Southern resident killer whales, found along the northwest coast of North America, are in trouble mostly because of inadequate prey, the number vessels in their habitat has much less impact, according to work published in the open access journal PLoS ONE.

The researchers, led by Katherine Ayres, who completed the work while at University of Washington in Seattle, measured two different hormone levels, fecal thyroid and glucocorticoid, to distinguish between two different theories for the whale's decline.

Both measures supported the inadequate prey hypothesis, which suggests that the killer whale is primarily limited by the decrease in the population of Chinook salmon, its major food source, more than the vessel impact hypothesis, which suggests that the animals are psychologically stressed from the high number of vessels in the area.

Ayres explains, "The data support Chinook salmon being a more important driver of physiology than vessel traffic for the Southern resident killer whale population, however vessel traffic may cause added physiological stress during times of low prey availability." Researcher Samuel Wasser concurs, adding, "Recovering their Chinook salmon prey is critical to assure long-term killer whale recovery. Everything, including boats and toxins, matters more when prey is low."

Both nutritional and psychological stress lead to an increase in glucocorticoid levels, while only nutritional stress affects thyroid hormone levels, so measuring both of these levels allowed the researchers to identify which of the two models is correct. The results suggest that whale conservation efforts should focus on salmon population recovery, the authors write.

Citation: Ayres KL, Booth RK, Hempelmann JA, Koski KL, Emmons CK, et al. (2012) Distinguishing the Impacts of Inadequate Prey and Vessel Traffic on an Endangered Killer Whale (Orcinus orca) Population. PLoS ONE 7(6): e36842. doi:10.1371/journal.pone.0036842