Southern Residents, salmon, and certainty in science
Examining the research that compares salmon abundance to Southern Resident killer whale survival and reproduction.
The Southern Resident killer whales (SRKW) are some of the Salish Sea’s most iconic inhabitants, but also among its most imperiled. While their Northern Resident neighbours are steadily increasing in population size, the SRKW have flatlined. After seemingly recovering from a decline in the late 1960s, when many calves and juveniles were removed for the live aquarium trade (or killed in the process), they reached a peak of 98 individuals in the 1990s. Since then, however, their numbers have decreased to a current total of 74.
Figuring out what is driving this concerning population trend has been the goal of a considerable volume of research over the last few decades, much of it focusing on the SRKW’s preferred prey species: Chinook salmon. Influential studies from the tail end of the 2000s showed that the annual abundance of Chinook was positively associated with not just survival, but also reproduction. For obvious reasons, both processes are intimately linked to population growth; if a lack of prey is causing more whales to die, and fewer whales to be born, numbers will decline.
New research published by Benjamin Nelson and colleagues from Oceans Initiative and NOAA aimed to update our understanding of the relationship between salmon abundance and SRKW survival and reproduction, adding an additional 15 years of data since this relationship was previously assessed. Previous studies had evaluated the effect of Chinook abundance on SRKW birth rate and survival separately. Nelson et al. took a new approach by modelling both demographic rates simultaneously to account for correlations between them (e.g. a higher death rate could reduce the number of births by removing reproductive-age females from the population or reducing genetic diversity in the population).
The study found that survival rates were positively correlated with salmon abundance, meaning that whales were more likely to die in years with fewer salmon. This finding is consistent with the results of the earlier studies, and suggests that prey availability should still be treated as a threat to the SRKW population.
However, in contrast to previous research, the authors did not find any link between salmon abundance and birth rates in the SRKW. Although they note a declining trend in fecundity over the years, with birth rates peaking in the mid-1990s then decreasing until the present, they found no evidence that this was related to salmon abundance.
They discuss various potential explanations for this outcome. One possibility is that the relationship between Chinook abundance and birth rate has changed since it was last directly assessed in 2009, perhaps because today’s Chinook are smaller and contain fewer calories than they did historically. Alternatively, the authors suggest they may simply lack statistical power to detect a relationship, even if one does exist. In discussing their results, they highlight an important but uncomfortable truth: as threatened populations dwindle, so does our ability to quantify the threats they face.
SRKWs are one of the most intensively studied whale populations in the world, with detailed demographic data available for every individual virtually from birth. Despite this, their inherently small sample size and low reproductive output (even in the best case scenario, females can only produce a single calf approximately every three years) mean that a few extra births or deaths in a given year can have a big influence on model outcomes.
Science rarely provides hard certainties
When making important decisions that affect many stakeholders, policymakers understandably want a high level of confidence that a specific action will lead to a desired outcome. But science rarely provides hard certainties, and, from an outside perspective (or even an inside perspective), it can be confusing when studies appear to show conflicting findings.
For me, it helps to remember that science is a process. With each new building block, we have to question our assumptions, evaluate the strength of evidence, and place the new information in the context of what has come before, continually updating our best understanding. This is especially important when working on very small populations where studies are more likely to be limited by statistical power.
So, what do we take from this newest puzzle piece? The most current science still supports a link between SRKW survival and Chinook salmon abundance, which means that increasing the number of salmon available to SRKW is a sensible target for recovery plans. The authors note that adult survival is already high, but the potential increase in calf survival would be welcomed, especially in light of the declining birth rates.
This doesn’t mean that increasing salmon abundance alone will be enough to meet population recovery targets. A new population viability analysis showed that the survival benefit conferred by increased salmon numbers is unlikely to substantially boost population growth unless it is combined with measures to reduce environmental contaminants, noise, and other sources of disturbance that prevent SRKW from foraging efficiently. Unfortunately, as is often the case in conservation biology, there is no single silver bullet we can rely on to save the Southern Residents – a suite of threats requires a suite of solutions.
While this study did not find a relationship between SRKW reproduction and salmon abundance, the overall body of evidence to date remains mixed. For example, recent work suggests that hormonal changes related to poor nutrition are linked to high rates of miscarriage. Even if we take the result found by Nelson et al. at face value, it raises an important question: if a lack of salmon isn’t causing the observed decline in birth rates among SRKW, what is?
Unfortunately, the list of potential candidates is long, including pollution, noise and disturbance from boats, and loss of genetic diversity. Given the importance of increasing birth rates for population growth, this question certainly warrants further study. But the reality is that this is often a slow, iterative process; science can play the long game, the Southern Residents likely can’t. Implementing policies to mitigate these known threats should not be delayed while we wait for certainty.
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