Terms of Reference

Distribution, Biological Characteristics, and Relative Abundance Information of Chinook Salmon Stocks within Southern Resident Killer Whale Critical Habitat

Regional Peer Review - Pacific Region

November 20-21, 2024

Nanaimo, BC

Chairperson: Cory Lagasse

Context

The Southern Resident Killer Whale (Orcinus orca) population is listed as endangered under Canada's Species at Risk Act (SARA). 'Resident' killer whale ecotypes feed primarily on Pacific salmon and Chinook salmon (Oncorhynchus tshawytscha) are the dominant component, particularly during early spring and summer (Ford et al. 1998, Hanson et al. 2010, Hanson et al. 2021). From early spring through early autumn, the Southern Resident Killer Whale (SRKW) population makes sustained use of the southern Strait of Georgia, Juan de Fuca Strait and Swiftsure Bank area (Ford and Ellis 2006, Thornton et al. 2022) and since 2008 areas of coastal and inland waters around Vancouver Island have been designated as critical habitat (Fisheries and Oceans Canada (DFO) 2018).

Several Chinook salmon populations that were most prevalent in SRKW diets have declined in recent years (Hanson et al. 2010, COSEWIC 2018). The abundance of specific Chinook salmon populations is correlated with SRKW demographic rates (Ward et al. 2009, Ford et al. 2010, Velez Espino 2015) and individual condition (Stewart et al. 2021), suggesting declines in prey availability may constrain SRKW recovery. Since 2018 Fisheries and Oceans Canada has introduced additional management measures to increase prey availability for SRKW including additional area-based fishery closures on Swiftsure Bank and in the Salish Sea, as well as increased production of hatchery-origin Chinook salmon. Environmental contaminants and vessel disturbance have also been identified as threats to recovery (DFO 2018) but are outside the scope of this analysis. Since 2019 Canadian and American agencies have restricted harvest and increased hatchery production of Chinook salmon to increase prey availability for SRKW.

Many different Chinook salmon populations migrate through SRKW critical habitat and therefore are potential prey; however, Chinook salmon populations likely differ in their suitability as prey for several reasons. First, the relative abundance of different Chinook salmon populations varies at small spatial scales due to population-specific seasonal migrations (Freshwater et al. 2021), so that certain populations will be more accessible to SRKW than others. Second, SRKW habitat use has changed in recent years (Stewart et al. 2023) and there is an improved understanding of SRKW foraging behaviour (Thornton et al. 2022), which may moderate how different Chinook populations are exposed to predation. Third, SRKW may target specific prey size classes so that their diet composition diverges from available prey (Ford and Ellis 2006).

Fisheries and Oceans Canada (DFO) Salmonid Enhancement Program and Fisheries Management has requested that Science Branch provide advice on Chinook salmon stock composition at monthly or seasonal temporal scales to support SRKW survival and recovery. Stock composition will be described based on presence in SRKW diet samples, spatio-temporal overlap with SRKW high use areas based on fisheries-dependent data, and ecological characteristics (e.g., body size).

The assessment and advice arising from this Canadian Science Advisory Secretariat (CSAS) Regional Peer Review (RPR), will be used to inform implementation of effective recovery measures^{Footnote 1}. The advice will be used by Fisheries Management to assist with decisions regarding SRKW prey availability measures and Chinook salmon harvest management for First Nations, recreational and commercial fisheries. This information will also be used for discussions with First Nations and stakeholders to support recommendations on prey availability management measures for the future. The advice will also be used by the Salmonid Enhancement Program to inform salmon habitat restoration activities and be used to guide future salmon enhancement.

Objectives

The following working paper will be reviewed and provide the basis for discussion and advice on the specific objectives outlined below.

• Freshwater, C., Luedke, W., Toews, S., Thornton, S.J. In prep. Distribution, biological characteristics, and relative abundance information of Chinook salmon stocks in Southern Resident Killer Whale Critical Habitat. Can. Sci. Advis. Sec. Res. Doc.

The specific objectives of this review are to:

1. Report seasonal changes in Chinook salmon size and stock composition, based on fisheries-dependent samples, within SRKW high-use areas of critical habitat, during May to October.
2. Report Chinook salmon age, stock composition, and hatchery origin (where possible) within SRKW prey samples.
3. Quantify differences between the prey field (as indexed by Chinook salmon fisheries) and SRKW prey samples based on results of (1) and (2).
4. Report on SRKW prey preference across multiple Chinook salmon ecological dimensions (age, size, and stock identity) within the bounds of critical habitat where data permits. Prey preference during this period will be discussed in the context of broader changes in Chinook salmon abundance and prey quality.
5. Examine and identify uncertainties in the data and methods.

Expected Publications

• Science Advisory Report
• Research Document

Expected Participation

• Fisheries and Oceans Canada (DFO) (Ecosystems and Oceans Science, Fisheries Management, Salmonid Enhancement Program)
• Carlton University
• Committee on the Status of Endangered Wildlife in Canada
• Ha’oom Fisheries Society
• Island Marine Aquatic Working Group
• Ka:’yu:’k’t’h’/Che:k’tles7et’h’ First Nations
• Lower Fraser Fisheries Alliance
• Maa-nulth Treaty Society
• National Atmospheric and Oceanic Administration
• Oceanwise
• Oregon State University
• Pacific Salmon Foundation
• Raincoast Conservation Foundation
• Sport Fishing Advisory Board
• University of British Columbia
• Upper Fraser Fisheries Conservation Alliance
• Uu-a-thulk Fisheries
• Washington Department of Fish and Wildlife

References

• COSEWIC. 2018. COSEWIC assessment and status report on the Chinook Salmon Oncorhynchus tshawytscha, Designatable Units in Southern British Columbia (Part One – Designatable Units with no or low levels of artificial releases in the last 12 years), in Canada. Ottawa.
• Fisheries and Oceans Canada (DFO). 2018. Amended recovery strategy for the Northern and Southern Resident Killer Whales (Orcinus orca) in Canada. Fisheries & Oceans Canada, Ottawa.
• Ford, J.K.B., Ellis, G.M., Barrett-Lennard, L.G., Morton, A.B., Palm, R.S., Balcomb, K.C., III. 1998. Dietary specialization in two sympatric populations of killer whales (Orcinus orca) in coastal British Columbia and adjacent waters. Canadian Journal of Zoology 76:1456-1471.
• Ford, J.K.B., and Ellis G.M. 2006. Selective foraging by fish-eating killer whales Orcinus orca in British Columbia. Marine Ecology Progress Series 316:185-199.
• Ford, J.K.B., Ellis, G.M., Olesiuk, P.F., Balcolmb, K.C., III. 2010. Linking killer whale survival and prey abundance: Food limitation in the oceans' apex predator? Biology Letters 6:139-142.
• Freshwater, C., Anderson, S.C., Beacham. T.D., Luedke, W., Wor, C., King, J. 2021. An integrated model of seasonal changes in stock composition and abundance with application to Chinook salmon. PeerJ 9:e11163.
• Hanson, M.B., Baird, R.W., Ford, J.K.B., Hempelmann-Halo, J., Doornik, D.M.V., Candy, J.R., Emmons, C.K., Schorr, G.S., Gisborne, B., Ayres, K.L., Wasser, S.K., Balcomb, K.C., Sneva, J.G., Ford, M.J. 2010. Species and stock identification of prey consumed by endangered southern resident killer whales in their summer range. Endangered Species Research 11:69-82.
• Hanson, MB, Emmons, C.K., Ford, M.J., Everett, M., Parsons, K., Park, L.K., Hempelmann, J., Van Doornik, D.M., Schorr, G.S., Jacobsen, J.K., Sears, M.F., Sears, M.S., Sneva, J.G., Baird, R.W., Barre, L., Hyrenbach, D. 2021. Endangered predators and endangered prey: Seasonal diet of southern resident killer whales. PLoS One 16:e0247031.
• Stewart, JD, Durban, J.W., Fearnbach, H., Barrett-Lennard, L.G., Casler, P.K., Ward, E.J., Dapp, D.R. 2021. Survival of the fattest: linking body condition to prey availability and survivorship of killer whales. Ecosphere 12:e03660.
• Stewart, JD, Cogan, J., Durban, J.W., Fearnbach, H., Ellifrit, D.K., Malleson, M., Pinnow, M., Balcomb, K.C. 2023. Traditional summer habitat use by southern resident killer whales in the Salish Sea is linked to Fraser River Chinook salmon returns. Marine Mammal Science 39:858-875.
• Thornton, SJ, Toews, S., Stredulinksy, E., Gavrilchuk, K., Konrad, C., Burnham, R., Noren, D.P., Holt, M.M., Vagle, S. 2022. Southern Resident Killer Whale (Orcinus orca) summer distribution and habitat use in the southern Salish Sea and the Swiftsure Bank area (2009 to 2020). Can. Sci. Advis. Sec. Res. Doc. 2022/037. 62 p.
• Vélez‐Espino, LA, Ford, J.K., Araujo, H.A., Ellis, G., Parken, C.K., Sharma, R.. 2015. Relative importance of chinook salmon abundance on resident killer whale population growth and viability. Aquatic Conservation: Marine and Freshwater Ecosystems 25: 756780.
• Ward, EJ, Holmes, E.E., Balcomb, K.C. 2009. Quantifying the effects of prey abundance on killer whale reproduction. Journal of Applied Ecology 46:632-640.