Wild salmon are getting smaller due to aquaculture

Researchers from Finland used genetic methods to determine if a fishery for an aquaculture fish food source, combined with changes in salmon fishing, are causing wild salmon sizes to reduce.

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

Salmon aquaculture

This indirect effect identified in the study highlights the true effects of salmon aquaculture. Some of the capelin fishery catch is used as fishmeal for salmon aquaculture feed, which suggests that a strong harvest and a declining capelin abundance can be an indirect way salmon aquaculture could influence wild salmon populations.

“The aquaculture industry has made important progress in finding alternative protein sources for aquaculture fish feed, and our study suggests that these efforts have not been in vain, as it seems capelin harvest may affect wild salmon populations,” explained Professor Craig Primmer at the University of Helsinki. “Globally, 18 million tonnes of wild fish such as capelin are harvested annually for domestic animal feed, so there is still work to do to further reduce the effects of aquaculture on wild fish populations.”

Linking environmental and human influences with evolutionary changes

“Our earlier research had shown that the age at which salmon were maturing in this river was getting younger, and consequently the size of salmon that are spawning was also getting smaller, illustrating ‘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,” added Primmer.

Dr Yann Czorlich, the first author of the study, commented: “This previous research could not tell us what environmental or human influences might be linked with the evolutionary changes. To understand this, we needed to link the yearly changes in the salmon DNA variation with annual changes in environmental and human-linked factors.”

“We gathered literally millions of data-points about factors, including yearly water temperature, salmon fishing effort, and commercial fisheries catches of the fish salmon eat in the ocean, and compared them with our data on DNA changes in our 40-year time series.”

Changes in salmon fishing affect the size of wild salmon

In addition to the indirect effect of capelin harvesting, the research team also identified a direct effect of salmon fishing in the river. “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,” said Jaakko Erkinaro, a Research Professor at Natural Resources Institute Finland.

The researchers analysed local indigenous knowledge to discover 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 continued.

Weir fishing has decreased in recent years, and simultaneously, the proportion of small, early maturing fish in spawning populations has increased. “Our finding that there are different forms of fishing acting in opposite directions at different phases of their life-cycle highlights new challenges for salmon management, but also the value of having unique long-term data series at our disposal,” noted Erkinaro.

A unique long-term scale salmon archive

The research team examined scale samples from salmon over a 40-year period and linked the variation of a gene that determines salmon reproduction age and size, with the effects of different fishing methods. The scale samples used for the study came from a unique long-term scale archive maintained by Luke.

The archive holds samples from more than 150,000 salmon individuals collected by trained, volunteer fishers since the 1970’s from River Teno, one of the most prolific salmon rivers in Europe. The scales were used to determine the age structure of the salmon population. They were also the source of DNA for genetic analysis.

The study was conducted in co-operation between the University of Helsinki, the Natural Resources Institute Finland (Luke), the University of Turku and the Norwegian Institute for Nature Research (NINA). Funding for this study was received from the Academy of Finland and the European Research Council.

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