Chinook salmon (Okanagan population) COSEWIC assessment and status report: chapter 10

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Limiting Factors and Threats

Limiting factors and threats associated with spawning and rearing habitat in the Okanagan basin in Canada have already been addressed under Habitat Trends. In addition, the National Marine Fisheries Service (1998) identified the section of the Okanagan River below McIntyre Dam as the highest priority within the Okanagan-Okanogan Basin for protection and restoration. They also identified the greatest habitat risk as the potential loss of suitable rearing area in Osoyoos Lake. Although the statement is for sockeye, it is also applicable to chinook. This rest of this section will focus on threats outside of the basin.

Broodstock collection for hatcheries (see Rescue Effect) and in-river and ocean fisheries on the hatchery-enhanced population are unquantified threats.

Chinook migrating to and from the Okanagan River face increased mortality due to predation or injury at each of the mainstem dams and their impoundments. There are nine mainstem hydroelectric dams that adult and juvenile chinook migrate through, four of which are federally operated (Bonneville, Dalles, John Day, and McNary) and five operated by Public Utility Districts (PUDs) (Priest Rapids, Wanapum, Rock Island, Rocky Reach, and Wells). Ferguson et al. (2004) estimated a survival rate of 80-85% for adult chinook migrating past 8 hydroelectric dams on the Columbia and Snake Rivers (tributary to the Columbia). While not specific to chinook in the Upper Columbia River basin, a similar estimate for chinook passing the nine Columbia River dams downstream of the Okanagan River may be appropriate. In the current sub-basin planning efforts for the Okanagan, survival rates for migrating smolts are estimated at between 86% and 91% per dam for stream-type chinook (Moore et al., 2004), which suggests that between 26% and 43% of the smolts that leave the Okanagan River make it through Bonneville Dam.

Fishery Impacts

Okanagan chinook likely migrate with the Upper Columbia summer chinook salmon, although direct observations to confirm this are not available. The Joint Chinook Technical Committee (CTC) (PSC 2003) uses coded-wire-tag (CWT) releases from Wells Dam Hatchery to monitor exploitation of Columbia River summer chinook (including Upper Columbia stocks), which is one of the 36 exploitation rate indicators monitored. CWT recoveries in all fisheries (including associated incidental mortality) and escapement are used to reconstruct cohort size by brood year for each indicator stock. Based on these data, total fishing mortalities by catch year and brood year exploitation rates are estimated.

Total fishing mortalities for Columbia River Summer chinook have been calculated to catch year 2003 (Table 2) (PSC 2003, CTC unpublished data from R. Sharma, 2005 personal communication). For years 1979-1980 and 1987-1990, total fishing mortality averaged 71% and 66.9% respectively. For 1991-1998, total fishing mortality has averaged 32.7%, with a high of 50.2% and a low of 17.6%. Since 1999, total fishing mortality has averaged 62.8%, and increased steadily from 46.1% in 1999 to 76.4% in 2003. In recent years, fishing mortality has been distributed approximately equally between Canadian, Alaskan and southern U.S. fisheries. Canadian exploitation has primarily occurred in the Northern BC and West Coast Vancouver Island troll fisheries, while U.S. exploitation occurs mainly in the Alaskan troll and southern U.S. troll and sport fisheries (which include in-river harvest).

The Columbia summer chinook mortality distributions described above are by catch year, with captured fish being three to six years old (i.e., mixed broods). Brood-year exploitation rates provide a measure of fishing impacts on each brood across all years during which fisheries harvest that brood. Currently, the most recent completed brood year for which such rates are available is 1999 (Figure 8). Exploitation rates for the 1975-1977 brood years ranged between about 60%-70%, subsequently decreasing from 70% in 1983 to 20% by 1991 and 1992. Exploitation rates have been considerably higher than this (70% to 80%) on more recent broods (1997-1999). In addition, there is an increase in the contribution of the Columbia River fisheries to the brood year exploitation rates on more recent broods (1997-1999).

From PSC, 2003; CTC unpublished data from R. Sharma, personal communication.

Figure 8: Brood Year Total Exploitation Rate for Columbia River Summer Run

Note that 1999 is still incomplete as five-year-old CWT data have not been processed.

However, further cohort analysis on total fisheries mortality conducted by the National Oceanic and Atmospheric Administration (NOAA) suggests that impacts specifically on Okanagan/Similkameen chinook may not be properly represented by Wells Dam hatchery stock and fisheries impacts may be less than predicted, but further analysis is required (NOAA 2005).

Prior to fisheries catch year 1999, the majority of fishing mortality is associated with the ocean fisheries, while mainstem Columbia River treaty ceremonial and sustenance harvest is estimated to have been less than 3% since 1986 (U.S. vs. Oregon Technical Advisory Committee 1999). However, in-river fisheries have increased since 2001 (Figure 9) due to the increase in escapement (Figure 10).

Figure 9: Terminal Harvest Rates of Upper Columbia Summer Chinook Management Group in the Columbia River

Data from CRITFC, Colville Tribes, and WDFW).

Figure 10: Summary of Upper Columbia Chinook Escapement Past Bonneville Dam and Before the Additional Upstream Fishery in the Columbia River (1979-2005)

The fishery in 2005 went through the following process. The escapement objective for the 2005 Upper Columbia Summer run at Bonneville Dam was decided to be 29 000 adults (hatchery plus wild). The projected run size for these fish was 62 400. Therefore, under the new management plan, the maximum allowable total harvest rate for this group was set at 47.6% (Treaty 23.8% and non-Treaty 23.8%) (Table 3).

The actual escapement was 60 173 summer chinook. Preliminary estimates (Washington Department of Fish & Wildlife - Columbia River Compact) indicate that the actual treaty harvest (Zone 6-Between Bonneville and McNary Dams) was about 7 642 summer chinook, or 12.7%, less than the 23.8% allocated (Matylewich, 2005 personal communication). The lower harvest rate was likely due to the greater numbers returning than had been predicted, but may also be due to a decrease in the number of treaty fishers during summer 2005. Commercial capture of summer chinook has been limited by the total annual allowable bycatch of endangered steelhead trout: the summer chinook fishery (and its incidental catch of steelhead) is constrained by the need to reserve some steelhead trout bycatch for the lucrative fall chinook fishery. Fishers attempt to use large-meshed gill netting gear to target chinook since this gear generally excludes steelhead trout; however, large steelhead individuals (called B-type) are still captured (Matylewich, 2005 personal communication).

For 2005, non-treaty harvest (23.8%) is allocated to several groups (commercial, sport, and non-treaty tribal fisheries. The 2005 sport fisheries in the Columbia River for Upper Columbia Summer chinook below Bonneville Dam (Zones 1-5) were changed from mark-selective to non-selective as of 1 July 2005 (i.e., both hatchery-origin and wild chinook could be retained). The non-treaty harvest below Bonneville Dam was estimated to be 4 174 individuals, or 7.0% (using the actual escapement return of 60 173 summer chinook). The non-treaty harvest above McNary Dam was estimated to be 5968 or 10.0% (Matylewich, 2005 personal communication). In summary, the Columbia River fishery for 2005 had an allocation of 47.6%, but actual harvest was approximately 18 196 chinook or 30.31% (Table 3). This is still an increase from previous years.