Thomas Mehner, Heinz Schultz, Dirk Bauer, Reglindis Herbst, Hanno Voigt & Jürgen Benndorf. 1. Introduction - PDF

ANN. Ann. Zool. ZOOL. Fennici FENNICI 33: Vol. 33 ISSN X 353 Helsinki 28 November 1996 Finnish Zoological and Botanical Publishing Board 1996 Intraguild predation and cannibalism in age-0

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ANN. Ann. Zool. ZOOL. Fennici FENNICI 33: Vol. 33 ISSN X 353 Helsinki 28 November 1996 Finnish Zoological and Botanical Publishing Board 1996 Intraguild predation and cannibalism in age-0 perch (Perca fluviatilis) and age-0 zander (Stizostedion lucioperca): Interactions with zooplankton succession, prey fish availability and temperature Thomas Mehner, Heinz Schultz, Dirk Bauer, Reglindis Herbst, Hanno Voigt & Jürgen Benndorf Mehner, T., Schultz, H., Bauer, D., Herbst, R., Voigt, H. & Benndorf, J., Institute of Hydrobiology, University of Technology, D Dresden, Germany Received 22 August 1995, accepted 6 September 1996 The preconditions for the ontogenetic diet shift from planktivory to piscivory in age-0 perch and age-0 zander were examined in the long-term biomanipulated Bautzen reservoir (Germany). Early piscivory already was observed in age-0 zander of approximately 20 mm total length, but occurred only in the largest ( 40 mm TL) perch specimens during early summer. The onset of piscivory was triggered by high water temperatures in late spring which enhanced the growth rates of the predators relative to their prey, and by the drastic decline in zooplankton abundances during that time. Due to the lack of other prey fish, cannibalism and mutual intraguild predation dominated in piscivorous zander and perch. Early piscivory in both species was additionally favoured by the biomanipulation experiment in Bautzen reservoir due to (1) an abundant Daphnia population in spring which increased the growth rates of the predators, and (2) the coexistence of perch and zander in high densities which is uncommon in naturally-structured waters. 1. Introduction Ontogenetic diet shifts are widespread in sizestructured populations (Werner & Gilliam 1984). In many piscivorous fishes, there is a succession from zooplanktivory as larvae and small juveniles, to benthivory as juveniles, and finally to piscivory as yearlings and adults (Willemsen 1977, Werner 1986, Fox 1989, Persson & Greenberg 1990). Zooplanktivory in adult fish may influence significantly the dynamics of their prey (Gliwicz & Pijanowska 1989), and this predation pressure may cascade down the whole aquatic foodweb (Carpenter et al. 1987, Mills et al. 1987, Benndorf 1995). But planktivorous age-0 fish also can drive the zooplankton succession in years of high recruitment (Cryer et al. 1986, Qin & Culver 1995). Thus the age-0 group may be the trophic link be- 354 Mehner et al. ANN. ZOOL. FENNICI Vol. 33 tween piscivorous fish and zooplankton (Mills et al. 1987, Mehner et al. 1996a). However, the switch from planktivory to piscivory in age-0 fish also may be important, since then the trophic cascade is regulated from the top. A diet shift to piscivory early in life could be advantageous for a single individual, since a piscivorous fish can directly reduce the density of potential intraspecific (cannibalism) and interspecific competitors (intraguild predation, Polis et al. 1989). In Central Europe, piscivory in zander (Stizostedion lucioperca) has been documented during the first year of life by Nagiec (1977), Van Densen (1985a) and Buijse and Houthuijzen (1992). Similar observations were made in the closely-related age-0 walleye (Stizostedion vitreum vitreum) in North America (Forney 1966, Kempinger & Carline 1977, Johnson et al. 1992). It was assumed that either the decline in zooplankton densities (Nagiec 1977) or the availability of suitable prey fish (Van Densen 1985a) were the main forces which induced the early switch in age-0 zander. The zander often coexists with another piscivorous percid species, the perch (Perca fluviatilis and P. flavescens, respectively; Van Densen 1985b, Johnson et al. 1992). However, the uptake of fish prey was found only occasionally in young-of-the-year perch (Spanovskaya & Grigorash 1977, Van Densen 1985b, Thiel 1989). We hypothesized that the onset of piscivory in age-0 perch may be influenced by the relative availability of zooplankton and fish prey. Declining densities of large zooplankton species are a very common phenomenon in lakes in Europe during the summer (Sommer et al. 1986). However, the vulnerability of prey fish for age-0 perch may be rather limited in many waters, since the mouth gape of perch is smaller than that of the zander, thus resulting in a higher predator:prey size ratio than for zander (Collette et al. 1977). This difference also holds true for perch and zander larvae and juveniles (M. Plewa, unpubl. results). The diet switch to piscivory in age-0 perch additionally could be positively stimulated by a high water transparency, since, in contrast to the zander, the visual system of the perch is insufficiently adapted to low light intensities (Ali et al. 1977, Wahl et al. 1993). In the long-term biomanipulated Bautzen reservoir (see below), (1) perch and zander are very abundant, (2) age-0 fish density is extraordinarily high, and (3) the population abundance of daphnids regularly declines in early summer. Consequently, the aim of our study was to examine whether both zander and perch become piscivorous during the first year of life, and determine the potential consequences of these early diet shifts on the development of the age-0 fish and zooplankton communities. 2. Material and methods 2.1. Study site Bautzen reservoir is a hypertrophic, polymictic, man-made lake 70 km northeast of Dresden (Saxony, Germany). The surface area of the lake is 533 ha, mean depth about 7.4 m, and maximum depth about 12 m. A long-term biomanipulation experiment started in 1981 has strongly influenced structure and food pathways within the fish community (Schultz et al. 1992, Benndorf 1995). Due to continuous stocking and catch restrictions to anglers since 1981, the stocks of adult piscivorous fish (mainly zander, perch, and pike, Esox lucius) increased to about 40 50% of total fish biomass in 1993 and 1994 (Schultz et al. 1992; and unpubl. results). Considering the hypertrophic state of Bautzen reservoir, the fish community should be dominated by cyprinids (Persson et al. 1991, Kubecka 1993). But, as a consequence of direct and indirect predation effects of the high number of piscivores, the stocks of roach, Rutilus rutilus, and bream, Abramis brama, have drastically been depressed during the last five years (Schultz 1996). Therefore, the fish community of Bautzen reservoir represents a transient, enjuvenated stage of ichthyocoenosis succession with a fast-growing piscivorous perch stock and high densities of zander and ruffe (Gymnocephalus cernuus) (Kubecka 1993). Due to the declining stocks of cyprinids, the main zooplanktivorous fish are now the age-0 groups of the percids (Mehner et al. 1996a). Piscivorous 0+ groups occur in extraordinarily high densities ( ind. ha 1 in September, Schultz et al. 1992, and unpubl. results). In addition, population density of the dominant cladoceran species Daphnia galeata has peaked in spring and autumn and declined in summer, during all years of investigation except for the ANN. ZOOL. FENNICI Vol. 33 Intraguild predation and cannibalism in perch and zander 355 period (Köthe & Benndorf 1994, Mehner et al. 1995, Köthe et al. 1996) Age-0 fish Fish larvae and age-0 juveniles were sampled between June and the middle of September in 1993 and In 1993, mainly gill nets with mesh sizes of 6, 7, 10 and 13 mm were used (Table 1). Fleets of nets were set in several regions of the reservoir in different water depths (2 to 10 m). Fish larvae were additionally caught in littoral regions with a lift net (1 m 2, mesh size 1 mm). In 1994, a conical plankton tow net (0.35-m diameter, 2.5-m length, mesh size 780 µm) was used to sample the larvae. Later, age-0 fish were caught close to the shore by using a small seine net (15 m long, 4-mm mesh size; Table 1), and in deeper ( 2 m) regions by using fleets of gill nets as in 1993 (Table 1). Since catch-per-unit-effort data were not directly comparable between the different gears, the abundance of fish was estimated in relative terms only (Table 1). All fish larvae were fixed in 3% formaldehyde solution immediately after capture. Larger fish were frozen ( 18 C). In the laboratory, species were identified. The total length (mm, TL) of all specimens was measured and the mean length of each species was computed from the complete catch on one date. Where available, the stomach content of at least 10 specimens of both age-0 perch and zander was inspected (Table 1). The frequency of fish prey in the stomachs was determined, and the numerical composition of the prey fish species was calculated Zooplankton Zooplankton was sampled in vertical series (2.5-m depth interval, 2-l Ruttner-sampler) from the deep- Table 1. Overview of the gear used to sample age-0 fish in Bautzen reservoir in 1993 and The abundance of fish is indicated in relative terms (xxx = dominant, xx = present, x = rare, 0 = not present). The number of stomachs examined in age-0 perch and age-0 zander is given in brackets. Date Gear Relative abundance of fish species Perch Zander Ruffe Roach June Lift net xxx (10) June Lift net, gill nets xxx (19) x (8) June Gill nets xxx (20) xx (10) x x 23 June Gill nets xxx (23) xx (48) x x 29 June Gill nets xxx (10) xx (3) xx 0 9 July Gill nets xx (26) xx (7) xxx 0 15 July Gill nets xxx (45) xx (14) xx x 27 July Gill nets xxx (20) 0 xx x 24 August Gill nets xxx (43) x (3) xx x 2 September Gill nets xxx (57) xx (25) xx x 7 September Gill nets xxx (10) xx (20) xx x 9 September Gill nets xxx (32) x (1) xx x 14 September Gill nets xxx (25) x (5) xx June Conical net xxx (43) xx (47) x xx 16 June Seine net xxx (25) xx (24) x xx 28 June Gill nets, seine xxx (31) 0 x x 12 July Gill nets, seine xx (100) x (6) xxx x 27 July Gill nets, seine xx (83) x (1) xx xxx 9 August Gill nets, seine xxx (151) 0 xx xx 1 September Gill nets, seine xxx (118) 0 xx xx 13 September Gill nets, seine xxx (121) 0 xx xx Total: (1 012) (222) 356 Mehner et al. ANN. ZOOL. FENNICI Vol. 33 Fig. 1. Mean epilimnetic (0 5-m water depth) water temperatures ( C) in Bautzen reservoir from May until October, 1993 and est part of the lake approximately twice a month between the middle of May and the middle of October. Secchi depth and mean epilimnetic water temperature were determined at 1-m intervals on the same dates. All zooplankton subsamples were concentrated through a 55-µm net. Large zooplankton (daphnids 1.1 mm, Leptodora kindtii, Chaoborus flavicans) were collected by vertical hauls with two different plankton nets (500 and 750 µm, opening diameter 25 cm, flowmeter inside). The samples were immediately preserved in 3% formaldehyde solution containing 30 g saccharose l 1 (Haney & Hall 1973). Counts from all depth intervals were separately performed using a Kolkwitz chamber by numbering at least 50 specimens of each important species. These subsamples were averaged and expanded to obtain total zooplankton densities. Individual zooplankters were measured and grouped into 50-µm size classes by projecting them onto a screen which was connected with the microscope. Biomasses were calculated using length weight regressions (Bottrell et al. 1976, Mehner et al. 1995). 3. Results There were differences in the patterns of mean epilimnetic water temperatures in spring and summer 1993 and 1994 (Fig. 1). In 1993, water temperatures were significantly higher at the end of May and during the first weeks of June (about C) than in 1994 (about C; Fig. 1). Fig. 2. Zooplankton biomass (mg wet weight l 1 ) and water transparency (Secchi depth in m) in Bautzen reservoir from May until October 1993 (upper graph) and 1994 (lower graph). Daphnids are nearly exclusively Daphnia galeata, other crustaceans include Bosmina longirostris, Chydorus sphaericus, calanoid and cyclopoid copepods, Leptodora kindtii and Chaoborus flavicans. In contrast, higher water temperatures were observed during summer 1994, than in the preceding summer (21 C vs. 17 C, Fig. 1). The development of the zooplankton community showed a similar pattern in spring of both years (Fig. 2). High biomasses of Daphnia galeata (8 11 mg wet weight l 1 ) in May were followed by a drastic decline of the population densities in June. In July and August, small cladocerans (Bosmina longirostris, Chydorus sphaericus) dominated instead, but their biomass never exceeded 4 mg wet weight l 1. In 1993, a second population peak of Daphnia galeata was observed in autumn, whereas the population density of this large cladoceran species remained near zero in September and October 1994 (Fig. 2). The density of calanoid and cylopoid copepods, and of carnivorous cladocerans always was lower than 0.3 mg w.w. l 1. The highest Secchi depths ( 5 m) were ANN. ZOOL. FENNICI Vol Intraguild predation and cannibalism in perch and zander Fig. 3. Mean total lengths (TL, in mm) of age-0 fish in Bautzen reservoir between June and September 1993 (upper graph) and 1994 (lower graph). measured during the spring peaks of D. galeata in both years (Fig. 2). In summer and autumn, water transparency was always 1.5 m owing to strong blooms of algae (Benndorf 1995). Age-0 perch dominated the catch at nearly all sampling dates in both years (Table 1). Ruffe also were present, but in lower abundances than the perch. Age-0 zander frequently were caught in 1993, but occurred in lower numbers in June and July In August and September 1994, no zander were sampled with either seine net or gill nets (Table 1). Age-0 roach regularly were caught in 1993, but in extremely low numbers. However, this species was very abundant in 1994 (Table 1). The mean TL of perch increased from about mm in June to about 55 mm in September in both years (Fig. 3). Growth was substantially higher in June than during summer. A similar growth pattern was observed in ruffe, although the length increase was slightly higher than in the perch during the summer Length of roach Fig. 4. Frequency of occurrence of fish prey (in % of fish analyzed) in the stomachs of age-0 zander and age-0 perch from Bautzen reservoir in 1993 (upper graph) and 1994 (lower graph). was lower than that of the perch in June in both years, but the roach reached higher final TLs than did the perch at the end of the season (about 80 mm in 1993, and 70 mm in 1994; Fig. 3). The most rapid growth was observed in age-0 zander. Within 6 weeks, the mean TL increased from about 20 mm to mm in the middle of July (Fig. 3). From mid-july to August, zander lengths remained constant, but again strongly increased in September Age-0 perch were piscivorous only for a limited period in both years. Maximum frequencies of fish consumed by perch (20 50% of the fish analyzed) were observed during the drastic decline of daphnids at the end of June in both years (Fig. 4), corresponding to a fish size of about 40 mm TL. From the middle of July onwards, perch did not consume fish. In contrast, frequencies of fish prey consumed by age-0 zander increased steadily from zero in June to 100% in July in both years (about mm TL). From the end of July until the end of August 1993, zander piscivory was drastically depressed, but increased again in 358 Mehner et al. ANN. ZOOL. FENNICI Vol. 33 Fig. 5. Species composition (in % of abundance) of fish prey in the stomachs of piscivorous age-0 perch and age-0 zander from Bautzen reservoir in 1993 (upper two graphs) and in 1994 (lower two graphs). September Values were not available for the same period in The main prey species for piscivorous age-0 perch were its own smaller conspecifics (cannibalism) and smaller zander in 1993 (Fig. 5). The degree of cannibalism in age- 0 perch was higher in July than in June. In contrast, zander were almost exclusively cannibalistic in June, but preyed additionally on perch and ruffe later in the season in 1993 (Fig. 5). In June 1994, juvenile roach were eaten by both piscivorous age-0 percids (Fig. 5). In July of this year, however, the degree of cannibalism was close to 100% in both species. 4. Discussion In this study, we have analyzed the interactions between piscivory in age-0 perch and age-0 zander, and the seasonal succession of the zooplankton community in Bautzen reservoir during two years. We have found that the summer decline of large daphnids coincided with the onset of piscivory in both percid species. However, the high availability of prey fish of suitable length appears to be the second prerequisite for these ontogenetic diet shifts. High water temperatures in spring stimulated piscivory in perch due to large size differences between predator and prey. In addition, the growth rate was higher during the piscivorous phase than during the predominantly zooplanktivorous phase of both zander and perch. In both years, we observed a rapid decline in D. galeata biomass at the end of June. Since daphnids are the main prey items of small age-0 perch and zander (Spanovskaya & Grigorash 1977, Van Densen 1985ab, Hartmann 1986, Fox 1989), this decrease forced the fish to shift to another food source. In age-0 zander, the early onset of piscivory is a very common phenomenon. Van Densen (1985a) found that the zander predated on fish (mainly smelt, Osmerus eperlanus) already at TL = 20 mm in Lake Tjeukemeer (The Netherlands). Buijse and Houthuijzen (1992) observed the diet switch to piscivory in 60 and 70 mm TL classes of zander in Lake Ijssel (The Netherlands). In Oneida Lake (USA), age-0 walleyes started feeding on age-0 perch when walleyes had reached a length of mm TL (Mills et al. 1987). In Bautzen reservoir, the sole prey fish of suitable length for age-0 zander were its own smaller conspecifics in late spring 1993 (Fig. 3). Consequently, the degree of cannibalism was nearly 100% (Fig. 5). Later in the year, the mean size of the zander was more than twice the length of the other age-0 percid species (Fig. 3), and, as a consequence, we found mainly ruffe and perch in the stomachs of zander in September 1993 (Fig. 5). According to Van Densen (1985a), 0+ zander can ingest prey fish which are even larger than 50% of the predator size. In 1994, small roach juveniles of that size were available to zander as prey fish for a short period (June 16, Fig. 3). But, due to the fast growth of roach and the large size of ruffe and perch relative to zander lengths in late June and July, the zander was forced to become cannibalistic (Fig. 5). This behaviour may have contributed to the strong decline in abundance of age-0 zander, which was reflected by the lack of these fish in the gill-net catches from August 1994 onwards (Table 1), although canni- ANN. ZOOL. FENNICI Vol. 33 Intraguild predation and cannibalism in perch and zander 359 balism by adult zander additionally may be important for this reduction (Mills et al. 1987, Mehner et al. 1996a). The relative availability of zooplankton and fish prey also strongly influenced the degree of piscivory in age-0 perch. In contrast to zander, age-0 perch often switch to benthic food as a consequence of reduced zooplankton densities (Persson & Greenberg 1990, Wu & Culver 1992). The occasional uptake of fish prey by age-0 perch has been described only in a few studies from Europe (Spanovskaya & Grigorash 1977, Van Densen 1985b, Thiel 1989). In late June and July of both years, the majority of age-0 perch did feed on chironomids and small cladocerans (Mehner et al. 1995, 1996b). But, the largest specimens of the population became piscivorous. This diet shift obviously was not triggered by high water transparencies, because the Daphnia decline occurred simultaneously with a drastic reduction in Secchi depths (Fig. 2). The piscivorous age-0 perch preyed predominantly on juvenile zander (Figs. 4 and 5, and Mehner et al. 1995). However, the rapid growth of the zander led consequently to decreasing piscivory and increasing cannibalism in perch (Fig. 3), since no other prey fish of suitable lengths were available during the rest of the season. In June 1994, the length differences between all species wer
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