Evolution induced by fishing may explain why the bodies of Alaska’s sockeye salmon have shrunk by 5 per cent since the 1940s.

Neala Kendall at the University of Washington in Seattle sorted through data from canneries on the size and age at maturation of the sockeye (Oncorhynchus nerka) fished from five fisheries in Bristol bay, Alaska, since 1943. She found that the fish had become, on average, 14 millimetres shorter. Moreover, the number of fish that spent two rather than three years in the ocean before returning to fresh water to spawn increased by 16 per cent.

Fishing for the largest sockeyes is the likely culprit, says Kendall, because fishing targets larger fish. Many of these are breeding females, and removing them from the population also eliminates their eggs. Body size is a heritable trait, so the genes within those eggs – some of which are associated with late maturing and large body size – are also lost.

Such fishing-induced evolutionary changes appear to explain why the population of Atlantic cod (Gadus morhua) in the southern Gulf of St Lawrence has been unable to rebound since the fisheries there collapsed almost three decades ago. As with sockeye, cod fishers harvested the largest fish, including breeding females.


If current trends continue, the lucrative Bristol bay sockeye industry – which brought in $5 billion between 1950 and 2008 – could become less profitable.

Net tightens

Even without proof of fishing-induced genetic changes, other salmon fisheries in Alaska have begun trying to prevent such problems. Until recently, most fishers along Alaska’s Yukon river caught Chinook salmon (Oncorhynchus tschawytscha), the largest salmon breed, using gill nets designed to catch only the very largest fish – those with a girth greater than 215 millimetres. Last year, the Alaska Department of Fish and Game mandated a change of design to avoid these large fish and net only those that are 190 millimetres or less.

Those changes may have come too late, says Chris Stark, a fisheries biologist at the University of Alaska in Fairbanks. The fish have already shrunk so much that 190 millimetres “has become the optimal size for catching big Chinooks”, he says.

Kendall presented her findings at the second International Marine Conservation Congress in Victoria, British Columbia, Canada, last month.