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Seabird mortality on longline fisheries in the western Mediterranean: factors affecting bycatch and proposed mitigating measures
Biological Conservation 98 (2001) 357±363
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Seabird mortality on longline ®sheries in the western Mediterranean: factors aecting bycatch and proposed mitigating measures E.J. Belda *, A. SaÂnchez SEO/BirdLife, c/Melquiades Biencinto, 34, E-28053, Madrid, Spain Received 24 February 2000; received in revised form 14 September 2000; accepted 25 September 2000
Abstract Seabird mortality on longlines in the Western Mediterranean around Columbretes Islands (Spain) was studied by observers in 1998±1999, and ringing recoveries obtained from 1992 to 1998. Average numbers varied between 0.16 and 0.69 birds/1000 hooks set, and incidental mortality aected seven dierent seabird species. Bycatch occurred mainly around sunrise and from mid afternoon until dusk. This was related to seabird abundance but not with ®shing eort (number of hooks set). About 656±2829 birds were estimated to be killed annually, of which 66% were Cory's shearwaters (Calonectris diomedea). In this species mortality was mainly among adult birds and aected reproductive success. Taking all this into account, bycatch on longlines may be a risk for the conservation of the populations of Cory's shearwater in the area. The results show that setting of longlines at night, and from one hour after sunrise until mid afternoon may reduce seabird bycatch in the area. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Longline ®shing; Bycatch; Shearwater; Mediterranean; Conservation
1. Introduction Seabird mortality on longlines is one of the world's most serious threats for pelagic seabirds (Weimerskirch and Jouventin, 1987; Brothers, 1991; Dunn 1997). The problem is well known in the southern hemisphere where it is putting many albatross species into chronic decline (Croxall and Prince, 1990; Brothers, 1991; Murray et al., 1993; Weimerskirch et al., 1997). However, although longlining is growing in European waters (Dunn, 1994), little information is available about seabird mortality, threats to populations of scavenging seabirds and possible mitigating measures. Data on seabird mortality associated with longliners in European waters is largely anecdotal. The main bycatch in Portuguese waters is reported to be gannets (Sula bassana; Dunn, 1994), fulmars (Fulmarus glacialis) in Norwegian waters (Follestad and Strann, 1991; Lùkkeborg, 1996), and Manx shearwaters (Punus yelkouan) o northern Spain (Diego et al., 1988). In Mediterranean waters the species known to be implicated in incidental bycatch on longlines are Cory's * Corresponding author at: Department of Biology, University of Oulu, PL 3000 90401 Oulu, Finland. E-mail address: [email protected] (E.J. Belda).
shearwater (Calonectris diomedea), Balearic shearwater (P. mauretanicus), Audouin's gull (Larus audouinni), and yellow-legged gull (L. cachinnans; Aguilar, 1998; SaÂnchez and Belda, 1998). However, there is no quantitative information either on the magnitude of seabird mortality on longlines or which species may be threatened by longlining. The areas where this bycatch has been reported are around the Balearic Islands, but mainly around the Columbretes Islands (see Methods for precise location). The aims of this study were: (1) to evaluate the rate of seabird bycatch by longline vessels in the Western Mediterranean around the Columbretes Islands; (2) to discover which species are aected and for which of them longlines may pose a higher risk; (3) to study factors that in¯uence seabird mortality rate; and (4) to propose mitigating measures to reduce seabird mortality in the study area. 2. Methods 2.1. Study area The study was conducted around the Columbretes Islands (39 540 N, 0 410 E, Mediterranean Sea, Spain).
0006-3207/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(00)00178-6
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Around the archipelago there is an important commercial ®shing ¯eet from Valencia, the CastelloÂn and Catalonia. The islands are situated about 50 km o CastelloÂn coast and 150 km o the Balearic Islands. The observations were done in vessels ®shing within a radius of 40 km around the Columbretes Islands (Fig. 1). The archipelago is formed by four groups of islands (19 ha in total). The islands were declared as a Natural Park in 1988, and Marine Reserve (4400 ha) in 1990. In the islands there are breeding colonies of Cory's shearwater (100 breeding pairs), Audouin's gull (c. 200±400 pairs), yellow-legged gull (500 pairs), and shag (Phalacrocorax aristotelis; 5±10 pairs; SaÂnchez and Castilla, 1997; own unpublished data). 2.2. Fishing methods used in the area Both bottom and pelagic longlines are used in the area. Pelagic longlines are used for ®shing sword®sh (Xiphias gladius), and bottom longlines target hake (Merluccius merluccius), red common sea bream (Pagrus pagrus), red sea bream (Pagellus bogavareo) and toothed bream (Dentex dentex). We grouped all the data from bottom longline ®sheries, although there are small differences in the longlines used for ®shing each of the ®sh species targeted. The ¯eet was composed of small vessels and setting of longlines was manual. The usual number of longline vessels ®shing in the area were 19 pelagic longliners, 13 bottom longliners and 11 vessels setting both sorts of
lines. The lines were set at 4±5 knots in the case of bottom longlines and about 7±9 knots in pelagic longlines. Bottom longline used hooks of size 3 (3.42 cm) while pelagic longline used hook of size 5 (5.42 cm). The hooks were baited with European pilchard (Sardina pilchardus) and round sardinella (Sardinella aurita) of size between 10 and 15 cm in the case of bottom ®shery, and between 20 and 25 cm in pelagic longlines. The number of hooks by set varied between 114 and 2000 (mean= 543265 hooks for bottom longlines; mean=1687346 hooks for pelagic longlines). Hooks were baited manually one by one, so the amount of ®sh potentially available to seabirds is identical to the amount of hooks set. Bottom longline setting took place between 03.00 and 19.30 h, and the mean length of the setting process was 3824 min (range 9±130 min; n=105). Pelagic longline setting occurred between 11.00 and 20.00 h, and the process lasted between 51 and 201 min (mean 10340 min, n=24). Hauling took place during the morning of the following day in the case of pelagic longlines and between 2 and 6 h after setting in the case of bottom longlines. There were no discards or oal in either of the two types of longlines. 2.3. Data sources and analysis Observations on seabird-longline interactions were made during 129 setting and hauling operations between 1998 (n=50: 26 bottom longlines and 24 pelagic longlines) and 1999 (n=79; all of them bottom
Fig. 1. The study area, showing the area where seabirds were caught on longlines (bycatch area), and colonies of origin of the ringed birds recovered.
E.J. Belda, A. SaÂnchez / Biological Conservation 98 (2001) 357±363
longlines). Two observers were placed on vessels between 15 July and 14 October in 1998 and between 25 May and 19 August in 1999. The data recorded came from eight dierent longline vessels (®ve bottom longliners, one pelagic longliner, and two vessels set both sorts of lines). During the ®shing operations, the observers recorded the number of hooks set, the number and species of birds caught on longlines, times of starting and ®nishing setting and hauling operations and the presence of moonlight during setting. Counts of seabirds attending the vessels were made every 10 min during setting operations. During night setting the distance at which seabirds could be observed was limited by the lights used for setting. These lights allowed recording of birds up to a distance of about 75 m. The numbers of birds caught must be considered as minimum ®gures as some birds were too far behind the vessel to be recorded and may have been lost before hauling. In the case of the Cory's shearwater the study was coincident with its breeding season. We considered that this could in¯uence the rate of bycatch of this species, so we compared the numbers caught at dierent stages of the breeding cycle, i.e. during the laying period (from 20 May until 5 June), incubation (until 12 July), nestling period (until mid October), and post-¯edging (from mid October until migration to winter quarters). The dierent periods were determined as averages of years 1993±1995 in the study conducted by SaÂnchez and Castilla (1997). To analyse the in¯uence of time of setting on seabird mortality, we considered three times intervals for bottom longline setting: night time, 1 h before sunrise to 1 h after sunrise (hereafter sunrise), and daytime. In the case of pelagic longlines, setting operations were always ®nished before darkness, so we analysed the variation on an hourly basis. The eect of time of setting on seabird mortality was analysed comparing the number of birds caught at different periods of the day. Expected values of birds caught in each period were estimated taking into account the number of hooks set in each period [expected no bird caught in a given period=(total no birds caught/total no hooks set) hooks set in the period]. Because in some cases the dierence between expected values and observed values was larger than observed values we used the log-likelihood ratio test instead of the Chi-square test (Williams, 1976). The variation in seabird mortality at dierent stages of the breeding cycle was analysed in a similar way. Statistical analyses were done according to Zar (1996). Information about the age of birds caught and their colonies of origin was obtained from 49 ringing recoveries between January 1992 and June 1998. These recoveries came from seabirds caught on longlines. Only for birds ringed as pulli was it possible to estimate the precise age when they were recovered dead. In the case
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of birds that were already adults when ringed, we considered in the analyses that adult birds were at least 2 years old when were ringed. This may cause a bias in the estimate of the age of birds caught, underestimating the real age at death. 3. Results 3.1. Seabird mortality In 1998, 12 birds were caught on 17,400 hooks set on bottom longlines, and 10 birds caught on 40,088 hooks set on pelagic longlines. The average bycatch rate on bottom longlines was 0.691.78 birds per 1000 hooks (n=26 setting operations), and on pelagic longlines it was 0.250.5 birds per 1000 hooks (n=24 setting operations). The dierence in the numbers of birds caught by the two longlining methods, weighted by the number of hooks set was signi®cant (G1=4.57, P<0.05). In 1999, ®ve seabirds were caught out of 31,324 hooks set by bottom longline vessels. The average bycatch rate was 0.160.49 birds caught per 1000 hooks (n=79 setting operations). When compared with the number of birds caught on bottom longlines in 1998, the dierences were signi®cant (G1=19.3, P<0.01). No seabird mortality was observed during hauling operations. Cory's shearwater was the most abundant species caught, followed by yellow-legged gull, Audouin's gull and gannet (Table 1). The most frequent modes of incidental capture were swallowing of the hooks (50%) and being hooked up by the wing (41%). Other species were caught on longlines during 1998 and 1999 during ®shing trips without observers: one shag, one great skua (Stercorarius skua) and four Balearic shearwaters (Punus mauretanicus). 3.2. Age of birds caught on longlines Out of 38 ringing recoveries of Cory's shearwater between 1992 and 1998 attributable to longlining, 10 were ringed as chicks and age at death was between 4 and 13 years old (mean=7.7 years). For the 28 birds ringed when they were >1 year old, age at death was between 2 and 15 years old (mean=7.9 years). More than 60% of the birds killed were 7 or more years old. All the Audouin's gulls recovered were ringed as chicks, and were between 4 and 13 years old when caught (mean=7.9 years, n=9 birds). One of the yellow-legged gulls caught was 3 years old and the other one was at least 4 years old. In 1999 one yellow-legged gull and one Audouin's gull caught during the observed operations had been ringed as chicks. Of the 38 Cory's shearwater recovered, 47.4% were ringed in the Columbretes Islands, and the other in the
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Table 1 Numbers of seabirds attending setting operations by longline vessels around Columbretes Islands (Spain), frequency of taking baits, and numbers caught Numbers attendinga
Frequency of occurrenceb
Seabird mortalityc
n
MeanS.D.
n
%
1998
Cory's shearwater Audouin's gull Yellow-legged gull Balearic shearwater Gannet Common tern (Sterna hirundo) Great skua Total
656 227 73 18 14 11 1 1000
5.0914.1 1.764.7 0.571.8 0.140.7 0.111.1 0.060.5 0.0070.0 ±
70 53 32 10 3 5 1
54.3 41.1 24.8 7.8 2.3 3.9 0.7
17 0 3 ± 2 ± ± 22
1 3 1 ± ± ± ± 5
38 9 2 ± ± ± ± 49
Number of setting operations
129
50
79
±
a b c
129
1999
1992±1998
Sum of maximum counts and mean counts during setting operations. Number of setting occasions (out of 129) when a species was seen. 1998 and 1999 data from observers on vessels; 1992±1998 data from ringing recoveries.
Balearic Islands (dierent colonies from Majorca and Minorca, see Fig. 1). All the recoveries of Audouin's gulls (n=9) came from the Ebro Delta colony, while all the yellow-legged gulls (n=2) were ringed in the Columbretes Islands. 3.3. Factors aecting seabird mortality 3.3.1. Seabird abundance during setting operations Seven dierent species were recorded trying to take baits during setting operations (Table 1). Cory's shearwater constituted 65.6% of the total number of birds (n=1000) attending the vessels during setting operations, and Audouin's gull 22.7%. During setting operations for bottom longlines, 77.5% (n=608) of the birds attempting to take baits were observed during sunrise, 15% during daytime hours and 7.5% at night. The number of seabirds at night, sunrise and daytime weighted by number of hooks set at each period were signi®cantly dierent (G2=190.8, P<0.05). In pelagic longlines only 32.5% of the total number of seabirds following the vessels during setting operations were observed during the ®rst 5 h of setting activities and 67.5% afterwards. This dierence was signi®cant (G1=60.8, P<0.05; weighted by the number of hooks set at each period). 3.3.2. Time of setting the longlines There were variations in the number of seabirds caught during the day both in 1998 and 1999. In the case of bottom longlines in both years most of the incidental captures of seabirds occurred around sunrise and in neither year were any birds caught during daytime (Fig. 2). The number of seabirds caught at night, sunrise and daytime weighted by number of hooks set at each period were signi®cantly dierent in both 1998 and 1999 (G2=9.6, P<0.01; G2=11.03, P<0.01, respectively). All the incidental captures of seabirds that occurred
Fig. 2. Number of birds caught on bottom longlines and number of hooks set at night, around sunrise (1 h before and after sunrise), or during the day in (a) 1998 and in (b) 1999.
before sunrise (n=3) were in nights with moonlight during setting and were Cory's shearwaters. In the case of pelagic longlines there were no incidental captures of seabirds during the ®rst 5 h of setting activities in spite of the fact that 52.7% of the hooks (n=40,338 hooks set) were set during that period (Fig. 3). There were signi®cant dierences in the number of seabirds caught at dierent hours weighted by the number of hooks set at each hour (G10=20.03, P<0.05). The number of seabirds caught by bottom and pelagic longlines per number of birds attending the vessels during setting operations was similar (2.73/100 birds following bottom longlines vessels and 2.79/100 birds attending pelagic longline vessels).
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Fig. 3. Number of birds caught on pelagic longlines and number of hooks set at dierent hours. Hours standardised to the hour of sunset, sunset hour=0.
3.3.3. Relationship to breeding cycle of Cory's shearwater The number of Cory's shearwaters caught was signi®cantly dierent during the dierent stages of the breeding cycle (G3=11.14, P<0.05; weighted by the number of hooks set). Most of the incidental captures took place during the chick rearing stage, while no mortality was observed during the incubation stage (Fig. 4). 4. Discussion 4.1. Seabird mortality and population impacts This is the ®rst work reporting bycatch rates of seabirds by longline ®sheries in the Mediterranean. The average bycatch rate of seabirds found in this study (0.16±0.69 birds per 1000 hooks set) is within the range of catch rates reported from other longline ®sheries in the southern hemisphere (0.02±5.3 birds per 1000 hooks; Barnes et al., 1997). This problem may be important, as other studies have shown that this source of mortality is the main cause of the decline of albatross populations and is likely to aect other seabird species (Weimerskirch and Jouventin, 1987; Weimerskirch et al., 1997). Thus, in the Mediterranean longline ®sheries may cause the decline of some seabird populations, especially if these ®shing techniques are expanded. The total number of hooks set by the 43 vessels of the longline ®shing ¯eet around Columbretes Islands can be obtained from the mean number of hooks set in each setting operation and the average number of ®shing days (pelagic longlines=60 days/year; bottom longlines vessels=210 days/year; mixed longlines vessels=60 days/year pelagic longlines+150 days/year bottom longlines). Thus, we estimated that 2.3 million pelagic hooks and 1.8 million bottom hooks are set each year. From the mortality rate found in this study, we there-
361
Fig. 4. Number of Cory's shearwaters on longlines and number of hooks set in relation to dierent stages of the breeding cycle of this species.
fore estimate that c. 656±2829 birds are killed annually around the Columbretes Islands. From this total estimate of birds caught annually around the Columbretes Islands, we may estimate that about 437±1867 Cory's shearwaters are killed annually around the Columbretes islands. Aguilar (1998) suggested that about 1300 Cory's shearwaters are caught annually by longlines in Balearic waters during the breeding season. The population in these two areas is about 11,000 breeding pairs (Diaz et al., 1996). Taking into account that 60% of the Cory's shearwaters killed were birds that have already reached the reproductive age, mortality on longlines may be aecting 4±6% of the breeding population in the area. Because procellariiforms are long-lived, show delayed maturity, have a low reproductive rate and high adult survival, even small increases in adult mortality may produce step population declines (Lebreton and Isenmann, 1976; Croxall et al., 1984). In the Columbretes Islands there has been a decline of 45% in the number of breeding pairs of Cory's shearwater in the last 2 years, associated with a decline in adult survival rates (Sanchez et al., 1999). Mortality aected mainly birds that have already reach the reproductive age (7 years; Mougin et al., 1986; Ristow et al., 1990), and it occurred during the breeding season, aecting specially during the period of rearing the chicks. We know from ringing recoveries that some breeding failures were due to mortality of one of the parents on longlines: in 1997, a male, which was previously ringed in the nest during incubation, was killed on longlines. The female deserted the nest few days later. There was a similar situation again in 1998. We have also found parents alive at the nests with hooks in its beak. Thus, not only adult survival rate is aected by mortality on longlines, but also reproductive success of Cory's shearwaters is reduced due to incidental mortality of adult birds, at least in the colonies of the Columbretes and Balearic Islands. Taking all these points into account, there are reasons enough to be concerned about populations of this species, especially in small
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populations, given the results of this study. Long-term monitoring of breeding colonies and incidental bycatch is needed to assess the extent of longline mortality in the western Mediterranean population. Audouin's gull was formerly considered Endangered (SPEC 1; Collar et al., 1994; Tucker and Heath, 1994), but in recent years the population has increased (del Hoyo et al., 1996; Hagemeijer and Blair, 1997). Some birds caught in this study came from the most important colony for the species, the Ebro Delta, which has also increased in the last years (c. 14,000 breeding pairs; Oro et al., 1996). We may estimate that the number of birds killed annually on longlines was between 72 and 311 (a maximum of 1.1% of the breeding population of the Ebro Delta). Therefore we suggest that longline ®sheries do not pose a risk for the conservation of this species at present. Observers did not record any mortality of Balearic shearwater during our study in 1998 and 1999. Most of the population is in the Atlantic during this period of the year normally (Yesou, 1986; Le Mao and Yesou, 1993). Nevertheless, this is a globally endangered species with a total population of c. 3000 pairs breeding in the Balearic islands. It has been caught on longlines and the population shows a declining trend (Aguilar, 1998) so more information on the impact of ®sheries is needed. 4.2. Factors aecting seabird mortality The results suggest that the most important factor in explaining the variation in seabird incidential captures is time of setting the longline, as has been pointed out in other studies (Brothers, 1991; Murray et al., 1993; Ashford et al., 1995; Cherel et al., 1996; Barnes et al., 1997). In our study area, there was a peak in the bycatch rate around sunrise (in bottom longline ®sheries) and another from mid afternoon until dusk (when pelagic longlines are working). The peak in the bycatch rate is associated with the peak in the number of birds attending longlines vessels (around sunrise and from mid afternoon until dusk). Thus, the in¯uence of time of setting may be due to the foraging activity of seabirds attending longline vessels. This may also explain the dierences in mortality found between pelagic and bottom longlines (mortality was higher in bottom longlines). These dierences in bird mortality disappeared when seabird abundance was taken into account. We have no evidence that other factors such as speed of the vessel during setting operations, dierences in hook or bait size in¯uence bird mortality. Cory's shearwater is considered as a nocturnal feeder (Cramp and Simmons, 1977) but our results suggest that it may be mainly a crepuscular feeder, at least in the study area. The other species observed taking baited hooks are mainly diurnal. All the species observed in this study attempting to take baits from longlines have
been observed trying to get discards from trawlers, and from sardine ®shing vessels (personal observation). In the study area, trawlers start ®shing activities one or two hours after sunrise and ®nish about mid-afternoon and sardine ®shing vessels throw discards early in the morning in their way from ®shing areas to harbour. It may be that discards from trawlers and sardine ®shing vessels in¯uence the abundance of seabirds attending longline vessels, and therefore bycatch rate. This interaction between dierent ®sheries needs more research. 4.3. Recommendations for reducing seabird bycatch Our results, and previous work (Vaske, 1991; Murray et al., 1993; Imber, 1994; Cherel et al., 1996; Barnes et al., 1997) show that setting the longlines outside the periods of sunrise and sunset will reduce seabird mortality. The measure of setting at night or during daytime is starting to be used by most of the ®shermen in the study area. Fishermen in the area have tried other mitigating measures. They used a towed line as a bird-scaring device but birds get used to it and it loses its eectiveness and, secondly, it interferes with the longline. A bird scaring line with suspended streamers, mounted on a pole on the stern of the vessel to gain sucient height above the sea (Brothers, 1994) have not been used in the area. These bird scaring lines have proved to be ecient in reducing seabird bycatch in other ®sheries (Brothers, 1991; Ashford et al., 1995; Lùkkeborg, 1996). Correct height also prevents the ®shing line interfering with the bird line. The mounting position must ensure that the bird line with streamers attached is towed astern directly above the area where baits enter the water (Brothers et al., 1999). We therefore recommend their use in the Mediterranean longline ®sheries, although studies on the eciency of this measure in small vessels should be conducted. Acknowledgements This work is a contribution to a project conducted by SEO/BirdLife and funded in 1998 by the Secretaria General de Pesca Maritima (Ministerio de Agricultura, Pesca y AlimentacioÂn) and in 1999 by the Project LIFE: ``ZEPAS's insulares en la Comunidad Valenciana'' of the Conselleria de Medio Ambiente, Generalitat Valenciana. This study bene®ted most from the kind-cooperation of all the skippers and the crew of vessels that participated, but especially to vessels Arrogante, Jubemar, Katan and Panollo I.E. Dunn and E. MõÂnguez helped us with the design of the study. E. Barba, E. Dunn, B.N.K. Davis and an anonymous referee made useful suggestions on an earlier version of the manuscript and R. Ribes helped us with the English. This
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work will not have been possible without the help at dierent stages of the study of M. GimeÂnez, J.J. JimeÂnez, T. Lamperez, R. MartõÂ, E. Revenga, and J.J. Vicent. References Aguilar, J.S., 1998. BiologõÂa y ConservacioÂn de la Pardela Balear Punus mauretanicus. Documents TeÁcnics de Conservacio (II ep) 2. Govern Balear, Palma de Mallorca. Ashford, J.R., Croxall, J.P., Rubilar, P.S., Moreno, C.A., 1995. Seabird Interactions with Longlining Operations for Dissostichus eleginoides around South Georgia, April to May 1994. CCAMLR Science 2, 111±121. Barnes, K.N., Ryan, P.G., Boix-Hinzen, Ch., 1997. The impact of the hake Merluccius spp. longline ®shery o South Africa on Procellariform seabirds. Biological Conservation 82, 227±234. Brothers, N., 1991. Albatross mortality and associated bait loss in the Japanese longline ®shery in the southern ocean. Biological Conservation 55, 255±268. Brothers, N.P., 1994. Principles of Bird Line Construction and Use to Reduce Bait Loss and Bird Deaths during Longline Setting. CCAMLR WG-IMALF 94/19. Brothers, N.P., Cooper, J., Lùkkeborg, S., 1999. The Incidental Catch of Seabirds by Longline Fisheries: Worldwide Review and Technical Guidelines for Mitigation. FAO Fisheries Circular No. 937. Cherel, Y., Weimerskirch, H., Duhamel, G., 1996. Interactions between longline vessels and seabirds in Kerguelen waters and a method to reduce seabird mortality. Biological Conservation 75, 63± 70. Collar, N.J., Crosby, M.J., Statters®eld, A.J., 1994. Birds to Watch 2: The World List of Threatened Birds. BirdLife Conservation, Series No. 4. Cramp, S., Simmons, K.E.L. (Eds.), 1977. The Birds of the Western Palearctic, Vol. 1. Oxford University Press, Oxford. Croxall, J.P., Evans, P.G.H., Schreiber, R.W., 1984. Editor's introduction: status and conservation or the world's seabirds. ICBP Technical Publication 2, 1±4. Croxall, J.P., Prince, P.A., 1990. Recoveries of wandering albatrosses Diomedea exulans ringed at South Georgia. Polar Rec. 29, 143±145. del Hoyo, J., Elliot, A., Sargatal, J. (Eds.), 1996. Handbook of the Birds of the World, Vol. 3. Hoatzin to Auks. Lynx Edicions, Barcelona. DõÂaz, M., Asensio, B., TellerõÂa, J.L. 1996. In J.M. Reyero (Ed.), Aves IbeÂricas. I. No Paseriformes. Madrid. Diego, J.A., Prieto, J.R., Fernandez, M.B. 1988. Captura de aves marinas en artes de pesca en Astrurias. In: Lopez, C. (Ed.), Aves Marinas GIAM Formentera, GOB Palma de Mallorca, pp. 17±31. Dunn, E., 1994. Interactions between Fisheries and Marine Birds: Research Recommendations. RSPB report, November 1994. Dunn, E., 1997. Sustainable ®sheries and seabirds. RSPB Conservation Review 11, 44±50. Follestad, A., Strann, K.B., 1991. Seabirds and ®shing nets: the scope and nature of the problem in Norway. NINA Oppdragsmelding 78, 1±14. Hagemeijer, E.J.M., Blair, M.J., 1997. The EBCC Atlas of European Breeding Birds: Their Distribution and Abundance. T and AD Poyser, London.
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Imber, M.J., 1994. Report on a Tuna Longline Fishing Voyage aboard Southern Venture to Observe Seabird By-catch Problems. Science and Research Series, 65. NZ Department of Conservation, Wellington. Le Mao, P., YeÂsou, P., 1993. The annual cycle of Balearic shearwaters and western-Mediterranean Yellow-legged gulls: some ecological considerations. In: Aguilar, J.S., Monbaillou, X., Paterson, A. (Eds.), Estatus y Conservacion de Aves Marinas: Actas del II Simposium MEDMARAVIS. SEO, Madrid, pp. 135±146. Lebreton, J.D., Isenmann, P., 1976. Dynamique de la population camarguaise de Mouette Rieuse: un modeÁle matheÂmatique. Terre Vie 30, 529±549. Lùkkeborg, S., 1996. Sea-bird Bycatch and Bait Loss in Longlining using Dierent Setting Methods. CCAMLR Science Committee Working Group on Environmental Monitoring and Management, Bergen, Norway, August 1996. Mougin, J.L., Jouanin, C., Despin, B., Roux, F., 1986. The age of ®rst breeding of cory's shearwater on Selvagem Grande South Atlantic ocean and problems of ring loss. Ringing & Migration 7, 130±134. Murray, T. E., Bartle, J.A., Kalish, R., Taylor, P.R., 1993. Incidental capture of seabirds by Japanese blue®n tuna longline vessels in New Zealand waters, 1988±1992. Bilogical Conservation, 3, 181±210. OroÂ, D., Genovart, X., Ruiz, X, JimeÂnez, J., Garcia-Gans, J., 1996. Dierences in diet, population size and reproductive performance between two colonies of Audouin's Gull Larus audouinii aected by a trawling moratorium. Journal of Avian Biology 27, 245±251. Ristow, D., Feldmann, F., Scharlau, W., Wink, M., 1990. Population structure philopatry and mortality of cory's shearwater (Calonectris diomedea diomedea). Die Vogelwelt 111, 172, - 181. SaÂnchez, A, Barba, E., Belda, E.J., 1999. Variation of survival rates of Cory's shearwater in Columbretes Islands. 100 Years Ringing Symposium, Helgoland, Germany, September, 1999. Sanchez, A., Belda, E.J., 2000. Incidental catch of seabirds in longline ®sheries around Columbretes Islands, Mediterranean Sea (Spain). In: YeÂsou, P., Sultana, J. (Eds.), Monitoring and Conservation of Birds, Mammals and Sea-turtles of the Mediterranean and Black Seas. Environment Protection Department, Floriana, Malta, pp. 133±134. SaÂnchez, A., Castilla, A., 1997. La Pardela Cenicienta (Calonectris diomedea) en las Islas Columbretes. BiologõÂa y ConservacioÂn, Ayuntamiento de CastelloÂn. Tucker, G.M., Heath, M.F., 1994. Birds in Europe: Their Conservation Status. Birdlife International, Cambridge. Vaske, T., 1991. Seabird mortality on tuna longline ®shery in southern Brazil. Ciencia e Cultura 43, 388±390. Weimerskirch, H., Jouventin, P., 1987. Population dynamics of the wandering albatross Diomedea exulans of the Crozet Islands: causes and consequences of the population decline. Oikos 49, 315±322. Weimerskirch, H., Brothers, N., Jouventin, P., 1997. Population dynamics of wandering albatross Diomedea exulans and Amsterdam albatross D. Amsterdamensis in the Indian Ocean and their relationships with longline ®sheries: conservation implications. Biological Conservation 79, 257±270. Williams, K., 1976. The failure of Pearson's goodness of ®t statistic. Statistician 25, 49. YeÂsou, P., 1986. Balearic shearwaters summering in Western France. In: Medmaravis, X., Monbailliu, X.X. (Eds.), Mediterranean Marine Avifaune. Springer Verlag, Heidelberg, pp. 513±517. Zar, J.H., 1996. Biostatistical Analysis, 3rd Edition. Prentice Hall, NJ.
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Seabird mortality on longline ®sheries in the western Mediterranean: factors aecting bycatch and proposed mitigating measures E.J. Belda *, A. SaÂnchez SEO/BirdLife, c/Melquiades Biencinto, 34, E-28053, Madrid, Spain Received 24 February 2000; received in revised form 14 September 2000; accepted 25 September 2000
Abstract Seabird mortality on longlines in the Western Mediterranean around Columbretes Islands (Spain) was studied by observers in 1998±1999, and ringing recoveries obtained from 1992 to 1998. Average numbers varied between 0.16 and 0.69 birds/1000 hooks set, and incidental mortality aected seven dierent seabird species. Bycatch occurred mainly around sunrise and from mid afternoon until dusk. This was related to seabird abundance but not with ®shing eort (number of hooks set). About 656±2829 birds were estimated to be killed annually, of which 66% were Cory's shearwaters (Calonectris diomedea). In this species mortality was mainly among adult birds and aected reproductive success. Taking all this into account, bycatch on longlines may be a risk for the conservation of the populations of Cory's shearwater in the area. The results show that setting of longlines at night, and from one hour after sunrise until mid afternoon may reduce seabird bycatch in the area. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Longline ®shing; Bycatch; Shearwater; Mediterranean; Conservation
1. Introduction Seabird mortality on longlines is one of the world's most serious threats for pelagic seabirds (Weimerskirch and Jouventin, 1987; Brothers, 1991; Dunn 1997). The problem is well known in the southern hemisphere where it is putting many albatross species into chronic decline (Croxall and Prince, 1990; Brothers, 1991; Murray et al., 1993; Weimerskirch et al., 1997). However, although longlining is growing in European waters (Dunn, 1994), little information is available about seabird mortality, threats to populations of scavenging seabirds and possible mitigating measures. Data on seabird mortality associated with longliners in European waters is largely anecdotal. The main bycatch in Portuguese waters is reported to be gannets (Sula bassana; Dunn, 1994), fulmars (Fulmarus glacialis) in Norwegian waters (Follestad and Strann, 1991; Lùkkeborg, 1996), and Manx shearwaters (Punus yelkouan) o northern Spain (Diego et al., 1988). In Mediterranean waters the species known to be implicated in incidental bycatch on longlines are Cory's * Corresponding author at: Department of Biology, University of Oulu, PL 3000 90401 Oulu, Finland. E-mail address: [email protected] (E.J. Belda).
shearwater (Calonectris diomedea), Balearic shearwater (P. mauretanicus), Audouin's gull (Larus audouinni), and yellow-legged gull (L. cachinnans; Aguilar, 1998; SaÂnchez and Belda, 1998). However, there is no quantitative information either on the magnitude of seabird mortality on longlines or which species may be threatened by longlining. The areas where this bycatch has been reported are around the Balearic Islands, but mainly around the Columbretes Islands (see Methods for precise location). The aims of this study were: (1) to evaluate the rate of seabird bycatch by longline vessels in the Western Mediterranean around the Columbretes Islands; (2) to discover which species are aected and for which of them longlines may pose a higher risk; (3) to study factors that in¯uence seabird mortality rate; and (4) to propose mitigating measures to reduce seabird mortality in the study area. 2. Methods 2.1. Study area The study was conducted around the Columbretes Islands (39 540 N, 0 410 E, Mediterranean Sea, Spain).
0006-3207/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(00)00178-6
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Around the archipelago there is an important commercial ®shing ¯eet from Valencia, the CastelloÂn and Catalonia. The islands are situated about 50 km o CastelloÂn coast and 150 km o the Balearic Islands. The observations were done in vessels ®shing within a radius of 40 km around the Columbretes Islands (Fig. 1). The archipelago is formed by four groups of islands (19 ha in total). The islands were declared as a Natural Park in 1988, and Marine Reserve (4400 ha) in 1990. In the islands there are breeding colonies of Cory's shearwater (100 breeding pairs), Audouin's gull (c. 200±400 pairs), yellow-legged gull (500 pairs), and shag (Phalacrocorax aristotelis; 5±10 pairs; SaÂnchez and Castilla, 1997; own unpublished data). 2.2. Fishing methods used in the area Both bottom and pelagic longlines are used in the area. Pelagic longlines are used for ®shing sword®sh (Xiphias gladius), and bottom longlines target hake (Merluccius merluccius), red common sea bream (Pagrus pagrus), red sea bream (Pagellus bogavareo) and toothed bream (Dentex dentex). We grouped all the data from bottom longline ®sheries, although there are small differences in the longlines used for ®shing each of the ®sh species targeted. The ¯eet was composed of small vessels and setting of longlines was manual. The usual number of longline vessels ®shing in the area were 19 pelagic longliners, 13 bottom longliners and 11 vessels setting both sorts of
lines. The lines were set at 4±5 knots in the case of bottom longlines and about 7±9 knots in pelagic longlines. Bottom longline used hooks of size 3 (3.42 cm) while pelagic longline used hook of size 5 (5.42 cm). The hooks were baited with European pilchard (Sardina pilchardus) and round sardinella (Sardinella aurita) of size between 10 and 15 cm in the case of bottom ®shery, and between 20 and 25 cm in pelagic longlines. The number of hooks by set varied between 114 and 2000 (mean= 543265 hooks for bottom longlines; mean=1687346 hooks for pelagic longlines). Hooks were baited manually one by one, so the amount of ®sh potentially available to seabirds is identical to the amount of hooks set. Bottom longline setting took place between 03.00 and 19.30 h, and the mean length of the setting process was 3824 min (range 9±130 min; n=105). Pelagic longline setting occurred between 11.00 and 20.00 h, and the process lasted between 51 and 201 min (mean 10340 min, n=24). Hauling took place during the morning of the following day in the case of pelagic longlines and between 2 and 6 h after setting in the case of bottom longlines. There were no discards or oal in either of the two types of longlines. 2.3. Data sources and analysis Observations on seabird-longline interactions were made during 129 setting and hauling operations between 1998 (n=50: 26 bottom longlines and 24 pelagic longlines) and 1999 (n=79; all of them bottom
Fig. 1. The study area, showing the area where seabirds were caught on longlines (bycatch area), and colonies of origin of the ringed birds recovered.
E.J. Belda, A. SaÂnchez / Biological Conservation 98 (2001) 357±363
longlines). Two observers were placed on vessels between 15 July and 14 October in 1998 and between 25 May and 19 August in 1999. The data recorded came from eight dierent longline vessels (®ve bottom longliners, one pelagic longliner, and two vessels set both sorts of lines). During the ®shing operations, the observers recorded the number of hooks set, the number and species of birds caught on longlines, times of starting and ®nishing setting and hauling operations and the presence of moonlight during setting. Counts of seabirds attending the vessels were made every 10 min during setting operations. During night setting the distance at which seabirds could be observed was limited by the lights used for setting. These lights allowed recording of birds up to a distance of about 75 m. The numbers of birds caught must be considered as minimum ®gures as some birds were too far behind the vessel to be recorded and may have been lost before hauling. In the case of the Cory's shearwater the study was coincident with its breeding season. We considered that this could in¯uence the rate of bycatch of this species, so we compared the numbers caught at dierent stages of the breeding cycle, i.e. during the laying period (from 20 May until 5 June), incubation (until 12 July), nestling period (until mid October), and post-¯edging (from mid October until migration to winter quarters). The dierent periods were determined as averages of years 1993±1995 in the study conducted by SaÂnchez and Castilla (1997). To analyse the in¯uence of time of setting on seabird mortality, we considered three times intervals for bottom longline setting: night time, 1 h before sunrise to 1 h after sunrise (hereafter sunrise), and daytime. In the case of pelagic longlines, setting operations were always ®nished before darkness, so we analysed the variation on an hourly basis. The eect of time of setting on seabird mortality was analysed comparing the number of birds caught at different periods of the day. Expected values of birds caught in each period were estimated taking into account the number of hooks set in each period [expected no bird caught in a given period=(total no birds caught/total no hooks set) hooks set in the period]. Because in some cases the dierence between expected values and observed values was larger than observed values we used the log-likelihood ratio test instead of the Chi-square test (Williams, 1976). The variation in seabird mortality at dierent stages of the breeding cycle was analysed in a similar way. Statistical analyses were done according to Zar (1996). Information about the age of birds caught and their colonies of origin was obtained from 49 ringing recoveries between January 1992 and June 1998. These recoveries came from seabirds caught on longlines. Only for birds ringed as pulli was it possible to estimate the precise age when they were recovered dead. In the case
359
of birds that were already adults when ringed, we considered in the analyses that adult birds were at least 2 years old when were ringed. This may cause a bias in the estimate of the age of birds caught, underestimating the real age at death. 3. Results 3.1. Seabird mortality In 1998, 12 birds were caught on 17,400 hooks set on bottom longlines, and 10 birds caught on 40,088 hooks set on pelagic longlines. The average bycatch rate on bottom longlines was 0.691.78 birds per 1000 hooks (n=26 setting operations), and on pelagic longlines it was 0.250.5 birds per 1000 hooks (n=24 setting operations). The dierence in the numbers of birds caught by the two longlining methods, weighted by the number of hooks set was signi®cant (G1=4.57, P<0.05). In 1999, ®ve seabirds were caught out of 31,324 hooks set by bottom longline vessels. The average bycatch rate was 0.160.49 birds caught per 1000 hooks (n=79 setting operations). When compared with the number of birds caught on bottom longlines in 1998, the dierences were signi®cant (G1=19.3, P<0.01). No seabird mortality was observed during hauling operations. Cory's shearwater was the most abundant species caught, followed by yellow-legged gull, Audouin's gull and gannet (Table 1). The most frequent modes of incidental capture were swallowing of the hooks (50%) and being hooked up by the wing (41%). Other species were caught on longlines during 1998 and 1999 during ®shing trips without observers: one shag, one great skua (Stercorarius skua) and four Balearic shearwaters (Punus mauretanicus). 3.2. Age of birds caught on longlines Out of 38 ringing recoveries of Cory's shearwater between 1992 and 1998 attributable to longlining, 10 were ringed as chicks and age at death was between 4 and 13 years old (mean=7.7 years). For the 28 birds ringed when they were >1 year old, age at death was between 2 and 15 years old (mean=7.9 years). More than 60% of the birds killed were 7 or more years old. All the Audouin's gulls recovered were ringed as chicks, and were between 4 and 13 years old when caught (mean=7.9 years, n=9 birds). One of the yellow-legged gulls caught was 3 years old and the other one was at least 4 years old. In 1999 one yellow-legged gull and one Audouin's gull caught during the observed operations had been ringed as chicks. Of the 38 Cory's shearwater recovered, 47.4% were ringed in the Columbretes Islands, and the other in the
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Table 1 Numbers of seabirds attending setting operations by longline vessels around Columbretes Islands (Spain), frequency of taking baits, and numbers caught Numbers attendinga
Frequency of occurrenceb
Seabird mortalityc
n
MeanS.D.
n
%
1998
Cory's shearwater Audouin's gull Yellow-legged gull Balearic shearwater Gannet Common tern (Sterna hirundo) Great skua Total
656 227 73 18 14 11 1 1000
5.0914.1 1.764.7 0.571.8 0.140.7 0.111.1 0.060.5 0.0070.0 ±
70 53 32 10 3 5 1
54.3 41.1 24.8 7.8 2.3 3.9 0.7
17 0 3 ± 2 ± ± 22
1 3 1 ± ± ± ± 5
38 9 2 ± ± ± ± 49
Number of setting operations
129
50
79
±
a b c
129
1999
1992±1998
Sum of maximum counts and mean counts during setting operations. Number of setting occasions (out of 129) when a species was seen. 1998 and 1999 data from observers on vessels; 1992±1998 data from ringing recoveries.
Balearic Islands (dierent colonies from Majorca and Minorca, see Fig. 1). All the recoveries of Audouin's gulls (n=9) came from the Ebro Delta colony, while all the yellow-legged gulls (n=2) were ringed in the Columbretes Islands. 3.3. Factors aecting seabird mortality 3.3.1. Seabird abundance during setting operations Seven dierent species were recorded trying to take baits during setting operations (Table 1). Cory's shearwater constituted 65.6% of the total number of birds (n=1000) attending the vessels during setting operations, and Audouin's gull 22.7%. During setting operations for bottom longlines, 77.5% (n=608) of the birds attempting to take baits were observed during sunrise, 15% during daytime hours and 7.5% at night. The number of seabirds at night, sunrise and daytime weighted by number of hooks set at each period were signi®cantly dierent (G2=190.8, P<0.05). In pelagic longlines only 32.5% of the total number of seabirds following the vessels during setting operations were observed during the ®rst 5 h of setting activities and 67.5% afterwards. This dierence was signi®cant (G1=60.8, P<0.05; weighted by the number of hooks set at each period). 3.3.2. Time of setting the longlines There were variations in the number of seabirds caught during the day both in 1998 and 1999. In the case of bottom longlines in both years most of the incidental captures of seabirds occurred around sunrise and in neither year were any birds caught during daytime (Fig. 2). The number of seabirds caught at night, sunrise and daytime weighted by number of hooks set at each period were signi®cantly dierent in both 1998 and 1999 (G2=9.6, P<0.01; G2=11.03, P<0.01, respectively). All the incidental captures of seabirds that occurred
Fig. 2. Number of birds caught on bottom longlines and number of hooks set at night, around sunrise (1 h before and after sunrise), or during the day in (a) 1998 and in (b) 1999.
before sunrise (n=3) were in nights with moonlight during setting and were Cory's shearwaters. In the case of pelagic longlines there were no incidental captures of seabirds during the ®rst 5 h of setting activities in spite of the fact that 52.7% of the hooks (n=40,338 hooks set) were set during that period (Fig. 3). There were signi®cant dierences in the number of seabirds caught at dierent hours weighted by the number of hooks set at each hour (G10=20.03, P<0.05). The number of seabirds caught by bottom and pelagic longlines per number of birds attending the vessels during setting operations was similar (2.73/100 birds following bottom longlines vessels and 2.79/100 birds attending pelagic longline vessels).
E.J. Belda, A. SaÂnchez / Biological Conservation 98 (2001) 357±363
Fig. 3. Number of birds caught on pelagic longlines and number of hooks set at dierent hours. Hours standardised to the hour of sunset, sunset hour=0.
3.3.3. Relationship to breeding cycle of Cory's shearwater The number of Cory's shearwaters caught was signi®cantly dierent during the dierent stages of the breeding cycle (G3=11.14, P<0.05; weighted by the number of hooks set). Most of the incidental captures took place during the chick rearing stage, while no mortality was observed during the incubation stage (Fig. 4). 4. Discussion 4.1. Seabird mortality and population impacts This is the ®rst work reporting bycatch rates of seabirds by longline ®sheries in the Mediterranean. The average bycatch rate of seabirds found in this study (0.16±0.69 birds per 1000 hooks set) is within the range of catch rates reported from other longline ®sheries in the southern hemisphere (0.02±5.3 birds per 1000 hooks; Barnes et al., 1997). This problem may be important, as other studies have shown that this source of mortality is the main cause of the decline of albatross populations and is likely to aect other seabird species (Weimerskirch and Jouventin, 1987; Weimerskirch et al., 1997). Thus, in the Mediterranean longline ®sheries may cause the decline of some seabird populations, especially if these ®shing techniques are expanded. The total number of hooks set by the 43 vessels of the longline ®shing ¯eet around Columbretes Islands can be obtained from the mean number of hooks set in each setting operation and the average number of ®shing days (pelagic longlines=60 days/year; bottom longlines vessels=210 days/year; mixed longlines vessels=60 days/year pelagic longlines+150 days/year bottom longlines). Thus, we estimated that 2.3 million pelagic hooks and 1.8 million bottom hooks are set each year. From the mortality rate found in this study, we there-
361
Fig. 4. Number of Cory's shearwaters on longlines and number of hooks set in relation to dierent stages of the breeding cycle of this species.
fore estimate that c. 656±2829 birds are killed annually around the Columbretes Islands. From this total estimate of birds caught annually around the Columbretes Islands, we may estimate that about 437±1867 Cory's shearwaters are killed annually around the Columbretes islands. Aguilar (1998) suggested that about 1300 Cory's shearwaters are caught annually by longlines in Balearic waters during the breeding season. The population in these two areas is about 11,000 breeding pairs (Diaz et al., 1996). Taking into account that 60% of the Cory's shearwaters killed were birds that have already reached the reproductive age, mortality on longlines may be aecting 4±6% of the breeding population in the area. Because procellariiforms are long-lived, show delayed maturity, have a low reproductive rate and high adult survival, even small increases in adult mortality may produce step population declines (Lebreton and Isenmann, 1976; Croxall et al., 1984). In the Columbretes Islands there has been a decline of 45% in the number of breeding pairs of Cory's shearwater in the last 2 years, associated with a decline in adult survival rates (Sanchez et al., 1999). Mortality aected mainly birds that have already reach the reproductive age (7 years; Mougin et al., 1986; Ristow et al., 1990), and it occurred during the breeding season, aecting specially during the period of rearing the chicks. We know from ringing recoveries that some breeding failures were due to mortality of one of the parents on longlines: in 1997, a male, which was previously ringed in the nest during incubation, was killed on longlines. The female deserted the nest few days later. There was a similar situation again in 1998. We have also found parents alive at the nests with hooks in its beak. Thus, not only adult survival rate is aected by mortality on longlines, but also reproductive success of Cory's shearwaters is reduced due to incidental mortality of adult birds, at least in the colonies of the Columbretes and Balearic Islands. Taking all these points into account, there are reasons enough to be concerned about populations of this species, especially in small
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populations, given the results of this study. Long-term monitoring of breeding colonies and incidental bycatch is needed to assess the extent of longline mortality in the western Mediterranean population. Audouin's gull was formerly considered Endangered (SPEC 1; Collar et al., 1994; Tucker and Heath, 1994), but in recent years the population has increased (del Hoyo et al., 1996; Hagemeijer and Blair, 1997). Some birds caught in this study came from the most important colony for the species, the Ebro Delta, which has also increased in the last years (c. 14,000 breeding pairs; Oro et al., 1996). We may estimate that the number of birds killed annually on longlines was between 72 and 311 (a maximum of 1.1% of the breeding population of the Ebro Delta). Therefore we suggest that longline ®sheries do not pose a risk for the conservation of this species at present. Observers did not record any mortality of Balearic shearwater during our study in 1998 and 1999. Most of the population is in the Atlantic during this period of the year normally (Yesou, 1986; Le Mao and Yesou, 1993). Nevertheless, this is a globally endangered species with a total population of c. 3000 pairs breeding in the Balearic islands. It has been caught on longlines and the population shows a declining trend (Aguilar, 1998) so more information on the impact of ®sheries is needed. 4.2. Factors aecting seabird mortality The results suggest that the most important factor in explaining the variation in seabird incidential captures is time of setting the longline, as has been pointed out in other studies (Brothers, 1991; Murray et al., 1993; Ashford et al., 1995; Cherel et al., 1996; Barnes et al., 1997). In our study area, there was a peak in the bycatch rate around sunrise (in bottom longline ®sheries) and another from mid afternoon until dusk (when pelagic longlines are working). The peak in the bycatch rate is associated with the peak in the number of birds attending longlines vessels (around sunrise and from mid afternoon until dusk). Thus, the in¯uence of time of setting may be due to the foraging activity of seabirds attending longline vessels. This may also explain the dierences in mortality found between pelagic and bottom longlines (mortality was higher in bottom longlines). These dierences in bird mortality disappeared when seabird abundance was taken into account. We have no evidence that other factors such as speed of the vessel during setting operations, dierences in hook or bait size in¯uence bird mortality. Cory's shearwater is considered as a nocturnal feeder (Cramp and Simmons, 1977) but our results suggest that it may be mainly a crepuscular feeder, at least in the study area. The other species observed taking baited hooks are mainly diurnal. All the species observed in this study attempting to take baits from longlines have
been observed trying to get discards from trawlers, and from sardine ®shing vessels (personal observation). In the study area, trawlers start ®shing activities one or two hours after sunrise and ®nish about mid-afternoon and sardine ®shing vessels throw discards early in the morning in their way from ®shing areas to harbour. It may be that discards from trawlers and sardine ®shing vessels in¯uence the abundance of seabirds attending longline vessels, and therefore bycatch rate. This interaction between dierent ®sheries needs more research. 4.3. Recommendations for reducing seabird bycatch Our results, and previous work (Vaske, 1991; Murray et al., 1993; Imber, 1994; Cherel et al., 1996; Barnes et al., 1997) show that setting the longlines outside the periods of sunrise and sunset will reduce seabird mortality. The measure of setting at night or during daytime is starting to be used by most of the ®shermen in the study area. Fishermen in the area have tried other mitigating measures. They used a towed line as a bird-scaring device but birds get used to it and it loses its eectiveness and, secondly, it interferes with the longline. A bird scaring line with suspended streamers, mounted on a pole on the stern of the vessel to gain sucient height above the sea (Brothers, 1994) have not been used in the area. These bird scaring lines have proved to be ecient in reducing seabird bycatch in other ®sheries (Brothers, 1991; Ashford et al., 1995; Lùkkeborg, 1996). Correct height also prevents the ®shing line interfering with the bird line. The mounting position must ensure that the bird line with streamers attached is towed astern directly above the area where baits enter the water (Brothers et al., 1999). We therefore recommend their use in the Mediterranean longline ®sheries, although studies on the eciency of this measure in small vessels should be conducted. Acknowledgements This work is a contribution to a project conducted by SEO/BirdLife and funded in 1998 by the Secretaria General de Pesca Maritima (Ministerio de Agricultura, Pesca y AlimentacioÂn) and in 1999 by the Project LIFE: ``ZEPAS's insulares en la Comunidad Valenciana'' of the Conselleria de Medio Ambiente, Generalitat Valenciana. This study bene®ted most from the kind-cooperation of all the skippers and the crew of vessels that participated, but especially to vessels Arrogante, Jubemar, Katan and Panollo I.E. Dunn and E. MõÂnguez helped us with the design of the study. E. Barba, E. Dunn, B.N.K. Davis and an anonymous referee made useful suggestions on an earlier version of the manuscript and R. Ribes helped us with the English. This
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work will not have been possible without the help at dierent stages of the study of M. GimeÂnez, J.J. JimeÂnez, T. Lamperez, R. MartõÂ, E. Revenga, and J.J. Vicent. References Aguilar, J.S., 1998. BiologõÂa y ConservacioÂn de la Pardela Balear Punus mauretanicus. Documents TeÁcnics de Conservacio (II ep) 2. Govern Balear, Palma de Mallorca. Ashford, J.R., Croxall, J.P., Rubilar, P.S., Moreno, C.A., 1995. Seabird Interactions with Longlining Operations for Dissostichus eleginoides around South Georgia, April to May 1994. CCAMLR Science 2, 111±121. Barnes, K.N., Ryan, P.G., Boix-Hinzen, Ch., 1997. The impact of the hake Merluccius spp. longline ®shery o South Africa on Procellariform seabirds. Biological Conservation 82, 227±234. Brothers, N., 1991. Albatross mortality and associated bait loss in the Japanese longline ®shery in the southern ocean. Biological Conservation 55, 255±268. Brothers, N.P., 1994. Principles of Bird Line Construction and Use to Reduce Bait Loss and Bird Deaths during Longline Setting. CCAMLR WG-IMALF 94/19. Brothers, N.P., Cooper, J., Lùkkeborg, S., 1999. The Incidental Catch of Seabirds by Longline Fisheries: Worldwide Review and Technical Guidelines for Mitigation. FAO Fisheries Circular No. 937. Cherel, Y., Weimerskirch, H., Duhamel, G., 1996. Interactions between longline vessels and seabirds in Kerguelen waters and a method to reduce seabird mortality. Biological Conservation 75, 63± 70. Collar, N.J., Crosby, M.J., Statters®eld, A.J., 1994. Birds to Watch 2: The World List of Threatened Birds. BirdLife Conservation, Series No. 4. Cramp, S., Simmons, K.E.L. (Eds.), 1977. The Birds of the Western Palearctic, Vol. 1. Oxford University Press, Oxford. Croxall, J.P., Evans, P.G.H., Schreiber, R.W., 1984. Editor's introduction: status and conservation or the world's seabirds. ICBP Technical Publication 2, 1±4. Croxall, J.P., Prince, P.A., 1990. Recoveries of wandering albatrosses Diomedea exulans ringed at South Georgia. Polar Rec. 29, 143±145. del Hoyo, J., Elliot, A., Sargatal, J. (Eds.), 1996. Handbook of the Birds of the World, Vol. 3. Hoatzin to Auks. Lynx Edicions, Barcelona. DõÂaz, M., Asensio, B., TellerõÂa, J.L. 1996. In J.M. Reyero (Ed.), Aves IbeÂricas. I. No Paseriformes. Madrid. Diego, J.A., Prieto, J.R., Fernandez, M.B. 1988. Captura de aves marinas en artes de pesca en Astrurias. In: Lopez, C. (Ed.), Aves Marinas GIAM Formentera, GOB Palma de Mallorca, pp. 17±31. Dunn, E., 1994. Interactions between Fisheries and Marine Birds: Research Recommendations. RSPB report, November 1994. Dunn, E., 1997. Sustainable ®sheries and seabirds. RSPB Conservation Review 11, 44±50. Follestad, A., Strann, K.B., 1991. Seabirds and ®shing nets: the scope and nature of the problem in Norway. NINA Oppdragsmelding 78, 1±14. Hagemeijer, E.J.M., Blair, M.J., 1997. The EBCC Atlas of European Breeding Birds: Their Distribution and Abundance. T and AD Poyser, London.
363
Imber, M.J., 1994. Report on a Tuna Longline Fishing Voyage aboard Southern Venture to Observe Seabird By-catch Problems. Science and Research Series, 65. NZ Department of Conservation, Wellington. Le Mao, P., YeÂsou, P., 1993. The annual cycle of Balearic shearwaters and western-Mediterranean Yellow-legged gulls: some ecological considerations. In: Aguilar, J.S., Monbaillou, X., Paterson, A. (Eds.), Estatus y Conservacion de Aves Marinas: Actas del II Simposium MEDMARAVIS. SEO, Madrid, pp. 135±146. Lebreton, J.D., Isenmann, P., 1976. Dynamique de la population camarguaise de Mouette Rieuse: un modeÁle matheÂmatique. Terre Vie 30, 529±549. Lùkkeborg, S., 1996. Sea-bird Bycatch and Bait Loss in Longlining using Dierent Setting Methods. CCAMLR Science Committee Working Group on Environmental Monitoring and Management, Bergen, Norway, August 1996. Mougin, J.L., Jouanin, C., Despin, B., Roux, F., 1986. The age of ®rst breeding of cory's shearwater on Selvagem Grande South Atlantic ocean and problems of ring loss. Ringing & Migration 7, 130±134. Murray, T. E., Bartle, J.A., Kalish, R., Taylor, P.R., 1993. Incidental capture of seabirds by Japanese blue®n tuna longline vessels in New Zealand waters, 1988±1992. Bilogical Conservation, 3, 181±210. OroÂ, D., Genovart, X., Ruiz, X, JimeÂnez, J., Garcia-Gans, J., 1996. Dierences in diet, population size and reproductive performance between two colonies of Audouin's Gull Larus audouinii aected by a trawling moratorium. Journal of Avian Biology 27, 245±251. Ristow, D., Feldmann, F., Scharlau, W., Wink, M., 1990. Population structure philopatry and mortality of cory's shearwater (Calonectris diomedea diomedea). Die Vogelwelt 111, 172, - 181. SaÂnchez, A, Barba, E., Belda, E.J., 1999. Variation of survival rates of Cory's shearwater in Columbretes Islands. 100 Years Ringing Symposium, Helgoland, Germany, September, 1999. Sanchez, A., Belda, E.J., 2000. Incidental catch of seabirds in longline ®sheries around Columbretes Islands, Mediterranean Sea (Spain). In: YeÂsou, P., Sultana, J. (Eds.), Monitoring and Conservation of Birds, Mammals and Sea-turtles of the Mediterranean and Black Seas. Environment Protection Department, Floriana, Malta, pp. 133±134. SaÂnchez, A., Castilla, A., 1997. La Pardela Cenicienta (Calonectris diomedea) en las Islas Columbretes. BiologõÂa y ConservacioÂn, Ayuntamiento de CastelloÂn. Tucker, G.M., Heath, M.F., 1994. Birds in Europe: Their Conservation Status. Birdlife International, Cambridge. Vaske, T., 1991. Seabird mortality on tuna longline ®shery in southern Brazil. Ciencia e Cultura 43, 388±390. Weimerskirch, H., Jouventin, P., 1987. Population dynamics of the wandering albatross Diomedea exulans of the Crozet Islands: causes and consequences of the population decline. Oikos 49, 315±322. Weimerskirch, H., Brothers, N., Jouventin, P., 1997. Population dynamics of wandering albatross Diomedea exulans and Amsterdam albatross D. Amsterdamensis in the Indian Ocean and their relationships with longline ®sheries: conservation implications. Biological Conservation 79, 257±270. Williams, K., 1976. The failure of Pearson's goodness of ®t statistic. Statistician 25, 49. YeÂsou, P., 1986. Balearic shearwaters summering in Western France. In: Medmaravis, X., Monbailliu, X.X. (Eds.), Mediterranean Marine Avifaune. Springer Verlag, Heidelberg, pp. 513±517. Zar, J.H., 1996. Biostatistical Analysis, 3rd Edition. Prentice Hall, NJ.