Switching to marine prey leads to unprecedented mercury concentrations in a population of coastal Alaska wolves

Methylmercury (MeHg) bioaccumulates in organisms and biomagnifies in food webs, resulting in elevated concentrations in tissues of apex predators that may negatively impact health. As MeHg is mainly produced by aquatic microbes, predators feeding in aquatic food webs tend to have higher mercury (Hg) concentrations in their tissues than those feeding in terrestrial food webs. In a region of coastal Southeast Alaska, wolves switched from a terrestrial to marine-based diet specializing on recently recovered sea otters. We hypothesized that this prey switch would lead to higher Hg concentrations in wolf tissues. Therefore, we quantified total Hg (THg) concentrations in wolf hair (n = 25) and THg and MeHg in wolf liver (n = 7), muscle (n = 3), kidney (n = 2), and brain (n = 2) tissues from two wolf packs - a marine foraging island pack (located on Pleasant Island), and an adjacent mainland pack (located on the Gustavus Forelands) with a predominantly terrestrial diet. We paired this information with carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analyses of wolves (n = 65) collected from 2000 to 2023 to determine the proportional contribution of marine subsidies and infer trophic positions of wolves in the two packs. For comparison to wolves with a completely terrestrial prey diet, we quantified THg and MeHg in tissues from Interior Alaska wolves. Liver THg concentrations in the Pleasant Island wolves (mean = 17.59 ppm; range = 0.63-64.30) categorized individuals at 'high risk' and 'severe risk' and were 7 and 278 times higher than other coastal and Interior Alaska wolves, respectively, and 11 to >2000 times higher than concentrations reported in wolves globally. THg concentrations in wolf hair increased with both δ¹³C and δ¹⁵N indicating that foraging for marine and higher trophic position species exposes wolves to a level of Hg which may have health consequences.