In recent years, diets of Steller sea lions have been primarily determined by identifying undigested hard parts of prey recovered from scats. However, this method is subject to a number of limitations. For example, some soft-bodied prey may not be represented by hard parts, or large or spiny prey may be under-represented if sea lions only consume the fleshy parts (e.g., salmon bellies). Some hard parts may also be preferentially regurgitated (e.g., cephalopod beaks), while others may experience different rates of digestion and loss depending on the size and type of prey eaten.

In addition to bones, scats contain the DNA of prey consumed by sea lions. The presence of prey DNA in scat is unlikely to be as susceptible to the limitations of passage of hard parts. The genetic technology also has an added benefit of being able to identify species of salmon and rockfish which can only be identified to family using the hard part technique.

A collaborative study to assess whether diet could be determined using molecular genetic techniques was undertaken by Bruce Deagle and colleagues from the University of Tasmania and the Australian Antarctic Division with Drs. Tollit and Trites from UBC. The study used captive sea lions fed four different species of prey in different amounts over 2 months. PCR techniques (using primers that excluded the predator’s DNA) were used to identify the mitochondrial DNA of the prey present in sub-samples of scat collected from the captive sea lions.

The study showed that species consumed in small numbers were detected as reliably as other species eaten in greater numbers (see Deagle et al. 2005), and suggests that the DNA technique is a promising means to determine what sea lions eat in the wild.

Figure 2. PCR products amplified using fish specific primers and separated on a denaturing gradient gel. Bands amplified from genomic DNA: Sample 1 contains DNA from five fish prey. Samples 2-6 show bands from individual species (herring, smelt, pollock, salmon and capelin)

Researchers from Drs Tollit and Trites collaborating with DFO, NMML and UAF have begun testing the DNA technique on scats collected from Steller sea lions across British Columbia and Alaska. The Genetics Laboratory at the Pacific Biological Station in Nanaimo (DFO) has developed 16S rDNA primer sets to amplify potential prey items. They used separate primer sets to amplify the DNA of fish, cephalopods, and crustaceans. Primer sets (different colors) were run together (multiplexed) in the same lane for each scat, and each gel was run under two conditions to ensure adequate resolution among prey species. Recovery of prey DNA from scats (even from old dried scats) has been high (>80%). The next step is to compare the species identified using the DNA technique with those identified from the hard prey remains found in the scat.