A comparative study on the feeding of eel, Anguilla anguilla (L.), bream, Abramis brama (L.) and ruffe, Gymnocephalus cernuus (L.) in Lake Võrtsjärv, Estonia

Abstract

The stomach contents of eel, bream and ruffe were examined in order to detect the diet overlap and possible food competition between these benthophagous fishes in the eutrophic Lake Vortsjarv. The frequency of occurrence, mean number and mean restored weight of food items per individual were used for the description of their diet. Overlap in diet of bream (standard length >15 cm), eel (SL>30 cm) and ruffe (SL>4.5 cm) appeared mostly in respect of Chironomus plumosus (L.) larvae and pupae, which may lead to food competition between these fishes. However, these fishes can eat different chironomid instars. Unlike eel, who takes mainly large fourth-instar larvae and pupae of C. plumosus, ruffe and bream eat, in addition, small third-and second-instar larvae of this species as well as other small-sized chironomids, mainly Einfeldia carbonaria (Meig.), Microchironomus tener (Kieff.) and Procladius spp. Obviously, even if the overlap was pronounced, serious competitive interactions would arise only in case of limited food resources in the years with a low biomass of C. plumosus. A significant correlation (r= 0.88; P < 0.05) was found between the mean biomass of fourth-instar larvae of C. plumosus in the lake sediment and in the stomach of ruffe in different years. The diet composition of ruffe was more diverse (including a wide range of invertebrates, fish ova and detritus) than that of eel and bream. Evidently, omnivorous mode of feeding provides ruffe with an advantage over other benthophagous fishes living in the lake. Unlike eel and bream, ruffe continued to take food in late autumn and winter when water temperature in L. Vortsjary decreased to 2–0.2°C.