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Freshwater (2002) 47, 2425–2434

Comparison of losses of planktivorous fish by and seine-fishing in a lake undergoing long-term biomanipulation

KLAUS WYSUJACK and THOMAS MEHNER Department of Biology and of Fishes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany

SUMMARY 1. stocking at artificially high densities and are the two common approaches to reduce the amount of planktivorous and benthivorous fish in lake biomanipulation programmes. Both measures have advantages and disadvantages, but their relative efficacy has not previously been directly compared. 2. We calculated the average annual catch of roach and bream in a lake undergoing long- term biomanipulation (Feldberger Haussee, Germany) by seining each year between 1992 and 1998. We compared this value with a bioenergetics estimate of annual consumption rates of the dominant cohorts of , pikeperch and pike, in 1997 and 1998. We also determined composition and length distribution of fish in stomachs of the piscivores. 3. Roach was the dominant prey species of both pikeperch and pike, whereas bream was rarely taken by either piscivorous species. Seining removed on average larger specimens of roach than were found in the stomachs of the piscivores. 4. Based on stocking densities of the piscivores, published mortality rates, and individual consumption rates, feeding of pikeperch and pike on roach exceeded the manual removal of roach by seining by a factor of 4–15 () in 1997 and 1998. 5. Based on these results, a combination of fishing and piscivore enhancement is recommended. Whereas the stocks of adult roach and bream have to be reduced mainly by fishing, the predation of piscivores should be directed predominantly towards the juvenile zooplanktivorous fish. Therefore, small size-classes of piscivorous fish should be promoted by , including stocking and harvest regulations.

Keywords: biomanipulation, diet, piscivorous fish, predation, seining

lakes is to enhance herbivorous , Introduction in particular, by reducing their principal predators, Biomanipulation is a widely accepted for planktivorous fish. More daphnids are expected to improving water quality of eutrophicated lakes. either reduce biomass or to shift the Recent reviews (McQueen, 1998; Hansson et al., size structure of the algal , such that 1998; Drenner & Hambright, 1999) list a large number ultimately the water becomes clearer than before the of fully or partially successful whole-lake experi- manipulation. Unwanted planktivorous and benthi- ments. The central aim in manipulating food webs in vorous fishes are removed either manually by seining or trawling with large nets or by increasing the Correspondence: Klaus Wysujack, Department of Biology and predation pressure exerted by piscivores in the lake. Ecology of Fishes, Leibniz-Institute of Freshwater Ecology and Both strategies have been shown to work effectively. Inland Fisheries, PO Box 850119, 12561 Berlin, Germany. Manual fish removal by seining was an important E-mail: [email protected] measure in restoring water quality of shallow lakes in

2002 Blackwell Science Ltd 2425 2426 K. Wysujack and T. Mehner the Netherlands (Meijer et al., 1999), the United in the lake. Based on consumption data, we estimated Kingdom (Moss et al., 1996), Denmark (Jeppesen et al., how many piscivorous fish were required to balance 1990), and Sweden (Bergman, Hamrin & Romare, the annual removal of by seining, which 1999). Food-web manipulation by removing cyprinids has been regularly conducted in the lake since 1985. In with a trawl was successful in a deeper Finnish lake addition, we calculated potential densities and the (Horppila et al., 1998). At least partially successful annual population consumption rates of the dominant biomanipulations by intensive piscivore stocking predator groups by using their stocking numbers and were reported from Germany (Benndorf, 1995), Den- published mortality rates. mark (Berg, Jeppesen & Søndergaard, 1997), Norway (Brabrand, Faafeng & Nilssen, 1990), Poland (Prejs Methods et al., 1997), the U.S.A. (Kitchell, 1992), and Canada (Ramcharan et al., 1995). However, the overall success The eutrophic, dimictic Feldberger Haussee (136 ha, rate of biomanipulations was substantially lower 6.4 m mean depth) is located about 150 km north of when relying on piscivore stocking only compared Berlin in the Baltic lake district, Germany. It has been with a combination of piscivore stocking and manual biomanipulated since 1985 by regularly removing removal of planktivorous fish (Hansson et al., 1998; planktivorous and benthivorous fish, mainly roach, Drenner & Hambright, 1999). Rutilus rutilus (L.), and bream, Abramis brama (L.), Both manual removal and stocking with piscivores with a large seine net (Mehner et al., 2001). If pisci- have advantages and disadvantages. They differ in vores were caught, they were returned to the lake. The labour and cost intensity and in their susceptibility to net covered an area of about 5 ha and the mesh-size of the concerns of commercial and recreational fisheries, the cod-end ranged from 16 to 20 mm. Seining was nature conservation and welfare. Removal of conducted 15–25 times per year at changing locations planktivorous fishes by seining has been found to be in all parts of the lake. Since 1988, the lake has also effective in shifting shallow lakes from the turbid to been stocked with variable numbers and sizes of the clear-water state only if at least 75% of all fish piscivorous fish (Table 1). Stocking included pike- could be harvested within one or two seasons (Perrow perch, Sander lucioperca (L.), pike, Esox lucius L., et al., 1997; Hansson et al., 1998; Meijer et al., 1999). European catfish, Silurus glanis L., Eurasian perch, Achieving this goal is very labour intensive, partic- Perca fluviatilis L., and occasionally , Anguilla ularly in large lakes (>100 ha). Stocking with pisci- anguilla (L.). Between 1995 and 1997, the fisheries for vores, in contrast, is relatively fast and involves little piscivores had been closed completely, and currently labour investment, but it requires repeated interven- perch and catfish are still fully protected from fisheries. tions over many years and costs of stocking material For pike and pikeperch, the fishing size limit has been can be exceedingly high (e.g. about 5 e per 500 g of increased to 60 cm. pikeperch in Germany). However, piscivores are the As the fish community composition in the seine net favoured target species of recreational fisheries in has been analysed in detail only since 1992, we many countries (Bogelius, 1998; Wolos, Teodorowicz concentrated our comparison on the period between ) & Brylski, 1998; Lathrop et al., 2002), making piscivore 1992 and 1998. Average annual catches (kg haul 1)of stocking often an attractive option also from a fisheries roach and bream, and of the piscivores, pikeperch, management point of view. Thus, to adopt the best pike, perch >15 cm in length, and catfish, were strategy in a given situation, it is important to know calculated directly from the seine net catches. which of the approaches is more effective in removing Population structure, diet composition and con- planktivorous fish, and whether they can replace or sumption of piscivorous fish were studied in 1997 and complement each other. 1998. In addition to catches from the seine net, We tackled these questions by comparing seining piscivores were caught by a small beach seine (littoral yield and predation losses of planktivorous and zone), a purse seine (), gill-netting (lit- benthivorous fishes in a eutrophic, medium-sized toral zone and open water) and by electrofishing lake undergoing long-term biomanipulation. We esti- () from March to October. Consequently, mated consumption of planktivorous fish by analy- fish of each piscivorous species for the diet analyses sing the diet composition of the dominant piscivores were caught throughout the year in all of the

2002 Blackwell Science Ltd, Freshwater Biology, 47, 2425–2434 Seining and piscivore stocking as biomanipulation measures 2427

Table 1 Overview of fish removal by )1 seining and stocking with juvenile Stocking numbers (ind. ha ) Seining hauls piscivorous fish in Feldberger Haussee )1 Year (no. year ) Pikeperch Pike Perch Catfish between 1992 and 1998. Total length of fish stocked: pikeperch: 8–10 cm; pike: 1992 19 220.6 – – – 25–30 cm; perch: 22–24 cm; catfish: 1993 18 220.6 – – – 40–50 cm 1994 20 – 5.9 – 0.6 1995 16 – 14.7 – 4.4 1996 18 36.8 19.1* 22.1 4.2 1997 20 – – 20.6 – 1998 15 – 8.9 2.4 –

) *Additional stocking with pike fingerlings of 7–8 cm total length (47.8 ind. ha 1). lake in which they occurred. Fish were measured, for both species. Corresponding average wet weight weighed and some scales were removed for age (ww in g) increments were subsequently estimated readings. Approximately half of all analysed food from length–weight regressions (pike: ww ¼ 0.0053 samples were collected by stomach flushing. For that TL3.046, n ¼ 408, R2 ¼ 0.988; pikeperch: ww ¼ 0.0047 purpose, the fish were anaesthetised with MS 222 and TL3.162, n ¼ 497, R2 ¼ 0.993). released to the lake after the procedure. Stomach Individual consumption for the period 1 March contents were frozen and later analysed in the until 31 October 1997 and 1998 was calculated based laboratory. Prey fish were identified to species, their on the estimated weight increments for the cohorts lengths determined using bone fragments (Mann & using the computerised version of a bioenergetics Beaumont, 1980; Mehner, 1990), and weighed (wet model (Hanson et al., 1997). This model sums up the weight). determined weight increment and the energetic costs During 1997 and 1998, the pike population was of basal metabolism, activity, reproduction and waste dominated by the 1995 and 1996 year-classes, which losses, and thus calculates consumption rates by comprised 52 and 18% by number in 1997 and 40 and taking fluctuations in water temperature into account. 41% in 1998, respectively, according to summarised The model has been tested and frequently used for catches with active gears. Age-classes could be easily Sander and Esox species (Fox, 1991; Heikinheimo & distinguished because the fish were marked at stock- Korhonen, 1996; Salonen, Helminen & Sarvala, 1996). ing. In the same years, the pikeperch population was For a more detailed description, see Kitchell, Stewart dominated by the 1993 and 1995 year-classes. The two & Weininger (1977) and Hanson et al. (1997). cohorts could be distinguished from the length– Physiological parameters for pikeperch and pike frequency distribution. However, discriminating were used as defined from original studies in the between pikeperch stocked in 1992 and 1993 from computer model (Hanson et al., 1997). In case of the length–frequency distribution was sometimes pikeperch, data mainly originate from studies on the difficult, and reading of opercular bones was restric- closely related North American walleye, Stizostedion ted to 101 fish. Consequently, the older cohort (mainly vitreum vitreum (Mitchill). However, as tempera- the 1993 year-class) may have included also some ture tolerance differs between Eurasian and North pikeperch from the 1992 year-class. In 1997, the 1993 American species, temperature-dependent parame- and 1995 year-classes comprised 46 and 48% of the ters were modified according to published values. population, compared with 33 and 52% in 1998. Thus, we increased optimum and maximum tempera- Average annual growth rates (1 March until 31 Octo- ture for consumption in pikeperch to 27 and 33 C, ber 1997 and 1998) for an individual of each of the respectively, and optimum and maximum tempera- four cohorts of pike and pikeperch were calculated ture for respiration to 33 and 36 C, respectively from total lengths (TL in cm) derived from scale (133 (Salonen et al., 1996). For pike, the optimum and pike; Wysujack et al., 2001) and opercular bone (101 maximum temperature for consumption were pikeperch) readings by assuming that 95% of the decreased to 20 and 30 C, respectively (Raat, 1990), annual growth occurred between March and October. whereas for the temperature dependence of respir- Lengths were calculated using the Fraser–Lee method ation a slightly steeper slope was used (increase from

2002 Blackwell Science Ltd, Freshwater Biology, 47, 2425–2434 2428 K. Wysujack and T. Mehner 0.055 to 0.070; Brett & Groves, 1979; Heikinheimo & Korhonen, 1996). Energy densities were determined ) for fish from Feldberger Haussee as 5380 J g 1 ww in ) ) pike, 5840 J g 1 in pikeperch, and 5500 J g 1 in roach (unpublished data). Water temperature during the study period was calculated as the average from recordings at 1-m depth intervals of the upper 5 m of the lake (i.e. the zone fully oxygenated throughout the year) at approximately biweekly intervals in 1997 and 1998. Individual consumption rates were balanced by biomass with the average annual catch of roach and bream from the seine net in order to estimate how many piscivores would have been needed to equal manual removal. Unfortunately, and biomass estimates for the piscivores were not avail- able. Therefore, to check the reliability of our comparisons of manual removal and predation, we calculated a range of potential densities of pike and pikeperch based on stocking numbers and published annual mortality rates. In pike and pikeperch, total instantaneous mortality rates in natural waters may ) vary between 0.3 and 0.6 year 1, equivalent to an annual survival of 55–74% (Craig, 1987, 1996). This Fig. 1 high survival may be realistic considering that both Average annual catches by seining of roach and bream (a), and of the piscivores pikeperch, pike, catfish and perch pike and pikeperch were stocked at the end of their (b), in Feldberger Haussee between 1992 and 1998. Data repre- first year after the fingerling stage (Table 1), and sent arithmetic means + SD of 15–20 seining hauls per year. that fisheries mortality should have been negli- gible because fishing of piscivores was prohibited between 1995 and 1997. By using the above outer 1998. The relative proportions of the four most limits of the mortality ranges, we calculated a important piscivorous species were not constant, but minimum and maximum annual consumption rate pikeperch and pike dominated in most years of the piscivores to allow for a comparison with the (Fig. 1b). manual removal. Diet of the two year-classes each of pike and pikeperch that dominated in 1997 and 1998 was predominantly composed of roach (Fig. 2). Perch Results was preyed upon in much smaller proportions. Bream The average annual catch of fish with the seine net was taken only by the 2+ pike in both years, and fluctuated considerably among years between 1992 constituted <4% of prey biomass. As catfish and and 1998. Roach was the dominant species in 5 of perch were less important than pike and pikeperch in ) 7 years (97–262 kg haul 1), whereas bream was dom- the seine net catches, and their diet contained a larger ) inant in 1993 and 1998 (32–200 kg haul 1) (Fig. 1a). fraction of crayfish, Orconectes limosus Raf., they were The mean annual fish removal between 1992 and not considered in further analysis. The length distri- ) ) 1998 was 157 kg haul 1 for roach and 110 kg haul 1 bution of roach in pike and pikeperch stomachs for bream, corresponding to 2.82 tonnes of roach differed considerably, both among the predator and 1.95 tonnes of bream for the whole lake. Total groups, and between the predators and the seine catches of piscivorous fish (pikeperch, pike, catfish, net. Because of the mesh size of the net, most of the perch >15 cm) increased steadily between 1992 and caught roach were >16 cm (Fig. 3). This size group ) 1998, reaching the highest value (79.4 kg haul 1)in was also well represented in the diet of 4+ pikeperch

2002 Blackwell Science Ltd, Freshwater Biology, 47, 2425–2434 Seining and piscivore stocking as biomanipulation measures 2429

Fig. 2 Diet composition of the four dominant piscivore cohorts in Feldberger Haussee in 1997 and 1998. in 1997 and 5+ pikeperch in 1998. In contrast, the other predator groups fed predominantly on smaller roach with a TL of 10–16 cm (Fig. 3). The piscivores fed roach in amounts equivalent to 1.1–4.7 times of their average weight (Table 2). The highest consumption of roach by an individual was calculated for a 3+ pike in 1998, which had consumed about 3360 g roach between March and October (Table 2). The lowest consumption was calculated for a 2+ pikeperch in 1997 with about 950 g of roach Fig. 3 Length distribution of roach (2 cm TL classes) caught by seining (total catches in 1995–98) and in the stomachs of the consumed. Consequently, to compensate for the dominant piscivore cohorts in Feldberger Haussee in 1997 and 2.82 tonnes of roach caught annually by seining (see 1998. The arrows denote the fish length above which roach in the above), 838 pike of age class 3+, or up to 2959 pikeperch seine net were caught representatively. of age class 2+ would have been needed (Table 2). Given an assumed instantaneous of was estimated at 4.1–15.1 times the average manual ) 0.3–0.6 year 1, the stocking density of 0+ pikeperch at removal of roach by seining in the period from 1992 to the end of the first year in 1993 (Table 1; total of about 1998 (Fig. 4). 30 000 individuals) should have resulted in approxi- mately 2700–9000 4+ pikeperch in the lake in 1997 Discussion (Table 3). Values for the other piscivore groups calculated in the same way ranged from 600 to 1100 The scenarios presented in this study for Feldberger (2+ pike), 1400–1900 (1+ pike) and 2800–9400 (2+ Haussee in 1997 and 1998 suggest that the average pikeperch) in 1997. Values for 1998 were calculated in annual removal of roach by seining was markedly the same way (Table 3). lower than the predation of these fish by piscivores. Total consumption of roach as the product of Even if the mortality rates of piscivores were above ) individual consumption (Table 2) and number of the assumed upper limit of 0.6 year 1, the stocking piscivores (Table 3) varied between 9.4 and 34.4 ton- densities of pike and pikeperch would have been high ) nes year 1, depending on the assumed mortality rate enough to exert a predation pressure on roach that (Fig. 4). Annual consumption of planktivorous fish exceeded the seining yield. This result points to the

2002 Blackwell Science Ltd, Freshwater Biology, 47, 2425–2434 2430 K. Wysujack and T. Mehner

Table 2 Initial and final wet weight, and total fish and roach consumption (wet weight) between 1 March and 31 October for an average individual of the four dominant piscivore groups present in Feldberger Haussee in 1997 and 1998. Also given are the numbers ) of piscivorous fish of each group needed to compensate the average annual removal of roach by seining (2.82 tonnes year 1), and the ratio between annual roach consumption and mean weight of piscivores

Piscivore Initial Final Total fish Total roach Number of Roach consumption Year group weight (g) weight (g) consumption (g) consumption (g) fish required per mean weight

1997 Pikeperch 2+ 196 593 1136 953 2960 2.80 Pikeperch 4+ 1566 1872 2220 1869 1510 1.32 Pike 1+ 142 565 1552 1332 2120 4.70 Pike 2+ 857 1521 3707 3208 880 2.81 1998 Pikeperch 3+ 614 967 1425 1133 2490 1.47 Pikeperch 5+ 1888 2540 3044 2416 1170 1.10 Pike 2+ 587 1231 2929 2600 1090 3.06 Pike 3+ 1556 1792 3768 3365 840 2.02

Table 3 Estimated numbers of the dom- Number of Low-mortality High-mortality )1 )1 inant piscivore groups present in Feld- Year Piscivore group stocked fish* scenario (0.3 year ) scenario (0.6 year ) berger Haussee in 1997 and 1998, based on 1997 Pikeperch 2+ † – 9400 2800 stocking numbers and published instan- Pikeperch 4+ 30 000 9000 2700 taneous mortality rates Pike 1+ 2600 1900 1400 Pike 2+ 2000 1100 600 1998 Pikeperch 3+ † – 10 400 2300 Pikeperch 5+ 30 000 6700 1500 Pike 2+ 2600 1400 800 Pike 3+ 2000 800 300

*See Table 1 for size. †Because stocking numbers are unknown, the density was estimated from the relative ratio of this year-class to the 1993 year-class in the catches.

however, that stocking alone is insufficient to build up a high piscivore biomass fast enough to improve water quality in the short term. For example, pike- perch was first stocked in 1988, but a high biomass was not observed until 1996, suggesting that the total protection of piscivores from fishing between 1995 and 1997 contributed substantially to the steep increase in the density of piscivores observed in 1996 (Mehner et al., 2001). Exclusive stocking of piscivores to biomanipulate a lake has rarely been performed. In a recent review, Drenner & Hambright (1999) report that 17% (7 of 41 Fig. 4 Average annual removal (+SD) of roach by seining in case studies) of all whole-lake manipulations 1992–98 and estimated annual consumption of roach by the four involved only stocking measures. In addition, the dominating piscivore cohorts in Feldberger Haussee in 1997 and 1998. Consumption rates are based on either high piscivore exclusive stocking strategy was found to be the least ) mortality rates (Z ¼ 0.6 year 1) or low mortality rates successful approach of all biomanipulation measures. ) (Z ¼ 0.3 year 1) during the years between stocking and analyses. However, in their review, the authors did not distinguish between different approaches of piscivore great potential of stocking piscivores as the sole or enhancement. Benndorf (1990) stated that a diverse main biomanipulation measure. The delayed increase piscivore community (species and size-classes) is of piscivore biomass in the seining hauls reveals, needed for long-term success in biomanipulation.

2002 Blackwell Science Ltd, Freshwater Biology, 47, 2425–2434 Seining and piscivore stocking as biomanipulation measures 2431 Piscivores stocked in stratified lakes were predomin- stocking from an early of pikeperch to a antly pikeperch or walleye (Kitchell, 1992; Benndorf, dominance of perch and the littoral predator pike may 1995). Pikeperch is a pelagic predator, naturally well be an effective strategy for long-term biomani- occurring mainly in eutrophic or even hypertrophic pulation in stratified lakes. lakes (Craig, 1987). Besides the direct predatory If piscivores are present in sufficient densities, they impact, stocking by pikeperch has been found to consume large amounts of planktivorous species. In affect the behaviour of planktivorous fish such that Feldberger Haussee, an average piscivorous fish of immigration to the pelagic zone is hampered about 1 kg consumed 1.1–4.7 kg of roach per year (Brabrand & Faafeng, 1993; Ho¨lker et al., 2002). (Table 2). To keep planktivorous fish at a given Therefore, zooplankton feeding may be suppressed biomass level, their net production must be removed. by the simple presence of piscivorous fish. However, If a production-to-biomass ratio in roach of 0.5 (mean pikeperch show strong interannual fluctuations in value of data for planktivorous fish summarised in and hence in population densities Barthelmes, 1981) can be assumed and a roach (Barthelmes, 1988; Buijse & Houthuijzen, 1992). consumption of 2.4 times the mean weight of the Additionally, there is evidence that the piscivores (average of values in Table 2), a piscivore efficiency and activity of pikeperch in clear lakes of biomass of about 21% of the biomass moderate trophic state is substantially lower than in would be sufficient to prevent net growth of the turbid lakes (Ryder, 1977; Greenberg, Paszkowski & planktivorous fish population. The proportion by Tonn, 1995). Consequently, if biomanipulation is weight of piscivores in the seine net catches increased initially successful, pikeperch should eventually be in Feldberger Haussee from 5% in 1992 to 20% in 1997 replaced by other piscivorous species that are and 1998. Pike and pikeperch alone accounted for 14% naturally common in lakes with higher water of the total fish biomass in 1997 and 1998. These transparency, to sustain the desired effects estimates suggest that both feeding by piscivores and in the long term (Wysujack et al., in press). seining contributed to maintaining the roach popula- In shallow lakes, pike has been the favoured species tion over the last years at a rather constant level. for stocking, provided submerged macrophytes are However, there were notable differences in the present in sufficient density (Berg et al., 1997; Meijer & effects of seining and piscivory on the species Hosper, 1997). The most critical function of dense composition and length distribution of the target macrophyte beds in pike recruitment is to afford fish. Seining removed a large amount of bream, a protection from predatory threat to juvenile pike by species that was rarely encountered in the stomachs their larger conspecifics (Bry, 1996; Skov & Berg, of the piscivores. This observation could be the result 1999). Therefore, in stratified lakes such as Feldberger of the relatively small size of the piscivores in Haussee, where macrophytes colonisation is restricted Feldberger Haussee and the greater body height to a narrow littoral zone, pike recruitment may be of bream compared with roach of equal length limited. The only other frequently occurring native (Lammens et al., 1992; Nilsson & Bro¨nmark, 2000), species available for effective biomanipulation in affording young bream an earlier size from Central may be perch, a dominant piscivore predation than other fishes, including roach. This particularly in mesotrophic and slightly eutrophic interpretation is in line with the claim by Lammens lakes (Persson et al., 1991). Although perch has been et al. (1992) that because of the size refuge of larger stocked regularly in Feldberger Haussee since 1996, bream, fish in hypertrophic shallow its recruitment is still very limited, probably because lakes tend to develop towards a codominance of predation by pikeperch, in particular, has caused high pikeperch and bream. The only way to prevent this mortality of the young perch (Laude et al. 2000; see situation is to remove bream by fishing. Seining is also Fig. 2), as has been observed in other biomanip- also effective at removing large roach, which even- ulated lakes (Post & Rudstam, 1992; Do¨rner, Wagner tually may reach a similar size refuge as larger & Benndorf, 1999). However, as the lake becomes bream. Thus, seining and piscivore stocking com- clearer and pikeperch performs increasingly less well, plement each other as biomanipulation measures, perch may ultimately become most effective in particularly in lakes where bream is an important suppressing planktivores. Thus, shifting piscivore component of the fish community.

2002 Blackwell Science Ltd, Freshwater Biology, 47, 2425–2434 2432 K. Wysujack and T. Mehner Whereas fishing usually removes the larger fish and References thus reduces the reproductive potential of the popu- Barthelmes D. (1981) Hydrobiologische Grundlagen der lation with implications on the pressure of Binnenfischerei, p. 252. Gustav Fischer, Jena. daphnids on phytoplankton in the long term, con- Barthelmes D. (1988) Fish predation and reac- sumption of a similar biomass of smaller planktivorous tion: biomanipulation background data from fisheries fish directly reduces the mortality of daphnids. research. Limnologica, 19, 51–59. Therefore, in lakes where fishing as the sole bioma- Benndorf J. (1990) Conditions for effective biomanipula- nipulation strategy is ineffective, a combination of tion; conclusions derived from whole-lake experiments fishing adult planktivorous and benthivorous fish and in Europe. Hydrobiologia, 200 ⁄⁄ 201, 187–203. promoting smaller size classes of piscivores may be an Benndorf J. (1995) Possibilities and limits for controlling alternative strategy to fostering a diverse piscivore eutrophication by biomanipulation. Internationale Revue community in terms of both species and age-classes der Gesamten Hydrobiologie, 80, 519–534. Berg S., Jeppesen E. & Søndergaard M. (1997) Pike (Esox (cf. Benndorf, 1990). lucius L.) stocking as a biomanipulation tool 1. Effects Piscivores are favoured target species of both on the fish population in Lake Lyng, Denmark. recreational and commercial fisheries in Central Eur- Hydrobiologia, 342 ⁄⁄ 343, 311–318. ope and parts of North America (Johnson & Staggs, Bergman E., Hamrin S. & Romare P. (1999) The effects of 1992; Bogelius, 1998; Wolos et al., 1998; Lathrop et al., cyprinid reduction on the fish community. Hydrobiolo- 2002). Therefore, stocking with piscivores facilitates gia, 404, 65–75. participation of potential lake users in restoration Bogelius A. (1998) National survey of recreational fish- programmes. However, for fishing for piscivores in eries in Sweden. In: Recreational Fisheries: Social, Economic biomanipulated lakes in the long term requires and Management Impacts (Eds P. Hickley & H. Tomp- enhanced size limits and reduced bag limits, at least kins), pp. 24–26. Fishing News Books Blackwell, for anglers (Benndorf, 1990; Johnson & Staggs, 1992; Oxford. Lathrop et al., 2002). Nevertheless, if anglers and Brabrand A. & Faafeng B. (1993) shift in roach (Rutilus rutilus) induced by pikeperch (Stizostedion professional fishermen are involved in decisions lucioperca) introduction: predation risk versus pelagic about stocking practices and harvest regulations as behaviour. Oecologia, 95, 38–46. an adaptive management strategy (Walters & Hilborn, Brabrand A., Faafeng B. & Nilssen J.P.M. (1990) Relative 1976), this kind of biomanipulation may be a sustain- importance of phosphorus supply to phytoplankton able combination of water quality and inland fisheries production: fish excretion versus external loading. management (Holland, 1995; Iyer-Raniga & Treloar, Canadian Journal of Fisheries and Aquatic Sciences, 47, 2000). 364–372. Brett J.R. & Groves T.D.D. (1979) Physiological ener- getics. In: . Vol. VIII. Bioenergetics and Acknowledgments Growth (Eds W.S. Hoar, D.J. Randall & J.R. Brett), pp. We wish to thank A. Tu¨ rck, C. Helms, U. Laude, 279–352. Academic Press, New York. M. Sachtleben, R. Degebrodt, U. Frankiw and Bry C. (1996) Role of vegetation in the cycle of pike. In: Pike. Biology and Exploitation (Ed. J.F. Craig), pp. 45– H. Rosengarten for support during field sampling. 67. Chapman & Hall, London. K. Duis, H. Do¨rner, F. Ho¨lker, H. Brach, and two Buijse A.D. & Houthuijzen R.P. (1992) Piscivory, anonymous reviewers made helpful comments on the growth, and size-selective mortality of age 0 pikeperch manuscript. R. Koschel and P. Kasprzak provided (Stizostedion lucioperca). Canadian Journal of Fisheries and earlier seining data. Many thanks to S. Poynton for Aquatic Sciences, 49, 894–902. linguistic improvements. The first author was sup- Craig J.F. (1987) The Biology of Perch and Related Fish, p. 333. ported by a grant of the German Environmental Croom-Helm, London & Sydney. Foundation (Deutsche Bundesstiftung Umwelt). Craig J.F. (1996) , predation and the Financial support was also provided by the State role in the community. In: Pike: Biology and Exploitation Department of Environment and Nature of Neubran- (Ed. J.F. Craig), pp. 201–217. Chapman & Hall, denburg and by the Ministry of Building, Country London. Development and Environment of Mecklenburg- Do¨rner H., Wagner A. & Benndorf J. (1999) Predation by piscivorous fish on age-0 fish: spatial and temporal Vorpommern.

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