Research Document - 2007/065
Scientific Information in Support of Recovery Potential Analysis for Chinook Salmon Okanagan Population, Oncorhynchus tshawytscha
By C. Davis, H. Wright, T. Brown, B. Phillips, R. Sharma, C. Parken
Abstract
The Okanagan chinook population is the last remaining Columbia basin stock that resides within Canada and it is geographically and genetically distinct from chinook populations elsewhere in Canada. The Canadian Okanagan population consists of anadromous salmon that migrate to and from the Pacific Ocean through the Columbia River, to the area bounded by McIntyre Dam at the outlet of Vaseux Lake. Ancestral Columbia River Chinook salmon population have been estimated at 2-4 million fish with the historic abundance in the Upper Columbia in the hundreds of thousands. Historically the Okanagan Chinook population was large enough to support an important food and commercial/economic trade fishery prior to non-native human settlement. However, downstream fishing combined with high inter-dam mortalities for migrating salmon has led to reduced numbers. Rapid human development in the river basin has led to wide spread degradation of habitat. Loss of habitat has also been attributed to irrigation and water withdrawal, logging, mining, transportation corridors, and other human activities, which have reduced the quantity, quality, and capacity of spawning and rearing areas. The annual number of chinook spawning in Canada is less than 50 adults.
There is a high degree of interrelatedness for chinook found within the Okanagan River. A close familial relationship among Okanagan Chinook presents strong evidence for the successful out migration, return and survival of a few families of Okanagan Chinook. Yet the level of genetic diversity in the small population and recovery of a few tagged fish indicates that it is currently receiving strays from a larger population. The lack of significant differentiation in allele frequency between the Similkameen and Okanagan River samples indicates that the Similkameen population is likely the source of strays.
The amount of spawning and rearing habitat available within the Canadian portion of the Okanagan River was estimated to be 16km². Anadromous species may use the Columbia River for rearing, and must use it as a migration corridor. Juveniles rear and grow to adults in the Pacific Ocean. Adults spawn over a patchy range of habitat. Total spawning capacity estimates range from 2,440 to 8,680 fish with a defensible estimate of 1460 spawning pairs. These estimates are based on watershed areas, known habitats, and behavioural characteristics of Okanagan Chinook. The most northern accessible portion of the Okanagan River contains reaches that are suitable for spawning and rearing. The naturalised upper sections contain a variety of complex habitats, while the lower channelized section lacks habitat complexity. It has no backwater pools, primary pools, undercut banks, pool tail-out glides, and has little groundwater influence. Following their emergence in April or May, the exact rearing locations of Okanagan Chinook fry are unknown. We suspect they rear in the Okanagan River for a short time and in Osoyoos Lake, but they may also rear downstream in the mainstem Columbia River.
The life history of the Canadian portion of the Okanagan Chinook population has never been examined as a unique entity. We suspect their life history is similar to the life history of other Upper Columbia River summer stocks that have been examined in more detail. Juvenile Chinook move downstream through the Columbia River throughout the year and pass through the estuary to the ocean. Fish may remain in the estuary for periods ranging from weeks to months.
Chinook in Canada have been adversely impacted by human induced changes in the environment. These threats include; water withdrawals, construction of dams that limit and exclude passage or entrain/harm migrating fish, channel modification and introduction of non-native fish species. American Columbia River habitat impacts can be severe. These alterations have resulted in reductions in habitat complexity, slower water velocities and higher water temperatures with the Columbia and Okanagan Rivers. Chinook salmon populations are also impacted by fisheries and large scale hatchery supplementation.
We employed a parameter estimation and sensitivity analysis, using stochastic and deterministic elements to evaluate population trajectories under baseline conditions and explored the potential impacts of multiple management alternatives (Appendix C). Our population viability analysis (PVA) model indicates that juvenile survival downstream through the hydro-power system limits population persistence. The same is true for adult survival, which is likewise constrained by upstream passage mortality through the hydropower system. Ocean survival is another influential parameter, but values used in our simulations were derived from a period when ocean survivals were among the highest recorded (i.e., the late 1990’s). Thus, it is likely that observed rates of decline would exceed those observed in our simulations. While fishing mortality also contributes to the decline, even complete cessation of harvest and corresponding reduction in mortality was found to be insufficient to recover the stock.
Given the uncertainty that managers can dramatically improve juvenile and adult survival through the gauntlet of American hydro dams and reservoirs, it appears that the only alternative that can feasibly forestall extirpation in the near-term is via hatchery production. However, the magnitude of artificial production required to meet escapement goals is immense and would require a large program (approximately 1.75 million smolts annually). A program of that magnitude would be accompanied by its own array of risks.
The long-term recovery objective should be a secure and viable Canadian chinook population within the Okanagan Basin. The short-term objective should be to maintain this run of chinook through hatchery supplementation. The longer-term objective would require a viable naturally spawning Canadian chinook population. The minimum population size of this spawning population based on a scenario from the population viability analysis was an average of 295 individuals over four brood years. It was speculated that this could be achieved by 2050.
Immediate action needs to be taken to prevent the Canadian population from being extirpated. First and foremost would be the implementation of a hatchery program to supplement the current population. Second would be investigation into provisions for fish passage at facilities currently limiting access. Third is determining and mitigating the impacts of predation/competition with exotics fish species is required. Fourth, reducing fisheries impacts should be investigated. Lastly, investigation into how Canada can contribute to improve downstream survival through mainstem hydroelectric dams should be conducted. To support the existing wild population, measures should be taken to ensure that the required habitat features are maintained, enhanced, or restored in the Canadian portion of the Okanagan.
A high degree of uncertainty exists in establishing recovery goals, recovery targets, and in defining critical habitats. A clear understanding of the life-cycle characteristics of the Canadian Okanagan population is required. Three biological scenarios exist based on possible differences in degree of isolation and degree of uniqueness of the population. Each of these scenarios has different implications for recovery time, recovery goals, and delineation of critical habitats. Continuation of studies to clarify these uncertainties is recommended.
Accessibility Notice
This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.
- Date modified: