RELEASES OF PIKEPERCH (STIZOSTEDION-LUCIOPERCA (L)) FINGERLINGS IN LAKES WITH NO ESTABLISHED PIKEPERCH STOCK

One-summer-old pikeperch (Stizostedion lucioperca (L.)) fingerlings (4 -8 cm) were released in ten small and medium-sized (175-3 600 ha) Finn ish lakes with no permanent pikeperch stock in five successive years. Mean stocking density ranged 10-40 fingerlings/ha. In the following 5- 7 years, data were collected to estimate the growth and survival rates of the fish and to assess the total yield and profitability of the re leases in each lake. Catch samples were obtained from co-operating loc al fishermen. Fishing statistics were obtained by mailed fishing quest ionnaires. Reference data on growth and survival were collected from t he two parent populations, which belong to the most abundant in Finlan d. Large annual and between-lake variation was found in the growth and survival rates of the fish released. In most year-classes, the stocke d fish were smaller than one-year-old pikeperch in the parent populati ons. In two lakes, subsequent growth rates were faster than in the par ent populations. Similarities in year-class patterns in parent populat ions and annual survival patterns of stocked fish suggest that common factors contribute to the variation in both cases. Year-class indices for the parent populations were, however, not correlated with summer m ean temperatures or with the mean size of one-year-old fish. The yield of the releases varied in different lakes from 0.1 to 34 kg per thous and fingerlings (mean 11.5 kg). In two lakes, the estimated economic o utput exceeded the direct costs of the releases. An onset of natural r eproduction was recorded in two other lakes. In an attempt to explain the variable results, the quality of the stocking material, the limnol ogical features of the lakes, as well as the differences in the fish c ommunities and in the fisheries are considered.