Science Advisory Report 2019/030

Cumulative Effects Assessment for Northern and Southern Resident Killer Whale (Orcinus Orca) Populations in the Northeast Pacific


  • Southern Resident (SRKW) and Northern Resident Killer Whales (NRKW) were listed as Endangered and Threatened, respectively, in 2003 under the Species at Risk Act (SARA). A Recovery Strategy for these populations was completed in 2008 (most recently amended in 2018; DFO 2018) and an Action Plan was completed in 2017 (DFO 2017a).
  • The Species at Risk Act (SARA) Recovery Strategy (amended) identifies three principal threats to Resident Killer Whales – reduced prey availability, acoustic and physical disturbance, and environmental contaminants (DFO 2018).
  • The current work aims to address the high priority recovery measure (RM #11) in the Action Plan which identifies the need to "Assess cumulative effects of potential anthropogenic impacts on Resident Killer Whales using an appropriate impact assessment framework for aquatic species".
  • In the current analysis, potential mitigation measures and management actions have not been addressed. Future changes in anthropogenic activities have not been included. The population level effects of low probability-high impact events, such as catastrophic oil spills, were out of scope for this assessment.
  • The cumulative effects assessment framework developed consisted of two phases, the first being a Pathways of Effects (PoE) conceptual model that described the linkage pathways between threats and Resident Killer Whale mortality and birth rates, including potential interactions between threats.
  • The second phase of the cumulative effects assessment framework was a Population Viability Analysis (PVA) model that quantitatively assessed the effects of the priority threats on the population trajectories of Resident Killer Whales.
  • Seven of the 13 linkage pathways identified in the PoE could be quantified in the second phase of the assessment framework (PVA model) based on current data and knowledge.
  • A number of key assumptions were made in the PVA modeling. The consequence of exposure to threats is assumed to be the same for both populations while the exposure to the threats is assumed to be population-specific. Quantification of linkage pathways to mortality rate and birth rate was based on information mostly obtained in the Salish Sea area in the summer/fall period but was assumed to represent threat conditions throughout the range and throughout the year. Chinook salmon abundance, irrespective of size selectivity by killer whales, was assumed to represent preferred prey available to Resident Killer Whales.
  • Population models were constructed using demographic and genealogy data from DFO and Center for Whale Research, and a previously published population model for SRKW (Lacy et al. 2017).
  • 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. Individual and cumulative threat scenarios were each tested by running 10,000 model simulations. 
  • Individual threat scenarios used both updated and newly available parameters for Chinook salmon abundance, vessel noise/physical presence, vessel strike, and polychlorinated biphenyl (PCB) contamination. Taken one at a time, the modeled effects of individual threats did not replicate the observed population trajectories for NRKW and SRKW populations, suggesting that a cumulative model could better replicate the trends in Resident Killer Whale populations.
  • When the threats are considered together (Chinook salmon abundance, vessel noise/physical presence, vessel strike and PCB contamination), the output of the PVA model closely replicated the observed population trajectories for the two populations, suggesting it is a useful model for the assessment of cumulative effects.
  • The cumulative effects PVA model was then used to project population trajectories for NRKW and SRKW into the future (10,000 model simulations), based on recent threat levels, best available knowledge, and the assumption that no future mitigation will take place. The model outputs indicate that the average modelled NRKW population trajectory increases to the carrying capacity set in the model within 25 years. In contrast, the average modelled SRKW population trajectory declines, 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 75-97 years.
  • The model highlights the importance of considering threats collectively. Specifically, within the cumulative effects PVA assessment, Chinook salmon abundance and its interactions with vessel noise and PCBs strongly influenced modelled killer whale population dynamics.
  • The cumulative effects 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, and this approach has the potential for use in other populations and species.

This Science Advisory Report is from the March 12-13, 2019 Cumulative effects assessment for Northern and Southern Resident Killer Whale populations in the Northeast Pacific. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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