The ecological status of Lake Lappajärvi (South Ostrobothnia, Western Finland) has decreased and is now classified as moderate. Eutrophication, declined vendace (Coregonus albula) stock in 1990´s and socio-economical changes have decreased the amount of fishermen and catch. In aim to improve the ecological status of the lake and to increase commercial fishing, a project ”Kalastus elinkeinoksi Lappajärvellä” was carried out in 2001–2004. The impacts of the project were monitored during 2001–2006.

Total catch was 1655 tons (appr. 29 kg/ha/yr). Even if target (2100 tons) was not reached, the project had plenty of positive, even if sometimes weak or dubious, effects on fish populations and the ecological status of the lake. CPUE of pikeperch (Zander lucioperca) and big-sized perch (Perca fluviatilis) increased first in fishermen´s catch. Later in the project, the changes of CPUE were more contradictory. Pelagic coarse fish populations, especially smelt (Osmerus eperlanus) declined clearly. The vendace population, instead, increased and stayed abundant. The increase of pikeperch and big-sized perch populations, were explained mainly by warm summers and in the case of pikeperch, by successful introductions. Predators, especially pikeperch, are able to control the stocks of plankton-feeders in the pelagial of the L. Lappajärvi and on the other hand, can prevent the migration of roach to the pelagial. After the project, there were indications about the return of the populations to their starting-point. In the littoral zone of the lake, the impacts of the project were much slender. Zooplankton densities increased slightly in surface waters, but after the project, densities started to decline again. The blooms of blue-green algae seemed to decrease during the project, but great fluctuations of the blooms made assessing difficult. Biomanipulation and decreasing of nutrient load from catchment diminished P concentration in the lake, helped by dry years with less runoff. The declining of P concentration continued also after the project. The ascending trend of chlorophyll-a, prevailing before the project, stopped and the concentration stabilized. Chl-a/P relationship as well as the internal nutrient load was decreased by the project. N concentration, color of the water or transparency of the lake were not impacted.

The ecological status of the L. Lappajärvi can be improved by biomanipulation, but the fishing must be continuous. Target catch could be around 20-30 kg/ha/yr, but at the same time, the decreasing of external nutrient load is necessary. Fishing effort should concentrate both on commercially valuable and less-valuable fish species. At the same time, the populations of predator species should be reinforced.