Diet composition and fate of contaminants in subsistence harvested northern sea otters (Enhydra lutris kenyoni) from Icy Strait, Alaska

Abstract

Northern sea otters (Enhydra lutris kenyoni) in Southeast Alaska have experienced a significant population increase since their successful reintroduction to the area after previous near extirpation owing to historic fur trading. The purpose of this study was to examine sea otter diet and metals contamination in an area of Southeast Alaska with the most robust increases in sea otter numbers, Glacier Bay/Icy Strait, with the intent of gathering baseline data for a healthy population of sea otters and as a reflection of the local coastal environmental health of the area. This research was a collaborative effort with Alaska Native subsistence hunters and the Alaska Department of Environmental Conservation. In Chapter 1, sea otter stomachs (n=25) were obtained in April 2015 and April 2016 from Alaska Native subsistence hunters in Icy Strait, Alaska. There were no differences in sea otter diet between years. Bivalves dominated the sea otter diet. Northern horsemussels (Modiolus modiolus) made up the greatest proportion of the diet (0.46 ± 0.48). Fat gaper clams (Tresus capax) and northern horsemussels were found in the highest proportion of stomachs (0.64 and 0.60, respectively). There was not an apparent trend between sea otter age and the minimum number of total prey items, stomach contents mass, or mean frequency of occurrence of the top four prey species. Sea otters from this study are likely to be dietary generalists throughout their lives. In Chapter 2, brain, gonad, kidney, and liver tissues, as well as stomach contents were analyzed for arsenic, cadmium, copper, lead, total mercury, and selenium for the 2015-harvested sea otters that were also referenced in Chapter 1 (n=14). In general, arsenic and lead had the highest concentrations in stomach contents, cadmium and selenium were highest in the kidneys, and copper and total mercury were highest in the livers. While brains and gonads had the lowest metals concentrations of any tissue, the metal with the greatest concentration within the brain was copper, and within the gonads was selenium. Concentrations of arsenic, cadmium, total mercury, and lead demonstrated a relationship with sea otter length. In general, all the mean metals concentrations for these sea otters were below published effects threshold values for marine mammals. Only total mercury demonstrated biomagnification from the stomach contents (i.e., the prey) to all higher-level tissues. Selenium health benefit values were positive in all sea otter tissue types analyzed in the present study, indicating that concentrations of selenium had an overall health benefit in protecting those tissues against mercury toxicity. Evaluating how contaminants concentrate and get distributed in tissues of top trophic levels provides an indication for potential exposure to humans and demonstrates how these keystone species act as indicators of local coastal ecosystem health. The results of studies on dietary exposure and metals contamination in top trophic level consumers such as sea otters can be used in monitoring the health of sea otter populations and the local environment that they inhabit.