Science Advisory Report 2019/052

Recovery Potential Assessment - Okanagan Chinook Salmon (Oncorhynchus Tshawytscha) (2019)

Summary

• Summer, ocean-type, Okanagan Chinook are the Designatable Unit (DU) of Chinook Salmon (Oncorhynchus tshawytscha) that spawn in the Okanagan River above Osoyoos Lake in British Columbia (BC).  This DU was assessed by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) as Endangered in April 2017. No other life history forms, such as spring stream-type Okanagan Chinook, were assessed.
• Area-under-the-curve escapement estimates for Okanagan Chinook averaged nine non-adipose clipped individuals from 2009 – 2012, then averaged 50 individuals from 2013-2017.  In 2018, the escapement estimate was 10 individuals.
• Genetic analysis indicates the Okanagan Chinook are part of the upper Columbia Summer Chinook meta-population from which wild and US hatchery-origin fish are known to stray into Canada. However, DNA analysis of samples collected from Okanagan Chinook indicate that some of the spawning fish in Canada result in progeny that return to Canadian spawning sites.
• High mortality rates are a major factor that inhibit the summer Okanagan Chinook DU's ability to recover.  High riverine temperatures, passage at multiple dams, and interception in recreational and commercial fisheries all limit survival and production of the DU.
• Little is known specifically about habitat use in Canada; instead, much of the understanding must be inferred from what is known about the state of habitat for Sockeye Salmon in the Okanagan River and summer Chinook habitat use in the upper Columbia River.  Spawning redds within the Okanagan River are defined as the residence for this DU.
• The main anthropogenic threats and limiting factors identified for summer Okanagan Chinook include: modifications to water ways (through dams, channelization, impoundment, water management, and temperature effects), fishery related mortality (largest component associated with US in-river fisheries), predation/competition from invasive species, land use changes in riparian zones, varying freshwater and ocean conditions. Climate change is anticipated to exacerbate the impact of these threats.
• A recovery target based on a 4-year geometric mean of 1,000 spawners was identified in the Recovery Potential Assessment (RPA) report.  This recovery target represents a minimal viable population number for which maintenance of a trend toward positive population growth will be crucial.
• The population trajectory and probability that summer Okanagan Chinook reach the DU recovery target was explored using stock-recruit models and forward simulation. The results showed that the probability of reaching the recovery target in 12 years under current conditions is not likely.
• Simulation model results suggest the scenario that had the highest likelihood of reaching the recovery target was supplementing the population with hatchery Chinook from the upper Columbia River.  Improved survival rates through freshwater habitat improvements would further support supplementation mitigation efforts (≥ 90% probability).
• Due to the limited information about this DU, significant sources of uncertainty include: meta population dynamics, fishery mortalities (i.e. targeted and bycatch), spawning habitat limitations, freshwater carrying capacity, life history parameters, and magnitude of reduced productivity in relation to specific threats/limiting factors.
• All sources of harm should be reduced to the maximum extent possible.  No fishery related mortality is currently sustainable in Canadian waters.  Although stock assessment, research, and mitigation actions are sources of potential mortalities, they are required to both facilitate and determine whether positive population grow towards the recovery target occurs.
• Although not quantified here, population growth of anadromous salmon is known to be particularly sensitive to habitat quality; therefore, human-induced impacts to freshwater habitat of Okanagan Chinook Salmon (i.e. downstream migration mortality through dams and reservoirs) should also be considered in addition to fishery mortality when considering allowable harm. 

This Science Advisory Report is from the May 28-30, 2019 regional peer review on Recovery Potential Assessment – Okanagan Chinook (Oncorhynchus tshawytscha). Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2019/052 (PDF, 570 KB)