Stocking, Population Development and Food Composition of Pike Esox Lucius in the Biomanipulated Feldberger Haussee (Germany) - Implications for Fisheries Management

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Limnologica 31 (2001) 45-51 LIMNOLOGICA http ://www.urbanfischer.de/j ournals/limno © by Urban & Fischer Verlag

Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Department of Biology and Ecology of Fishes, Berlin, Germany

Stocking, Population Development and Food Composition of Pike Esox lucius in the Biomanipulated Feldberger Haussee (Germany) - Implications for Fisheries Management

KLAUS WYSUJACK, UWE LAUDE, KARL ANWAND & THOMAS MEHNER

With 2 Figures and 3 Tables

Key words: Biomanipulation, pike, diet composition, growth, fisheries management

Abstract

Management of the piscivorous fish stocks plays a major role in com- necessary to maintain a high predation pressure on plankti- bining water quality management and sustainable fisheries manage- vorous fish. ment. From 1994 to 1996 and in 1998, young-of-the-year (YOY) pike Pike of most age groups are strongly associated with Esox lucius were stocked in different numbers in the course of a aquatic vegetation. The youngest individuals live nearest to biomanipulation project in Lake Feldberger Haussee. Since 1997, the shore, and only larger individuals are also able to use the population development, growth and food composition of pike have open water (CHAPMAN & MACKAY 1984; BRY 1996; GRIMM & been studied. The fish stocked in 1995 were dominant within the pop- KLINGE 1996). In a shallow Swedish lake, pike length was ulation. Pike showed rapid growth, indicating a high food consump- inversely related to vegetation density (EKLOV 1997). tion. The predation pressure was mainly directed towards roach Ru- tilus rutilus and small perch Percafluviatilis, the main planktivores. According to GRIMM (1981, 1989), pike biomass in a lake is Natural recruitment was, however, low and the pike catches have limited to about 110 kg per ha vegetated area. Pike popula- drastically declined since 1996. Low recruitment is a problem that tions are known to be strongly self-regulated by cannibalism, may require new stocking measures. Removal of larger pike by fish- and stocking measures sometimes may fail due to intra- ing would improve the survival of newly stocked fish. The application specific predation (GRIMM 1983; GRIMM & KLINGE 1996). of stocking, protection and fishing in lake management is discussed. This should be considered in fisheries management. Increasing the number of predatory fish does not only lead to a reduction of planktivorons fish biomass but also to a Introduction transformation into more valuable fish biomass (BERG et al. 1997), except for countries or regions where angling for Enhancing the number and biomass of piscivorous fish is re- cyprinids is very popular. If pike (or piscivores in general) are garded as a tool, or at least as a supplementary technique, in more valuable for both commercial and recreational fisheries, water quality management (PERROW et al. 1997). Since pike this should make biomanipulation by stocking with pisci- is a common and successful predator in fresh waters of the vores attractive for fisheries. In this case, maintaining a stable northern hemisphere, it is often used in biomanipulation population and simultaneously a stable (and high) yield (BERG et al. 1997; PERROW et al. 1997). The investigations of should be the objective of an effective management of the SONDERGAARD et al. (1997) indicated that it is possible to cre- pike stock. ate a top-down control of phytoplankton biomass by intro- In the present study, population development, growth and ducing high numbers of 0+ pike in shallow lakes. However, food composition of pike in a long-term biomanipulated lake the number of pike at the end of the summer is strongly regu- (Feldberger Haussee, Germany) were investigated. Based on lated by the area of suitable habitat (BERG et al. 1997). There- the results, measures for a sustainable fisheries management fore, regular stocking measures over several years could be are suggested.

0075-9511/01/31/01-045 $15.00/0 45 Study site and were stocked as YOY fish. To reduce the risk of cannibalism they were distributed along the vegetated areas. Fish were caught using a range of active and passive fishing Feldberger Haussee, a dimictic, eutrophic hardwater lake, is located about 100 km north of Berlin in the Baltic take district of Germany gears. An 800 m seine net with 16 mm mesh size of the bag was used between 15 and 20 times per year from March to November in 1995, (13 ° 28' E and 53 ° 20' N). It has a surface area of 136 ha, a maximum 1996, 1997 and 1998. Electrofishing (direct current, 270-300 V, depth of 12 m and a mean depth of 6.4 m. In the 1960s and 1970s, the 4 kW) was regularly conducted in the littoral zone in 1997 (June to lake became heavily eutrophicated through high sewage input. In November) and 1998 (February to November). Gill nets with mesh 1980, the waste water was removed from the catchment area, this led sizes of 20-75 mm were used monthly from June to November in to a reduction of the external nutrient load by 90%. Nevertheless, due 1997 and twice per month from April to November in 1998. The gill to an enormous internal nutrient pool, the water quality did not im- nets were set for about 4 hours at dusk. Additionally, we fished with a prove significantly until 1985 (KASPRZAKet al. 1988, 1995; KOSCHEL small beach seine (38 m total length, 6 mm mesh size of the bag), but et al. 1993). Biomanipulation measures started in 1985 with a substantial de- without a regular rhythm. Scales of 133 pike were used for determination of age and crease in the numbers and biomasses of planktivorous and benthivo- growth. Back-calculation of length was carried out using the Fraser- rous species, mainly roach and bream, Abramis brama, by fishing. Lee-method (DEVRIES & ERIE 1996). Samples for diet analyses were Additionally, piscivorous fish (pikeperch Stizostedion Iucioperca, taken from June to November in 1997 and from February to Novem- European catfish Silurus gIanis, eel Anguilla anguilla, perch and ber in 1998. The majority of the food samples was collected by stom- pike) were stocked yearly in different intensities between 1988 and ach flushing of fish, which were anaesthetised with MS 222 and re- 1998 (apart from 1990). However, a drastic increase in the propor- leased to the lake after the procedure. Stomach contents were stored tion of piscivores only occurred between 1995 and 1997, after the frozen. Prey organisms were identified as exactly as possible. Origi- catch of predatory fishes by commercial fisheries and anglers had been stopped. In 1998, limited angling was permitted again (mini- nal lengths were calculated using relationships between standard mum size for pike and pikeperch 60 cm, daily catch limit one fish length or fork length and total length, which were previously obtained from potential prey fish. If prey items were in a higher stage of per person). digestion, pharyngeal bones (cyprinids), opercular bones (percids), After the beginning of the biomanipulation measures, phosphorus vertebrae and scales were used for identification. For the most impor- concentrations decreased abruptly. The trophic state and the plank- tant prey species, roach, the original length was calculated by mea- tonic community changed considerably. The biomass of total crus- suring the pharyngeal bones according to MANN & BEAUMONT tacean plankton and of Daphnia spp., as well as average Secchi depth clearly increased (KoscHEL et al. 1993; KASPRZAKet al. 1995). Nev- (1980). The relationships to total length were also obtained from ertheless, coverage of Feldberger Haussee by macrophytes was low; roach previously caught in Feldberger Haussee. in 1996, only 3 ha (2.2% of the lake area) were covered by macro- Stomach contents were evaluated as follows: the percentage com- phytes (PALM 1998). position by number (percentage of each food type of the total number of food items eaten), the percentage composition by weight (percentage of each food type of the total weight of all food items eaten) and the frequency of occurrence (percentage of fish that had eaten a spe- Materials and Methods cial food type) were calculated. Empty stomachs were excluded from these calculations. Due to the limited usefulness of each single Detailed data on stocking measures with pike during the biomanipu- method we calculated the Relative Importance index RI (GEORGE & lation period between 1994 and 1998 are given in Table 1. Prior to HADLEY 1979). This index is derived from the Absolute Importance the stocking measures, almost no pike were present in the lake. In index for food type a (AI,) in the following way: 1995 and 1996, some stocked pike were marked by clipping of a ven- AI~ = % frequency of occurrence + % total numbers + % total weight tral fin (Table 1). Generally, pike originated from fish farms (ponds) (for food type a) tt RI~= 100 AI~ / ~ AI~ (n = number of different food types) Table I. Stocking of pike in Lake Feldberger Haussee (136 ha) dur- a=l ing biomanipulation. Fish were marked by clipping of a ventral fin, except for 1994 when alcian blue was used.

Date Number of Fish mark- Stocking Mean total Results stocked pike ed (%) density length (cm) (number The pike population in Feldberger Haussee was dominated ha -1) by the stocked fish. The proportion (by number) of the fish stocked in 1995 and 1996 in the total pike catches (gill net November 1994 800 100,0 5.9 25-30*) catches excluded) was 67.9% and 68.1% in 1997 and 1998, November 1995 2000 50.0 14.7 35.4 respectively. With proportions of 48.6% (1997) and 40.8% June 1996 6500 10,0 47.8 7.7 (1998) the fish stocked in 1995 were dominant, whereas the November/ 2600 79,3 19.1 26.9 December 1996 pike that were stocked later were less important although November 1998 1212 100.0 8.9 26.5 they had been stocked in higher numbers. In 1994, 100% of the stocked pike were marked with *) More exact data are not available. alcian blue using a Dermo-Jet inoculator (MEDIMEX,

46 Limnologica 31 (2001) 1 Table 2. Mean back-calculated total lengths (cm) of pike in Feld- Table 3. Diet composition of pike (273 pike; 127 stomachs containing berger Haussee (numbers of fish given in parentheses). food; 159 prey organisms). Samples were obtained from June to November in 1997 and from February to November in 1998. Year class Total length at age (cm) Prey species Composition Composition Frequency Relative I II III by by of occur- Import- number (%) biomass (%) fence (%) ance (%) 1997 27.2 (5) 1996 28.4 (52) 45.3 (27) Roach 57.2 86.1 59.1 65.2 1995 36.4 (71) 51.3 (71) 62.4 (16) Bream 4.4 1.5 4.7 3.4 1994 29.9 (5) 45.5 (5) 56.9 (5) Other 10.6 4.0 13.4 9.0 cyprinids*) Average 32.7 (133) 49.6 (103) 61.1 (21) Perch 20.1 3.9 24.4 15.6 Pike 0.6 1.0 0.8 0.8 Eel 1.9 3.0 2.4 2.4 Others 5.2 0.5 5.6 3.6 kpr. 1997 20 *) This group includes rudd Scardinius erythrophthabnus, tench Tinca tinca, white bream Abramis bjoerkna, crucian carp Carassius >, 0 carassius and prey fish wbich were identified as cyprinids but not 20 down to the species level. 0"=o q-

> 0 t newly stocked pike Sept.- Nov. 1998 25 20

20 .." ..% 0 i i i - i i i ~o to ¢e ~o co ¢o oa ¢e

Total length [cm] Fig. 1. Length frequency distribution of pike in Lake Feld- ,-- 10 O berger Haussee (1997/1998, gill net catches excluded). re . 5

0 -- I-- I I I -- Hamburg) but in the following years no catch of any of these 0 20 40 60 80 1 O0 pike was recorded. Therefore, the success of the stocking Pike length [cm] measure in 1994 could not be evaluated. Fig. 2. Relationship between the length of the predator Seine net catches declined from 16.3 pike per haul (1996) (pike) and the length of the main prey fish (roach). Lroaoh = to 11.2 (1997) and 8.4 (1998), but due to a high variability of 2.32" L °'46'R2 = 0.152; p < 0.001; n = 89. the catches no significance could be shown (ANOVA). The pike catch per unit effort (CPUE) of electrofishing (pike per hour electrofishing) decreased significantly from 3.6 in 1997 to 1.4 in 1998 (t-test, p < 0.05). For this reason, about 1200 juvenile pike were stocked in November 1998 (Table 1, Fig. 1). The stomach contents of 273 pike caught in 1997 and The length distribution of pike in our catches is given in 1998 were analysed. The overall proportion of empty stom- Fig. 1. Because of their high size selectivity, gill net catches achs was 53.5% with the highest values in August 1997 were excluded. A linear relationship with an intercept para- (74.2%) and July 1998 (69.2%). In the remaining 127 pike, meter of 5.1 cm was calculated between scale radius and fish 159 prey items were found. Results are shown in Table 3. total length (r2= 0.88). During the first years after stocking, The main prey fish was roach, which was the dominant pike of the strongest year classes (1995 and 1996) showed a planktivore. Small perch also were of considerable impor- rapid growth (Table 2). For the year classes 1994 and 1997, tance. Bream, which also occurred frequently in the lake, sample sizes were too small to assess growth. The year were rarely taken. There was no evidence for a high propor- classes 1995 and 1996 had significantly different lengths at tion of cannibalism. A regression analysis between length of the respective age (Table 2, t-test for age 1 and 2, p < 0.001). pike and length of the main prey species, roach, showed a Nevertheless, a comparison of the length increment in the weak but significant relationship. The increase of prey size second year of life did not show a significant difference with predator size was best described by a power function (t-test). (r2= 0.152, p < 0.001, Fig. 2).

Limnologica 31 (2001) 1 47 Discussion spring (at the end of the annual growing season or at the time of annulus formation, respectively) were too low to verify the It has previously been shown that the survival rate of the ini- results for age 2 and 3. tially stocked pike is higher than that of subsequently stocked Pike showed a rapid growth during the first years after fish. This has been interpreted as a consequence of cannibal- stocking. Compared with a growth standard for pike popula- ism (BEYERLE 1971), and it may explain the low proportion tions from 82 circumpolar water bodies (CASSELMAN 1996), of the pike stocked in 1996 in our catches. The major part of growth of pike in Lake Feldberger Hanssee amounted to the cannibalism had probably occurred before the beginning 148% and 139% at age 2 and 3, respectively. On the one of the food analyses in June 1997 and was therefore not de- hand, the high length at the respective age is partly caused by tected. The population development in Feldberger Haussee is the relatively large length at the time of stocking. This applies similar to that recorded in the Bautzen Reservoir where the particularly to the pike stocked in 1995 (Table 1). On the first strong year class dominated the population over several other hand, the high biomasses of cyprinids provide good years (SCHULTZ et al. 1992). The total protection from fish- feeding conditions. It has to be noticed that the absolute eries in Feldberger Haussee between 1995 and 1997 led to a length increment in the second and third year is also higher dominance of larger pike. Similarly, it was shown by GRIMM than in the above mentioned growth standard. The rapid (1983) that the biomass of smaller pike is negatively correlat- growth indicates a high food consumption which, in addition ed to that of larger ones. In a long-term study in Windermere, to the high proportion of roach and small (mainly planktivo- intensive netting for larger pike (>55 cm) led to a dominance rous) perch in the food, points towards the positive role of of younger and smaller pike but not to a decrease in the total pike in the biomanipulation of Feldberger Haussee. numbers (FROST & KIPLING1967; KIPLING& FROST 1972). The proportion of empty stomachs was in the same range The high proportion of stocked fish indicates a low natural as found in other studies (LAWLER 1965; HEI~NHEIMO & recruitment which can be considered as the reason for the de- KORHONEN 1996; KANGUR& KANGUR 1998). Several authors clining pike catches since 1996. Low recruitment may be a described this proportion to be highest in summer (LAWLER consequence of the low coverage with macrophytes, which 1965; MANN 1976); this was also the case in our study. CRAIG leads to a lack of spawning and/or nursery habitats. A consid- & BABALUK(1989) observed a growth depression of pike dur- erable proportion of the sparse vegetation in Feldberger ing summer which was explained by low food consumption in Haussee consists of Phragmites australis (PALM 1998). this season. Furthermore, in a study on angling vulnerability, Phragmites and Typha habitats are neither frequently used as no pike were caught during midsummer when water tempera- spawning sites (FRANKLIN& SMITH 1963; FROST & KIPLING ture ranged between 21 and 30 °C (WEITHMAN& ANDERSON 1967; McCARRAHER& THOMAS 1972) nor commonly used 1976). This could also be interpreted as a decrease in food in- by small pike (EKL(3V 1997; SKOV & BERG 1999). The mor- take at high temperatures. Of course, the high proportion of phology of the lake with its few shallow areas probably pre- empty stomachs in summer may also reflect more rapid diges- vents a quick recovery of aquatic macrophytes, although tion at higher temperatures as was noted by MANN (1976). Secchi depth considerably increased during the previous However, this could only be an explanation for the high pro- years. It can therefore be supposed that the natural recruit- portion of empty stomachs in the present study, if more rapid ment of pike will remain low, at least for the next few years. digestion did not result in a higher frequency of feeding. Since most fish were not older than 3 years and a high pro- The importance of roach in the diet of pike corresponds to portion of them was marked at stocking, age determination the numerical dominance of roach in the littoral and sub-lit- from scales was practicable. CASSELMAN(1996) noted that the toral zones of the lake (LAUDE, unpubl, results). Neverthe- scale method is valid for determining the age of fast-growing less, the low abundance of 0+ pike leads to a low predation or young pike. Pseudoannuli could be recognised according to pressure of pike on 0+ and 1+ cyprinids. Furthermore, the use the results of former studies (see the overview on scale read- of 0+ pike in 0+ cyprinid control is restricted to clear waters ing in pike by CASSELMAN1996). The Fraser-Lee-method was with sufficient aquatic vegetation (RAAT 1990). Therefore, appropriate for describing the growth of pike from Haussee. the present pike stock in Feldberger Haussee is not capable of For the 1995 year class, the back-calculated mean length for controlling the juvenile cyprinids. Similar situations with age 1 was 36.4 cm. The fish were stocked in the middle of numbers of stocked predatory fish that were insufficient to November in 1995 with a mean length of 35.4 cm. Assuming control the 0+ cyprinids were described by MEIJER et al. that growth in winter is very low and that the annulus forma- (1995) and GOLDYN et al. (1997). In a study of BERG et al. tion occurs in early spring, back-calculated and measured (1997), stocking with juvenile pike led to a significant de- length are in good agreement. This also applies for the 1996 crease in the CPUE of juvenile planktivorous fish in the lit- year class, although it must be noticed that a comparison is toral zone but not in the pelagic area. only possible with the length data from the pike stocked in Of course, predation of pike is not the only factor influenc- late autumn in 1996. In general, our observations on growth of ing the year class strength of juvenile cyprinids. Other abiotic marked pike confirm the results of the back-calculation. How- and biotic factors, like water temperature and food availabili- ever, the numbers of marked pike caught in winter or early ty, are also of great importance. To recapitulate, it can be stat-

48 Limnologica 31 (2001) 1 ed that the low predation of the present pike stock in lake fence of cannibalism, especially in autumn when vegetation is Haussee on juvenile cyprinids increases the chances for the breaking down. It has to be accepted that the carrying capacity cyprinids to build up strong year classes. With regard to for pike in a lake may change with the season. Therefore, also biomanipulation, this is clearly unfavourable. when stocking in autumn, the stocking density has to be ap- Assuming that the pike biomass in a lake is limited by the propriate to the habitat area suitable for pike because canni- area covered by macrophytes and similar structures (GRIMM balism does not only affect pike in their first summer and au- 1981) and that the biomass of smaller pike is negatively corre- tumn. Additionally, the predation on juvenile cyprinids is very lated to that of large ones (GRIMM 1983), a population may be important for biomanipulation, especially when fishing for dominated by a high number of small pike or a low number of the adult cyprinids is part of the management strategy. large ones. It has been shown that smaller pike consume more To improve the survival rates of newly stocked pike, a pre- food in relation to their body weight than larger ones (SALAM ceding fishing for larger pike would be useful (PREJS et al. & DAVIES 1994; HEIKINHEIMO& KORttONEN 1996). Therefore, 1997). The removal of larger pike also leads to increased it can be supposed that, at the same total pike biomass in a numbers of young pike when natural recruitment is sufficient lake, a pike stock consisting of a high number of small fish (KIPLIN~ & FROST 1972; MANN 1982; KY0SCHE 1996). For the will consume more food than a small number of large ones. In same reason GRIMM (1989) stated that intensive exploitation addition, this higher predation pressure will be directed to- of pike stocks may indirectly result in a sufficient predation wards the juvenile cyprinids and the small perch (EKL0V & on cyprinids. In addition, if fishermen and anglers could har- HAMRIN 1989) and therefore possibly (with the above men- vest the larger pike, their compliance to cooperate in such pro- tioned prerequisites) prevent the cyprinids from forming jects should increase. There will also be an economic benefit strong year classes (GRIMM 1989). Another important fact is (sale of fish or fishing licenses). Even if it is considered that that high numbers of these cyprinids would be preyed upon be- larger pike also use the open water (CHAPMAN~; MACKEY fore they start feeding on pelagic zooplankton. 1984), it is not necessary to protect them from fisheries, be- Pike are known to be highly susceptible to angling pres- cause pikeperch, an effective predator of the open water of eu- sure. In a study of WEITHMAN & ANDERSON (1976), northern trophic lakes, is also abundant in Feldberger Haussee. pike were 4.2 times as vulnerable to angling as muskellunge Besides, a complete prohibition of fisheries over a long (E. masquinongy) and 3.2 times as vulnerable as tiger muskie period is not desirable in a region where tourism is an impor- (F1 hybrid of pike and muskellunge). Therefore, the decline tant branch of the economy. Therefore, it is recommended to of the pike stock in 1998 might be a consequence of the re- apply a strategy consisting of intervals of stocking, protec- opening of the lake for angling. tion, fishing, stocking etc. In the sense of an adaptive management, the success of the measures must be scrutinised regularly and, if necessary, the measures have to be adapted Management implications (WALTERS & HILBORN 1976). It is also possible to provide artificial structures for spawning and/or as habitats for juve- Although the protection from fisheries from 1995 to 1997 led niles (McCARRAHER & THOMAS 1972; BRY 1996; SKOV & to a high predation of pike especially on roach, the present BERt 1999), but it is doubtful whether this is practicable in a tendency in stock development is unfavourable. Since natural lake with a large area like Feldberger Haussee. recruitment is low and there is no indication of a change in this situation, subsequent stocking will be necessary. This Acknowledgements: The authors wish to thank C. HELMS, conclusion does not contradict the results of PIERCE et al. A. TORCK, H. ROSENGARTEN,C. POLLEICHTNER,C. LEVIN, P. BEECK (1995), who stated that pike stocking is no longer a conven- and U. BAADEfor assistance in the field. We are grateful to the local tional management tool in Minnesota lakes. The pike popula- fisherman U. FRANKIWfor good cooperation and to M. VALENT~Nfor tions in their study were characterised by high densities of assistance in the laboratory. We wish to thank K. Du~s and two small fish, hence the situation was opposite to that in Feld- anonymous referees for helpful comments on the manuscript. Finan- berger Haussee. Our results show that stocking with pond- cial support was provided by the Ministry of Building, Country De- reared YOY pike in autumn can be successful, especially if velopment and Environment of Mecklenburg-Vorpommern and by pike density prior to stocking has been low. Nevertheless, the State Department of Environment and Nature of Neubrandenburg (STAUN). The first author is supported by a scholarship of the only little predation of these pike is directed towards 0+ Deutsche Bundesstiftung Umwelt. cyprinids and the period in which they feed mainly on juvenile (0+/1+/2+) cyprinids is relatively short. Because of the low stock of 0+ pike no sound data on the diet of pike in their References first summer could be obtained in this study. Nevertheless, it may be expected that this age group mainly forages on 0+ and BERG, S., JEPPESEN, E. ~; SONDERGAARD,M, (1997): Pike (Esox lu- 1+ prey fish. Therefore, it should be of advantage to stock Cius L.) stocking as a biomanipulation tool: 1. Effects on the fish pike fry or fingerlings in spring in comparison with stocking population in Lake Lyng, Denmark. Hydrobiologia 342/343: in autumn. It is probably not possible to prevent the occur- 311-318.

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JAEGER(eds.), by a top aquatic predator, Esox lucius L. J. Fish Biol. 25: 109-115. Verfahren zur Sanierung und Restanrierung stehender Gew~isser, CRAIG, J. F. & BABALU~,J. A. (1989): Relationship of condition of pp. 274-279. Stuttgart. walleye (Stizostedion vitreum) and northern pike (Esox lucius) to KIPLING, C. & FROST, W. E. (1972): Certain effects of an annual re- water clarity, with special reference to Dauphin Lake, Manitoba. moval of pike (Esox lucius) from Windermere. Verh. Internat. Can. J. Fish. Sci. 46: 1581-1586. Verein. Limnol. 18: 1122-1127. DEVRmS, D. R. & FRIE, R. V. (1996): Determination of age and KNOsc~, R. (1996): Anmerkungen zur Hechtwirtschaft in Seen. growth. In: B. R. MURPHY & D. W. WILLIS (eds.), Fisheries tech- Fischer und Teichwirt 4: 137-142. niques, 2rid ed., pp. 483-512. American Fisheries Society. Bethes- KOSCHEL, R., KASPRZAK, P., KRIENITZ, L. & RONNEBERGER, D. da, Maryland, USA. 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50 Limnologica 31 (2001) 1 SCHULTZ,H., WIELAND,E & BENNDORF,J. (1992): Raubfischbesatz WALTERS, C. J. & HILBORN, R. (1976): Adaptive control of fishing zur Regulation des Fischbestandes in einer hypertrophen systems. J. Fish. Res. Bd. Can. 33:145-159. Talsperre. In: M. VON LUKOWICZ:Die Bedeutung der fischerei- WE~THNAN,A. S. & ANDERSON, R. O. (1976): Angling vulnerability lichen Bewirtschaftung fiir die aquatischen Lebensr/iume. Arbei- of Esocidae. Proc. Ann. Conf. SE Assoc. Game Fish Commission- ten des Deutschen Fischereiverbandes 55: 57-92. ers 30: 99-102. SKOV, C. & BERG, S. (1999): Utilization of natural and artificial habitats by YOY pike in a biomanipulated lake. Hydrobiologia 41}8/409:115-122. Author's address: KLAUS WYSUJACK, Leibniz-Institute of Fresh- SONDERGAARD,M., JEPPESEN, E. & BERG, S. (1997): Pike (Esox lu- water Ecology and Inland Fisheries, Department of Biology and cius L.) stocking as biomanipulation tool: 2. Effects on lower Ecology of Fishes, MiJggelseedamm 310, D - 12587 Berlin, Germany; trophic levels in Lake Lyng, Denmark. Hydrobiologia 342/343: Tel.: +49(0)30 64181-654; Fax: +49(0)30 64181-750; 319-325. e-mail: [email protected]

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