A new study (co-authored by NOAA scientists) analyzed the population trend of the Southern Resident killer whales against Chinook salmon abundance. It’s of concern that they failed to find a clear link between this endangered population’s low birth rate and the decline of Chinook salmon. The authors also appeared to shift the baseline for a killer whale’s lifespan without considering the significant role of mature males and wise grandmothers in this population’s survival.
What was the study?
Scientists created computer models to “update their understanding of a potential causal relationship between prey availability and Southern Resident killer whale population dynamics.” Simply put, is there a link between how much food is available each year for these Chinook-dependent killer whales and the number of births and deaths?
What did they find?
“Results suggest that Southern Resident killer whale mortality rates are more strongly associated with Chinook salmon abundance than birth rates.”
What did the models show about birth rates?
The models showed “minimal evidence of an association” between Southern Resident killer whales’ birth rates and Chinook salmon abundance. In other words, they show no apparent link between the number of pregnancies and the number of prey these killer whales can find.
They noted that their birth rate peaked in the mid-1990s when, as a NOAA scientist once said, “It was a good time to be a killer whale.” The study found that since that time, their birth rate “appears to have been declining to the present day.” Little wonder, as Chinook salmon abundance peaked in the same decade.
Despite this, the study proposed “an urgent need to investigate factors other than prey limitation that might explain later-term pregnancy loss and the associated low birth rates.” They did not elaborate on these factors, only alluding to other studies on potential infectious diseases and environmental stress that may affect pregnancy in cetaceans. Yet a 2017 study co-authored by Dr. Giles attributed a miscarriage rate of 69% to seasonal Chinook salmon shortages.
Here’s a weakness in these models: They only include successful pregnancies. Since 2022, there have been five known births, but this does not account for all pregnancies. Other females identified as pregnant by scientists over the last few years subsequently miscarried. To quote Dr. Giles, “This suggests that getting pregnant is not the problem; staying pregnant is.”
Wild Orca’s Southern Resident killer whale health monitoring program works to identify pregnancies in the endangered population through the measurement of hormones in feces.
What did the study show about survival rates?
Half the models showed “at least a 95% probability of a positive correlation between salmon abundance and survival.” Simply put, you are more likely to die prematurely without enough salmon.
To assess survival, they divided the Southern Resident killer whales into four categories, from young to old. They found that “young males” aged 10-21 and “young females” aged 10-42 had the highest average survival rates, and “old males” over 21 had the lowest.
Yet, by classifying males over 21 as old, this study appears to shift the baseline, as NOAA’s website states, “The average lifespan for male killer whales is about 30 years, but they can live up to at least 60 years.”
Why would they live so long? Females rarely select males under 21 as mates; they prefer older, larger males, as evidenced by DNA matches to fathers of 53 calves born since 1974. We know that 85% of these calves had a father over 21, and more significantly, half were over 30.
Surely NOAA jeopardizes this population’s future by prematurely writing off “old” males? Lowering the average lifespan from 30 to 21 would also increase the likelihood of inbreeding. With fewer choices of older males, females may have to select a closely related, younger male.
Likewise, extending the lifespan of “old” females (defined here as over 42) must be prioritized. As the holders of ecological knowledge, they find food for their pods, increasing the survival chances of their daughters’ calves and their adult sons and ensuring he’s fit and healthy for mating. Being old in this population is an evolutionary benefit, not a drain.
Sadly, it would not be a government study without another scapegoat. “Our analysis also suggests Northern Resident killer whale abundance could also be affecting Southern Resident killer whales…possibly through competition over shared prey resources.” Yet, according to this study, in the past, Chinook salmon likely fed 100 more killer whales than in the current combined populations.
This suggests that a decline in abundance and size of prey due to overfishing and habitat loss are more likely factors. Indeed, the study noted that as Chinook salmon are now smaller, killer whales need to find more to survive. So why does NOAA allow the catch of immature salmon before they can mature to reproduce or become calorie-rich killer whale prey?
What are the solutions?
While this study offers little, notably, two of its authors contributed to a recent study that found some. For example, this study stated, “Our model suggested annual survival rates of non-calf animals were very high, which limits the potential for population recovery by improving survival.”
The newer study found that “a single birth or death represents an annual population growth or decline of 1.4%, underscoring the value of each individual in preventing the disappearance of a population.” They proposed that increasing male survivorship could play a key role.
While this study’s authors suggested that “even if a causal relationship does exist between Southern Resident killer whale survival rates and Chinook salmon abundance, this does not necessarily ensure that salmon recovery alone would be a panacea.” With an updated model, the newer study found “protecting these killer whales appears to be impossible without restoring diminished populations of Chinook salmon.”
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Why would two papers published in the same year, with some shared authors, reach such different conclusions? NOAA Fisheries scientists who work for the Department of Commerce co-authored the first. This conflict of interest maximizes Chinook salmon fishing despite the Endangered Species Act charging the agency to protect these species. Conservation biologists authored the second, with an understanding of the need for an ecosystem-based approach.
Numerous studies show that the Southern Resident killer whales could thrive again if we increased the number of older and larger Chinook salmon in the ecosystem. Better-fed whales could live longer, with more calves surviving. Bigg’s killer whales in the Salish Sea are steadily increasing despite being subjected to environmental contaminants, vessel noise, and disturbance. The difference? Their prey is abundant. Actions to recover Chinook salmon can still turn this around.
