Nutrient variation in prey consumed by Chinook salmon may be playing a critical role in their growth and survival, according to research compiled by the Alaska Fisheries Science Center (AFSC) in Juneau.
The study compiled by research biologist Wesley Strasburger notes that there is evidence that vitamin B1 — otherwise known as thiamine — deficiency can affect survival and recruitment of western Alaska Chinook salmon and that with climate change these fish are not consuming adequate amounts of prey containing this critical vitamin.
The Chinook may also, in fact, be consuming prey with thiaminase, an enzyme that destroys vitamin B1 — an essential dietary nutrient for these fish — Strasburger notes in his report, which is included in the latest newsletter of the North Pacific Anadromous Fish Commission, in Vancouver, Canada.
Researchers at the AFSC have not yet branched out beyond the Yukon/western Alaska area and some hatcheries in Southeast Alaska, Strasburger said. He also noted that sockeye salmon tend to have less of an issue with vitamin B1 levels due to the difference in their diets relative to the more piscivorous, or fish-eating, species.
“We have compared Chinook to chum eggs within the same location and generally find that it is a larger issue within Chinook,” Strasburger said.
Thiaminase is found in high levels in certain small pelagic forage fisheries which are common prey for juvenile and maturing Chinook. It is also thought to be produced in certain bacteria, and likely translated up the food chain, where it is found predominantly in the internal organs of certain fish species.
Vitamin B1 deficiency was first linked to reproductive failure of wild fish back in the mid-1990s.
Strasburger notes that understanding factors that influence the survival of salmon is important as salmon play a key role in the ecological and socioeconomic framework of many Alaska communities. Recent changes in Arctic temperatures have important implications for coastal ecosystems of the northern Bering Sea and Yukon River Chinook salmon, he said.
Chinooks returning to western Alaska have shown markedly decline since the late 1990s, most notably with king returns to the Yukon River declining by about half of their 1982-1997 historical size.
In western Alaska, as much as 60% of sampled Chinook eggs were below the 95% survival threshold and up to 80% were at levels thought to be associated with secondary effects of low vitamin B1 levels. The average egg vitamin B1 concentration in Yukon River Chinook also displays a decreasing pattern with increasing migratory distance, Strasburger said. The same pattern was observed in eggs collected on the Yukon in previous years.
“This data suggests a cost associated with in river spawning migration dynamics that can be stock specific,” Strasburger wrote.
Researchers also sampled juvenile muscle and liver tissue and found that juvenile Chinook and coho salmon had lower muscle and liver B1 concentration than chum or pinks, and the highest annual proportion of thiaminase-positive prey in their diet at around 60%, followed by coho at around 40%.
Chinook and coho are much more selective for fish as prey than any other Pacific salmon, Strasburger noted.
In Southeast Alaska the researchers have observed variable diet composition in Chinook diet, primarily based on the region of foraging.
The production of both vitamin B1 and thiaminase under varying environmental conditions is not well understood, Strasburger said. Both likely have bacterial and phytoplankton origins.
“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,” he said.
