Depleted Food Supply Could Mean Extinction for Southern Resident Killer Whales
According to study released in October 2017 in the journal Scientific Reports, there is a 25 percent chance that Southern resident killer whales, also called Orcas will disappear in 100 years. The author, led by Robert C. Lacy of the Chicago Zoological Society, hope that their research will inform effective recovery plans.
According to the published paper, the critically endangered, Southern Resident killer whale (SRKW) population of the northeastern Pacific Ocean faces numerous threats. These include a dwindling Chinook salmon population, anthropogenic noise, and high levels of stored contaminants, including PCBs.
The researchers analyzed the killer whale population to look at its long-term viability under varying levels of anthropogenic stressors. They wrote that if things remain as they are, there will be no population growth and that more calves must be born to increase the population. However, a decline is expected if new or increased threats are imposed.
By conducting a “population viability analyses” (PVA), the researchers were able to explore “scenarios representing the three main anthropogenic threats – prey limitation, acoustic and physical disturbance, and PCBs – that might worsen with increased development, or could be mitigated through management. Across the ranges of threat levels that we examined, reduction of the prey base was the single factor projected to have the largest effect on depressing population size and possibly leading to extinction, although either higher levels of noise and disturbance or higher levels of PCB contamination are sufficient to push the population from slow positive growth into decline.”
Mean projected SRKW population sizes for scenarios with (from top to bottom): no anthropogenic noise or contaminants; current Chinook abundance, noise, and PCBs; reduced Chinook, increased noise, and additional threats of oil spills and ship strikes as estimated for low level impacts of future industrial development; and these increased and additional threats with higher level impacts of development. Graph credit: Robert C. Lace, et al. (see Source, below).
If any additional or increasing anthropogenic threats combine with the predicted decline in Chinook abundance due to climate change, “then the population could decline by as much as 1.7% annually, have a 70% probability of declining to fewer than 30 animals, and have a 25% chance of complete extirpation within 100 years.”
They paper’s authors write, “Prey limitation is the most important factor affecting population growth. However, to meet recovery targets through prey management alone, Chinook abundance would have to be sustained near the highest levels since the 1970s. The most optimistic mitigation of noise and contaminants would make the difference between a declining and increasing population, but would be insufficient to reach recovery targets. Reducing acoustic disturbance by 50% combined with increasing Chinook by 15% would allow the population to reach 2.3% growth.”
The author’s note that they may be overly optimistic in their projections, if recent declines in fecundity (2011-16) are more than short-term a fluctuation. They recommend that recovery efforts focus on fecundity, because:
“There is simply more potential for improving reproduction than for improving adult survival when survival is already close to 1. Even complete elimination of adult mortality in the SRKW (not a biological possibility) would result in a population growth rate of 1.8%, still below the recovery goal of 2.3% growth. Although recovery cannot be achieved solely by improving adult survival, any decline in adult survival caused by new or exacerbated threats could have serious consequences for the population.”
Mean population growth for SRKW achieved by mitigation of anthropogenic threats. Threat reductions are scaled on the x-axis from no reduction to the maximum reductions tested: Chinook abundance increased up to 1.3x the long-term mean; noise disturbance during feeding was reduced from 85% to 0; and PCBs were reduced from accumulation rates of 2 ppm/y to 0. The top line shows growth rates under a combination of varying levels of improved Chinook abundance plus mitigation of noise to half the current level. Graph credit: Robert C. Lace, et al. (see Source, below).
While anthropogenic noise is a part of this research paper, one of the paper’s authors, Kenneth C. Balcomb III, a principal investigator at the Center for Whale Research on San Juan Island, Washington, told the The Tyee that he does not consider noise to be the primary issue. He says, “The whales don’t give a shit about noise, they want fish. Noise does not interfere with them catching fish. The whales are not catching fish because the fish aren’t there. The whales will ride the bow waves of oil tankers. Noise is not the issue.”
Lacy, Robert C; Williams, Rob; Ashe, Erin; Balcomb III, Kenneth C; Brent, Lauren J. N. Clark, Christopher W; Croft, Darren P; Giles, Deborah A; MacDuffee, Misty; & Paquet, Paul C. Evaluating anthropogenic threats to endangered killer whales to inform effective recovery plans. Scientific Reports 7, Article number: 14119 (2017), doi:10.1038/s41598-017-14471-0.