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Benefits from fish stocking—experiences from stocking young-of-the-year pikeperch, Stizostedion lucioperca L. to a bay in the Baltic Sea
ELSEVIER
Fisheries Research 32 (1997) 123-132
Benefits from fish stocking -experiences from stocking young-of-the-year pikeperch, Stizostedion lucioperca L. to a bay in the Baltic Sea Sture Hansson
*, Fredrik Arrhenius
‘, Sture Nellbring
2
Dept. Systems Ecology, Stockholm University, S-106 91 Stockholm, Sweden
Accepted 26 May 1997
Abstract To evaluate the economic viability of fish stocking, it must be possible to identify the stocked fish in the wild. To find a suitable marking method, young-of-the-year (YOY, length 8-10 cm) pikeperch (Stizostedion Z~cioperca) were tagged in different ways and stocked in ponds. Over a 185-day winter period, there was no significant size-dependent mortality, but clear differences in mortality among fish marked in different ways. Fingerling and streamer-tagged fish had significantly lower survivorship than fin-clipped (left pelvic fin) and colour-marked (Alcian Blue) fish, which were not different from controls. Seventeen thousand, seven hundred sixty-six YOY pikeperch were fin-clipped and released in a large bay of the Baltic Sea on the Swedish east coast in October 1991. The following year, 23% of all 1-yr-old fish caught came from this stocking. The proportion of marked fish was considerably higher close to the release site, showing that they were relatively stationary during their first year of life. A simple economic analysis demonstrates that pikeperch stocking can be very profitable. Based on the capital invested in the stocked YOY, the economic yield corresponds to an annual interest rate of 43%. 0 1997 Elsevier Science B.V. Keyword.vt Baltic Sea; Fish; Pikeperch; Stizostedion
lucioperca; Tagging; Stocking: Management
1. Introduction Pikeperch (Stizostedion lucioperca L.) occurs in many Swedish lakes and is among the more expensive fish species in Sweden (Fig. 1). In the 1995
_ Corresponding author. Fax: +46-8-158417; e-mail: [email protected] ’ Present address: Institute of Marine Research, Box 4, S-453 21 Lysekil, Sweden. * Present address: HZrb&rgesvPgen 2”, S-126 48 HPgersten, Sweden. 01657836/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO165-7836(97)00050-7
commercial freshwater fishery, pikeperch contributed the majority (23%) of the total catch value (Anon., 1996). In Swedish coastal waters, substantial pikeperch populations occur in several localities in the Baltic Sea archipelagos (Lehtonen et al., 1996) and their distribution has probably increased due to eutrophication (Hansson, 198.5; Hansson and Rudstam, 1990). There is a long tradition in Sweden of stocking pikeperch to lakes (Aim, 1920; Nilsson et al., 1987) and coastal areas. In the latter case, the goal is usually to supplement populations in areas with self-reproducing pikeperch, and most of the fish are
S. Hansson et al./Fisheries
124 -p&perch
salmon
---b
perch
2.1. Tagging methods
O_t’ 94101
94107
95m2
95rn8
96/03
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96/09
date, year/month Fig. 1. Prices paid for six fish species caught in the Baltic Sea. Herring, cod and salmon are mainly caught in offshore areas, while pikeperch, perch and pike are caught only in the coastal fishery. Data summarised from monthly numbers of the journal Yrkesjiskaren (‘The commercial fisherman’) (Anon., 1996).
stocked in autumn as young-of-the-year (YOY). The costs for these stockings are normally covered by angler associations, commercial fishermen and sometimes by governmental subsidies. Despite the high value of pikeperch and considerable investment in coastal stockings, they have never been economically evaluated. In this paper we report an economical analysis of a coastal stocking, from the viewpoint of the anglers/fishermen financing the activities. We will also discuss possible ecological benefits from pikeperch stockings. To evaluate stocking results, it must be possible to evaluate how the stocked fish contribute to the population abundance of the concerned species. To achieve this, the stocked fish must be marked in a way that does not decrease their survivorship. We will first report on experiments made to evaluate mortality rates caused by handling and different marking methods, and then report results from a stocking experiment in a Baltic Sea bay.
2. Material and methods The YOY pikeperch that we used were bought from commercial freshwater fish farms and of a size range generally used in stocking. Typical of this type of aquaculture in Sweden, the parental stock originated from lakes and not from brackish water.
We compared five treatments in experiments to evaluate mortalities from handling and different fish-marking methods: 1. removalof the left pelvic fin (Churchill, 1963) 2. colour marking with Alcian Blue, applied with a jet inoculator (Hart and Pitcher, 1969) on the left side of the fish, between the pelvic and the anal fin 3. tagging with fingerling tag FTF-69, purchased from Floy Tag and Manufacturing. This tag is a numbered 3.2 X 4.8 mm plastic oval, and was attached to the fish with a vinyl thread under the anterior dorsal fin 4. tagging with streamer tag FTSL-73, purchased from Floy Tag and Manufacturing. This tag is a numbered polyethylene film, 3 X 50 mm in size, and was applied through the musculature under the anterior dorsal fin. 5. In addition to tagged fish, we also used control fish which, except for the tagging, were handled in the same way as those tagged All fish were measured to the nearest mm (total length, Lagler, 1978) both before and after the experiments, after being anaesthetised with MS222. The three tagging experiments were performed at the Institute of Freshwater Research at Drottningholm, where the fish were kept in filtered (250 pm) flowthrough water of ambient temperature from Lake Malaren (approximately 59.9”N, 17.5”E). The size distribution of the fish used in these experiments is shown in Fig. 2. Mortalities are given as annual mortality rates (m> estimated from the equation Nt, = Nt, . exp[ - m(days/365)]. Nt, and Nt, is the number of fish at day t, and t,, respectively. 2.1.1. Experiment 1 This experiment was done to test for acute mortality caused by handling and tagging of fish. In total, 125 YOY pikeperch (25 per marking method, including control animals) were incubated overnight (9-10 October 1989) in a tank with a volume about 1 m3. 2.1.2. Experiment 2 To test for mortality differences among fish-marking methods over an intermediate time period, 500 fish (100 per treatment) were stocked for 42 days (10
S. Hansson et al. /Fisheries
Fig. 2. Length distribution of pikeperch used in the three marking experiments and in the stocking in Himmerfj’arden. The fish used for stocking were slightly larger than those used in the experiments.
Research 32 (I 997) 123- 132
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The stocking area, Himmerfjarden, is a relatively large (10 X 3 km) and deep (average _ 18 m, maximum 37 m) bay, eutrophied by discharge from a municipal sewage treatment plant. The phytoplankton primary production in the area is about 60% higher than in a nearby reference area (average annual primary production during 1977-1988, 206 and 132 g m-’ yr-‘, respectively, Johansson, 1992). Normally, the summer water temperature reaches _ 20°C above the thermocline, which usually is situated at a depth of around 10 m. The salinity is N 6%c in the surface and N 7%0 at depth > 30 m.
October to 21 November 1989) in a 11 X 28 m pond, with a maximum depth of 1.2 m and an approximate volume of 200 m3. 2.1.3. Experiment 3 Long term and overwinter mortality was studied in an experiment started on 10 October 1989, and ended after 185 days on 12 April 1990. As in the previous experiment, 500 fish (100 per treatment) were stocked in a pond. The size of the pond was 29 x 30 m, with a maximum depth of 1.2 m and an approximate volume of 680 m3. 2.2. Stocking experiment Based on the results from the pond experiments, we could use either fin-clipping or colour to mark stocked fish. Of these methods, fin-clipping was chosen because that method was fastest and hence more appropriate for mass-marking. Furthermore, we were concerned that the colour mark would be ‘diluted’ and hence difficult to detect as the fish grow. On 15-16 October 1991, 17766 YOY pikeperch were fin-clipped (left pelvic fin) and stocked in Himmerfj’arden, a bay in the northern Baltic proper (Fig. 3). The average size of these fish was 94 + 6 mm (standard deviation, IZ= 293) and 4.5 k 1.9 g (n = 781, which was slightly larger than in the three marking experiments in 1989 (Fig. 2). In the summer of 1992, we fished with gill nets (mesh sizes: 8, 10 and 12 mm knot-to-knot) to capture both the stocked and wild 1 + -yr-old pikeperch.
Fig. 3. The study area divided into sub-areas (A-E). The area we refer to as Himmerfarden in this paper includes sub-areas C and D. In October 1991, 17766 fin-clipped YOY pikeperch were released in the area marked in dark grey in the northern part of area C and in 1992 we fished for I-yr-old fish. Proportions of marked fish in different areas are shown in pie charts. Numbers in ovals, below the sub-area letters, show average catches in August, in numbers of YOY pikeperch per gill net per night.
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Research 32 (1997) 123-132
Secchi disk readings are 3-4 m in summer (JuneAugust). More information on environmental conditions in the area can be found in Hansson et al. (1990), Rudstam et al. (1992) and Johansson (1992).
used was 91 f 6 mm, and there was no significant length difference between the five treatments (analysis of variance, SPSS, 1993, p > 0.10). No mortality occurred.
2.3. Economic calculations
3.1.2. Experiment 2 After the 42-day incubation, 16% of the fish had died, but there was no significant difference in mortality among treatments (Table 1). There was no difference in length of pikeperch between the start of the experiment and after 42 days and no significant interaction indicating different size-selective mortality among treatments (Table 2). The general size difference among treatments was an experimental artefact and does not influence the interpretations above.
To simplify the economic analyses, we assumed that all pikeperch are caught at age 5 yrs in Himmerfjkden. At this age, the average size is > 40 cm, which is the Swedish minimum legal length for pikeperch (growth data from Hansson et al., 1997b). Because of high fishing pressure in Himmerfjkden by both commercial fishermen and anglers, it is reasonable to assume that most pikeperch are caught at this age. If the average fish is older that 5 yrs when caught, this is likely to increase the profitability of the stocking, as described in Section 3.3. The total economic costs of stocking includes both the cost of the stocked fish and the interest on this money that would be earned until the fish are caught and their value realised. The total stocking cost can thus be calculated as PC1 + i)‘, where P is the price paid for the fish, i is the annual interest rate and y is the number of years from stocking to capture. Using data from 1995, the cost for stocking 17 766 YOY pikeperch can be calculated as 70000 SEK, based on P = 48 000 SEK, i = 8% and y = 5 yrs. The value of pikeperch depends of the perspective of the person catching it. An angler may be willing to invest much time and expensive equipment, but it is difficult to convert these investments into a value per kg of fish. The fish value that we will use in evaluating the economy of the stocking is the average price paid to pikeperch fishermen in 1995 (32 SEK per kg, Fig. 1). Using this price in the economic analysis, the catch of stocked fish after 5 yrs must exceed 2200 kg to cover the stocking cost.
3. Results and discussion 3. I. Tagging methods 3.1.1. Experiment 1 This experiment was designed to study acute, overnight, mortality. The average size of the fish
3.1.3. Experiment 3 In the long-term experiment (185 days) the average mortality were 32%. The mortality varied significantly among the treatments, with particularly high mortalities among fingerling and streamer-tagged fish (Table 1). There was no difference in survivorship (x2 = 0.3, df = 2, p > 0.1) or in length (Table 3) among fin-clipped, colour marked and control fish. The average mortality rate (m) of fin-clipped, colour marked and control fish was significantly higher in the 42-day experiment than in the 185-day experiment (average 1.45 & 0.06 and 0.56 + 0.04, respectively, p < 0.001, ANOVA). This shows that there was an increased initial mortality, caused by Table 1 Number of pikeperch that died and survived in pond experiments. Homogeneity in frequencies was tested through the X*-test Treatment Control
Finclipped
Colourmarked
Fingerling tags
Experiment 2: Fish kept in a pond for 42 days (10 October to 21 November) 82 No. of alive 85 84 85 16 15 18 No. of dead 15 df=4;
x*=0.50;
Streamer tags
83 17
p>O.l
Experiment 3: Fish kept in a pondfor 185 days (10 October to 12 April) 75 74 65 No. of alive 77 25 26 35 No. of dead 23 df = 4; x2 = 24.6; p < 0.01
49 51
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Research 32 Cl9971 123- 132
Table 2 Results of a two-way ANOVA on length data from tagging experiment 2, where fish were kept in ponds for 42 days. The factors are time (start and termination of experiment) and treatment (the four marking methods plus the control). The lowest number of measures in any category was 82 ( = number of fish with fingerling tags at the termination of the experiment), and to balance the analyses 82 randomly drawn measures were used for each of the other categories F
Source of Variation
SS
df
MS
within + residual time treatment time X treatment
24263 13.2 3389 121
810
30.0 13.2 84.5 30.3
1 4 4
0.44 2.82 1.01
Significance of F
0.51 0.02 0.40
the transportation of the fish from the rearing ponds and/or the handling of the fish during the start of the experiments. Excluding this initial mortality from experiment 3, the estimated overwinter mortality rate from 2 1 November 21 to 12 April for fin-clipped, colour marked and control fish was 0.30 + 0.05. Experiment 3 also made it possible to study overwinter mortality. It has been suggested that the survivorship among YOY fish is size-dependent and
121
Table 3 Results of a two-way ANOVA on length data from experiment 3, where fish were kept in ponds for 185 days. The factors are time (start and termination of experiment) and treatment (fin-clipped. colour-marked and control). The lowest number of measures in any category was 74 ( = number of streamer tagged fish at the termination of the experiment), and to balance the analyses 74 randomly drawn measures were used for each of the other categories F
cif MS
Source of Variation
SS
within + residual time treatment time X treatment
12231 438 27.9 11.4 I 11.4 0.41 0.52 73.8 4 36.9 1.32 0.27 4.6 4 2.3 0.08 0.92
Significance
of F
that smaller individuals suffer from a higher, starvation-related, winter mortality (e.g., Kempe, 1962; Henderson and Holmes, 1988; Johnson and Evans, 1990, 1991; Post and Evans, 1989; SvBjdson, 1976). The conditions in experiment 3 were excellent to study this, as the pond had no predators which otherwise could have biased the results. Furthermore, feeding conditions were probably poorer than in nature, as there were hardly any benthic fauna or larger zooplankton in the pond. There are no data on
SO 70
-
* unmarked
-
the data can be described
X marked
-
fitting
this equation
by the relation:
wei,~ht=a”;lcnXht”
to our data gives:
“n-marked
length
X
. . marked
(mm) of age I pikeperch
Fig. 4. Relation between length and weight of 1-yr-old pikeperch from Himmerfj~den. Inserted in the graph are equations describing the relationship for the two groups of fish. Confidence intervals (95%) for the constants are not overlapping and the two groups hence different.
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the temperature, but the pond was ice covered during midwinter and it is reasonable to assume that the winter temperature was slightly above 0°C. Despite these harsh feeding and temperature conditions the size distribution of the fish at the start and at the end of the experiment was the same (Kolmogorov-Smirnov Z = 0.44, p > 0.5, SPSS, 1993). Individual growth of the numbered fingerling and streamer tagged fish actually showed a small decrease in length (95% CI for change in length, -0.92 to -0.03 mm). Our results, and those presented by Willemsen (1977), do not support the theory of starvation-related size-dependent mortality among 0 + pikeperch, suggested by Svkdson and Molin (1973). It is possible that size-selective predation may reduce the survival of small individuals. However, this is not a process specific for the winter, and the minimum size of surviving individuals will vary with the predation intensity. Size-selective winter mortality could have been used to explain the observed condition (weight to length relationship) differences between stocked and wild 1-yr-old pikeperch recaptured in our stocking experiment (Fig. 4). The argument would be as follows: the stocked YOY were smaller than those naturally produced, and winter mortality had hit small and thin individuals particularly. The pond result contradicts this explanation, as we found no increased mortality of small fish. Other factors (e.g., genetic or diet differences) must be considered possible explanations for this condition difference. Gut content analyses did, however, not indicate major diet differences between the two groups of fish (Hansson, Arrhenius and Nellbring, unpubl.) and there was no difference in the frequency of empty stomachs between the two groups ( x2 = 2.20, df= 1, p > 0.1). There are no data available to evaluate genetic differences between the stocked and the wild fish. 3.2. Stocking experiment In 1992, 506 fish were caught in the fishery for 1 + -yr-old pikeperch. Of these, 115 (23%) had no left pelvic fin. As no fish without the right pelvic fin were caught, we assumed that all these 115 specimens were stocked. The average size of these fish was 132 + 21 mm compared to 152 ). 32 mm for the
Table 4 Catches of pikeperch with ages 1 + yr in Himmerfj’tiden 1992. Effort is the number of gill net nights fished and CPE is the catch per effort Number of fish caught
May
June
July
August
marked unmarked effort CPE
0 0 24 0
3 123 204 0.62
9 18 80 0.34
103 2.50 256 1.38
wild (nonstocked) 1 + -yr-old pikeperch (statistically significant size different at p < 0.001, ANOVA). In the beginning of the stmrmer, catches of 1 + yr-old pikeperch were low and tagged fish constituted only a small proportion of the total catch. Later, both catches and the proportion of tagged fish increased (Table 4), with catch rate peaking in August. Overall in August, 29% of the 1-yr-old pikeperch were fin-clipped, but there was a considerable geographic variation (Fig. 3). The highest proportion of tagged fish (61%) was recorded in the area where the fish were stocked and this proportion decreased with increasing distance from the stocking area. The spatial distribution of marked fish indicated that they were relatively stationary during their first year of life. Other studies in the area indicate that adult fish are also relatively stationary within Himmerfjarden (Hansson et al., 1997c). The average catch rate, expressed as the number of 1 + -yr-old pikeperch per gill net per night, was lowest in the stocking area. We do not know to what extent this reflects a lower abundance in this area, but the result is certainly influenced by our capture methods. Particularly in this area, we fished in different habitats to identify the ‘typical’ sites for young pikeperch and we were consequently more efficient when fishing in the other areas. The typical habitat used by the 1 + -yr-old pikeperch was sheltered or moderately exposed, l-3 m deep bottoms, vegetated by macrophytes (c.f. Urho et al., 1990). When evaluating gill net catch data, it must be remembered that they do not represent unbiased estimates of the fish abundance, but are influenced by the catchability of the fish (cf. Hansson and Rudstam, 1995; Rudstam et al., 1984). We have clear indications that small pikeperch are more difficult to catch than larger fish. Despite considerable
S. Hansson et al./Fisheries
earlier efforts with mesh sizes as small as 6 mm (Hansson, unpubl.), we have caught only five YOY pikeperch in Himmerfjtiden. Our second indication that small pikeperch are more difficult to catch is based on the proportion of stocked fish in the catches. These fish were on average smaller than the naturally produced pikeperch, and their proportion in the catches increased more than 10 times between June and August (Table 4), at the same time as their average length increased from 95 to 135 mm. When evaluating the economy of the stocking, data of August will be used, as then the marked fish were recruited into the catchable population of 1 + -yr-old pikeperch. However, as the stocked 1-yr-old pikeperch were significantly smaller than those naturally produced, gill net catches may still underestimate the proportion of stocked fish. The economic analysis in Section 3.3.1 may thus be conservative with regards to the profitability of pikeperch stocking. 3.3. Possible benefits of pikeperch
stocking
3.3.1. Economic analysis There are two different causes by which each pikeperch eventually disappear from the population; natural mortality and fishery mortality. In our economic calculations, we have assumed that all pikeperch are caught at the age of 5 yrs, which implies that those not caught in the fishery die from natural causes at this age. This assumption simplifies the analyses but produces conservative economic predictions. If pikeperch survived to older age, some of them would also occur in the catches and would have to be included in the economic analysis. Actually, if fish are caught at older age than assumed in our calculations, the expected profitability of the stocking would increase as long as the interest rate on invested money is lower than the product of the specific growth of the fish and the proportion surviving from natural mortality. To determine to what pikeperch age the profitability of the stocking will increase, we assumed an annual adult fish natural mortality rate of 0.10 (Lehtonen, 1979; Lehtonen, 19831, that the growth follows the von Bertalanffy curve found by Hansson et al. (1997b) ( Lage = 538{ 1 - exp[ - 0.273(age + 0.195)]), L is length in mm and age is in years) and
Research 32 (1997) 123-132
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that the length (mm) to weight (gram) relation is weight = 3.19 + lop6 . length3.17. Under these circumstances, and an economic interest rate of 8%, the profitability will increase for fish up to almost 8 yrs of age. If the annual natural mortality is 0.05 or 0.2, corresponding ages are almost 9 and 7 yrs. After these ages have been reached, the profitability will decrease, but the age at capture must increase yet another couple of years before the profitability will be lower than when catching the fish at an age of 5 yrs. With the present intensive fishery for pikeperch, there is only a very small part of the population that survives 9 yrs or more (cf. Hansson et al., 1997a,b). This means that our economic analysis, based on catches only of 5-yr-old fish, is likely to underestimate the stocking profitability. Based on the economic assumptions (value of pikeperch 32 SEK per kg and actual stocking costs 70 000 SEK, see Section 21, the weight of the stocked fish caught after 5 yrs must exceed 2200 kg (3524 fish) to cover the stocking cost. Based on the latter figure and the 17 766 pikeperch stocked to the area, the maximum allowable annual mortality rate is 0.32, which is a considerably higher mortality than the 0.05-0.2 suggested for age 3 yrs and older Baltic Sea pikeperch by Lehtonen (1979, 1983). It is reasonable to assume, however, that the natural mortality is higher for younger fish and Deelder and Willemsen (1964) in their review, found that mortality is high during the first summer but then declines considerably. Our fish were stocked after this period of high mortality, but still the mortality is likely to be higher than for age 3 yrs and older fish. We have not found relevant mortality estimates for these younger fish, but a maximum mortality can be estimated. If the annual natural mortality is 0.1 for fish of age 3 yrs and older, the corresponding mortality from stocking at the age of 3 yrs can be as high as 0.62, and the required 3524 fish of age 5 yrs would still be produced. This mortality is higher than the overwinter mortality we recorded for the age 0 + fish (Experiment 3). These estimations hence show that there are good chances that a stocking like ours can be economically profitable. Another approach to the economical analysis is based on the pikeperch catches by professional fishermen and anglers in Himmerfj’lirden, and the propor-
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S. Hansson et al. / Fisheries Research 32 (1997) 123-132
tion of marked 1 + pikeperch in our 1992 fishery. On average, the total catch of pikeperch in Himmerfjarden is about 15 000 kg per year, and the value of these fish is 480000 SEK. To be profitable, the stocking must increase this value by 70000 SEK, which also means that stocked fish must constitute at least 13% of the catch. A comparison between this percentage and the total proportion of marked fish in August (29%), or the proportion of marked fish in Himmerfj’sirden (39%, Fig. 31, suggest that pikeperch stocking has a very strong economic potential. If 30% of a year class is derived from a stocking of the size of ours and the natural recruitment is average, this stocking will increase catches by 43% or 6400 kg. The value of these fish is 206000 SEK and the annual return on the invested capital will be 34%. In economical calculations like these, many different assumptions have to be made. It should be remembered that our data are from one year only, and we have assumed that this year is representative of average conditions. However, the early summer 1991 was cool and the natural recruitment in Himmerfj’arden might have been below average (cf. Buijse and Houthuijzen, 1992; Colby and Lehtonen, 1994; Deelder and Willemsen, 1964; Lappalainen and Lehtonen, 1995; Lehtonen and Lappalainen, 1995; Rundberg, 19771, in which case the value of the stocking is overestimated. For another Baltic Sea area at the same latitude, the Parnu Bay in Estonia, Lappalainen et al. (1995) found a significant negative relationship between year class strength and water temperature in June and they also describe the 1991 year class as weak; about l/3 of average. If the 1991 year class strength in our study area was also l/3 of average, one would expect three times higher catches of wild pikeperch under average conditions and consequently decreased proportions of marked fish (18% in Himmerfjarden, area C + D in Fig. 3, and 12% of all 1 + pikeperch). The stocking in Himmerfj’tiden would be profitable even under this scenario, although the margin would be decreased. As discussed above, however, the size selectivity of the gill nets might have made us underestimate the proportion of marked fish and hence underestimate the economic value of the stocking. Another conservative factor in our analysis is that we have assumed that all fish are caught at an age of 5 yrs, while it is reasonable that the fish are caught be-
tween ages 5-8 yrs, resulting in an increased profitability. 3.4. Other benefits of pikeperch stocking There are other effects which might be economically beneficial, but which are not included in our analyses. An increased pikeperch population probably decreases the efforts needed for a given catch, decreasing costs for manpower and equipment. Stocking may also increase the pikeperch spawning stock biomass, which potentially may increase the natural reproduction and hence also increase the value of stockings. As discussed in another article (Hansson et al., 1997a1, there are also other potentially beneficial effects of pikeperch stocking in Himmerfj’tiden. The intensive fishery in this bay has reduced the pikeperch population and also the predation pressure on herring. Increasing the pikeperch population by stocking could restore the intensity of this food web interaction to a more ‘natural’ level. A consequence of this would be a reduced herring population in the bay and this change could cascade through the food web and result in increased zooplankton abundances, increased grazing pressure on phytoplankton and decreased densities of phytoplankton (cf. the classical paper on biotic interactions by Hairston et al., 1960 and the trophic cascade concept of Carpenter et al., 1985). A negative correlation between the zooplankton biomass and the phytoplankton chlorophyll a concentration (Johansson, 1992) indicates that zooplankton may have this structuring potential in Himmerfj’arden. Pikeperch stocking could thus improve the water quality in Himmerfj’arden, which would add to the economic value of this management activity. In conclusion, our results imply that is possible to mark YOY pikeperch without increasing their mortality rate, which will make it possible to evaluate stocking operations. Our data also indicate that pikeperch stocking in coastal Baltic Sea waters can be economically profitable. Stockings may be needed to maintain both an intensive fishery and a ‘natural’ food web structure (Hansson et al., 1997a). Increased pikeperch populations may also help mitigate eutrophication (biomanipulation, c.f. Carpenter et al., 1995; Krienitz et al., 1996; Persson et al., 1993).
S. Hansson er af. / Fisheries Research 32 C19971 123- 132
Despite that this stocking study has been done for one year only, our results clearly show that pikeperch stockings may have a strong potential in natural resource management.
Acknowledgements This study was supported by a grant from the Swedish Council for Forestry and Agricultural Research. The tagging experiments were conducted in the facilities of the Institute of Freshwater Research at Drottningholm and made possible through the support from its staff. We are very much indebted to the Ame Asplund family for practical support in stocking and catching fish. Helen Bjurulf, Bjijm Klinga, Marcus Nikula and Emil Rydin helped with the field work. Daniel Duplisea, Carl Folke, BengtOwe Jansson and Daniel Schindler made valuable comments on earlier versions of this manuscript.
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ture, population control, and competition. Am. Natural. 94, 421-425. Hansson, S., 1985. Effects of eutrophication on fish communities, with special reference to the Baltic Sea-a literature review. Rep. Inst. Freshw. Res., Drottningholm 62, 36-56. Hansson, S., Rudstam, L., 1990. Eutrophication and Baltic fish communities. Ambio 19, 123-125. Hansson, S., Rudstam, L.G., 1995. Gillnet catches as an estimate of fish abundance: A comparison between vertical gillnet catches and hydroacoustic abundances of Baltic Sea herring (&pea harengus) and sprat (Sprattus spruffus). Can. J. Fish. Aquat. Sci. 52, 75-83. Hansson, S., Larsson. U., Johansson, S., 1990. Selective predation by herring and mysids, and zooplankton community structure in a Baltic Sea coastal area. J. Plankt. Res. 12, 1099-1116. Hansson, S., Arrhenius, F., Nellbring, S., 1997a. Food web interactions in a Baltic Sea coastal area. Forage Fishes in Marine Ecosystems. Alaska Sea Grant Report, in press. Hansson, S., Arrhenius, F., Nellbring, S., 1997b. Diet and growth of pikeperch (Srizosredion luciopercal in a Baltic Sea area. Fish. Res. Hansson, S., Elmgren, R., Hobbie, J.E., Larsson, U., Fry, B., Johansson, S.. 1997~. The stable nitrogen isotope ratio as a marker of food web interactions and fish migration. Ecology. Hart, P.J.B., Pitcher, T.J., 1969. Field trials of fish-marking using a jet inoculator. J. Fish Biol. 1, 383-385. Henderson, P.A., Holmes, R.H.A.B.R., 1988. Size-selective overwintering mortality in the sand smelt, Arherina boyeri Risso, and its role in population regulation. J. Fish Biol. 33, 221-233. Johansson, S.. 1992. Regulating factors for coastal zooplankton community structure in the northern Baltic proper. PhD thesis, Dept. of Zoology, Stockholm Univ., Stockholm. Johnson, T.B.. Evans, D.O., 1990. Size-dependent winter mortality of young-of-the-year white perch: Climate warming and invasion of the Laurentian Great Lakes. Trans. Am. Fish. Sot. 119, 301-313. Johnson, T.B., Evans, D.O., 1991. Behaviour, energetics. and associated mortality of young-of-the-year white perch (Morone americana) and yellow perch (Perca flavescens) under simulated winter conditions. Can. J. Fish. Aquat. Sci. 48, 672-680. Kempe, 0.. 1962. The growth of the roach (Leuciscus rutilus L.) in some Swedish lakes. Rep. Inst. Freshw. Res., Drottningholm 44. 42-104. Krienitz, L., Kasprzak, P., Koschel. R., 1996. Long term study on the influence of eutrophication, restoration and biomanipulation on the structure and development of phytoplankton communities in Feldberger Haussee (Baltic Lake District Germany). Hydrobiologia 330, 89- 110. Lagler, K.F.. 1978. Capture, sampling and examination of fishes. In: Bagenal, T. (Ed.), Methods for Assessment of Fish Production in Fresh Waters. IBP handbook no. 3. Blackwell, Oxford, pp. 7-47. Lappalainen, J., Lehtonen, H., 1995. Year-class strength of pikeperch (Stizosredion lucioperca L.1 in relation to environmental factors in a shallow Baltic Bay. Ann. Zool. Fen. 32, 41 l-419.
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Lappalainen, J., Erm, V., Lehtonen, H., 1995. Pikeperch, Stizosfedion lucioperca CL.), catch in relation to juvenile density and water temperature in F%rnu Bay, Estonia. Fish. Manage. Ecol. 2, 113-120. Lehtonen, H., 1979. Stock assessment of pikeperch (Stizostedion Zucioperca L.) in the Helsinki sea area. Finn. Fish. Res. 3, 1-12. Lehtonen, H., 1983. Stocks of pikeperch (Stizostedion lucioperca L.) and their management in the Archipelago Sea and in the Gulf of Finland. Finn. Fish. Res. 5, 1-16. Lehtonen, H., Lappalainen, J., 1995. The effects of climate on the year-class variations of certain freshwater fish species. In: Beamish, R.J. (Ed.), Climate Change and Northern Fish Populations. Can. Spec. Publ. Fish. Aquat. Sci. 121, 37-44. Lehtonen, H., Hansson, S., Winkler, H., 1996. Biology of pikeperch, Stizosfedion Zucioperca (L.) in the Baltic Sea. Ann. Zool. Fen. 33, 525-535. Nilsson, N.-A., Nyman, L., Fbrst, M., Johansson, N., Steffner, N., 1987. Utplantering av fisk och kraftdjur-rekommendationer. Information f&n Sotvattenslaboratoriet, Drottningholm 10, l90. Persson, L., Johansson, L., Andersson, G., Diehl, S., Hamrin, S.F., 1993. Density-dependent interactions in lake ecosystems: Whole lake perturbation experiments. Oikos 66, 193-208. Post, J.R., Evans, D.O., 1989. Size-dependent overwinter mortality of young-of-the-year yellow perch (Perca jlauescens): Laboratory, in situ enclosure, and field experiments. Can. J. Fish. Aquat. Sci. 46, 1958-1968.
Rudstam, L.G., Magnuson, J.J., Tonn, W.T., 1984. Size selectivity of passive fishing gear: A correction for encounter probability applied to gill nets. Can. J. Fish. Aquat. Sci. 41, 1252-1255. Rudstam, L.G., Hansson, S., Johansson, S., Larsson, U., 1992. Dynamics of planktivory in a coastal area of the northern Baltic Sea. Mar. Ecol. Progr. Ser. 80, 159-173. Rundberg, H., 1977. Trends in harvests of pikeperch (Stizostedion lucioperca), Eurasian perch (Perca Juuiatilis), and northern pike (&ox lucius) and associated environmental changes in lakes Malaren and Hj’almaren, 1914-1974. J. Fish. Res. Board Can. 34, 1720-1724. SPSS, 1993. SPSS for Windows, Base System User’s Guide, Release 6.0. SPSS, Chicago, 828 pp. Sv’tidson, G., 1976. Interspecific population dominance in fish communities of Scandinavian lakes. Rep. Inst. Freshw. Res., Drottningholm 55, 144-171. Sv%rdson, G., Molin, G., 1973. The impact of climate on Scandinavian populations of the sander, Stizostedion lucioperca CL.). Rep. Inst. Freshw. Res., Drottningholm 53, 112-139. Urho, L., Hilden, M., Hudd, R., 1990. Fish reproduction and the impact of acidification in the Kyrijnjoki River estuary in the Baltic Sea. Environ. Biol. Fish. 27, 273-283. Willemsen, J., 1977. Population dynamics of percids in Lake IJssel and some smaller lakes in the Netherlands. J. Fish. Res. Board Can. 34, 1710-1719.
Fisheries Research 32 (1997) 123-132
Benefits from fish stocking -experiences from stocking young-of-the-year pikeperch, Stizostedion lucioperca L. to a bay in the Baltic Sea Sture Hansson
*, Fredrik Arrhenius
‘, Sture Nellbring
2
Dept. Systems Ecology, Stockholm University, S-106 91 Stockholm, Sweden
Accepted 26 May 1997
Abstract To evaluate the economic viability of fish stocking, it must be possible to identify the stocked fish in the wild. To find a suitable marking method, young-of-the-year (YOY, length 8-10 cm) pikeperch (Stizostedion Z~cioperca) were tagged in different ways and stocked in ponds. Over a 185-day winter period, there was no significant size-dependent mortality, but clear differences in mortality among fish marked in different ways. Fingerling and streamer-tagged fish had significantly lower survivorship than fin-clipped (left pelvic fin) and colour-marked (Alcian Blue) fish, which were not different from controls. Seventeen thousand, seven hundred sixty-six YOY pikeperch were fin-clipped and released in a large bay of the Baltic Sea on the Swedish east coast in October 1991. The following year, 23% of all 1-yr-old fish caught came from this stocking. The proportion of marked fish was considerably higher close to the release site, showing that they were relatively stationary during their first year of life. A simple economic analysis demonstrates that pikeperch stocking can be very profitable. Based on the capital invested in the stocked YOY, the economic yield corresponds to an annual interest rate of 43%. 0 1997 Elsevier Science B.V. Keyword.vt Baltic Sea; Fish; Pikeperch; Stizostedion
lucioperca; Tagging; Stocking: Management
1. Introduction Pikeperch (Stizostedion lucioperca L.) occurs in many Swedish lakes and is among the more expensive fish species in Sweden (Fig. 1). In the 1995
_ Corresponding author. Fax: +46-8-158417; e-mail: [email protected] ’ Present address: Institute of Marine Research, Box 4, S-453 21 Lysekil, Sweden. * Present address: HZrb&rgesvPgen 2”, S-126 48 HPgersten, Sweden. 01657836/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO165-7836(97)00050-7
commercial freshwater fishery, pikeperch contributed the majority (23%) of the total catch value (Anon., 1996). In Swedish coastal waters, substantial pikeperch populations occur in several localities in the Baltic Sea archipelagos (Lehtonen et al., 1996) and their distribution has probably increased due to eutrophication (Hansson, 198.5; Hansson and Rudstam, 1990). There is a long tradition in Sweden of stocking pikeperch to lakes (Aim, 1920; Nilsson et al., 1987) and coastal areas. In the latter case, the goal is usually to supplement populations in areas with self-reproducing pikeperch, and most of the fish are
S. Hansson et al./Fisheries
124 -p&perch
salmon
---b
perch
2.1. Tagging methods
O_t’ 94101
94107
95m2
95rn8
96/03
Research 32 (1997) 123-132
96/09
date, year/month Fig. 1. Prices paid for six fish species caught in the Baltic Sea. Herring, cod and salmon are mainly caught in offshore areas, while pikeperch, perch and pike are caught only in the coastal fishery. Data summarised from monthly numbers of the journal Yrkesjiskaren (‘The commercial fisherman’) (Anon., 1996).
stocked in autumn as young-of-the-year (YOY). The costs for these stockings are normally covered by angler associations, commercial fishermen and sometimes by governmental subsidies. Despite the high value of pikeperch and considerable investment in coastal stockings, they have never been economically evaluated. In this paper we report an economical analysis of a coastal stocking, from the viewpoint of the anglers/fishermen financing the activities. We will also discuss possible ecological benefits from pikeperch stockings. To evaluate stocking results, it must be possible to evaluate how the stocked fish contribute to the population abundance of the concerned species. To achieve this, the stocked fish must be marked in a way that does not decrease their survivorship. We will first report on experiments made to evaluate mortality rates caused by handling and different marking methods, and then report results from a stocking experiment in a Baltic Sea bay.
2. Material and methods The YOY pikeperch that we used were bought from commercial freshwater fish farms and of a size range generally used in stocking. Typical of this type of aquaculture in Sweden, the parental stock originated from lakes and not from brackish water.
We compared five treatments in experiments to evaluate mortalities from handling and different fish-marking methods: 1. removalof the left pelvic fin (Churchill, 1963) 2. colour marking with Alcian Blue, applied with a jet inoculator (Hart and Pitcher, 1969) on the left side of the fish, between the pelvic and the anal fin 3. tagging with fingerling tag FTF-69, purchased from Floy Tag and Manufacturing. This tag is a numbered 3.2 X 4.8 mm plastic oval, and was attached to the fish with a vinyl thread under the anterior dorsal fin 4. tagging with streamer tag FTSL-73, purchased from Floy Tag and Manufacturing. This tag is a numbered polyethylene film, 3 X 50 mm in size, and was applied through the musculature under the anterior dorsal fin. 5. In addition to tagged fish, we also used control fish which, except for the tagging, were handled in the same way as those tagged All fish were measured to the nearest mm (total length, Lagler, 1978) both before and after the experiments, after being anaesthetised with MS222. The three tagging experiments were performed at the Institute of Freshwater Research at Drottningholm, where the fish were kept in filtered (250 pm) flowthrough water of ambient temperature from Lake Malaren (approximately 59.9”N, 17.5”E). The size distribution of the fish used in these experiments is shown in Fig. 2. Mortalities are given as annual mortality rates (m> estimated from the equation Nt, = Nt, . exp[ - m(days/365)]. Nt, and Nt, is the number of fish at day t, and t,, respectively. 2.1.1. Experiment 1 This experiment was done to test for acute mortality caused by handling and tagging of fish. In total, 125 YOY pikeperch (25 per marking method, including control animals) were incubated overnight (9-10 October 1989) in a tank with a volume about 1 m3. 2.1.2. Experiment 2 To test for mortality differences among fish-marking methods over an intermediate time period, 500 fish (100 per treatment) were stocked for 42 days (10
S. Hansson et al. /Fisheries
Fig. 2. Length distribution of pikeperch used in the three marking experiments and in the stocking in Himmerfj’arden. The fish used for stocking were slightly larger than those used in the experiments.
Research 32 (I 997) 123- 132
125
The stocking area, Himmerfjarden, is a relatively large (10 X 3 km) and deep (average _ 18 m, maximum 37 m) bay, eutrophied by discharge from a municipal sewage treatment plant. The phytoplankton primary production in the area is about 60% higher than in a nearby reference area (average annual primary production during 1977-1988, 206 and 132 g m-’ yr-‘, respectively, Johansson, 1992). Normally, the summer water temperature reaches _ 20°C above the thermocline, which usually is situated at a depth of around 10 m. The salinity is N 6%c in the surface and N 7%0 at depth > 30 m.
October to 21 November 1989) in a 11 X 28 m pond, with a maximum depth of 1.2 m and an approximate volume of 200 m3. 2.1.3. Experiment 3 Long term and overwinter mortality was studied in an experiment started on 10 October 1989, and ended after 185 days on 12 April 1990. As in the previous experiment, 500 fish (100 per treatment) were stocked in a pond. The size of the pond was 29 x 30 m, with a maximum depth of 1.2 m and an approximate volume of 680 m3. 2.2. Stocking experiment Based on the results from the pond experiments, we could use either fin-clipping or colour to mark stocked fish. Of these methods, fin-clipping was chosen because that method was fastest and hence more appropriate for mass-marking. Furthermore, we were concerned that the colour mark would be ‘diluted’ and hence difficult to detect as the fish grow. On 15-16 October 1991, 17766 YOY pikeperch were fin-clipped (left pelvic fin) and stocked in Himmerfj’arden, a bay in the northern Baltic proper (Fig. 3). The average size of these fish was 94 + 6 mm (standard deviation, IZ= 293) and 4.5 k 1.9 g (n = 781, which was slightly larger than in the three marking experiments in 1989 (Fig. 2). In the summer of 1992, we fished with gill nets (mesh sizes: 8, 10 and 12 mm knot-to-knot) to capture both the stocked and wild 1 + -yr-old pikeperch.
Fig. 3. The study area divided into sub-areas (A-E). The area we refer to as Himmerfarden in this paper includes sub-areas C and D. In October 1991, 17766 fin-clipped YOY pikeperch were released in the area marked in dark grey in the northern part of area C and in 1992 we fished for I-yr-old fish. Proportions of marked fish in different areas are shown in pie charts. Numbers in ovals, below the sub-area letters, show average catches in August, in numbers of YOY pikeperch per gill net per night.
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Research 32 (1997) 123-132
Secchi disk readings are 3-4 m in summer (JuneAugust). More information on environmental conditions in the area can be found in Hansson et al. (1990), Rudstam et al. (1992) and Johansson (1992).
used was 91 f 6 mm, and there was no significant length difference between the five treatments (analysis of variance, SPSS, 1993, p > 0.10). No mortality occurred.
2.3. Economic calculations
3.1.2. Experiment 2 After the 42-day incubation, 16% of the fish had died, but there was no significant difference in mortality among treatments (Table 1). There was no difference in length of pikeperch between the start of the experiment and after 42 days and no significant interaction indicating different size-selective mortality among treatments (Table 2). The general size difference among treatments was an experimental artefact and does not influence the interpretations above.
To simplify the economic analyses, we assumed that all pikeperch are caught at age 5 yrs in Himmerfjkden. At this age, the average size is > 40 cm, which is the Swedish minimum legal length for pikeperch (growth data from Hansson et al., 1997b). Because of high fishing pressure in Himmerfjkden by both commercial fishermen and anglers, it is reasonable to assume that most pikeperch are caught at this age. If the average fish is older that 5 yrs when caught, this is likely to increase the profitability of the stocking, as described in Section 3.3. The total economic costs of stocking includes both the cost of the stocked fish and the interest on this money that would be earned until the fish are caught and their value realised. The total stocking cost can thus be calculated as PC1 + i)‘, where P is the price paid for the fish, i is the annual interest rate and y is the number of years from stocking to capture. Using data from 1995, the cost for stocking 17 766 YOY pikeperch can be calculated as 70000 SEK, based on P = 48 000 SEK, i = 8% and y = 5 yrs. The value of pikeperch depends of the perspective of the person catching it. An angler may be willing to invest much time and expensive equipment, but it is difficult to convert these investments into a value per kg of fish. The fish value that we will use in evaluating the economy of the stocking is the average price paid to pikeperch fishermen in 1995 (32 SEK per kg, Fig. 1). Using this price in the economic analysis, the catch of stocked fish after 5 yrs must exceed 2200 kg to cover the stocking cost.
3. Results and discussion 3. I. Tagging methods 3.1.1. Experiment 1 This experiment was designed to study acute, overnight, mortality. The average size of the fish
3.1.3. Experiment 3 In the long-term experiment (185 days) the average mortality were 32%. The mortality varied significantly among the treatments, with particularly high mortalities among fingerling and streamer-tagged fish (Table 1). There was no difference in survivorship (x2 = 0.3, df = 2, p > 0.1) or in length (Table 3) among fin-clipped, colour marked and control fish. The average mortality rate (m) of fin-clipped, colour marked and control fish was significantly higher in the 42-day experiment than in the 185-day experiment (average 1.45 & 0.06 and 0.56 + 0.04, respectively, p < 0.001, ANOVA). This shows that there was an increased initial mortality, caused by Table 1 Number of pikeperch that died and survived in pond experiments. Homogeneity in frequencies was tested through the X*-test Treatment Control
Finclipped
Colourmarked
Fingerling tags
Experiment 2: Fish kept in a pond for 42 days (10 October to 21 November) 82 No. of alive 85 84 85 16 15 18 No. of dead 15 df=4;
x*=0.50;
Streamer tags
83 17
p>O.l
Experiment 3: Fish kept in a pondfor 185 days (10 October to 12 April) 75 74 65 No. of alive 77 25 26 35 No. of dead 23 df = 4; x2 = 24.6; p < 0.01
49 51
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Table 2 Results of a two-way ANOVA on length data from tagging experiment 2, where fish were kept in ponds for 42 days. The factors are time (start and termination of experiment) and treatment (the four marking methods plus the control). The lowest number of measures in any category was 82 ( = number of fish with fingerling tags at the termination of the experiment), and to balance the analyses 82 randomly drawn measures were used for each of the other categories F
Source of Variation
SS
df
MS
within + residual time treatment time X treatment
24263 13.2 3389 121
810
30.0 13.2 84.5 30.3
1 4 4
0.44 2.82 1.01
Significance of F
0.51 0.02 0.40
the transportation of the fish from the rearing ponds and/or the handling of the fish during the start of the experiments. Excluding this initial mortality from experiment 3, the estimated overwinter mortality rate from 2 1 November 21 to 12 April for fin-clipped, colour marked and control fish was 0.30 + 0.05. Experiment 3 also made it possible to study overwinter mortality. It has been suggested that the survivorship among YOY fish is size-dependent and
121
Table 3 Results of a two-way ANOVA on length data from experiment 3, where fish were kept in ponds for 185 days. The factors are time (start and termination of experiment) and treatment (fin-clipped. colour-marked and control). The lowest number of measures in any category was 74 ( = number of streamer tagged fish at the termination of the experiment), and to balance the analyses 74 randomly drawn measures were used for each of the other categories F
cif MS
Source of Variation
SS
within + residual time treatment time X treatment
12231 438 27.9 11.4 I 11.4 0.41 0.52 73.8 4 36.9 1.32 0.27 4.6 4 2.3 0.08 0.92
Significance
of F
that smaller individuals suffer from a higher, starvation-related, winter mortality (e.g., Kempe, 1962; Henderson and Holmes, 1988; Johnson and Evans, 1990, 1991; Post and Evans, 1989; SvBjdson, 1976). The conditions in experiment 3 were excellent to study this, as the pond had no predators which otherwise could have biased the results. Furthermore, feeding conditions were probably poorer than in nature, as there were hardly any benthic fauna or larger zooplankton in the pond. There are no data on
SO 70
-
* unmarked
-
the data can be described
X marked
-
fitting
this equation
by the relation:
wei,~ht=a”;lcnXht”
to our data gives:
“n-marked
length
X
. . marked
(mm) of age I pikeperch
Fig. 4. Relation between length and weight of 1-yr-old pikeperch from Himmerfj~den. Inserted in the graph are equations describing the relationship for the two groups of fish. Confidence intervals (95%) for the constants are not overlapping and the two groups hence different.
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the temperature, but the pond was ice covered during midwinter and it is reasonable to assume that the winter temperature was slightly above 0°C. Despite these harsh feeding and temperature conditions the size distribution of the fish at the start and at the end of the experiment was the same (Kolmogorov-Smirnov Z = 0.44, p > 0.5, SPSS, 1993). Individual growth of the numbered fingerling and streamer tagged fish actually showed a small decrease in length (95% CI for change in length, -0.92 to -0.03 mm). Our results, and those presented by Willemsen (1977), do not support the theory of starvation-related size-dependent mortality among 0 + pikeperch, suggested by Svkdson and Molin (1973). It is possible that size-selective predation may reduce the survival of small individuals. However, this is not a process specific for the winter, and the minimum size of surviving individuals will vary with the predation intensity. Size-selective winter mortality could have been used to explain the observed condition (weight to length relationship) differences between stocked and wild 1-yr-old pikeperch recaptured in our stocking experiment (Fig. 4). The argument would be as follows: the stocked YOY were smaller than those naturally produced, and winter mortality had hit small and thin individuals particularly. The pond result contradicts this explanation, as we found no increased mortality of small fish. Other factors (e.g., genetic or diet differences) must be considered possible explanations for this condition difference. Gut content analyses did, however, not indicate major diet differences between the two groups of fish (Hansson, Arrhenius and Nellbring, unpubl.) and there was no difference in the frequency of empty stomachs between the two groups ( x2 = 2.20, df= 1, p > 0.1). There are no data available to evaluate genetic differences between the stocked and the wild fish. 3.2. Stocking experiment In 1992, 506 fish were caught in the fishery for 1 + -yr-old pikeperch. Of these, 115 (23%) had no left pelvic fin. As no fish without the right pelvic fin were caught, we assumed that all these 115 specimens were stocked. The average size of these fish was 132 + 21 mm compared to 152 ). 32 mm for the
Table 4 Catches of pikeperch with ages 1 + yr in Himmerfj’tiden 1992. Effort is the number of gill net nights fished and CPE is the catch per effort Number of fish caught
May
June
July
August
marked unmarked effort CPE
0 0 24 0
3 123 204 0.62
9 18 80 0.34
103 2.50 256 1.38
wild (nonstocked) 1 + -yr-old pikeperch (statistically significant size different at p < 0.001, ANOVA). In the beginning of the stmrmer, catches of 1 + yr-old pikeperch were low and tagged fish constituted only a small proportion of the total catch. Later, both catches and the proportion of tagged fish increased (Table 4), with catch rate peaking in August. Overall in August, 29% of the 1-yr-old pikeperch were fin-clipped, but there was a considerable geographic variation (Fig. 3). The highest proportion of tagged fish (61%) was recorded in the area where the fish were stocked and this proportion decreased with increasing distance from the stocking area. The spatial distribution of marked fish indicated that they were relatively stationary during their first year of life. Other studies in the area indicate that adult fish are also relatively stationary within Himmerfjarden (Hansson et al., 1997c). The average catch rate, expressed as the number of 1 + -yr-old pikeperch per gill net per night, was lowest in the stocking area. We do not know to what extent this reflects a lower abundance in this area, but the result is certainly influenced by our capture methods. Particularly in this area, we fished in different habitats to identify the ‘typical’ sites for young pikeperch and we were consequently more efficient when fishing in the other areas. The typical habitat used by the 1 + -yr-old pikeperch was sheltered or moderately exposed, l-3 m deep bottoms, vegetated by macrophytes (c.f. Urho et al., 1990). When evaluating gill net catch data, it must be remembered that they do not represent unbiased estimates of the fish abundance, but are influenced by the catchability of the fish (cf. Hansson and Rudstam, 1995; Rudstam et al., 1984). We have clear indications that small pikeperch are more difficult to catch than larger fish. Despite considerable
S. Hansson et al./Fisheries
earlier efforts with mesh sizes as small as 6 mm (Hansson, unpubl.), we have caught only five YOY pikeperch in Himmerfjtiden. Our second indication that small pikeperch are more difficult to catch is based on the proportion of stocked fish in the catches. These fish were on average smaller than the naturally produced pikeperch, and their proportion in the catches increased more than 10 times between June and August (Table 4), at the same time as their average length increased from 95 to 135 mm. When evaluating the economy of the stocking, data of August will be used, as then the marked fish were recruited into the catchable population of 1 + -yr-old pikeperch. However, as the stocked 1-yr-old pikeperch were significantly smaller than those naturally produced, gill net catches may still underestimate the proportion of stocked fish. The economic analysis in Section 3.3.1 may thus be conservative with regards to the profitability of pikeperch stocking. 3.3. Possible benefits of pikeperch
stocking
3.3.1. Economic analysis There are two different causes by which each pikeperch eventually disappear from the population; natural mortality and fishery mortality. In our economic calculations, we have assumed that all pikeperch are caught at the age of 5 yrs, which implies that those not caught in the fishery die from natural causes at this age. This assumption simplifies the analyses but produces conservative economic predictions. If pikeperch survived to older age, some of them would also occur in the catches and would have to be included in the economic analysis. Actually, if fish are caught at older age than assumed in our calculations, the expected profitability of the stocking would increase as long as the interest rate on invested money is lower than the product of the specific growth of the fish and the proportion surviving from natural mortality. To determine to what pikeperch age the profitability of the stocking will increase, we assumed an annual adult fish natural mortality rate of 0.10 (Lehtonen, 1979; Lehtonen, 19831, that the growth follows the von Bertalanffy curve found by Hansson et al. (1997b) ( Lage = 538{ 1 - exp[ - 0.273(age + 0.195)]), L is length in mm and age is in years) and
Research 32 (1997) 123-132
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that the length (mm) to weight (gram) relation is weight = 3.19 + lop6 . length3.17. Under these circumstances, and an economic interest rate of 8%, the profitability will increase for fish up to almost 8 yrs of age. If the annual natural mortality is 0.05 or 0.2, corresponding ages are almost 9 and 7 yrs. After these ages have been reached, the profitability will decrease, but the age at capture must increase yet another couple of years before the profitability will be lower than when catching the fish at an age of 5 yrs. With the present intensive fishery for pikeperch, there is only a very small part of the population that survives 9 yrs or more (cf. Hansson et al., 1997a,b). This means that our economic analysis, based on catches only of 5-yr-old fish, is likely to underestimate the stocking profitability. Based on the economic assumptions (value of pikeperch 32 SEK per kg and actual stocking costs 70 000 SEK, see Section 21, the weight of the stocked fish caught after 5 yrs must exceed 2200 kg (3524 fish) to cover the stocking cost. Based on the latter figure and the 17 766 pikeperch stocked to the area, the maximum allowable annual mortality rate is 0.32, which is a considerably higher mortality than the 0.05-0.2 suggested for age 3 yrs and older Baltic Sea pikeperch by Lehtonen (1979, 1983). It is reasonable to assume, however, that the natural mortality is higher for younger fish and Deelder and Willemsen (1964) in their review, found that mortality is high during the first summer but then declines considerably. Our fish were stocked after this period of high mortality, but still the mortality is likely to be higher than for age 3 yrs and older fish. We have not found relevant mortality estimates for these younger fish, but a maximum mortality can be estimated. If the annual natural mortality is 0.1 for fish of age 3 yrs and older, the corresponding mortality from stocking at the age of 3 yrs can be as high as 0.62, and the required 3524 fish of age 5 yrs would still be produced. This mortality is higher than the overwinter mortality we recorded for the age 0 + fish (Experiment 3). These estimations hence show that there are good chances that a stocking like ours can be economically profitable. Another approach to the economical analysis is based on the pikeperch catches by professional fishermen and anglers in Himmerfj’lirden, and the propor-
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tion of marked 1 + pikeperch in our 1992 fishery. On average, the total catch of pikeperch in Himmerfjarden is about 15 000 kg per year, and the value of these fish is 480000 SEK. To be profitable, the stocking must increase this value by 70000 SEK, which also means that stocked fish must constitute at least 13% of the catch. A comparison between this percentage and the total proportion of marked fish in August (29%), or the proportion of marked fish in Himmerfj’sirden (39%, Fig. 31, suggest that pikeperch stocking has a very strong economic potential. If 30% of a year class is derived from a stocking of the size of ours and the natural recruitment is average, this stocking will increase catches by 43% or 6400 kg. The value of these fish is 206000 SEK and the annual return on the invested capital will be 34%. In economical calculations like these, many different assumptions have to be made. It should be remembered that our data are from one year only, and we have assumed that this year is representative of average conditions. However, the early summer 1991 was cool and the natural recruitment in Himmerfj’arden might have been below average (cf. Buijse and Houthuijzen, 1992; Colby and Lehtonen, 1994; Deelder and Willemsen, 1964; Lappalainen and Lehtonen, 1995; Lehtonen and Lappalainen, 1995; Rundberg, 19771, in which case the value of the stocking is overestimated. For another Baltic Sea area at the same latitude, the Parnu Bay in Estonia, Lappalainen et al. (1995) found a significant negative relationship between year class strength and water temperature in June and they also describe the 1991 year class as weak; about l/3 of average. If the 1991 year class strength in our study area was also l/3 of average, one would expect three times higher catches of wild pikeperch under average conditions and consequently decreased proportions of marked fish (18% in Himmerfjarden, area C + D in Fig. 3, and 12% of all 1 + pikeperch). The stocking in Himmerfj’tiden would be profitable even under this scenario, although the margin would be decreased. As discussed above, however, the size selectivity of the gill nets might have made us underestimate the proportion of marked fish and hence underestimate the economic value of the stocking. Another conservative factor in our analysis is that we have assumed that all fish are caught at an age of 5 yrs, while it is reasonable that the fish are caught be-
tween ages 5-8 yrs, resulting in an increased profitability. 3.4. Other benefits of pikeperch stocking There are other effects which might be economically beneficial, but which are not included in our analyses. An increased pikeperch population probably decreases the efforts needed for a given catch, decreasing costs for manpower and equipment. Stocking may also increase the pikeperch spawning stock biomass, which potentially may increase the natural reproduction and hence also increase the value of stockings. As discussed in another article (Hansson et al., 1997a1, there are also other potentially beneficial effects of pikeperch stocking in Himmerfj’tiden. The intensive fishery in this bay has reduced the pikeperch population and also the predation pressure on herring. Increasing the pikeperch population by stocking could restore the intensity of this food web interaction to a more ‘natural’ level. A consequence of this would be a reduced herring population in the bay and this change could cascade through the food web and result in increased zooplankton abundances, increased grazing pressure on phytoplankton and decreased densities of phytoplankton (cf. the classical paper on biotic interactions by Hairston et al., 1960 and the trophic cascade concept of Carpenter et al., 1985). A negative correlation between the zooplankton biomass and the phytoplankton chlorophyll a concentration (Johansson, 1992) indicates that zooplankton may have this structuring potential in Himmerfj’arden. Pikeperch stocking could thus improve the water quality in Himmerfj’arden, which would add to the economic value of this management activity. In conclusion, our results imply that is possible to mark YOY pikeperch without increasing their mortality rate, which will make it possible to evaluate stocking operations. Our data also indicate that pikeperch stocking in coastal Baltic Sea waters can be economically profitable. Stockings may be needed to maintain both an intensive fishery and a ‘natural’ food web structure (Hansson et al., 1997a). Increased pikeperch populations may also help mitigate eutrophication (biomanipulation, c.f. Carpenter et al., 1995; Krienitz et al., 1996; Persson et al., 1993).
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Despite that this stocking study has been done for one year only, our results clearly show that pikeperch stockings may have a strong potential in natural resource management.
Acknowledgements This study was supported by a grant from the Swedish Council for Forestry and Agricultural Research. The tagging experiments were conducted in the facilities of the Institute of Freshwater Research at Drottningholm and made possible through the support from its staff. We are very much indebted to the Ame Asplund family for practical support in stocking and catching fish. Helen Bjurulf, Bjijm Klinga, Marcus Nikula and Emil Rydin helped with the field work. Daniel Duplisea, Carl Folke, BengtOwe Jansson and Daniel Schindler made valuable comments on earlier versions of this manuscript.
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