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.
Highlights
The inland waters of Washington State and southern British Columbia, an area known as the Salish Sea, are home to two ecotypes of killer whale (Orcinus orca)
We developed a bioenergetic model of expected harbor seal consumption by individual transient killer whales in the Salish Sea between 1 January 2017 and 31 December 2017
Unlike previous time periods, which had one or two discrete peaks, transient occurrences were steadily high from April through September and higher overall throughout the year compared with historical time periods
Summary
The inland waters of Washington State and southern British Columbia, an area known as the Salish Sea, are home to two ecotypes of killer whale (Orcinus orca). The Southern Residents are fish-eating whales feeding predominantly on salmonids; West Coast Transients ( known as Bigg’s killer whales), are marine mammal eaters, preying upon species such as seals, sea lions, and porpoises (Ford et al, 1998). Increased presence of mammal-eating killer whales in the Salish Sea with implications for predator-prey dynamics. Anecdotal observations by the authors and many others have revealed that when in the same waterway, the mammal-eating killer whales often change course to avoid a direct interaction with the fish-eating killer whales
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