Causes of Decline among Southern Resident Killer Whales

Killer whales have awed and inspired humans since the beginning of recorded history. They are an icon of strength, beauty, intelligence, family and the mysteries of the ocean. The “Southern resident” killer whale (SRKW) community of Puget Sound fuels a multi-million dollar tourist industry in both the U.S. and Canada. The SRKWs are readily spotted because they travel in large, conspicuous groups and make regular trips into the relatively accessible inland waters of Washington and British Columbia (a.k.a. the Salish Sea).

The SRKW experienced an unexplained 20.4% decline between 1995-2001, setting off alarm bells that this population is likely threatened by anthropogenic changes to their environment. The population remained stable from 2001-2005. However, five individuals (5%) were lost in 2006, and seven individuals appear to have died (another 8% of the population) in 2008 (Center for Whale Research, pers. comm.). This is a devastating rate of decline for an already small population. In fact, the population could go extinct in our lifetime if innovative solutions are not provided quickly.

The Center for Conservation Biology is using noninvasive hormone measures of stress (glucocorticoids: GC’s) and nutrition (triiodothyronine: T3) in feces to examine the relative importance of the two largest of the proposed threats to the SRKW: Disturbance from private and commercial whale watching vessels and decline in their primary prey, Chinook salmon. Hormone measures in feces are ideal for investigating these pressures. We use specially trained scat detection dogs to help us attain the sample sizes needed to address multiple hypotheses. Use of dogs also allows us to acquire samples at greater distances from the killer whales to minimize any potential impact of our research vessel (see below).

Hypotheses

SRKWs occur most often in the inland water study area from Mid-May to Mid-October. Dynamic changes in vessel traffic and prey abundance occur over the study season (Figure 1. Tourist boat numbers begin to increase in May when the whales return to the area, peak around July and August and then decline into October. Short-term spikes in vessel traffic occur on the weekends and even larger spikes during holidays (Memorial Day, 4th of July and Labor Day). Chinook salmon abundance begins a steady increase starting around June, which peaks around August as returning salmon head up the Fraser River to spawn. Chinook abundance then begins a steady decline into October. We monitor changes in fecal stress (GCs) and nutrition (T3) hormones in response to these dynamic changes in boat and prey densities (Figure 1).

GCs increase during times of physiological stress and T3 decreases during times of nutritional deficits. Thus:

1. The boat stress hypothesis predicts that changes in GCs will positively correspond to the number of boats in proximity to the whales (Figure 1), while T3 concentrations remain unchanged.
2. The reduced prey hypothesis predicts that thyroid hormone will decrease and GCs will increase in response to nutritional stress resulting from reduced prey abundance. Therefore, T3 is predicted to follow the trend of the “Chinook abundance” line in Figure 1 while GCs should follow the opposite trend.
3. If High vessel traffic also impairs foraging ability of SRKWs, times of relatively high vessel traffic should be marked by a reduction in T3 and enhanced elevation of GCs for any given level of prey abundance.

Methodology

We rely on Tucker, a trained scat detection dog, 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 Kong toy as soon as the sample is collected. Once spotted, the sample (Figure 3a) is collected with a net or specially designed poop scooper (Figure 3b). Each sample is extracted and assayed for GCs, T3, sex-specific reproductive hormones (testosterone, estrogens and progestins). 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 (Figure 1).

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, 0% in 2007 and so far an 8% mortality rate 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).

 

Some whales were notably emaciated in 2008, based on concavity of the area behind their blowhole. This was particularly striking in whale, L67, right before she disappeared and was presumed dead (Center for Whale Research pers. comm). Combined, these data strongly suggest that there may not be enough prey to sustain the entire population during some years.
Preliminary results also suggest that vessels may play a role on short time scales. We hope to confirm this when 2008 vessel traffic data become available over the next few months.

Concluding Remarks

Multiple chinook salmon populations crashed on the west coast this past season in 2008, before the SRKW experienced the 8% population decline (Center for Whale Research unpubl. data). Boat traffic actually decreased during this season, most likely due to high gas prices and a recessing economy that discouraged tourism. These patterns and our preliminary data suggest that declining prey abundance may explain the high mortality in 2008, although interactions with vessel traffic must still be explored in greater detail. For now, it seems clear that mitigation efforts to increase number 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 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:

*NOAA, Northwest Fisheries Science Center
*The Canadian Consulate General
*The Center for Conservation Biology
*The Northwest Science Association