185 FOOD OF CORMORANTS Phalacrocorax carbo sinensis WINTERING IN BAVARIA, SOUTHERN GERMANY THOMAS KELLER ABSTRACT Following the spectacular population increase in conti­ nental Cormorants, numbers of wintering birds on lakes and rivers in Bavaria, southern Germany, have risen sharply. Because of a growing .concern among commercial fishermen and anglers, a diet study has been carried out at two major roosts: one at a lake (lake Chiemsee) and one at a dammed river (lower Inn river). Main prey species were cyprinids (Rudd, Roach, Chub and Bream), but at lake Chiemsee the commercially most important Whitefish also appeared in the diet during spawning in December/January. Mean daily intake per bird increased in the course of winter, because of larger specimens taken. Possible causes and effects of this phenomenon are discussed. Assuming an average daily uptake of 400 g fresh mass, it was estimated that at lake Chiemsee 3.3% of the total annual fish production was taken by Cormorants (vs. 28% by commercial fish­ ermen). The total catch of Whitefish by Cormorants amounted to a mere 3.2% of the total commercial catch of this species. Larger proportions were scored for Eel (22.3%) and Pike (6.2%). At the lower Inn river it was esti­ mated that Cormorants took 21 % of the total annual fish production. Both in view of species composition in the Cormorants' diet and of the consumption estimates specified above, it is considered unlikely that the birds impose a serious threat to commercial fisheries. Interference with recreational angling (e.g. for Grayling) may, however, occur. Weinbergstrasse 9, D-97753 Karlstadt-Stetten, Germany. INTRODUCTION & Hafner 1984), in The Netherlands (Marteijn & Dirksen 1991, Platteeuw et at. 1992, Dirksen et at. As a consequence of the increase of the Conno­ 1995) and in Austria (Trauttmansdorff 1992). rant Phatacrocorax carbo sinensis as a breeding Most of these have shown cyprinids to be the bird in NW-Europe since the early 1980s (Van main prey items. This paper provides a brief over­ Eerden & Gregersen 1995), the numbers of view of the food choice of the birds from two of wintering individuals in Bavaria (southern the major Bavarian roosts, based on the analysis Gennany) have increased substantially. Details of pellets. on the development of this wintering population are presented by Wtist (1981), Leibl & Vidal (1983), Bezzel & Engler (1985), Zach (1987), STUDY AREA AND METHODS Franz & Sombrutzki (1991) and Bezzel (1989, 1992). This increase has caused concern among The two roosts were selected for their suitability both commercial fishennen and anglers. Food for pellet collection. One of them was located on studies in the wintering regions of the Connorant the smallest of three islets in lake Chiemsee have mainly been carried out in other areas within (47°52' N, 12°25' E), the largest Bavarian lake Europe, i.e. Switzerland (EAWAG 1975, 1979, (79.9 km2), the other on the banks of the lower Ruhle 1985, Imfeld et at. 1986, MUller 1986, Inn river (48° 18' N, 13° 11' E), which is dammed Bundesamt fur Umweltschutz 1987, Morel & for power generation (Fig. 1). At each of the two Hausmann 1989, Suter 1991a, b, c, d), France (1m roosts pellets were collected on five consecutive ARDEA 83: 185-192 186 ARDEA 83(1), 1995 mass should be considered as minimum values, since (partial) digestion may cause fish remains to wear or even disappear (EAWAG 1975, 1979, Muller 1986, Worthmann & Spratte 1987, Morel & Hausmann 1989, Marteijn & Dirksen 1991, Zijlstra & Van Eerden 1995). No attempt for correction of wear was made. RESULTS At lake Chiemsee the average monthly numbers of Cormorants were 244 in November, 255 in December, 231 in January and 165 in February. In October and March 150 birds were recorded (Lohmann pers. comm.). At the lower Inn river the numbers of Cormorants were somewhat higher; average monthly Cormorant numbers being 288 in December, 302 in January and 211 in Fig. 1. Map of Gennany with the location of the February. No counts were made in October, study areas lake Chiemsee and lower Inn river. November and March. As the counts of the winter before (1989/90) showed very similar results (Franz & Sombrutzki 1991), the numbers of days each month throughout the winter of 1990/91 October 1989 (185), November 1989 (327) and (November-February at lake Chiemsee, Decem­ March 1990 (95) were used for calculation of bird ber-February at the lower Inn river). Pellet collec­ days. tion always took place in the early morning, Identifiable fish remains were found in 62% of before scavengers could get at them, and only all pellets (n =1758),20% only contained uniden­ fresh and complete ones were taken. A total of 'tifiable remains, while about 18% were comple­ 1758 pellets from both roosts were analysed. The tely empty. A total of 2303 individual prey items numbers of Cormorants present at the roosts were was found, belonging to 22 different species of recorded in the evenings before the pellet collec­ fish (Table 1). The length spectrum of the fish tions. At lake Chiemsee additional counts were caught ranged from 3 cm cyprinid fry to 70 cm made in October and March. Eel Anguilla anguilla (corresponding to a mass All identifiable fish remains (otoliths, pharyn­ spectrum of 1-900 g). The food composition of geal bones, opercula, eye lenses, vertebrae, etc.) the Cormorants was quite similar at the two were separated and analysed (for methods see roosts, consisting mainly of cyprinids (50-76% in EAWAG 1975, 1979, Jonsson 1979, Muller 1986, numbers, 31-64% in biomass at lake Chiemsee Worthmann & Spratte 1987, Morel & Hausmann and 64-90% in numbers, 58-82% in biomass at 1989, Marteijn & Dirksen 1991, Trauttmansdorff the lower Inn river; Fig. 2). At lake Chiemsee the 1992). Otoliths and pharyngeal bones were meas­ most important cyprinid species were Rudd ured in order to estimate length and mass of each Scardinius erythrophthalmus, Roach Rutilus individual prey item, according to regression rutilus and Chub Leuciscus cephalus. At the formulae (Berg 1985, Worthmann & Spratte 1987, lower Inn river mainly Bream Abramis brama R. Muller, pers. comm., own data). Estimated and Roach were caught. The economically very numbers of fish per pellet, fish length and fish important Whitefish Coregonus spp. were only Keller: DIET OF WINTERING CORMORANTS IN BAVARIA 187 Table 1. Prey species of Cormorants in the winter of 1990/91 at lake Chiemsee and the lower Inn river, southern Bavaria. Fish length, em Fish mass, g N Mean Min Max Mean Min Max Anguilla anguilla 45.6 23.2 70.4 322 32 900 57 Salmo trutta fario 24.5 12.0 34.3 300 18 700 5 Oncorhynchus mykiss 24.6 11.2 35.3 157 19 350 7 Coregonus spp. 28.6 13.0 40.8 277 16 636 130 Thymallus thymallus 27.5 13.2 42.2 221 24 646 87 Rutilus rutilus 27.3 9.3 35.0 202 6 632 133 Leuciscus leuciscus 24.7 18.2 31.2 204 55 398 5 Leuciscus cephalus 25.4 16.9 34.6 220 49 512 16 Leuciscus idus 19.3 69 1 Scardinius erythrophthalmus 24.5 10.0 33.0 276 15 517 45 Aspius aspius 37.2 32.7 41.6 623 420 825 2 Tinea tinea 27.4 25.2 31.2 286 213 416 3 Chondrostoma nasus 34.1 27.4 40.6 634 266 928 9 Barbus barbus 30.3 28.8 32.9 269 228 345 4 Albumus albumus 18.0 16.2 19.7 63 42 83 2 BUcca bjoerkna 22.7 18.6 26.8 145 59 230 2 Abramis brama 23.5 16.7 35.7 144 49 532 22 Cyprinidae, not identified 15.2 3.0 38.7 80 1 876 1348 Esox lucius 37.3 25.2 47.2 383 117 688 16 Lota Iota 29.8 17.9 41.0 204 42 490 60 Perea fiuviatilis 13.3 6.8 31.2 35 3 367 177 Stizostedion lucioperca 20.7 8.3 46.4 108 11 557 36 Gymnocephalus cemua 13.1 11.9 14.7 22 14 35 4 consumed in their spawning period at lake and up to 8% in numbers (15% in biomass) at the Chiemsee during the months of December and lower Inn river. The only other species reaching a January (up to 29% in numbers, 41 % in biomass; proportion of over 10% (either in numbers or in Fig. 2). Consumption of Eel (another economi­ biomass) in any month were Perch Perea fluviat­ cally important species) was also virtually ilis (in November 12% in numbers, 6% in restricted to lake Chiemsee, where only in biomass at lake Chiemsee and in December 23% November some large individuals (6% in in numbers, 9% in biomass at the lower Inn river; numbers and 31 % in biomass) were taken (Fig. Fig. 2) and Burbot Lota Iota (in November 12% 2). Grayling Thymallus thymallus, an important in biomass, 4% in numbers at lake Chiemsee). recreational species for anglers, were found at Two other species of recreational importance to both sites, but only in January and February at the anglers, Pike Esox lucius and Pikeperch onset of their spawning period (Fig. 2). In the Stizostedion lucioperca, were never found in latter month their proportion reached up to 23% proportions exceeding 1% of the prey items. in numbers (21 % in biomass) at lake Chiemsee Thus, except for the absence of the typically pre- 188 ARDEA 83(1), 1995 n=912 n=1318 .r:: III :;::: 20 o ~_.....d ~--=-lJL ~lJ ~c:lb.CL.._ eel whitefish grayling pike burbot perch pike-perch eel whitefish grayling cyprinids pike burbot perch pike-perch 100,-----------------------, DNov 1990 I Dec 1990 80 DJan 1991 III Feb 1991 ~ 60 <ll E n=85514 (g) n=189968 (g) ~ 40 .r:: III :;::: 20 ~[LCL-_ o ....