Pike (Esox lucius L) stocking as a biomanipulation tool .1. Effects on the fish population in Lake Lyng, Denmark

From 1990-1993 juvenile pike (Esox lucius) were stocked each spring in the eutrophic Lake Lyng (9.9 ha, max. depth 7.6 m, mean depth 2.4 m) in densities between 515 and 3616 pike ha(-1). In 1989-90 the fish population consisted mainly of roach (Rutilus rutilus), rudd (Scardinius erythrophthalmus), perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus), and total fish biomass was estimated at 477 kg ha(-1). Prior to stocking pike was not present in the lake. Following the first year of stocking, the density of roach, rudd and ruffe fry expressed as catch per unit effort decreased significantly by 64 to 97%. In 1991 ruffe disappeared completely. The pike stocking did not affect the density of perch significantly. The growth of pike was high and also the growth of perch increased significantly from 1990 to 1991 (p < 0.001) and from 1991 to 1994 (p < 0.001). We found a linear negative relationship between stocking density of pike in May or June and the abundance of juvenile planktivorous fish (r(2) = 0.85, p < 0.05) in the littoral zone in August. No relationship was found in the pelagic zone (r(2) = 0.21, p > 0.4). Pike survival was low in late August/early September. We suggest that growth of the piscivorous perch increased due to increased Secchi depth and a continuous high density of 0+ planktivores in the pelagic zone of the lake during the years of pike stocking, possibly caused by behaviourial changes and the forcing of the 0+ planktivores into the pelagic zone.