New publication now available: Tollit, D.J., L. Fritz, R. Joy, K. Miller, A. Schulze, J. Thomason, W. Walker, T. Zeppelin, T.S. Gelatt (2017). Diet of endangered Steller sea lions in the Aleutian Islands: New insights from DNA detections and bio-energetic reconstructions. Canadian Journal of Zoology 95(11): 853–868, year 2017, doi: 10.1139/cjz-2016-0253.

The western stock of Steller sea lions is endangered and continues to decline in the western Aleutian Islands part of their range.  This distinct population segment (DPS) was listed as ‘endangered’ in 1997 under the US Endangered Species Act due to a persistent decline in abundance since at least the mid-1970s.

Diet data have played an important role in trying to figure out why the population continues to decline. Diet and diet diversity in these sea lions are one way that scientists have been studying the impacts of fisheries and nutritional stress, but it is crucial to have accurate diet information.

Determining what sea lions have eaten has traditionally been done by inspecting and identifying bones and other hard parts such as squid beaks that are found in scats or the stomachs of dead animals. But just using hard parts can mean that some species of prey are impossible to identify in this way. Some hard parts will be difficult to find because of the effects of digestion or because the animal has regurgitated it. This means that the importance of some species of prey will be overestimated while the importance of others will be underestimated.

SMRU Consulting Canada researchers Dr. Dom Tollit (the project PI) and Dr. Ruth Joy collaborated with NOAA’s National Marine Mammal Laboratory in Seattle, Washington and DFO’s Molecular Genetic Laboratory in Nanaimo B.C., to use a combination of molecular techniques to identify species by their DNA and traditional hard part analysis to identify species from fish bones, otolith (fish ear bones) and octopus and squid beaks, from more than 600 non-breeding sea lion scats collected from the Aleutian Island region. This study formed part of a larger collaborative NOAA-led project (funded by the North Pacific Research Board) assessing Steller sea lion population decline, fisheries interactions and diet diversity.

Map showing range of the western Steller sea lion DPS. The circled area highlights the western Aleutian Islands were the population continues to decline, and where the scats used in this study were collected.

Using DNA and hard parts allowed Dom and Ruth and their co-authors to describe what this population of sea lions ate and how energetically important the different species of prey were in the diet. This study was the first to undertake a bio-energetic diet reconstruction using both prey hard parts and DNA for this population of sea lions in the Aleutian Islands.

DNA identification provides valuable comparative and complementary prey occurrence data for pinnipeds (seals and sea lions).  Considerable progress in DNA prey detection and DNA-based diet quantification techniques have been made and represent an important future direction of diet assessments for pinnipeds.

The diet of western Steller sea lions was found to be more diverse than previously reported. These sea lions fed on a greater variety of species when they were not breeding compared to when they were.

Diet contribution (a) % Split-sample frequency of occurrence (b) % frequency of occurrence of key prey types based on scats collected during the non-breeding season in the 2008 and 2012, as determined by prey hard parts, DNA, and a composite of both methods (n=number of scats). *significant increase in dietary contribution using composite methods compared to hard parts alone (all P<0.001).

Atka mackerel was confirmed as the most important (up to 27% by energy) prey for these sea lions during the non-breeding season. However, DNA analysis found that adult Pacific cod and likely giant Pacific octopus were also important prey for the western Steller sea lion. These species were identified more often by DNA techniques than they were from looking at hard part remains.  This key result tells us that previous diet analyses which had only used hard parts to determine diet had underestimated how important cod and giant Pacific octopus were in the diet of these non-breeding sea lions. Other important species included young adult smooth lumpsucker, small Irish lords and rockfish.

The study also compared the different methods of determining what the sea lions ate; i.e., only looking at hard parts (occurrence-based), only using molecular DNA techniques, and by combining hard parts and DNA. The ten most important prey determined from only prey hard parts were similar to diets determined from only using DNA, as well as to diets determined from combining hard parts and DNA. This is reassuring to results from previous prey hard part occurrence-based studies. Retention in the stomach or regurgitation of large cephalopod beaks, the removal of large cod heads and skeletal fragility of lumpsucker may explain up to two-fold differences between DNA and hard part detection rates.

The paper was published this week in Canadian Journal of Zoology .

Contact Dom for a copy of the paper.