Invasive species of crayfish native to America have been slowly spreading in the freshwater bodies within countries bordering the Baltic Sea, but now the problem has taken a critical turn. The invasive species, starting with signal crayfish but recently joined by at least three other nonnative species, are similar in many respects to the Baltic region’s native European crayfish (Astacus astacus) except in one potentially disastrous respect: they carry a plague to which they themselves are immune but causes a nearly 100% mortality rate for the European species. Scientists in Estonia are struggling to find preventive measures to save European crayfish. The largest problem is the signal crayfish (Pacifastacus leniusculus), which first appeared in Estonia in 2008. Another invasive species, the colourful marbled crayfish, is believed to have been released by pet owners, while other species may have invaded from ship ballast discharges. They are all immune to the plague.
Researchers at Estonia’s University of Life Sciences have fought the crayfish invaders—and the plague they carry— for years without success, using a variety of tools. Poison has been tried, but in flowing freshwaters that signal crayfish prefer, the poison is merely dissipated. Nets like those used to harvest local crayfish do not capture young invaders that escape. “Electro-fishing,” which sends jolts of energy into the water, doesn’t reach the crevices in rocks where crayfish dwell. More promising efforts include using eels to find and eat small crayfish in tiny hiding places, while the larger ones can continue to be trapped. But the trapping must be more intensive than now, the researchers say, and more public support is required. “The problem is becoming increasingly serious each year,” says Katrin Kaldre, a junior professor of aquaculture biology at the University of Life Sciences. “We practically discover a new signal crayfish outbreak every year.”
The project at the University of Life Sciences includes a public awareness campaign. “We taught in schools and at festivals how to distinguish between different crayfish species,” says Kaldre. “It’s important to raise awareness that not all crayfish are the same; you cannot simply move them from one body of water to another. Such actions have consequences.” People walking near water bodies are encouraged to photograph any crayfish they observe and report them through the national information phone line 1247.
