The volunteer-based protection association for Lake Köyliö (Köyliönjärven suojeluyhdistys) is doing important work to restore the lake, which is prone to eutrophication. The association’s stocking of pike perch fingerlings is helping to restore the ecological balance of the lake and improve water quality.
The decline of water fleas leads to increased algae growth
Lake Köyliö has suffered from significant blue-green algal blooms for decades, caused by the decline of the water flea population. Water fleas consume algae and thus help control its growth. However, the lake’s ecosystem is disrupted by so-called “trash fish,” small, non-edible cyprinids, which feed on water fleas.
“When water fleas disappear from the water, the cyprinids start searching for food by disturbing the lake’s muddy bottom. This activity releases phosphorus that has accumulated in the sediment over decades, further fueling the growth of blue-green algae,” explains Tapio Putko, a member of the association’s board.
Stocking pike perch removes trash fish from the food chain
Pike perch is a predatory fish that feeds on small fish, such as roach.
“As the populations of trash fish decrease due to predation by pike perch, the water flea population has the opportunity to grow and more effectively control the amount of algae. This reduces the nutrient load in the lake and improves water quality,” Putko continues.
Although the effects of pike perch stocking are not immediately visible, long-term efforts yield results. The improvement in Lake Köyliö’s water quality is the sum of many years of repeated actions. Improvements have already been observed, and the pike perch fingerlings are beginning to impact the ecosystem.
“Local residents and cottage owners are rediscovering the lake as a fishing spot, increasing local interest and commitment to its care,” Putko notes.
