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

The impact of whitefish (Coregonus lavaretus (L.)) on the trophic stru cture of eutrophic lakes was studied in Lake Ring, a small eutrophic D anish 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 bur bot (Lota lota (L.)) being the only other fish species present. A tota l of 10 993 0 + whitefish were stocked in the lake from October 1989 t o July 1990 and the structure of the fish, zooplankton and benthic inv ertebrate communities studied during the period 1989-91. Stomach conte nts analysis revealed that the whitefish mainly ate Daphnia and copepo ds in 1990-91, the proportion of copepods decreasing with increasing s ize of the fish and Daphnia being the overall most important food sour ce. The density of Daphnia in the lake decreased from 72 ind. l-1 in 1 989 to 9 ind. l-1 in 199 1; concomitantly the large species Daphnia ma gna and D. pulex almost disappeared and the density of cyclopoid copep ods increased from 72 to 101 ind. l-1, presumably because of improved food conditions, while that of calanoid copepods remained virtually un changed. 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). Cha nges were also observed in the benthic invertebrates; Chaoborus, oligo chaetes, and chironomids all decreased, whereas Pisidium increased. It is concluded that the stocking of whitefish in eutrophic lakes for co mmercial purposes may delay their recovery, or even lead to enhanced e utrophication.