New Scientist By Andy Coghlan

What’s good for farmed salmon may not be for natural ecosystems.

Small fish called wrasse painlessly peck sea lice from the skin of farmed salmon, enabling salmon farmers rely less on powerful pesticides that can harm other marine life. But there’s a catch: the wrasse are fished from the wild, and researchers have now found evidence that the practice may be depleting natural populations, with unforeseeable effects on marine ecosystems.

A team from Norway discovered a decline in wild wrasse populations after comparing catch rates at four marine protected areas – where fishing is banned – and at four regions where fishing is allowed. The sampling sites were all along the Skagerrak coast of southern Norway, a region where wrasse have been caught and shipped to salmon farmers for decades.

They found that the goldsinny wrasse was up to 65 per cent more abundant in samples from the non-fishing zones. The corkwing wrasse was 92 per cent more abundant in some samples from the non-fishing zones. “It’s telling us that fishing may be reducing the natural populations,” says Kim Halvorsen of the Norwegian Institute of Marine Research in Storebø, Norway, who led the team. “It’s never been shown before, but these numbers could indicate that fishing has had an impact.” 

Journal reference: Marine Biology Research, DOI: 10.1080/17451000.2016.1262042

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