Monday November 14, 2022

Too many salmon returning from the ocean would perhaps be a welcome problem for fisheries managers along the West Coast of the US, where many stocks of Pacific salmon are dwindling. In contrast, an increasing abundance of non-native Pacific pink salmon (Oncorhynchus gorbuscha) in many rivers in Norway and elsewhere in Scandinavia is raising concerns among fishery managers about their impacts on native Atlantic salmon (Salmo salar), ocean-run brown trout (Salmo trutta), and anadromous arctic char (Salvelinus alpinus). Atlantic salmon, in particular, are as iconic to the European North as Pacific salmon are to the Pacific Northwest. They are a cultural icon that embodies history and tradition, provides nourishment, and supports a large and economically valuable recreational fishing industry.

An invasive pink salmon from a river in Norway. Photo credit: Statsforvalteren i Troms og Finnmark

In the 1950s, Soviet fisheries managers first introduced pink salmon into rivers on Russia’s Kola Peninsula, which flow into the Barents Sea. Ever since then, pink salmon, which have among the highest straying rates of the Pacific salmon species, have been detected in Norwegian rivers. However, their numbers were exceedingly small and not considered a reason for concern until the 1980s, when a secondary introduction effort occurred. This second batch of pink salmon came from a different source population that purportedly had broader temperature tolerances, which led to the beginning of a rapid increase in invasive pink salmon abundance and geographic expansion. The sustained effort to annually stock Russian rivers with pink salmon eggs and fry was discontinued in 2001, and populations are now considered self-sustaining, and expanding, especially in recent years. In 2015, pinks were documented in 21 Norwegian rivers (less than 200 individuals total), but by 2017 they were present in at least 272 streams and numbered in the tens of thousands. Since then, populations have increased even further, prompting serious concern about their potential interactions with native fishes, especially Atlantic salmon.

Spawned out pink salmon decaying on the bank of a river in Norway. Photo credit: Statsforvalteren i Troms og Finnmark

Scientists are currently scrambling to investigate the potential effects of large pink salmon populations on cherished native Atlantic salmon, but disease transmission and competition for space and food (including redd superimposition), are among the chief concerns for fisheries managers and conservationists. Additional concerns include dead and rotting pink salmon, which introduce large amounts of nutrients that cause massive algal blooms and a severe reduction of oxygen in the river. However, one of the main challenges in evaluating the pinks’ impact is the lack of rivers without pink salmon for comparison, as just about every river in northern Norway now boasts a sizable run, particularly in odd years. While it may turn out that differences in run timing and overall life history minimize the pink salmon’s actual impact on Atlantic salmon, data to support this are currently lacking. Taking a precautionary approach, efforts are underway in many watersheds to drastically reduce the number of pink salmon spawners by setting traps and gillnets during their migration period. These methods, however, are not without drawbacks. Native species incidentally captured in gillnets may be harmed before they can be released, and most traps can only cover a small portion of the river channel and can be difficult to maintain during changing river flows. Consequently, in coordination with local fishing groups, the Norwegian government has decided to evaluate the efficacy of using resistance board weirs to intercept and trap migrating pink salmon in the early stages of their freshwater spawning migration. In the spring of 2022, FISHBIO was selected to fabricate two resistance board weirs for this purpose, which needed to be installed for a trial period by late summer. Now in operation far above the arctic circle, these two weirs are likely the northernmost weirs of their kind. But in a world still challenged by supply chain issues and international shipping problems, the process of getting the weirs built in our Fabrication Lab in Oakdale, California, and completing the installation in northern Norway was no mean feat. Stay tuned, the entire process from build to installation will be covered in an upcoming Fish Report.

This post was featured in our weekly e-newsletter, the Fish Report. You can subscribe to the Fish Report here.

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