 7.20 4.65 1.60
43 63 40 80 26 100
30 37 44 20 62 0
27 0 16 0 12 0
Pod sightings (n)
Mothercalf pairs (n)
33 70 10 15 6 0
4 13 1 6 0 0
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Fig. 2. Harbour porpoise sightings in block A during the aerial surveys in 1995 (n = 33) and 1996 (n = 70). The symbol size indicates group size of pod sightings. Map projection: Mercator.
Fig. 3. Harbour porpoise sightings in blocks B and C during the aerial surveys in 1995 (B, n = 10; C, n = 6) and 1996 (B, n = 15; C, n = 0). The symbol size indicates group size of pod sightings. Map projection: Mercator.
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Table 2 Aerial survey 1995 Block
Track no.
Pod sightings (n)
Relative coverage (%)
Pod abundance estimate for block
Weighted mean pod abundance estimate (95% CI)
Mean pod size estimate (CV)
Mean animal abundance (95% CI)
A
3 5 6 1 2 3 4 1 2 3 4 5
10 0 23 0 3 2 5 2 0 0 2 2
43 31 86 25 71 42 38 11 54 38 14 73
3639 0 4934 0 571 654 1871 2405 0 0 2225 556
3634 (1300–6000)
1.18 (.06)
4288 (1500–7800)
817 (300–2400)
1.20 (.11)
980 (360–2880)
514 (200–2300)
1.17 (.14)
601 (233–2684)
B
C
Pod abundance estimated for each block, mean of estimates weighted by the relative coverage based on 1995 surveys and derived mean animal abundance. CI = 95% Confidence interval; CV = Coefficient of variation.
possible to calculate a 95% confidence interval (CI). The larger the difference between tracks in terms of abundance, the larger the resulting confidence interval for the overall abundance. The last two columns show mean pod size and estimated mean animal abundance (Tables 2 and 3). The mean animal abundance in block A (North Sea) varied from 4288 (95% CI: 1500–7800) in 1995 to 7356 (95% CI: 5040–12 600) harbour porpoises in 1996. In the Baltic Sea, highest mean abundance was estimated for block B resulting in 980 (95% CI: 360– 2880) animals in the survey year1995 and 1830 (95% CI: 960–3 840) animals in 1996. The second survey block in the Baltic Sea (block C) yielded 601 (95% CI: 233–2 684) animals in 1995. There were no sightings in block C during the 1996 survey.
3.2. Incidental sightings 3.2.1. Platforms of opportunity From 1988 to 2002, 791 incidental sightings of harbour porpoise pods were recorded at the FTZ: 279 sightings were made in the Baltic Sea and 512 in the North Sea (Table 4). Sightings were mostly reported from coastal waters off Schleswig–Holstein (North and Baltic Sea) as well as along the coast of Mecklenburg–Vorpommern and around the Danish islands in the Baltic Sea. 3.2.2. Seasonal changes In the winter months (December–February, 1988– 2002), there were 50 incidental sightings in the North Sea, with a total of 88 harbour porpoises seen. Fig. 4 provides
Table 3 Aerial survey 1996 Block
Track no.
Pod sightings (n)
Relative coverage (%)
Pod abundance estimate for block
Weighted mean pod abundance estimate (95% CI)
Mean pod size estimate (CV)
Mean animal abundance (95% CI)
A
1 2 3 4 5 6 1 2 3 1 2 6
22 9 9 12 5 13 6 4 5 0 0 0
73 15 29 48 22 43 82 75 48 22 37 37
5417 13 353 5739 4807 4708 534 1126 844 1636 0 0 0
5838 (4000–10 000)
1.26 (.14)
7356 (5040–12 600)
1144 (600–2400)
1.60 (.10)
1830 (960–3840)
B
C
0 (0–0)
− (–)
− (–)
Pod abundance estimated for each block, mean of estimates weighted by the relative coverage based on 1996 surveys and derived mean animal abundance. CI = 95% Confidence interval; CV = Coefficient of variation.
U. Siebert et al. / Journal of Sea Research 56 (2006) 65–80 Table 4 Incidental sightings of harbour porpoises in the German North and Baltic Seas (1988 to 2002)
Number of sightings Number of porpoises Average group size (±SD) Maximum group size Number of calves Percentage of calves
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Table 5 Frequency of different group sizes sighted incidentally in the North and Baltic Seas (1988–2002)
North Sea
Baltic Sea
Group size
North Sea
Baltic Sea
512 1113 2.2 ± 3.2 40 59 5.3
279 689 2.5 ± 1.7 10 13 1.9
1 2 3–4 5–8 >8
235 (45.9%) 169 (33.0%) 87 (17.0%) 14 (2.7%) 7 (1.4%)
85 (30.5%) 100 (35.8%) 62 (12.1%) 27 (5.3%) 5 (1.0%)
information on the relative distribution of incidental sightings throughout the year. In July, the sightings (n=98; 26%) of harbour porpoises reached their maximum. 60% of the sightings, representing 53% of all counted porpoises, were noted between May and September. The distribution of observations in the Baltic Sea revealed a slightly different picture with a very low number of sightings in winter (Fig. 4). Only four sightings with a total of seven animals were reported. 85% of the sightings were noted from May to September. These sightings corresponded to 82% of the porpoises. In July and August, the number of sightings (n=73; n=56) and observed porpoises (n=114; n=130) reached its maximum, which is similar to the situation in the North Sea. 3.2.3. Group size and composition Platforms of opportunity accounted for 1802 harbour porpoises seen in the North Sea (n = 1113) and Baltic Sea (n = 689). Average group size was 2.5 (SD = 1.7) porpoises per sighting in the Baltic Sea and 2.2 (SD = 3.2) in the North Sea (Table 4).
The largest harbour porpoise group observed in the North Sea consisted of 40 animals on two occasions. Five groups of ten animals were seen in the Baltic Sea. In the whole period 59 calves were reported from the North Sea and 13 from the Baltic Sea (Table 4). Of the total number of 791 sightings of harbour porpoise groups in the North and Baltic Seas, 320 sightings consisted of a single animal, and in 269 cases two porpoises were seen. Groups of three to four animals (29.1%) were sighted comparatively often as well (Table 5). A difference in observed group size was detected between the North and Baltic Seas: most sightings (45.9%) in the North Sea consisted of a single animal, whereas most sightings in the Baltic Sea comprised two animals (35.8%). Most calves were reported in July (North Sea only), August and September (Fig. 5). Calves were registered in the North Sea more than twice as often as in the Baltic Sea in July and August. The number of calves started to decrease in August in the North Sea, while it peaked in the Baltic Sea during the same period (Fig. 5).
Fig. 4. Seasonal distribution of incidental sightings (%) of harbour porpoises in the German part of the Baltic and North Sea (1988–2002).
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Fig. 5. Seasonal occurrence of harbour porpoise calves observed from platforms of opportunity in the German part of the Baltic and North Sea (1988–2002).
3.2.4. Distribution The positions of incidental sightings in the period 1988 to 2002 are presented in Fig. 6. These encompass all sightings made from ships including a
position reference (North Sea: 443 sightings, Baltic Sea: 197 sightings). In the North Sea the majority of sightings were made around the islands of Sylt and Amrum (Fig. 6).
Fig. 6. Incidental sightings of harbour porpoises in the German North Sea (block A, n = 443) and Baltic Sea Sea (block B, n = 188; block C, n = 9), 1988 to 2002. Map projection: Mercator.
U. Siebert et al. / Journal of Sea Research 56 (2006) 65–80 Table 6 Strandings of harbour porpoise in the German North Sea and Baltic Sea (1990–2001) North Sea Baltic Sea Total Number of strandings and by-catches 1013 By-catches (%) 1.7 Calves (%) 43.1 Subadults (%) 22.2 Adults (%) 34.7 Males (%) 49.4 Females (%) 50.6
334 32.3 34.0 33.7 32.3 50.4 49.6
1347 9.3 40.7 25.2 34.1 49.6 50.4
In the Baltic block B, most porpoises were observed in the Little Belt and around the island of Fyn. In the German part of the Baltic Sea harbour porpoises were mostly observed between Flensburg Bight and Eckernförde as well as west and northwest of the island of Fehmarn (Fig. 6). In the eastern part of block B only sporadic sightings were registered and only nine sightings were reported from block C. 3.3. Strandings and by-catches 3.3.1. North Sea 996 harbour porpoises were found stranded along the coast of Schleswig–Holstein and 17 animals were
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identified as by-catch (1990–2001; Table 6). 457 were females and 446 were males. The rest (n = 110) remained unidentifiable with respect to sex. The proportions of females and males did not vary between years (χ2homogeneity test; 12 y; N = 903, χ 2 = 7.8, df = 11, P = 0.73). The total number of strandings was clearly larger in the North Sea (Binomial test of 1,013:334: z = 18.74, p < 0.0001). Fig. 7 shows that most of the stranded animals (n=517) were found on the North Frisian island of Sylt. The overall number of reported strandings decreased from north to south, as did the proportion of calves (Spearman's rank correlation coefficient, rS = 0.71, N = 11 islands, P = 0.015). Around the island of Sylt 237 (47.4%) of the stranded carcasses were calves, 98 (19.6%) were subadults and 165 (33.0%) were adults. The island south of Sylt, Amrum, showed a higher proportion of adults (46.6%; n = 34), while subadults made up 13.7% (n = 10) and calves accounted for 39.7% (n = 29) of all strandings (Fig. 7). Seventeen porpoises were identified as by-catches (six males and eleven females). They were all reported from 1991 to 1994 and in 1996 (Fig. 8). Six of these were calves, seven were subadults and four were adults. Fifteen animals were found dead on the beach as stranded by-catch either showing characteristic netmarks or
Fig. 7. Strandings of harbour porpoises on the German coast of the North Sea (1990–2001). The number next to the symbols shows the total number of stranded harbour porpoises. The symbol indicates the relative proportion of the three different age classes. S = Sylt, F = Föhr, A = Amrum, B = Büsum, H = Helgoland. Map projection: Mercator.
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Fig. 8. Temporal changes in the number of by-catches and strandings of harbour porpoises in the German part of the Baltic (BS) and North Sea (NS).
entangled in part of a net. Two harbour porpoises were found dead in a bottom-set gillnet near the island of Helgoland (January 1994). After 1996 no by-catches were identified in the German Bight (Fig. 8). The mean reported annual by-catch of harbour porpoises between 1990 and 2001 was 1.4 animals (SD = 2.1). Strandings were found year round. Along the North Sea coast, most animals were found in the summer months while the smallest number of strandings was discovered from November to March (Fig. 9). There was a clear periodicity in the number of animals found, indicating systematic seasonal variation (clear and significant peaks of linear auto-correlation-function of monthly data at twelve months, Pearson's correlation
coefficient rP = 0.50, N = 132mo, P < 0.001). Stranded calves were mainly found in June and July (Fig. 10). 3.3.2. Baltic Sea The annual number of strandings was much smaller in the Baltic than in the North Sea. From 1990 to 2001, a total of 229 harbour porpoises (81 females, 80 males) were found stranded along the Baltic shores of Schleswig–Holstein and Mecklenburg–Vorpommern and 105 animals (50 females, 53 males) were by-caught. The rest (n = 70) remained unidentifiable with respect to sex. Again, the proportions of females and males did not vary between years (χ2-homogeneity test; 12y; N = 264, χ2 = 11.36, df = 11, P = 0.41).
Fig. 9. Monthly distribution of strandings (1990–2001) in the German part of the Baltic and North Sea, shown as percentage (%) relative to the total number of strandings in the two areas.
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Fig. 10. Monthly distribution of calves on strandings (1990–2001) in the German part of the Baltic and North Sea, shown as relative amounts (%).
Most strandings occurred along the Fischland–Darss peninsula in Mecklenburg–Vorpommern (n = 40), near Flensburg (n = 37) and on the western shores of Fehmarn
in Schleswig–Holstein (n = 31). Relatively few calves, but a high proportion of subadults stranded along the Baltic coastline as compared to the North Sea (Fig. 11,
Fig. 11. Strandings of harbour porpoises on the German coast of the Baltic Sea (1990–2001). The number next to the symbols shows the total number of stranded harbour porpoises. The symbol indicates the relative proportion of the three different age classes. F = Flensburg, S = Schlei estuary, E = Eckernförde, K = Kiel, FH = Fehmarn, D = Fischland-Darss. Map projection: Mercator.
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Table 6). Proportions of age classes clearly differed between years in the North Sea (N = 712 individuals, χ2 = 63.77, df = 22, P < 0.0001) and tended to do so in the Baltic Sea (N = 299 individuals, χ2 = 32.16, df = 22, P = 0.075). The highest number and proportion of calves (n = 20, 54.1%) occurred near the outflow of the Schlei Fjord. Only a small number of calves stranded east of the island of Fehmarn. In Mecklenburg–Vorpommern, the strandings showed a fairly equal proportion of calves, subadults and adults. The proportion of by-caught harbour porpoises was clearly larger in the Baltic Sea than in the North Sea (χ2 = 280.4, df = 1, P < 0.0001). The by-caught porpoises, either reported by fishermen or coast guards (n = 55), were mostly found in bottom-set gillnets, on the beach with characteristic netmarks or entangled in part of a net (n = 50). Most animals taken incidentally were subadults (n = 64), but 27 adults und 12 calves were reported as by-catch as well. The mean annual bycatch rate of harbour porpoises between 1990–2001 based on the animals which were delivered or identified as by-catches was 8.8 animals (SD = 5.1). The by-catch of porpoises occurred in every year of the study period. The highest number of by-catches in relation to stranded animals occurred in 1990 and 1991. Subsequently, by-catch never exceeded the number of strandings (Fig. 8). Strandings were reported in every month. Comparison of the seasonal distribution of strandings (Figs. 9 and 10) indicated that the birth season in the Baltic Sea was about one month later than in the North Sea (maximum of cross-correlation function of monthly data at a lag of one month, all data: rP = 0.46, N = 134mo, P < 0.001; calves only: rP = 0.39, N = 134 mo, P < 0.001). 4. Discussion 4.1. Harbour porpoise in the German North and Baltic Seas The analyses of aerial surveys, incidental sightings and strandings all showed that the density of harbour porpoises in the Baltic was lower than in the North Sea study block. The number of mother-calf pairs was lower as well with a percentage of calves of 1.9 in the Baltic Sea compared to 5.3 in the North Sea. Furthermore, in the Baltic, the density of harbour porpoises decreased from west towards the east, confirming results from earlier surveys (Heide-Jørgensen et al., 1992, 1993; Benke et al., 1998; Hammond et al., 2002; Berggren et al., 2003; Scheidat et al., 2004). Information on incidental sightings from recreational boats (e.g. sailing
boats) was biased towards the west, where considerably more sailing takes place than in the east. The decline in abundance of harbour porpoises towards the east was, however, not obvious from strandings. Results of the stranding scheme indicated that similar numbers of porpoises were found along the coasts of Mecklenburg–Vorpommern and of Schleswig–Holstein. A high proportion of the strandings were calves. It is unclear where these porpoises came from. The direction and velocity of currents in the western Baltic suggest that some of these animals originated further north (e.g. Danish waters). Several hot spots of strandings were found in the Baltic Sea. These included the Schlei estuary, the area west of the island of Fehmarn in Schleswig–Holstein, which acted as a barrier to water pushed eastward by the prevailing westerly winds, and the area of the Fischland–Darss peninsula in Mecklenburg–Vorpommern. The highest number of stranded calves was found in the Schlei area, followed by the Fischland–Darss and Fehmarn areas. Most strandings of harbour porpoises along the shores of the North Sea occurred from June to August. In this period the proportion of calves in strandings was highest as well. In the same period the highest number of incidental sightings occurred. The high number of incidental sightings in this area as well as strandings around the islands of Sylt and Amrum confirmed that this area is often and regularly frequented and thus might be an important habitat for breeding harbour porpoises (Sonntag et al., 1999). This observation was reinforced by the high proportion of mother-calf pairs in the summer months, confirmed by the results of aerial surveys and incidental sightings. Recently conducted aerial surveys are complementing this picture with high densities of harbour porpoises observed close to the Danish border and in the area of Amrum Outerbank (Scheidat et al., 2004). In the 11-y study period, significantly more strandings of harbour porpoises occurred along the shores of the North Sea than the Baltic Sea. In the North Sea area most animals stranded on the northerly beaches of Schleswig–Holstein, especially on islands that may have functioned as a barrier to carcasses drifting northward with the residual coastal current (Otto et al., 1990). Only 10% of the strandings in the North Sea occurred south of 54°20′N. By-catch clearly dominated in the Baltic Sea. The number of porpoises incidentally taken in fisheries was higher in the western part of Kiel Bight than in its eastern part. However, fishermen in the western part were much more cooperative to report by-catches
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(Pfander, pers. comm., 2004). As reported earlier (Kinze, 1994; Benke et al., 1998) more subadults than adults were taken incidentally in fishing gear, for unknown reasons. Among strandings, adults and calves dominated in the North Sea, whereas mostly calves and subadults were found stranded in the Baltic Sea. Reliable information on by-catch in fishery can only be obtained if independent observer schemes are established (Northridge and Hammond, 1999). The by-catch of harbour porpoises in gillnet fisheries in the North Sea at large before 2000 was substantial. Catches in the range of several thousand harbour porpoises were reported in the Danish gillnet fishery (Vinther, 1999; Vinther and Larsen, 2004). The British gillnet fisheries took an estimated 400–800 porpoises per year (ASCOBANS, 2002). Catches were comparatively low with 30–50 porpoises annually in the small German gillnet fleet (Kock and Flores, 2003). The by-catch has probably declined since 2000 when the activities in the Danish and British gillnet fleets declined (Northridge and Hammond, 1999; Vinther and Larsen, 2004). The present level of by-catch is unknown. It appears that by-catch was the primary cause of death in the Baltic Sea while infectious diseases and perinatal death predominated in the North Sea (Siebert et al., 2001; Wünschmann et al., 2001). The identification of by-catches among strandings is difficult as some net types do not cause netmarks. Additionally, characteristics such as netmarks, haemorrhages (Kuiken et al., 1994) etc. disappear with progressing decomposition of the carcass. Taking into account the high by-catch rates from the North Sea at large (Northridge and Hammond, 1999; Vinther and Larsen, 2004) it is very likely that the number of by-catches is underestimated at necropsies because of the state of preservation of a large number of carcasses. The monthly distribution of strandings and incidental sightings in the Baltic suggested a strong seasonality with the highest occurrence of porpoises in summer. The same pattern is apparent in different areas of the North Sea (Benke et al., 1998; Lockyer, 2003; Hasselmeier et al., 2004). In the Baltic, the number of strandings and the occurrence of calves or juveniles peak about one month later than in the North Sea (Hasselmeier et al., 2004). Harbour porpoises of the North and Baltic Seas form distinct subpopulations with little genetic exchange (Tiedemann et al., 1996; Andersen et al., 1997; Huggenberger et al., 2002). Harbour porpoises in the Baltic Sea in turn belong to two subpopulations: the Baltic Proper (or central Baltic Sea) subpopulation, with the Darss sill as the western limit, and the Western Baltic
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subpopulation including the area of the Kattegat, Belt Seas, Øresund, Kiel Bight, Lübeck Bight and the Fehmarn Belt (Tiedemann et al., 1996; Andersen et al., 1997; Larsen et al., 2000; Huggenberger et al., 2002). Given the limited exchange between the subpopulations, the by-catch numbers reported for the Baltic Sea are at an unacceptably high level, threatening the survival of these populations (Berggren et al., 2002). First steps to reduce by-catch, such as improved reporting schemes and the use of pingers, have been taken at a European level. Success of these measures remains to be seen. 4.2. Comparison of methods 4.2.1. Aerial surveys in 1995 and 1996 Aeroplanes have often been used to conduct linetransect surveys of cetaceans. The method has its merits in areas of comparatively high density: it is effective in obtaining information on the distribution and abundance of harbour porpoises. The advantage of aerial surveys over ship-based surveys is that large areas can be covered in less time and that it is easier to react quickly to appropriate weather conditions. One disadvantage is that it is not possible to perform acoustic surveys or to collect biotic and abiotic data at the same time (e.g. salinity, sea surface temperature etc.). Additionally, the method has its shortcomings in regions where the abundance of animals is low, such as the central Baltic Sea. Other methods such as ship-based acoustic surveys with towed hydrophones or the deployment of a net of passive acoustic monitoring devices, so called ‘PODs’ (porpoise detectors) may be more informative in such areas. 4.2.2. Incidental sightings The incidental sightings recorded since 1988 are a valuable source of additional information to obtain a seasonal and temporal overview of the distribution of harbour porpoises along the German coast of the North and the Baltic Seas. However, a number of shortcomings need to be taken into consideration: sightings are qualitative measurements collected in an unsystematic way; operational constraints among boat operations vary (e.g. ferries, recreational boats); species, mother-calf pairs and group size may be misidentified. Mean group size may be overestimated because larger groups are easier to detect than single porpoises; the heterogeneity of these data precludes any rigid statistical analysis. The distribution of sighting effort is not equal for all areas and all seasons as areas of increased boat traffic exist in both seas (e.g. in the Wadden Sea),
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especially of recreational boats. Correcting for sighting effort is impossible as data on the frequency and distribution of recreational boats are missing. Nevertheless, patterns from incidental sightings are similar to those obtained from aerial surveys and the stranding network. 4.2.3. Strandings and by-catches Strandings data contain useful information on spatial and seasonal distributions of harbour porpoises (Polacheck et al., 1995). Furthermore, they allow the collection of additional information required for management and protection issues. This includes data on genetic and age structure, assessment of health and reproductive status, information on nutrition and chemical burden and mortality rates. Furthermore, the occurrence of other cetacean species may be noticed best from stranding records. Patrolling efforts for carcasses along various beaches have been following a standard procedure since 1990 and consequently have not varied between months and years. This is very important to ensure that yearly changes or trends are not artificially produced by varying efforts. Analysis of stranding records yield limited information on trends of abundance. For instance, the origin of the carcasses is unclear because strandings depend on various oceanographic features (e.g. currents and wind, which may vary between years). When westerly winds prevail, more dead animals are washed ashore on the German North Sea coast. Also, carcasses of harbour porpoises tend to sink to the bottom, and resurfacing time varies from a few days to several months depending on water temperature and currents (Moreno et al., 1992). Analysis of stranding information mirrored the analyses of the aerial surveys in that harbour porpoises are considerably more abundant around the North Frisian Islands than further south. In summary, the most appropriate methods for investigating abundance and distribution of cetaceans are dedicated aerial and ship surveys. Standard linetransect methods are used to estimate the number of animals in a given area based on the distance sampling data thus collected. Preferably, the survey should be designed for harbour porpoises only and not be combined with other surveys, at least for areas of high density. Good survey conditions are rare for the German Exclusive Economic Zones in the Baltic and North Sea. Aerial surveys can be conducted in a flexible way and even small time windows of low wind speeds can be used for survey flights. Therefore, aerial surveys were found to be the best method for survey work in the
coastal areas of the German waters. However, planes are limited in their range of action and thus can cover offshore areas only with difficulty. For such areas, e.g. the Dogger Bank, ship surveys may be more suited to conduct abundance estimates. To detect seasonal distribution, surveys should be conducted at least four times per year. To accurately interpret the results of local surveys it is necessary to conduct large-scale surveys that include the entire population of harbour porpoises. The SCANS survey conducted in 1994 was such a survey and a repeat took place in 2005. The use of the line-transect method is problematic in the Baltic Proper as harbour porpoise density is very low. New survey techniques have been developed for those areas covered under the current MINOS+ project (Marine warm-blooded animals in the North and Baltic Seas: Foundations for assessment of offshore wind farms; funded as part of the German government's research focus on renewable energies; http://www.minos-info.org) and tested in SCANSII in 2005. The methods developed and tested in SCANSII included the use of passive acoustics and vessels of opportunity, and recommended a suite of monitoring protocols modified according to species and area. Strandings data yield useful information on spatial and seasonal distribution and the occurrence of rare species. In addition, strandings networks provide a unique opportunity for data collection on the biology and health status parameters from tissue sampling. Other data such as genetic structure, reproductive status, food intake or anthropogenic impact are important for the management of harbour porpoise populations which, in combination with information on distribution and abundance, allow protection of the animals. Incidental sightings are valuable in areas where no or very limited specific surveys are conducted, such as the Mediterranean or Black Sea. In German waters they are a valuable additional source of information but of limited scientific potential. However, engaging interested individuals in this cost-effective reporting scheme helps foster public awareness for marine mammals and may result in stronger support for scientific study and political action to protect the sea. Acknowledgements We wish to thank Lex Hiby and Phil Lovell, Conservation Research Ltd. in Cambridge, for their help with the aerial survey data. Our thanks go to the pilots of the survey planes, especially Leif Petersen from the Danish Air Survey, and the observers. Thanks to Nils Koesters for rescuing some of the data and thanks
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to all contributors of the stranding network and the incidental sightings reporting scheme in Schleswig– Holstein. We thank Gerhard Schulze, German Oceanographic Museum, for collecting data on strandings of harbour porpoises in Mecklenburg–Vorpommern, as well as the Society for the Conservation of Marine Mammals (GSM) for collecting sighting data in the Baltic Sea. The comments of Krishna Das, Helena Herr, Ursula Verfuß, Franciscus Colijn and three anonymous reviewers are greatly appreciated. The various studies were funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, the German Federal Ministry for Education and Research, and the Ministry for Environment, Nature Conservation and Agriculture of Schleswig–Holstein. References Andersen, L.W., Holm, L.-E., Siegismund, H.R., Clausen, B., Kinze, C. C., Loeschcke, V., 1997. A combined DNA-micro-satellite and isozyme analysis of the population structure of the harbour porpoise in Danish waters and West Greenland. Heredity 78, 270–276. ASCOBANS, 2002. Incidental take of small cetaceans. Report of the Ninth Meeting of the Advisory Committee to ASCOBANS, Bonn 2002, unpublished. http://www.ascobans.org/files/finalres6.pdf. Benke, H., Siebert, U., Lick, R., Bandomir, B., Weiss, R., 1998. The current status of harbour porpoises (Phocoena phocoena) in German waters. Arch. Fish. Mar. Res. 46, 97–123. Berggren, P., Arrhenius, F., 1995. Densities and seasonal distribution of harbour porpoises (Phocoena phocoena) in the Swedish Skagerrak, Kattegat and Baltic Seas. In: Bjørge, A., Donovan, G. P. (Eds.), Biology of the Phocoenids. Rep. Int. Whal. Comm., Spec. Issue, vol. 16, pp. 109–121. Berggren, P., Wade, P.R., Carlström, J., Read, A.J., 2002. Potential limits to anthropogenic mortality for harbour porpoises in the Baltic region. Biol. Cons. 103, 313–322. Berggren, P., Hiby, L., Lovell, P., Scheidat, M., 2003. Abundance of harbour porpoises in the Baltic Sea from aerial surveys conducted in summer 2002. Working paper presented at the ASCOBANS meeting, Bonn, Germany, April 2003. 17 pp. Buckland, S.T., Anderson, D.R., Burnham, K.P., Laake, J.L., Borchers, D.L., Thomas, L., 2001. Introduction to Distance Sampling. Estimating Abundance of Biological Populations. University Press, Oxford. Das, K., Siebert, U., Fontaine, M., Jauniaux, T., Holsbeek, L., Bouquegneau, J.M., 2004. Ecological and pathological factors related to trace metal concentrations in harbour porpoises Phocoena phocoena from the North Sea and adjacent areas. Mar. Ecol. Prog. Ser. 281, 283–295. Hammond, P.S., Berggren, P., Benke, H., Borchers, D.L., Collet, A., Heide-Jørgensen, M.P., Heimlich, S., Hiby, A.R., Leopold, M.F., Øien, N., 2002. Abundance of harbour porpoise and other cetaceans in the North Sea and adjacent waters. J. Appl. Ecol. 39, 361–376. Hasselmeier, I., Abt, K.F., Adelung, D., Siebert, U., 2004. Stranding patterns of harbour porpoises (Phocoena phocoena) in the German North and Baltic Seas; when does the birth period occur? J. Cet. Res. Mang. 6, 259–263.
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