Harmful algal bloom toxins protect bivalve populations from sea otter predation

We demonstrate how harmful algal bloom (HAB) toxins can mediate the strength of consumer-prey interactions, and thus ecosystem level patterns and processes, by altering the foraging behavior of a keystone marine predator (sea otter Enhydra lutris). Our approach was to com- pare sea otter foraging behavior, diet, prey abundance and paralytic shellfish poisoning toxins (PSPT) across a range of sites within the Inside Passage of southeast Alaska recently occupied by sea otters. Sea otters in southeast Alaska modified their foraging behaviors depending on concentrations of PSPT in bivalve prey, especially their preferred prey, butter clams Saxidomus giganteus. At sites of intermediate prey toxicity (200 to 500 µg saxitoxin (STX) eq · 100 g -1 ), otters continued to forage on butter clams while discarding the most toxic body parts. At highly toxic sites (prey toxicity > 500 µg STX eq · 100 g -1 ) otters avoided butter clams and other large and abundant but toxic bivalve prey, and consumed smaller and/or less abundant non-toxic species. Butter clams were larger and more abundant in highly toxic feeding areas, supporting the hypothesis that PSPT toxicity provides a refuge from sea otter predation.