ENVIRONMENTAL-EFFECTS OF INTRODUCING WHITEFISH, COREGONUS-LAVARETUS (L), IN LAKE RING

The impact of whitefish (Coregonus lavaretus (L.)) on the trophic structure of eutrophic lakes was studied in Lake Ring, a small eutrophic Danish lake (22.5 ha, mean depth 2.9 m) in which the natural fish fauna is dominated by pike (Esox lucius L.), perch (Perca fluviatilis L.), and eel (Anguilla anguilla (L.)), roach (Rutilus rutilus (L.)) and burbot (Lota lota (L.)) being the only other fish species present. A total of 10 993 0 + whitefish were stocked in the lake from October 1989 to July 1990 and the structure of the fish, zooplankton and benthic invertebrate communities studied during the period 1989-91. Stomach contents analysis revealed that the whitefish mainly ate Daphnia and copepods in 1990-91, the proportion of copepods decreasing with increasing size of the fish and Daphnia being the overall most important food source. The density of Daphnia in the lake decreased from 72 ind. l-1 in 1989 to 9 ind. l-1 in 199 1; concomitantly the large species Daphnia magna and D. pulex almost disappeared and the density of cyclopoid copepods increased from 72 to 101 ind. l-1, presumably because of improved food conditions, while that of calanoid copepods remained virtually unchanged. As a result chl-a increased from 19 to 47 mug l-1 and Secchi depth decreased from 2.4 m to 1.7 m, despite there being no change in total P and total N (0.6 mg P l-1 and 1.3 mg N l-1, respectively). Changes were also observed in the benthic invertebrates; Chaoborus, oligochaetes, and chironomids all decreased, whereas Pisidium increased. It is concluded that the stocking of whitefish in eutrophic lakes for commercial purposes may delay their recovery, or even lead to enhanced eutrophication.