California Sea Grant College Program

Compound-specific isotope (CSI) analysis is rapidly growing tool in the field of ecology to assess the trophic position and foraging behavior of an animal. Only a handful of studies have examined the values and patterns of carbon and nitrogen amino acid isotopes ( δ 13C-AA and δ 15N-AA), and have primarily been done on plankton and other low trophic position organisms. Since AA data separate into unique biochemical groupings, much more detailed information is revealed than the widely used bulk isotope technique - such as source of diet (offshore vs. coastal), base of the food web, trophic positions, and an animals physiology. To examine the usefulness and power of the CSI method, we tested how  δ 13C-AA and  δ 15N-AA patterns and values appear in several populations of harbor seals (Phoca vitulina) off the California coast, primarily comparing seals from San Francisco Bay (SFB) and the Channel Islands (CI).

In Chapter 1, we compared bulk isotopes in captive seals from The Marine Mammal Center to wild seal populations. Our results indicated similar trophic transfer δ 15N values between predator and prey compared to other organisms. It concluded that weaner seals (< 1 year) from SFB and CI had recently weaned from their mother's milk, and were transitioning to a diet offish and invertebrates. Chapter 2 was the very first CSI study δ 15N-AA on marine mammals, where we examined the 'trophic' and 'source' AA grouping differences in captive seals. A smaller trophic enrichment factor (TEF) was shown compared to past studies on plankton. We hypothesize this is attributed to an animal's main form of nitrogen excretion (ammonia vs. urea), where urea-excreting animals exhibit this lower TEF value. Thus, we propose using a new multi-TEF trophic position equation to estimate the foraging ecology of wild harbor seals (Chapter 3). This equation provides more accurate predictions, suggesting they consume prey around 2.5 to 3 trophic positions. The coupled δ 13C-AA and δ15N-AA data suggest SFB yearlings (1–2 years) are nutritionally stressed, as they show higher than expected trophic position estimations. While seals in CI are consuming different types of prey (offshore vs. nearshore), but at expected trophic positions.