Year-Round Algal Toxin Exposure in Free-Ranging Sea Lions: Implications of Trophic Exposure for Declining Populations

Harmful algal bloom (HAB) toxins have led to illness and mortality of many species of marine mammals and seabirds, including species with declining populations. On the US West Coast, the two most common HAB toxins affecting both humans and wildlife are domoic acid and saxitoxin. In an effort to document baseline concentrations and to investigate factors that affect exposure to HAB toxins, I measured concentrations of domoic acid and saxitoxin in scats from Steller sea lions Eumetopias jubatus (n = 383 scats) and California sea lions Zalophus californianus (n = 125 scats) in Washington State over a two-year period. Toxin concentrations in the scat were compared to the prey remains in the scat and to concentrations in nearshore bivalves. Saxitoxin was detected in 45 % and domoic acid was detected in 17 % of all scats tested, and both toxins were detected in all seasons and months of the year. Saxitoxin in scat was variable by season, year, and location, whereas domoic acid levels were consistently higher in the summer and at the southern-most haulout complex. Both toxins were detected in scat in winter when it was not detected in nearshore bivalves, confirming for the first time that marine mammals can be exposed to algal toxins through their prey outside periods of active algal blooms, most likely through benthic to pelagic food web transfer of precipitated cells and resting cysts. This study also found that prey with low occurrence in the sea lions' diet, including walleye pollock Theragra chalcogramma, may act as vectors of significant algal toxin transfer up the food chain, a finding that could have profound implications for the endangered western distinct population segment of Steller sea lions because pollock are a dominant prey species in their diet. A variety of planktivorous, benthic, and pelagic fish were significantly associated with toxins in sea lion scat suggesting that multiple pathways through the marine food web lead to HAB toxin exposure in these top predators. In the face of increasing HABs worldwide, the finding that generalist predators, like sea lions, can be exposed to algal toxins year-round via multiple prey species may signal disproportionate impacts on declining populations already enduring multiple stressors.