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. The population remained stable from 2001-2005. However, five individuals (5%) were lost in 2006 and seven (8%) were lost in 2008 (Center for Whale Research, pers. comm.). This is an alarming rate of decline for an already small population.

Three primary hypotheses have been proposed to explain the decline:

1) Disturbance from private and commercial whale watching vessels;
2) Decline in the whales’ primary prey, Chinook salmon; and
3) 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 since toxicants that are stored in fat most likely enter into circulation when poor nutrition burns fat.

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 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.

Methodology

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. However, the specific orientation of the boat is varied with whale movements relative to wind direction so that the wind blows the scent from any scat in the water towards the dog: For whales moving in the direction of the wind-the boat moves immediately behind the whale(s) and conducts an upwind zigzag away from the whales. The width of the zigzag is the same as the area covered by the whales. For whales moving into the wind-the boat follows in a zigzag >100 meters behind the whales. For whales moving perpendicular to the wind-the boat positions at a 45° angle, again > 100 meters behind the group of whales.

Tucker indicates that he has detected a killer whale scat in the water by changing his behavior from passive (Figure 2a) to highly animated (Figure 2b). When this occurs, the handler directs the boat driver to steer into the wind. If the boat passes outside the scent cone emanating from the floating scat, the dog loses the scent and loses his animated posture. We respond by turning the boat perpendicular to the wind until the dog’s animation returns. The boat again turns into the wind. We repeat this process until we arrive at the sample.

The dog is rewarded by a game of tug-o-war with his WestPaw ball as soon as the sample is collected. Once spotted, the sample (Figure 3) is collected with a net or specially designed poop scooper. Each sample is extracted and assayed for stress, reproductive and nutrition hormones (glucocorticoids, GCs; tri-iodothyronine, T3, testosterone, estrogens and progestins), as well as for toxicants (PCBs and PBDEs). Scientists at the Northwest Fisheries Science Center also analyze our samples for prey DNA to see what the whale ate, and host DNA to determine the individual’s identity.

Preliminary Results

Thus far, the hormone data most strongly supports the reduced prey hypothesis.

Glucocorticoids (GCs): GC concentrations varied significantly over time (F0.05, 7, 73 = 5.74, p < 0.001; Figure 4), being relatively high in the spring, lowest in July and August and then high again in the fall and winter months. These seasonal trends are consistent with nutritional stress during times of low Chinook salmon abundance since GCs (Figure 4) vary conversely with prey abundance and are more strongly associated with prey availability than with boat abundance.

Thyroid Hormone (T3): T3 also varies with day of year (Julian Date) in a manner that tracks seasonal change in Chinook salmon abundance (F0.05, 3, 63 = 58.17, p < 0.001; Figure 5). However, the between year trends are particularly revealing as they also correspond to mortality patterns across years (Figure 5). The SRKW experienced a 5% mortality rate in 2006, 2% in 2007 and 8% in 2008. T3 levels were significantly lower in 2008 than in 2007 and intermediate in 2006, after controlling for day of the year (Figure 5). Most of these losses occurred prior to the SRKW arrival in May. This suggests that winter salmon may be particularly important to whale survival and that summer salmon runs may be insufficient to allow recovery from particularly harsh winters. The impacts of these varying levels of nutrition on toxicants in circulation is forthcoming.

Closing Remarks

Migitation 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.

Collaborators

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.

Funding

Support for this project is being provided by:

*The Washington Sea Grant, University of Washington, pursuant to NOAA Award No. NA10OAR417005
*NOAA, Northwest Fisheries Science Center
*The Canadian Consulate General
*The Center for Conservation Biology
*The Northwest Science Association

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