Algal Neurotoxins in Pink Salmon (Oncorhynchus gorbuscha) from Icy Strait and Icy Point, Alaska

Abstract

Blooms of algae in the genera Pseudo-nitzschia and Alexandrium produce neurotoxins that have the potential to harm humans and other mammals. While the effects of these toxins are best known for causing closures of shellfish fisheries, some finfish also consume toxic algal cells and can contain the algal toxins at levels that can be dangerous for mammals that frequently consume whole fish, such as sea lions. However, commercial finfish fisheries are rarely closed due to toxic blooms because most of the toxin remains in the gastrointestinal tracts of fish and is excreted fairly quickly, posing less risk to humans. In 2016, pink salmon returns in Alaska were disastrously low. The 2015 bloom of Pseudo-nitzschia was the largest on record and coincided with the coastal juvenile phase of the fish that would have been caught in the 2016 commercial fishery, suggesting that a mortality event may have occurred simultaneously with the Pseudo-nitzschia bloom, though direct mortality due to toxin exposure is an unlikely explanation for finfish mortality. I sought to determine whether mortality of juvenile pink salmon was associated with the Pseudo-nitzschia bloom, and whether exposure of juvenile pink salmon to algal neurotoxin levels in 2015 differed from previous years. I analyzed gut contents from juvenile salmon that had been collected from Icy Strait and Icy Point, Alaska, over the past twenty years for the presence of two algal neurotoxins. Both toxins were present at non-lethal levels in all years sampled, and domoic acid in gut contents from Icy Strait was significantly higher in 2015 than in any other year. Concentrations of both toxins were correlated with environmental conditions. Other salmonids have been demonstrated in laboratory settings to be highly resistant to environmental exposure to domoic acid, suggesting that the consumption of domoic acid did not directly harm the juvenile pink salmon. The anomalous environmental conditions in 2015 suggest a food web shift that may have affected juvenile salmon survival.