Freshwater Biology (2002) 47, 2425–2434 Comparison of losses of planktivorous fish by predation and seine-fishing in a lake undergoing long-term biomanipulation KLAUS WYSUJACK and THOMAS MEHNER Department of Biology and Ecology of Fishes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany SUMMARY 1. Piscivore stocking at artificially high densities and fishing 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 piscivores, pikeperch and pike, in 1997 and 1998. We also determined species composition and length distribution of prey 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 (biomass) 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 fisheries management, including stocking and harvest regulations. Keywords: biomanipulation, diet, piscivorous fish, predation, seining lakes is to enhance herbivorous zooplankton, Daphnia Introduction in particular, by reducing their principal predators, Biomanipulation is a widely accepted tool for planktivorous fish. More daphnids are expected to improving water quality of eutrophicated lakes. either reduce phytoplankton biomass or to shift the Recent reviews (McQueen, 1998; Hansson et al., size structure of the algal community, 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 Ó 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 planktivores 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 animal 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 eel, 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 (pelagic zone), 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- (littoral zone) 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 habitats of the Ó 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.