Cumulative effects assessment for West Coast Transient (Bigg’s) Killer Whales

The West Coast Transient (WCT) population of the Bigg’s Killer Whale (BKW) ecotype was listed as Threatened under the Species at Risk Act (SARA) in 2001. The SARA Recovery Strategy was completed in 2007 (DFO 2007) and is currently being amended. The SARA Recovery Strategy outlined the most pressing anthropogenic threats to WCT as chemical contaminants, and physical and acoustic disturbance (DFO 2007); and also described other threats: biological pollutants, trace metals, toxic spills, collision with vessels, and decline in prey availability or quality. Based on the Recovery Strategy, the current study focused on four identified threats: disturbance (acoustic); disturbance (physical); reduced prey availability; and contaminants. The current analysis uses an established cumulative effects framework that combines a Pathways of Effects (PoE) conceptual model with a Population Viability Analysis (PVA) to assess the cumulative effects of the four identified threats on killer whale vital rates. The study area focused on the Canadian portion of the Salish Sea (CSS) and the subset of WCT observed there during the years 2005 to 2022, to provide optimal overlap between knowledge of threats and population observations. Current knowledge of how threats affect mortality and fecundity rates of WCT in the CSS were synthesised into the overall PoE model consisting of 16 evidence-based linkages from identified threats to effects, eight from single threats and eight mediated through threat interactions. Future changes in anthropogenic activities and their potential threats, including those linked to climate change, were not included. The PoE linkages with sufficient knowledge to be quantified were retained to form a PVA specific PoE model consisting of six evidence-based linkages, four from single threats, and two from threat interactions, these quantifiable threats informed the inputs and structure of the PVA model. The PVA model explored the sensitivity and relative importance of quantifiable threats (vessel strikes, PCBs, vessel noise, reduced prey availability), threat interactions (between PCBs and prey, and vessel noise and prey), and cumulative effects on the population trajectories of the WCT in the CSS. In the PVA, each quantifiable threat and interaction was modelled individually and also together in a cumulative effects scenario. The cumulative effects model included impacts of prey availability on the population carrying capacity, masking of prey sounds by vessel noise, vessel strike mortality, and PCB contamination on calf mortality. The cumulative model, including all four threats, replicates the observed population trend closely with the CSS population size contained within the 90% distribution of the model estimates indicating good model fit. Reduced prey availability had the most influence on abundance trends for WCT in the CSS. The cumulative effects PVA model can be used to explore the impacts of different mitigation and management options for individual threats on the population trajectory, noting that it is sensitive to the value for carrying capacity (i.e., the maximum CSS abundance that the environment can sustain as a result of prey availability). Future projections showed a steady increase in the CSS population over the initial ten to twenty years and then stable population size through the rest of the simulation. The carrying capacity had the biggest effect on the simulated population trends, while the modeled prey trends had relatively small effects. The cumulative effects assessment framework used here, which combines a PoE with a PVA model, is an established approach that explicitly identifies and quantifies threat linkage pathways and associated uncertainties, with the potential for use in other populations and species.

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