Walruses are potentially exposed to paralytic shellfish toxin concentrations that impact their health during Alexandrium blooms in the Alaskan Arctic

Harmful algal bloom (HAB) activity of Alexandrium catenella, a dinoflagellate that produces paralytic shellfish toxins (PSTs), has increased in the Alaskan Arctic due to warming ocean conditions. Arctic wildlife exposure to PSTs due to these blooms remains unknown. The Pacific walrus (Odobenus rosmarus divergens) is a marine mammal that preys primarily on efficient vectors of PSTs (benthic invertebrates), making walruses an important model for assessing PST exposure risks. Here, we used paired Alexandrium data (Alexandrium cell and cyst densities and PST content), and PSTs measured in benthic invertebrates (clams, worms, and gastropods) to construct and validate trophic transfer models of PSTs in a critical Arctic food chain. Further, we used observed PSTs in walrus prey to estimate daily walrus oral PST doses (µg STX eq. kg^-1) during summers (2019 and 2022) with well-characterized and widespread Alexandrium blooms. Models were validated using various model metrics, however, predictions underestimated toxicity (STX eq.) of invertebrates. Observed PSTs in walrus prey resulted in 89 % (n= 47 of 53 total; 2019) and 67 % (n= 41 of 61; 2022) of PST doses exceeding thresholds posing a moderate health risk to walruses (> 11.6 μg STX eq. kg^-1). Ledyard Bay in the northeastern Chukchi Sea was identified as a PST exposure risk hotspot because it contained the highest estimated doses, while walruses simultaneously occupied the neighboring Point Lay haulout. Additional studies are required to assess how HAB activity will impact walrus health as well as food-security and food-safety for Indigenous communities that rely on walruses for subsistence.