A Southern Resident killer whale surfaces in choppy water in the Strait of Juan de Fuca.
Taken under NMFS permit #26288 | Wild Orca.

The Southern Resident killer whales are endangered due to lack of Chinook salmon. Yet each summer in the Salish Sea, they must hunt for their scarce prey in waters heavily trafficked by a wide variety of vessels. According to a 2012 study, this results in “behavioral changes, increased energy expenditure and foraging interference, resulting in psychological and/or nutritional stress.”

This study set out to determine if it’s possible to distinguish between the relative impacts of prey shortages and vessel traffic. By analyzing Southern Resident killer whale fecal samples collected from May to September over three years, scientists could track changes in their stress levels by analyzing two key stress hormones—glucocorticoids and thyroid.

What are these stress hormones and what do they do?

Glucocorticoid hormones are released from the adrenal gland and play a key role in maintaining glucose (blood sugar) levels. In “short-term nutritional emergencies” glucocorticoids increase to raise energy levels.

Thyroid hormones are released from the thyroid gland to control metabolism, the process of converting food into energy. In response to longer-term prey shortages, thyroid levels decrease “slowing metabolism to conserve energy stores.”

Thyroid hormones are “largely unaffected by psychological stress,” however glucocorticoid hormones will respond, and so their presence in fecal samples can indicate stressors such as vessel disturbance, in addition to nutritional stress.

Dr. Deborah Giles decants a Southern Resident killer whale fecal sample.
Photo by Wild Orca.

Wild Orca’s Dr. Deborah Giles processes fecal samples in preparation for hormone analysis.

Glucocorticoid stress hormones: what did they find?

The Southern Resident killer whales’ glucocorticoid hormones fluctuated according to Fraser River Chinook abundance, suggesting that “prey availability has a greater physiological impact than does vessel traffic.”

As Fraser River Chinook numbers gradually increased, the whales’ glucocorticoid hormones decreased accordingly as their nutritional stress levels lowered. When Fraser River Chinook were at their most plentiful in August, glucocorticoids were at their lowest level—despite peak vessel traffic—suggesting a minimal effect when prey was abundant. As Fraser River Chinook declined, stress levels rose again, “even though vessel numbers in proximity to the whales also markedly declined.”

Thyroid stress hormones: what did they find?

One surprising, yet important finding was that the Southern Resident killer whales’ thyroid hormones reflected Chinook abundance both in and outside the Salish Sea.

Their thyroid hormones were consistently highest on their return to inland waters each spring—indicating that they were well-fed. This led scientists to conclude that they were finding a rich food source on the outer coast—most likely Columbia River spring Chinook, as they’re “known to have particularly high fat content to sustain their long spawning migrations upstream to interior river systems.”

During the course of the summer, their thyroid hormones steadily decreased and this “suggests that the whales become somewhat food limited.” The scientists noted that this finding was unexpected, as “the more confined waterways of the Salish Sea, combined with large runs of salmon returning through the area would seem to provide easier foraging opportunities for the whales than the outer coast.”

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What do these findings show?

The scientists concluded that lack of prey had the greatest impact on the Southern Resident killer whales’ stress hormones.

Short-term prey shortages were reflected in high glucocorticoid hormone levels, while low thyroid hormones suggested a long-term prey deficit. The year with the greatest change in both stress hormone levels was followed by “the highest number of deaths and lowest number of births and surviving calves” during the study.

However, of concern is that the study could not rule out “a cumulative effect of vessel traffic on the overall stress hormone response, particularly during years of relatively low Fraser River Chinook abundance.” In fact, new models show that vessels and PCBs are magnifying the harmful impacts of insufficient Chinook, thus furthering the low birth rate and high early mortality in this endangered population.

These scientists’ stressed that “identification and recovery of strategic salmon populations in the Southern Resident killer whales’ diet are important to their recovery.” A government diet study identified key salmon rivers, yet dams still block access to spawning habitat, further endangering important Chinook populations.

It’s clear that this non-invasive method for evaluating the Southern Resident killer whales’ stress hormones is key to understanding the health impacts of insufficient prey. Wild Orca will continue this vital work to provide policymakers and fishery managers with timely science to take action to save these whales from extinction.