Condition and growth of ruffe Gymnocephalus cernuus (L.) in two large shallow lakes with different fish fauna and food recourse

Abstract

Ruffe is one of the most numerous fishes in Estonian large Lakes Peipsi (area 3555 km², mean depth 7.1 m) and Võrtsjärv (270 km², 2.8 m). Owing to its small size ruffe has little value for commercial or recreational fisheries today. However, due to their high abundance, ruffe play an important role in the food webs of both lakes. The aim of the investigation was to establish the proportion of ruffe in the fish community and to assess the wellbeing of this fish in the two lakes on the basis of growth rate and condition factor. According to experimental catches made in autumn 1998–2002, the average total catch of fishes per trawl-hour (CPUE) in L. Peipsi was about two times higher than in L. Võrtsjärv (204 kg and 105 kg, respectively). CPUE of ruffe made up 16 kg in L. Peipsi and 7 kg in L. Võrtsjärv. In these lakes the fish community composition and food resource appear to be different. Benthivorous fishes were dominating in L. Võrtsjärv and the biomass of macrozoobenthos was significantly lower in this lake than in L. Peipsi. In the first year, the growth rate of ruffe was higher in L. Peipsi where the state of zooplankton was better compared with L. Võrtsjärv. Differences in the linear growth rate of older fishes in L. Peipsi and L. Võrtsjärv were not significant. The lower condition factor of ruffe in L. Võrtsjärv reflects the limited food resource of abundant benthivorous fishes in this lake. Different trophic relationships in Estonian large lakes are better reflected in the ruffe's condition factor than in its linear growth rate.