Chinook salmon (Okanagan population) COSEWIC assessment and status report: chapter 8

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Biology

• Life Cycle and Reproduction
• Predation
• Physiology
• Dispersal/Migration
• Interspecific Interactions
• Adaptability

The general biology of chinook salmon has been well documented in North America. The following sections draw heavily from Healey (1991) and Myers et al. (1998). The characteristics of the Okanagan chinook population have only recently begun to be documented, apart from traditional ecological knowledge and sporadic past observations. The main source of current information on this population is a compilation of observations collected by the Okanagan Nation Alliance Fisheries Department (Wright and Long, 2005).

Life Cycle and Reproduction

Based on both traditional ecological knowledge (Ernst and Vedan, 2000) and recent observations on run timing (Wright and Long, 2005), Okanagan chinook spawn in the fall. Since there are accounts of chinook arriving in the river upstream of Osoyoos Lake in spring/early summer, there may have been a second population spawning in late-June/early July prior to elevation in river temperatures; alternatively, these early arrivals may be an early migrating segment of the same fall-spawning population. Historical Indian fisheries conducted in May, June and early July were likely for spring (stream-type) chinook (Moore et al., 2004). In addition, Canadian biologist Gartrell noted 100-300 spring chinook present on the spawning grounds upstream of Osoyoos Lake in May of 1936 (DFO unpublished files, 1936 Salmon Escapement Data Set file). Data on run timing have been collected for the U.S. Upper Columbia River chinook salmon ESU, including both ocean-type (U.S. Okanogan River) and stream-type (Methow River) chinook, and a summary of observed run times in the Okanagan River basin has been compiled (Figure 5). Recently, spawning by chinook upstream of Osoyoos Lake is observed in October, which is typical of ocean-type populations in the Upper Columbia River basin. Spawning is likely initiated by a reduction in temperatures to below 16 °C (Healey, 1991), which occurs in the Okanagan River in late September or early October (Hyatt and Rankin, 1999). While this spawn timing is typical of ocean-type chinook, conditions in the lake may permit a stream-type population to thermoregulate and delay spawning through to October, such as occurs elsewhere in the Columbia basin (Myers et al., 1998).

There is little information on the age distribution of spawners in the Canadian Okanagan River. However, assessments of the stock in the U.S. portion of the basin have identified approximately 21% as three-year-old males (i.e., jacks), 44% as four-year-olds, and 34% as five-year-olds (Howell et al., 1985, Chapman et al., 1994). (Age is measured from egg deposition.) No two-year-old (i.e., age 1+) spawners were recorded in the U.S. Okanogan basin, and only one percent of spawners were six-year-olds. In the Canadian Okanagan basin most of the small chinook that have been caught in Osoyoos Lake have been identified as two-year-olds (i.e., 1+) (ONAFD, unpublished data, 2005). Prior to 2005, seven full-sized chinook from the Canadian Okanagan basin were aged; one was a four-year-old (sex unknown), while the other six (three males and three females) were at least five years old (Wright and Long, 2005). For 2005, a total of 23 chinook were aged (DFO aging lab) in the Canadian Okanagan basin (ONAFD, unpublished data, 2005). The sex ratio was 43.5% males to 56.5% females. For 2005, 43.5% were three-year-olds (5 males and 5 females), 47.8% four-year-olds (4 males, 7 females), and 8.7% five-year-olds (1 male, 1 female). 

Figure 5: Summary of Historical and Recent Chinook Salmon Observations in the Canadian Okanagan Basin and Selected Historical Observations in the U.S. Portion of the Basin

Location (Date)

--- United States ---

--- Canada ---

¹ Myers et al. 1998,
² Smith 2002 (chinook spawning described as being near the end of sockeye spawning),
³ Smith 2003b,
⁴ Vedan 2002 (chinook fishery described as being in the fall, but prior to the chum salmon fishery in November),
⁵ MOE 1993,
⁶ Wright and Long 2005,
⁷ DFO SEDS unpublished files,
⁸ DFO SEDS correspondence files.

There are no fecundity data reported for Okanagan chinook. However, upper Columbia River chinook captured at Wells Dam, the nearest dam downstream of the Okanagan River confluence, showed a mean fecundity of 5041 eggs per female, with fish averaging 90.4 cm long (Hymer et al., 1992; Myers et al., 1998). Larger fish tend to produce more and larger eggs, but the relationship is non-linear (Myers et al., 1998). A variety of additional factors influence both the number and size of eggs including fish age, life history strategy, migration distance, and latitude (Myers et al., 1998).

Egg to fry survival in chinook is highly variable, but Healey (1991) placed an upper limit of 30% on eggs deposited and incubated under natural conditions. Egg survival can be relatively high where intra-gravel percolation is good and redds are not impacted by scour, desiccation, or deposition of fine particles (Healey, 1991). Where the density of spawners is high, established redds are often disturbed by subsequent spawners with a resultant loss of eggs.

Predation

Predators are commonly implicated as the principal agent of mortality among young juvenile chinook (Healey, 1991). Piscivorous birds and fish consume juvenile chinook in freshwater, estuarine, and marine environments. In addition, invertebrate predators have been observed to kill or injure juvenile salmon, but predation by invertebrate predators outside of hatchery conditions is not well documented (Healey, 1991). Mortality rates of 70%-90% among young juvenile Pacific salmon have been recorded in several river systems (Healey, 1991).

Among the piscivorous fish that likely prey on Okanagan chinook are numerous exotic species, 13 of which have been reported (Wright et al. 2002). Exotic species comprise 84% of the littoral fish population in Osoyoos Lake and are also found in the mainstem Okanagan River (Wright et al. 2002). Juvenile chinook in Osoyoos Lake may be preyed upon by bluegill (Lepomis macrochirus), black crappie (Pomoxis nigromaculatus), smallmouth bass (Micropterus dolomieui), yellow perch (Perca flavescens), and largemouth bass (Micropterus salmoides).

Predation mortality during downstream smolt migration has most likely increased in the Columbia River due to mainstem dams (Myers et al. 1998). This is evidenced by the fact that predator control measures have been conducted on the Columbia as a means of improving downstream smolt survival for salmon populations (Zimmerman 1999, Zimmerman and Ward 1999a, b).

Based on a review of several studies of mortality rates in ocean-rearing chinook salmon, Healey (1991) concluded that the marine mortality rate is likely less than 35% per year and probably closer to 20% per year. In addition, he concluded that mortality rates are likely higher during the first year or two at sea, and higher in coastal areas, which would result in relatively higher mortality rates for ocean-type chinook relative to stream-type chinook (which often complete extensive offshore migrations; Healey, 1983). Sources of mortality for chinook in the larger size classes include commercial and recreational fisheries, predation by large fish and mammals, disease, and adverse marine conditions in some years.

Physiology

The upper and lower temperatures for 50% pre-hatch mortality of chinook are 16 °C and 2.5-3.0 °C, respectively (Alderdice and Velsen, 1978 cited in Healey, 1991). The same authors identified the time to 50% hatch as about 159 days at 3 °C and 32 days at 16 °C, and concluded that a simple thermal sum model (development time = 468.7/T; where T is the average temperature during incubation) is adequate for predicting time to hatching.

Water percolation through spawning gravels is essential for egg and alevin survival, a requirement that can be severely compromised by siltation of spawning beds (Healey, 1991). Shelton (1955, cited in Healey, 1991) concluded that survival to hatching was greater than 97% at percolation rates of at least 0.03 cm/s, but that emergence was 13% or less from small gravel when percolation rates were less than 0.06 cm/s. Much higher emergence rates (87%) were recorded for chinook in large gravel with adequate intra-gravel flow.

Adults stop migration and seek temperature refuges when water temperatures exceed 22 °C (Alexander et al., 1998). The preferred temperature for chinook fry is 12-14 °C with the upper lethal temperature being 25.1 °C (Scott and Crossman, 1973).

Dispersal/Migration

Upstream migration of mature Okanagan chinook occurs mainly during daylight hours with few fish migrating upstream at night (Healey, 1991). Conversely, downstream movement of fry occurs mainly at night, generally concentrated around midnight, although small numbers of fry may move during the day (Healey, 1991). As presented in Figure 5, the timing of upstream migration into the Okanagan River corresponds with that in the U.S. portion of the basin.

The only observations of Okanagan chinook leaving Osoyoos Lake were obtained in a rotary screw trap set 300 m downstream of Zosel Dam (i.e., at the outlet of Osoyoos Lake) (Hansen, 1996a and b). Okanagan chinook were recorded as an incidental observation to the target species (sockeye smolts), and little information is provided other than that chinook fry (newly emerged) were captured in a majority of sampling sessions between April 17 and May 31. The chinook fry observed may not have been from spawning areas upstream of the lake, but rather from redds between the trap and Zosel Dam, where suitable spawning habitat is present but spawning has not been confirmed (C. Fisher, personal communication, 2005). Newly emerged fry were also captured upstream of Osoyoos Lake in April and May (Wright and Long, 2005). There are no records of Okanagan chinook smolts leaving Osoyoos Lake.

As previously mentioned, stream-type chinook from the Columbia basin perform extensive offshore migrations in the ocean before returning to spawn, while ocean-type chinook are more commonly found in nearshore waters along the coast of North America. The ocean behaviour of Okanagan chinook has not been studied.

Okanagan chinook returning to spawn in the Okanagan basin must either enter the Okanagan River before river temperatures are too high or wait in the Columbia River for temperatures to decrease to tolerable levels. If they enter the Okanagan Basin before temperatures are high, they may briefly hold in the river upstream of Osoyoos Lake before falling back to the lake as temperatures rise. Tagging studies have shown that summer-run (ocean-type) chinook enter the Okanagan River from the Columbia River through July until the Okanagan River water temperature reaches 22 °C, with the peak of Okanagan chinook migration into the Okanagan River occurring immediately after temperatures drop below this level in late-August (Alexander et al., 1998).

Interspecific Interactions

Predation on Okanagan chinook juveniles and adults has been discussed above and will not be covered in this section.

Okanagan chinook fry in freshwater feed on terrestrial insects, crustacea, chironomids, corixids, caddisflies, mites, spiders, aphids, corethra larvae, and ants (Scott and Crossman, 1973; Healey, 1991). The macrozooplankton community in Osoyoos Lake, upon which rearing Okanagan chinook feed in part, is dominated by cyclopoids and diaptomids, with substantial populations of Daphnia and Bosmina (Wright 2002). Okanagan chinook have also recently been found to be piscivorous, feeding on sockeye salmon fry (ONAFD, unpublished data, 2005). The degree of competition for food between cohabiting species of salmon rearing in freshwater is not known, but is presumably influenced by the degree of habitat segregation among species (Healey, 1991).

Young chinook salmon in the marine environment eat mainly fish, particularly herring, with invertebrates like squids, amphipods, shrimp, euphausiids, and crab larvae comprising the remainder of their diet (Scott and Crossman, 1973; Healey, 1991). The relative abundance of fish in the stomach contents of commercially caught chinook salmon increases with the size of the fish. In general, invertebrate taxa form a relatively small component of the diet of adult chinook salmon in the ocean, although there is considerable seasonal and regional variation in diet composition (Healey, 1991). The peak feeding periods for chinook salmon in the ocean appear to be spring and summer, with spring being the best period in the southern part of their North American range and summer the best period along the coast of Canada (Healey, 1991).

Adaptability

Chinook salmon exhibit a high degree of life history variation, as evidenced by the high degree of variability in the duration of freshwater and saltwater rearing stages, age at maturation, spawning habitat requirements, and rearing habitat requirements. The existence of this degree of variation suggests a high degree of adaptability in the species (Healey, 1991). 

Chinook salmon have been produced in hatcheries in North America for more than a century, with hatchery outplants introduced to a wide range of rivers with and without native chinook salmon populations (Myers et al., 1998). The species has also been successfully introduced into highly novel environments, including the Laurentian Great Lakes system and New Zealand rivers. However, there is currently considerable concern about the apparently low fitness of many hatchery outplants and the impacts this may have on naturally spawning populations (Berejikian and Ford, 2003).