Fish stocks of Lake Vesijaervi: from nuisance to flourishing fishery in 15 years

摘要

Original goals of the biomanipulation project of Lake Vesijarvi, southern Finland, were to eliminate blooms of cyanobacteria and re-establish a sustainable fishery (Keto & Sammalkorpi 1988). From 1989-93, blooms of cyanobacteria were eliminated with mass removal of 1018 metric tons coarse fish, mainly roach (52 %), Rutilus rutilus (L.) and smelt (27 %), Osmerus eperlanus (L.), from the Enonselka basin (2600 ha, Horppila & Peltonen 1994, Horppila et al. 1996, 1998). Fishing with seine and fyke nets to remove 60-86 metric tons of coarse fish annually from 1994-96 prevented new increase of roach stock (Peltonen et al. 1999a). Additionally, large-scale stockings of predatory fish, especially pikeperch, Sander lucioperca (L.), were done in 1987-91 to increase predation on coarse fish and to improve the value of fish catches (Kairesalo et al. 1999). Stocked pikeperch established a population in Lake Vesijarvi and the first strong natural year-class hatched in 1992 (Peltonen et al. 1999b). An attempt to increase the predation on cyprinids in Lake Vesijarvi by stocking pikeperch (Keto & Sammalkorpi 1988) was not successful, because the most important prey fishes of pikeperch in Lake Vesijarvi are perch, Perca fluviatilis L., and smelt (Peltonen et al. 1996). Only big pikeperch ( > 1kg) can feed on adult ( > 12cm) roach living in pelagial of the lake. Effective subsistence fishing with 35-45 mm (bar length) gillnets, together with small recruitment size, kept numbers of big pikeperch and size of spawning stock very low (Kairesalo et al. 1999). To improve pikeperch catches and natural recruitment, fishery region of Lake Vesijarvi forbade fishing with gillnets of mesh sizes 22-49 mm (bar length) in the Enonselka basin from the beginning of 1997 (Suoraniemi et al. 2000). A new three-year period of intensive stocking of pikeperch fingerlings occurred in 1997-99. Half of stocked fingerlings were bigger than usual to determine if survival of stocked pikeperch can be im- proved by stocking fingerlings of size equal to natural ones (Ruuhijarvi & Salminen 2004). The objective of this study is to evaluate how management of fish stocks and fishery of Lake Vesijarvi from 1997-2003 has succeeded in fullfilling original goals of the restoration project. Successful biomanipulation from 1989—1993 and management to present date offer a unique long-term time series to assess how biomanipulated lakes react and how they should be managed after mass removal of coarse fish.