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Research Document 2019/056

Cumulative Effects Assessment for Northern and Southern Resident Killer Whale Populations in the Northeast Pacific

By Murray, C.C., Hannah, L.C., Doniol-Valcroze, T., Wright, B., Stredulinsky, E., Locke, A., and R. Lacy

Abstract

The Northern and Southern Resident Killer Whale populations (NRKW and SRKW) that inhabit the waters of the Canadian Pacific coast are listed as Threatened (NRKW) and Endangered (SRKW) under the Species at Risk Act (SARA). The SARA recovery plan developed for these populations identified the assessment of the cumulative effects of anthropogenic threats impacting these populations as a high priority. To address this, a cumulative effects assessment framework was developed and applied comprising two components: a Pathways of Effects (PoE) conceptual model and a subsequent Population Viability Analysis (PVA) model. The PoE model summarises the current understanding of each priority threat (prey availability, disturbance and contaminants) and describes the structure of the threats in the assessment, including threat interactions and potential impacts to population parameters (fecundity and mortality). The PoE model forms the basis for the subsequent PVA model, which utilises the most recent available threat data to quantify the way threats impact population parameters and, together with demographic data, explore patterns of population growth and decline in different threat scenarios. The impacts of individual and cumulative threat scenarios on modelled SRKW and NRKW populations were compared to the observed population trajectories (2000-2017) in order to define a model that best captured the real world dynamics of the two populations. Of the various individual and combined threat models tested, the cumulative threats model, which incorporated all priority threats (Chinook salmon abundance, vessel noise/presence, vessel strike, and PCB contamination), predicted demographic rates closest to that observed for both populations. Population dynamics predicted by the model closely followed the observed demographics for NRKW and though it was the closest model to the observed population size for SRKW, it did not include the observed values within the bounds of uncertainty. However, when historical Chinook salmon model data were included in the model prediction, rather than a randomly chosen Chinook salmon index value, the fit improved for SRKW and the uncertainty bounds of both models included the observed values, suggesting that the cumulative model is a useful representation of the system.

The findings of this cumulative effects assessment highlight the importance of considering threats collectively. Specifically, within the cumulative effects PVA assessment, Chinook salmon abundance and its interactions with vessel noise/presence and PCBs strongly influenced modelled killer whale population dynamics. The cumulative effects PVA model was also used to project population trajectories for NRKW and SKRW into the future. The model outputs indicate that the mean modelled NRKW population trajectory increased to the carrying capacity set in the model within 25 years. In contrast, the mean modelled SRKW population trajectory declined, with a 26% probability of population extinction (defined in the model as only one sex remaining), and in those projections, extinction was estimated to occur after 86 (± 11) years. The cumulative effects assessment framework developed, that combines a PoE with a PVA model, is a novel approach that explicitly identifies and quantifies threat linkage pathways, and associated uncertainties. The framework is a potentially useful tool for managers and scientists and has been refined and tested with the latest threat information for these populations but could also be applied to other populations and species. It is cautioned that as model outputs are only as good as the model inputs, changes in exposure to natural and anthropogenic threats can affect the model’s accuracy. An iterative approach should be used so that model inputs and structure are regularly reviewed and updated to include new information about existing threats and the addition of new threats as knowledge is increased on these populations.

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