Increased presence of mammal-eating killer whales in the Salish Sea with implications for predator-prey dynamics

Abstract

The inland waters of Washington State and southern British Columbia, collectively known as the Salish Sea, comprise key habitat for two regional populations of killer whales (Orcinus orca): the mammal-eating West Coast Transients and the endangered fish-eating Southern Residents. These two populations are genetically distinct and may avoid each other. Transient killer whale usage of the Salish Sea has been previously assessed over two seven-year time periods, showing an increase from 1987 to 2010. We documented a continued significant increase in mammal-eating killer whale presence in the Salish Sea from 2011 to 2017, with intra- and inter-annual variability and with record sightings in 2017. This continued increase, likely in response to abundant marine mammal prey, is related to both a growing population and an increase in the number of West Coast Transients visiting the area. Additionally, a negative binomial regression shows that absence of Southern Residents is correlated to transient presence. Finally, both populations of killer whales have been linked to regional harbor seal populations; harbor seals are salmonid-eating competitors of the Southern Residents and are prey for the mammal-eating transients. With Southern Residents listed as endangered, culling harbor seals has been proposed as a measure to help in their recovery. With this in mind, we developed an energetic model to assess the minimum number of harbor seals consumed by transient killer whales. Using the actual number of whales present in each age-sex class for each day of the year, we estimate that, at a minimum, transients in the Salish Sea consumed 1090 seals in 2017. This is more than 2% of the 2014 estimated harbor seal population the Salish Sea. The population controlling effects of transient killer whale predation on harbor seals should be considered when evaluating any pinniped management actions in the Salish Sea.

Causes of Mortality in a Harbor Seal (Phoca vitulina) Population at Equilibrium

“…In our 17-year retrospective analysis, we only detected cases of predation-related trauma from mammal-eating killer whales in two adults and nine pups or weaned pups. We suspect a far higher level of killer whale-associated predation on harbor seals occurs in SJC (Shields et al, 2018), but this influence is not demonstrable via stranding data as most carcasses are likely consumed rather than beach cast. Even with limited available stranding data on predation, we hypothesize that the factors keeping the harbor seal population in SJC at equilibrium are probably a combination of bottom-up nutritional limitations and top-down predatory influences.…”

A long postreproductive life span is a shared trait among genetically distinct killer whale populations

“…From the survival model output, the survival patterns are similar for all three killer whale populations, with females expected to live both substantially longer than males and to ages of >50 years (Figure 2). Interestingly, the model predicts that male Bigg's killer whales may have a longer expected life span compared with resident males (Figure 2), a pattern that could be influenced by the differences in their environments, both social (Ellis, Franks, Nattrass, Cant, Weiss, et al, 2017;Lemaître et al, 2020) and ecological (i.e., prey availability (Ford et al, 2010;Shields et al, 2018)). However, this could also be an effect of male dispersal in Bigg's, resulting in more uncertainty for the model around the ages at death of males.…”

Estimating the stock size of harbor seals (Phoca vitulina richardii) in the inland waters of Washington State using line-transect methods

“…Transient or Bigg’s killer whales ( Orcinus orca ) are common in parts of the Salish Sea, and they feed almost exclusively on marine mammals [ 61 ]. Harbor seals are a major prey item of killer whales in this area [ 61 ], and since at least 2004, Bigg’s killer whales have been spending increasing amounts of time in the Salish Sea [ 62 , 63 ]. In fact, the scale of this increase in numbers and occupancy has steepened in recent years, with 2017 reaching a peak of over 200 occurrences [ 64 ].…”

“…Since mammal-eating killer whales are spending significantly more time in the northern inland waters of Washington, it is likely that harbor seal predation has also increased there. Shields et al [ 63 ] recently estimated that Bigg’s killer whales in the Salish Sea consumed well over 1,090 harbor seals in 2017. If these were mainly taken from the Washington Northern Inland Waters stock, then this could certainly have an impact on that stock, with little to no effect on the stocks in Hood Canal and Southern Puget Sound, where the killer whales rarely feed.…”