Pollock Conservation Cooperative Research Center
2001 Awarded Research Projects
An investigation into the possible relationship between killer whale (Orcinus orca) predation and the continuing decline of the Steller sea lion (Eumetopias jubatus) population
Markus Horning, Graham Worthy and Marilyn Dahlheim
Award: $27,142
Estimated completion: February 28, 2002
Abstract
In 1997, the western population of the Steller sea lion (Eumetopias jubatus) was classified as "endangered" and the eastern population as "threatened" under the Endangered Species Act. A number of reasons have been proposed as possible causes of this decline, but most have been eliminated from consideration. The leading hypothesis that is still being considered being a potential change in food availability (e.g., Castellini 1993, Merrick and Calkins 1996), however, despite numerous studies attempting to link the continuing decline in the size of the Steller sea lion population with nutritional stress, there has yet to be a definitive linkage made. An alternate cause for the continuing decline in the sea lion population has recently been proposed. While predation by transient killer whales (Orcinus orca) is not likely to have caused the Steller sea lion decline, it may now be a contributing factor.
Discerning the feeding habits of cetaceans can be difficult, however, there are several indirect methods of study available. In recent years a number of investigations have used stable isotopes of carbon and nitrogen to determine trophic relationships of a variety of birds and mammals (e.g., Schell et at. 1989, Hobson and Montevecchi 1991, Kurle and Worthy 2001, Kurle and Worthy In press). Isotope ratios are ultimately determined by the food that has'been incorporated into the animal over the past several weeks or months, and can give an overall idea of the average diet. With multiple types of food generally available, isotope ratios can indicate, but cannot prove, that a certain type of food was used; they can however, sometimes prove when a food has not been eaten and assimilated.
Isotope data derived from a pilot study on killer whales in Prince William Sound (PWS), AK suggested that some transient killer whales had been feeding at a higher trophic level than resident whales (Worthy and Abend 1998). This could imply that they had been feeding on marine mammals, but based only on isotopic data the possibility that they may have been feeding on piscivorous fish cannot be excluded. These stable isotope data do suggest that some transient whales were feeding at higher trophic levels than other transients or residents, however, it was considered unlikely that any whales were feeding solely on marine mammals (Worthy and Abend 1998).
Fatty acid signature analysis has emerged recently as a method that potentially has the resolution to determine individual prey species (Iverson et al. 1997). Marine food webs contain many long chain fatty acids (LCFA) that are specific to individual prey items, and are generally incorporated into marine mammal blubber with minimal modification (Iverson 1993). By examining the blubber layer, it is possible to use LCFA as indicators of possible prey items (Iverson 1993, Samuel and Worthy 1999). Ultimately, this approach will require knowledge of the fatty acid signatures of all likely prey species, but it has the potential to answer the question of whether a particular whale had been feeding on a specific species.
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