Gonadosomatic index of pelagic spawning female whitefish (Coregonus lavaretus L.) in Lake Constance-Upper Lake
Lake Constance-Upper Lake has undergone oligotrophication in recent years, with total phosphorus concentration decreasing from 33 μg l-1 in 1991 to 8 μg l-1 in 2007. Its fish community is routinely monitored. Pelagic gill nets with mesh sizes of 26mm–44 mm were used to capture lake whitefish (Coregonus lavaretus L. ) on a monthly or more frequent basis. In the years 1991 to 2007, the gonad development of female whitefish was investigated. From July/August until the beginning of spawning in December, five female whitefish were randomly sampled per mesh size on each sampling date for gonadosomatic analyses. Body wet weight and gonad wet weight were determined for each fish and scales were taken for age determination. The Gonadosomatic index (GSI) increased from values at about 2% in July to values slightly above 20% immediately before spawning. An analysis of covariance for all data showed a highly significant (P < 0. 0001) increase of GSI during the course of the year prior to spawning, a highly significant increase of GSI with the age of the fish, and a highly significant decrease of GSI over the study period. This indicates that gonad development of pelagic spawning female whitefish in Lake Constance is influenced by oligotrophication, and less energy is invested into reproduction in recent years.