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 June 6 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
Financial Disclosure: This work has been funded by the National Oceanic and Atmospheric Administration-Northwest Fisheries Science Center, the Joint Institute for the Study of the Atmosphere and Ocean, Washington Sea Grant No. NA10OAR417005, the Canadian Consulate General, Northwest Science Association, University of Washington Department of Biology, Lynn Riddiford and Jim Truman, Elizabeth Welch and the Center for Conservation Biology. M.J. Ford, M.B. Hanson, J. Hempelmann, and C. Emmons are employees of Northwest Fisheries Science Center. All authors were involved in the preparation of the manuscript. M.B. Hanson, J. Hempelmann, and C. Emmons were involved in data collection and analysis. K.L. Ayres, R.K. Booth are employees with the Center for Conservation Biology, a nonprofit directed by S.K. Wasser. K.L. Ayres and S.K. Wasser designed the study, applied for funding, collected data and did most of the analysis, decision to publish and preparation of the manuscript. R.K. Booth was involved in study design/methods and data analysis. The other funding agencies had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interest Statement: Authors Michael J. Ford and M. Bradley Hanson were partially responsible for funding decisions from Northwest Fisheries Science Center. Washington SeaGrant is also administered by NOAA, however the NOAA employees that are authors on this paper were not directly involved with SeaGrant funding decisions. Funding from this agency was a major part of funding this research. Authors Katherine Ayres, Rebecca Booth and Samuel Wasser work for the Center for Conservation Biology, which also partially funded this work. Author Kari Koski is an employee for the Soundwatch Boater Education program that provided the vessel traffic data for this research. Soundwatch is a boater education program that often consults on vessel regulations decisions based on their monitoring research. This does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials.
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