Rapu populations collapse in South Ostrobothnia as invasive species spreads

South Ostrobothnia’s native crayfish stocks have collapsed over the past decade, driven by crayfish plague and the spread of the invasive signal crayfish, according to the South Ostrobothnia Fisheries Centre.

The region’s traditional crayfishing grounds, including Lappajärvi, once supported commercial harvests that helped local families earn a living. Records from 50–70 years ago show catches large enough to buy mopeds, locals recall.

Today, the signal crayfish—carrying crayfish plague and outcompeting the native noble crayfish—has taken over. The plague was introduced decades ago via translocated signal crayfish, while the signal crayfish itself was stocked across Finnish waters from the 1960s onward in an attempt to boost crayfish numbers. Stocking has since been banned.

“There are only remnants left of the old noble crayfish populations,” said Marko Paloniemi, managing director of the South Ostrobothnia Fisheries Centre. “Elderly locals still talk about buying mopeds with their crayfish earnings.”

In Lappajärvi, one of the region’s most productive crayfishing lakes, signal crayfish were detected 12 years ago. A 2020 test catch confirmed the trend: signal crayfish numbers had increased over six years, while noble crayfish had nearly vanished. Only a couple of noble crayfish were caught.

“The noble crayfish is likely to disappear from Lappajärvi over time,” Paloniemi said. “The signal crayfish spread here through human activity—literally on bicycle wheels.”

Meanwhile, Lappajärvi’s water level this summer is 25 centimetres below average, raising concerns over algal blooms and fish kills. The lake’s surrounding municipalities—Evijärvi, Alajärvi and Vimpeli—are exploring measures to stabilize the situation.

Professional fisherman Sampo Savola, who checks traps weekly, said the low water levels complicate navigation but that water quality has improved in recent years. He suggested earlier spring flow reductions at the Halkosaari dam could have helped retain meltwater.