High-Tech Solutions Help Solve Dietary Puzzle

It is well known that environmental factors such as seasonal variations in prey abundance can trigger changes in the diet of many marine mammals. But exactly how animals such as Steller sea lions respond to this natural flux is still very much a mystery — and currently the subject of extensive research.

Scat analysis, or the examination of fecal matter, has been a very successful and non-invasive tool for helping to solve this mystery. Investigating the scat of a marine mammal helps researchers to better understand which types of prey are being eaten, how different types of food are passed, and which food sources are most abundant in the face of natural variations.

Although it can yield useful results, scat analysis may not provide the whole dietary picture. One of the limitations of traditional scat analysis is that it can easily miss certain key elements of the marine mammal diet that do not show up as hard remains in the feces.

For example, the sharp beak of a half-digested octopus may become lodged in the stomach lining of a marine mammal, essentially withholding the only ‘hard’ evidence that the octopus was ever consumed. Similarly, a marine mammal may choose to eat the soft belly of a salmon, ingesting none of the tell-tale bones that traditional scat analysis could easily detect. Even the hard exoskeletons of crustaceans such as shrimp can be completely broken down during digestion. Given these and other plausible scenarios, it is easy to see the limitations of traditional scat analysis.

In order to address the shortcomings of this technique, a recent collaborative study has attempted to investigate marine mammal diet by isolating prey DNA from scat samples. Bruce Deagle, a molecular biologist at the University of Tasmania, working with Drs Simon Jarman and Nick Gales of the Australian Antarctic Division, recently joined forces with Drs Dom Tollit and Andrew Trites at UBC to run a captive feeding study with two Steller sea lions at the Vancouver Aquarium Marine Science Centre.

For up to 34 days, the sea lions were fed a consistent diet of herring (47%), smelt (34%), salmon (13%) and squid (6%). Researchers regularly collected scat samples and subsequently amplified only DNA from soft prey remains.

Denaturing gradient gel electrophoresis separation of mitochondrial DNA amplification products. Lanes show amplification results for a combination of different fish species against a mix of all fish products. Lane 7, for example, shows a positive result for sockeye, smelt and herring.

The DNA analysis proved effective even on scat samples that had been left out in the sun for a few days. More importantly, the technique also showed a very high detection rate (>95%) for the presence of all four food types, even though the study animals ate a relatively high proportion of herring and smelt compared to that of salmon and squid.

Based on the initial success of this study, new genetic techniques such as clone libraries and Real Time PCR are now being explored to determine whether it is possible to accurately quantify the amount of prey DNA in scat samples.

Whether these proposed techniques are successful or not, this collaborative study has already yielded encouraging results. By combining two analytical approaches — the traditional examination of hard prey remains and the molecular identification of soft prey from scat — researchers have come one step closer to unlocking the subtle and complex mysteries of marine mammal diets.

22 November 2004