Causes of decline among Southern Resident killer whales
There are three resident killer whale populations found between Washington state and Alaska. These whales are called residents because they spend about half of the year foraging in inland waters, and rely almost exclusively on salmon as prey. The southern resident killer whales (SRKW) are the most threatened population. They experienced an unexplained 20.4% population decline between 1995-2001, but then showed periodic decline to their current number of 77 individuals. This is an alarming rate of decline for an already small population.
Three primary hypotheses have been proposed to explain the decline:
- Decline in the whales’ primary prey, Chinook salmon;
- Disturbance from private and commercial whale watching vessels; and
- Exposure to high levels of toxicants (e.g. PCB, PBDE and DDT), which are stored in the whales’ fat.
Hypotheses 2 and 3 likely interact with Hypothesis 1 since the impacts of boats and toxicants may be exacerbated by the lack of prey.
Understanding the relative impacts of these three pressures is vital to mitigating further SRKW losses given the considerable economic and political impacts associated with any one of them. The Center for Conservation Biology is partitioning these pressures by using a combination of noninvasive measures of stress and nutrition hormones as well as toxicants from feces. Scat samples are located by Conservation Canines (specially trained scat detection dogs) that are able to locate whale scat samples floating on the water from as far away as a nautical mile from the whales and, therefore, from distances unlikely to disturb the whales.
We rely on Tucker, a Conservation Canine, to help us acquire enough scat samples to test these hypotheses. The boat moves perpendicular to the wind with Tucker and his handler secure on the bow of the boat. The boat travels 200 – 400 meters behind or parallel to the whales depending on the direction of the wind. Tucker indicates that he has detected a killer whale scat in the water by changes in his behavior. When this occurs, the handler directs the boat driver to steer into the wind. The team follows Tucker’s changes until we arrive at the sample.
Each sample collected is extracted and assayed for DNA stress, reproductive and nutrition hormones, as well as toxicants (PCB, PBDE and DDT congeners).
These physiological products are then examined over time and in relation to independent measures of Fraser River Chinook salmon abundance (the primary prey of the killer whales while in the study area) and boat densities over time.
Both prey and boat densities are low when the whales arrive in the study area during late spring. Prey and boat densities peak around the same time in August. Glucocorticoids (GCs; also known as cortisol) rise in response to both psychological and nutritional stress, increasing mobilization of glucose to provide quick energy to respond to the immediate emergency. Temporal changes in GCs indicate that lack of prey is having the greatest impact on killer whales since GCs are lowest in August when prey and boats are most abundant and highest in late fall when prey and boats are at their lowest (Figure 3).
Thyroid hormone also corresponds to nutritional stress. It declines in response to nutritional stress, lowering metabolism so that animals will more conservatively use their remaining resources. This response is slower and more sustained than is the GC response. Also, unlike GCs, thyroid hormone is unresponsive to psychological stress. Interestingly, thyroid hormone is at its highest when the whales arrive in late spring, despite the relatively low abundance of Fraser River Chinook salmon at that time. This suggests that the whales arrive in our study area after feeding on a rich food source earlier that spring. The data indicate that prey source to be early spring runs of Chinook salmon, known to have exceedingly high fat content to sustain them for their long spawning trips upstream. The Columbia River is one likely source of such fish, indicating that their protection may be critical to ensure killer whale population stability. Thyroid hormone begins to decline upon killer whale arrival in the study area, reaching a plateau when the Fraser River Chinook peak, and then continues to decline as the Fraser River Chinook run subsides in the fall. These responses further corroborate the nutritional impacts on this population. Note also the between year variation in all of these measures (Figure 3).
The toxin work is just getting underway. Thus far, we have validated our ability to acquire PCB, PBDE and DDT congeners from killer whale scat. The relative concentrations of these congeners are similar to those measured in killer whale biopsy samples.
Mitigation efforts to increase the abundance and quality of available prey to Southern resident killer whales will be an important first step towards assuring SRKW recovery. Toxin work will further contribute to understanding the effects of environmental pressures on this population.
Ayres, Katherine L., Rebecca K. Booth, Jennifer A. Hempelmann, Kari L. Koski, Candice K. Emmons, Robin W. Baird, Kelley Balcomb-Bartok, M. Bradley Hanson, Michael J. Ford, Samuel K. Wasser. 2012. Distinguishing the Impacts of Inadequate Prey and Vessel Traffic on an Endangered Killer Whale (Orcinus orca) Population. PLoS ONE 7(6): e36842.
This work is being conducted as part of the dissertation research of Katherine Ayres and Jessica Lundin in our Center. Collaborators include the Center for Whale Research, the Whale Museum, the Northwest Fisheries Science Center and Cascadia Research.
Support for this project is being provided by: