Thiamine Deficiency Eyed as Link to Poor Returns of Western Alaska Chinooks

Image: North Pacific Anadromous Fish Commission.

A report from the North Pacific Anadromous Fish Commission (NPAFC) says thiamine (vitamin B1) deficiency may play an important role in poor returns of western Alaska Chinook salmon.

The report in the January 2023 edition of the NPAFC newsletter notes that thiamine deficiency causes abnormal neuromuscular signs and mortality in fish consuming diets lacking thiamine or containing thiaminase, an enzyme that destroys thiamine.

The thiaminase enzyme has been found in high levels in certain small pelagic forage fishes that are common prey for juvenile and maturing Chinook.

Diets high in thiaminase have been shown to be associated with depressed thiamine concentrations in the predator species, says the article’s author, research biologist Wesley Strasburger of the Alaska Fisheries Science Center in Juneau.

Thiamine deficiency was first linked to reproductive failure in wild fish in the mid-1990s. Pacific Chinook are known to feed on prey fish that contain thiaminase.

In addition to overt fry and adult mortality observed when fish consume prey fish containing thiaminase, low thiamine can limit spawning migration decrease growth, affect vision, reduce both predator avoidance and pray capture, and impair immune function, all of which negatively affect population recruitment, Strasburger said.

Thus, thiamine deficiency impacts multiple life stages and multiple physiological processes, he said.

Strasburger also cited the socioeconomic impact of poor Chinook returns to western Alaska since the late 1990s, in management actions that have severely restricted or closed commercial and sport fisheries. “Such restrictions significantly impact Alaskan fishermen and communities that depend on Chinook salmon for subsistence needs and economic opportunities,” he said.

The latest data available shows that in western Alaska, as much as 60% of sampled Chinook eggs were below the 95% survival threshold of thiamine for those eggs and up to 80% were at levels thought to be associated with secondary effects of low thiamine levels, he wrote.

The average egg thiamine concentration in Yukon River Chinook also showed a decreasing pattern with increasing migratory distance. The data suggests a cost associated with in river spawning migration dynamics that can be stock specific, which is particularly important for the Canadian-origin Chinook on the Yukon, he wrote.

Strasburger also noted that changing conditions in the Bering Sea tend to change distribution and abundance of forage fishes available to Chinook salmon. What with several recent warming events in the Bering Sea, pelagic forage fishes, especially capelin.

Distribution of capelin contracts northward during warm periods, drastically reducing their abundance in Chinook migration routes, while Pacific herring remain relatively constant in their distribution and abundance.

Current information does not provide a good understanding of production of thiamine and thiaminase under varying environmental conditions.

“We don’t yet know the extent of year-to-year variation in the thiamine/thiaminase status of each of these prey items, and we do not know the mechanism of production in our ecosystems, which is a likely avenue for future research,” Strasburger concluded.