MONITORING PELAGIC WHITEFISH (COREGONUS-LAVARETUS) AND VENDACE (COREGONUS-ALBULA) IN A HYDROELECTRIC RESERVOIR USING HYDROACOUSTICS

Stock assessment consisting of hydroacoustics each year and gill-netting every third year was performed in May 1986-1993 in the regulated Lake Osensjoen. Osensjoen, situated 438 m asl in south-eastern Norway, has a surface area of 45 km(2) and a mean depth of 37 m; the water level varies by 66 m and the pelagic fish stock consists of whitefish (Coregonus lavaretus L.) and vendace (Coregonus albula L.). The total density of large fish (greater than or equal to 28 cm) varied from 10.4 to 70.2 specimens/ha, with a mean value of 39.8 specimens/ha, corresponding to ca. 9 kg/ha. Owing to catches in pelagic gill-nets the fraction of whitefish was 41-68% in number of the estimated stock, and the mean weights of whitefish and vendace were 270-371 and 153-172 g, respectively, so whitefish constituted 62-76% of the biomass. The variation in total density and the variation between whitefish and vendace was caused by the fluctuating recruitment of both species. The mean lengths of two-year old whitefish and vendace showed reduced growth in years with strong recruitment. Strong year classes seemed to arise only in years when tapping off the lake, which is performed through a tunnel from the bottom, was minor about the time when larvae hatch in April-May. The yield was estimated in 1989 to be 1.0 kg/ha of whitefish and 0.12 kg/ha of vendace. Survival was estimated from stock assessment of large fish to be S = 0.55 for whitefish and S = 0.66 for vendace. The difference in survival rates was due to differences in exploitation, and the natural mortality and fishing mortality, respectively, were estimated to be M = 0.46 and F = 0.14 for whitefish and M = 0.36 and F = 0.05 for vendace. The best time for echosounding is May, before feeding activity results in the fish moving into the littoral zone, or closer to the surface in the pelagic zone. The fact that Osensjoen is regulated probably diminishes the fraction of fish in the littoral zone, especially immediately after filling of the lake.