Population Biology and Conservation

Our first encounter with the Southern Resident killer whales was on April 8, 1976, near Port Angeles in the Strait of Juan de Fuca. At this time very little was known about the population size and structure. During the preceding two decades marine parks had taken (or killed during capture) over 50 individuals from the population (Asper and Cornell 1977). Published reports suggested an abundance of animals and that individuals could continue to be sustainably taken from the population (Asper and Cornell 1977). In sharp contrast, using photographic ID techniques that were pioneered by Dr. Michael Bigg, we showed that this was a very small and vulnerable population. In 1976 we agreed with Dr. Bigg that there were approximately 68 animals remaining (Balcomb and Goebel 1976), with approximately 40% of the population having been taken into captivity or killed during capture. This research helped to end the live captures of killer whales from the Pacific Northwest. Both through our long-term monitoring of the population and through molecular work (e.g., Ford et al. 2011 and 2017) we have established that the population is demographically closed (there is no emigration or dispersal from the population and no gene flow with neighboring populations).

 

The detailed information collected by the Center for Whale Research on the population dynamics of the Southern Resident killer whales has been used to support management decisions in both Canada and the United States. In 2001, the Southern Resident killer whales were listed as endangered in Canada under the Species at Risk Act and, in 2005, they were designated endangered in the USA under the federal Endangered Species Act. Currently, the Southern Resident killer whales are one of the most critically endangered populations of marine mammals in the USA (NMFS 2017) and Canada (DFO, 2017).

 

Through our annual monitoring of the population, we have been able to track the changes in population size, which has fluctuated considerably over the last four decades (Krahn et al. 2002; Olesiuk et al. 2005; Balcomb et al. 1982). From a starting point of just 68 identified individuals in 1976 following the live captures (Balcomb et al. 1982), the population showed signs of recovery and grew to a peak size of 98 individuals by 1995. The population then went into a steep decline with only 78 individuals remaining in the population by July 2000. This decline was followed by a further period of moderate growth to 89 individuals in the population in July 2006. Since then the population has declined once again, and as of September 2017, there are only 76 individuals remaining in the population, which is getting very close to the population size following the depletion of the population by the live captures. The population is not recovering.

 

Our long-term monitoring of this population has provided key insights into the cause of this decline. In particular, over the years we have contributed to studies to quantify the foraging behavior of the Southern Resident killer whales (Heimlich-Boran 1986; Ford et al. 1998; Ford and Ellis 2005). This work has established that Chinook salmon (Oncorhynchus tshawytscha) is the predominant prey species in the diet of the Southern Resident killer whale population during the summer months (Ford et al. 1998; Ford and Ellis 2005) and that this dietary specialization is a major factor determining both their year-round distribution (Ford et al. 1998; Ford and Ellis 2005) and local habitat use (Heimlich-Boran 1986). This work has shown that periods of population decline (high mortality rates) are tightly associated with changes in Chinook salmon abundance, with higher mortality observed when salmon abundance is low (Ford et al. 1998; Ford and Ellis 2005). Moreover, our long-term data has shown that killer whale fecundity is highly correlated with the abundance of Chinook salmon with the probability of a female having a calf being 50% greater in years of high salmon abundance compared to years of low salmon abundance (Ward et al. 2009). These results highlight that the coast wide decline in Chinook abundance is likely to be the most significant factor contributing to the decline in the population.

 

In 2015 we provided written evidence for the hearing of the Canadian Trans Mountain Pipeline Expansion Project. Working with a team of international colleagues, we used population viability analysis (PVA) to assess the viability of the Southern Resident killer whale population and the impact that the proposed Trans Mountain Pipeline Expansion project would have on the population. This analysis showed that increased traffic and noise conditions would intensify existing threats, probably accelerating the rate of population decrease, possibly leading to complete loss of the population (Lacy et al. 2016).

 

It is often difficult to collect detailed information on individual animals in the marine environment, and we have worked with colleagues to develop new methods for collecting essential data to inform conservation and management decisions. These projects include using low power lasers to collect precise data on morphometrics and measure changes in the size and shape of the whales (Durban and Parsons 2006), and aerial images taken from a helicopter (Fearnbach et al. 2011. and 2017 in prep) to determine whale size, condition, and long-term growth trends. This data has been used to track the body condition and health of the whales over time. We are continuing to look for new ways of collecting data that will help both our understanding of these animals and inform their conservation and management, and in our future work, we will use an unmanned aerial vehicle (drone) to collect high-resolution video on the behavior of the whales.  

 

 

Social Organization

Because all individuals in the population are individually identifiable due to their unique markings, our long-term monitoring of the population over the last four decades has provided detailed data on the population social structure. We have established that neither sons nor daughters disperse from their mother and the population shows a stable matrifocal society composed of a hierarchy of social units. These matrilineal groups form three distinct pods (J, K, and L pods) that have persisted throughout the last four decades (Bigg 1990; Parsons et al. 2009; Foster et al. 2012b).

 

Although the overall pod structure has remained stable over decades, we have shown that the connectivity of the population has fluctuated and is also tightly linked to salmon abundance (Parsons et al. 2009; Foster et al. 2012b). Social groups are smaller in years of low salmon abundance (Parsons et al. 2009), and the population is less socially connected in years of low salmon (Foster et al. 2012b). Given the central importance of the social network for population processes such as mating dynamics and disease transmission, a change in social network structure caused by a change in food availability may have significant consequences for the long-term viability of the population.

 

By integrating our long-term data on the social structure of the population with molecular studies, we have been able to determine the mating dynamics in the population (Ford et al. 2011, and 2017 in review). This work has found high variance in male reproductive success with older and larger males having higher reproductive success. Although mating in the population often occurs in the same pod, current evidence suggests that mating between close relatives is avoided (Ford et al. 2017, in review) with rare exceptions.

 

 

The Evolution of Post Reproductive Females

One striking observation that emerged from our long-term studies is that much like humans, female resident killer whales typically have their last offspring in their late 30s or early 40s, but may live into their 80s and beyond (Olesiuk et al. 1990). This menopausal life history is very rare; indeed the only other mammals reported to exhibit this unusual life history strategy are humans and short-finned pilot whales (Croft et al. 2015). The question of why females of any species should stop reproduction before the end of life is a long-standing evolutionary puzzle. One hypothesis is that post-reproductive females play a key role in helping their relatives to survive and reproduce. Mathematical models suggest that in killer whales post-reproductive females should invest more time and energy to support their sons rather than their daughters - in this way grand offspring from sons (who mate with a female from a different social group) will not compete with the family group, and post-reproductive females can maximize the number of grand offspring that they have (Johnstone and Cant 2010). We tested this hypothesis and found that post-reproductive females provide significant survival benefits to both sons and daughters, but these effects are much more pronounced in sons. If a post-reproductive female dies, the risk of her adult son dying in the year following her death is up to 8 times greater. In contrast, their adult female offspring only have a 2.7 fold increased risk (Foster et al. 2012a).

 

Our long-term observations of the population have shown that one way that females boost the survival of their group is by acting as repositories for ecological knowledge (Brent et al. 2015). As outlined above, Southern Resident killer whales feed almost exclusively on salmon and move into the inshore waters to feed on the salmon as they migrate back to their natal streams to spawn. The timing here is everything - the whales need to know when and where the salmon will be. By observing patterns of group travel and analyzing over 750 hours of video we found that it is the experienced post-reproductive females that guide their group around the salmon foraging grounds and that they especially do this in times of low salmon (Brent et al. 2015). However, we know that reproductive females can also act as leaders and support their adult sons.

 

Why then do the females stop reproducing?

Our research has shown that it is a combination of the help that older females can provide the group and the costs of reproductive competition caused by reproducing at the same time as their daughters that drives the evolution of menopause in Southern Resident killer whales. These are the very same mechanisms that are hypothesized to drive the evolution of menopause in humans.