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In the 1960s, Sweden introduced signal crayfish to support fishing; the plague-carrying invaders helped cut native noble crayfish populations fourfold
In the 1960s, Sweden introduced signal crayfish to boost fishing. They carry a plague deadly to native noble crayfish, whose numbers have dropped fourfold.
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We all have made a seemingly smart choice at the time that later turned into a headache. Sweden did something like that in the 1960s. It brought in the signal crayfish, a species introduced to support fishing. According to the Swedish University of Agricultural Sciences (SLU), the move proved catastrophic for the native noble crayfish. Native populations have dropped by a factor of four since the newcomer arrived, and the newcomer carries a disease that is deadly to them.
A fishing boost that came with a catch
The signal crayfish did deliver on fishing. It has provided good fishing opportunities in southern Sweden, and built fishable populations in lakes Hjälmaren and Vättern. Commercial catches there have increased sharply, mostly because there is more fishing. But the signal crayfish is a chronic carrier of the crayfish plague. That means it carries the disease and often survives it, not like the noble crayfish.
How the plague travels through a lake
Crayfish plague is a fungal disease that affects only freshwater crayfish. It spreads through spores between animals. The noble crayfish dies quickly when infected; the signal crayfish often survives it and transmits it. Think of someone who carries a cold without getting sick, while everyone around them does. Signal crayfish can still get sick if stressed by pollution, poor food supply, crowding, or unfavorable water temperatures. Wet fishing gear, other equipment, bait fish, and spores in the water can also carry the plague.
Deep Dive
A native species on the edge
The decline of the noble crayfish started in the early 20th century, mainly due to the plague. SLU estimates up to 97 percent of its population has been wiped out in the past century. It is assessed as critically endangered on the Swedish Red List of Threatened Species. Acidification, pollution, and water regulation all contributed to the decline, but the signal crayfish accelerated it. Between 1994 and 2007, almost a thousand lakes, rivers and watercourses were affected by the plague, and only Norrbotten County has been completely free. The loss also hits the wallets. The noble crayfish is worth over twice as much as the signal crayfish on the market.
A problem bigger than Sweden
Sweden is not unique. In a review published in Knowledge and Management of Aquatic Ecosystems in 2009, David Holdich and colleagues found that non-native species now outnumber native species in Europe by two to one. Habitat loss, poor water quality, overfishing and climate change are also threats, but the most important are non-native crayfish and the crayfish plague. The aquarium trade is another route for new species. Some countries are setting up safe ‘ark’ sites to protect native crayfish.
What can Sweden and anglers do?
Sweden has tightened restrictions on crayfish introductions. SLU says illegal releases have persisted even though introductions were prohibited as early as 1994. The signal crayfish was designated an invasive species in 2016, and it is now illegal to farm the species or introduce it to new waters. Anglers can help too. SLU recommends completely drying boats, traps, boots, and other gear before moving between bodies of water, and only using bait fish from the water you are fishing in, or freezing it for one day. This story shows how moving one species can end up changing a whole ecosystem.
