Shortage of capelin shrinks River Teno Salmon

The study, published recently in journal Science, showed the shrinking size of Atlantic salmon in the River Teno in Northern Finland might not be due to directly fishing salmon. Rather, the impacts could stem from an indirect effect: the commercial fishery of one of wild salmons’ favorite foods in the ocean: a small omega-3 rich fish called capelin.

This indirect effect identified in the study puts the spotlight on salmon aquaculture. Some of the capelin fishery catch is used as fishmeal for salmon aquaculture food, suggesting strong harvest and declining capelin abundance can be an indirect way salmon aquaculture could influence wild salmon populations.

“Our earlier research had shown that the age at which salmon were maturing in this river was getting younger, and consequently also the size of salmon that are spawning was getting smaller, showing “evolution in action”. Important for demonstrating rapid evolution, there were also changes in their DNA at a gene known to be linked with maturation size and age”, explains professor Craig Primmer at the University of Helsinki.

Also changes in salmon fishing affect the size of salmon

In addition to the indirect effect of capelin harvest, the team also identified a direct effect of salmon fishing in the river, but with a twist. “We found that a special net type, a salmon weir, that accounts for majority of net catches, is capturing predominantly smaller fish, although net fishing is often assumed to catch larger fish”, says Jaakko Erkinaro, a Research Professor at Natural Resources Institute Finland (Luke). Indigenous knowledge provided an answer for this surprising result: “We discussed with local Sámi fishers who have fished with salmon weir for decades, and they explained that, compared to other net types, salmon weir has smaller mesh size, is used later in the season and mostly in shallower waters, which increases the catch of smaller salmon. This likely explains the result”, Erkinaro continues. Weir fishing has decreased in recent years, and simultaneously, the proportion of small, early maturing fish in spawning populations has increased.

News of the study, Natural Resources Institute Finland

Barentsportal ‒ Causes of capelin stock fluctuations

Massive Icelandic capelin quota of 869 600 tonnes ‒ Marine and Freshwater Research Institute of Iceland

Photo on the top: River Teno Salmon eat lot of capelin in the sea. Hopefully both salmon and capelin stocks will be strong in the future and fishing can be started again on River Teno. (Photo: Janne Rautanen)

(02.03.2022)