FOOD-WEB MANIPULATION IN A SMALL, EUTROPHIC LAKE WIRBEL, POLAND - EFFECT OF STOCKING WITH JUVENILE PIKE ON PLANKTIVOROUS FISH

In a four year experiment (1988-1991) carried out in a shallow, eutrop hic lake, an increase in young-of-the-year pike (Esox lucius) density was used to control planktivorous fish assemblages consisting mainly o f small-sized cyprinids: roach (Rutilus rutilus), Leucaspius delineatu s, and white bream (Blicca bjoerkna). Stocking with pike fry of ca 30 mm in three successive springs resulted in large-scale mortality among prey of vulnerable sizes. Rotenone treatment at the end of the fourth year showed that roach and white bream populations contained few 0 to 2 + fish, but very high numbers of 3 + to 6 + fish. By this time, s hort-lived, slow-growing L. delineatus, with no size refuge from preda tors, had been driven almost to extinction.