FISH ECOLOGY

Chinook Salmon

Chinook salmon are found along the Pacific Coast from the Ventura River in southern California to Point Hope, Alaska (Wydoski and Whitney 1979). In Washington, chinook salmon spawn in streams in the Columbia River Basin, Puget Sound, and coastal drainages (Wydoski and Whitney 1979). In the Lake Washington watershed, fall-run chinook salmon migrate through Lake Washington to reach spawning grounds in the Cedar and Sammamish River systems and in other Lake Washington tributaries. Washington Department of Fish and Wildlife (WDFW) hatchery staff allow returning progeny of the Issaquah Hatchery to migrate beyond the hatchery weir only if egg-take goals have been achieved. Occasional beach spawning within Lake Washington has been observed (Roberson 1967; Fresh, pers. comm., 28 March 2000).

Adults begin migrating into fresh water in June, peaking in August, and spawn from mid-August to mid-December (Myers et al. 1998). After spawning, females guard redds for up to three weeks before dying; males attempt to fertilize other redds before dying (U.S. Federal Register, 9 March 1998). Chinook salmon eggs hatch after 90 to 150 days, depending on water temperature (Wydoski and Whitney 1979).

Life History Strategies

The Puget Sound ESU exhibits an "ocean-type" life history (Myers et al. 1998). In general, ocean-type fish move relatively rapidly through fresh water into coastal or estuarine rearing areas, compared to their stream-type counterparts (U.S. Federal Register, 9 March 1998; Wydoski and Whitney 1979). The ocean-type chinook in the Lake Washington basin typically begin their downstream migration as sub-yearlings (Myers et al. 1998). Most chinook emigrate as fry after emerging from the gravel (Myers et al. 1998), reaching Lake Washington in early January to March (Fresh, pers. comm., 2 August 1999). A second wave of juvenile fingerlings enters Lake Washington in May and June (Fresh, pers. comm., 2 August 1999). Chinook fry are an average of 40 mm (1.6 in.) in length when they enter Lake Washington; chinook fingerlings are an average of 100 mm (4 in.) in length when they enter Lake Washington (Fresh, pers. comm., 2 August 1999).

In addition to the contribution of natural spawners, WDFW's Issaquah Creek hatchery has an annual production goal of releasing 2 million age-0+ chinook each May into Issaquah Creek, many of which enter Lake Washington via the Sammamish River. On average, that goal has been met over the last five years (Mahovlich, pers. comm., 7 June 2000). The University of Washington hatchery has an annual production goal, which is consistently met, of releasing 180,000 chinook smolts each May (Tetrick, pers. comm., 10 July 2000). The majority of chinook smolts leave Lake Washington in May and June (Fresh, pers. comm., 9 September 1999).

A study by Reimers (1971) in Sixes River, Oregon, demonstrated that juvenile migration timing within the "ocean-type" designation occurs as a continuum rather than a discreet event. Evidence that most juvenile chinook begin entering the lake in early January and are leaving Lake Washington as smolts by early July, suggests that juvenile chinook in the lake are exhibiting a "type-2" life history (as per Reimers 1971). However, the WDFW Cedar River fry trap that has provided most of the data on migration timing is typically operational only through the end of June or early July. Sampling at the mouth of the Cedar River has found that small numbers of juvenile chinook continue entering Lake Washington as late as 29 July (Fresh, pers. comm., 9 September 1999). Outmigrating chinook smolts have been observed at the Ballard locks in late August (Fresh, pers. comm., 7 June 2000).

Yearling and older chinook (monthly mean fork lengths ranging from 256-323 mm) were captured in littoral gill nets (2-8 m deep) in all regions of Lake Washington from January through October in 1984-1985 (Beauchamp, Univ. of Washington, unpubl. data). Tabor and Chan (1996) captured two juvenile chinook yearlings (234 and 280 mm fork length) in south Lake Washington in March 1995. Although it is not known whether these yearlings reared in the lake or in a tributary, their large size is typical of lake-rearing fish. The appearance of small numbers of age-1+ and age-2+ chinook juveniles in Lake Washington provides additional evidence that extended freshwater rearing occurs in the Lake Washington system (Fresh, pers. comm., 9 September 1999).

Haw and Buckley (1962) reported extended freshwater rearing of juvenile chinook in Lakes Washington and Sammamish, with age-I+, and -2+ smolts representing 21 percent and 12 percent respectively of sampled returning adults. The majority of age-0+ chinook juveniles in the Lake Washington watershed leave the lake by mid-summer; 66 percent of the returning adults sampled by Haw and Buckley (1962) had been age-0+ smolts. Reimers (1971) found relatively few juvenile fall chinook migrating as yearlings in Sixes River, Oregon, where yearling migrants represented only 3.1 percent of returning adults. Data from the Lake Washington Ecological Studies indicate that resident chinook up to adult size are in the lake at all times of the year; it is not clear whether these fish go to sea eventually or continue rearing in the lake until spawning (Warner, pers. comm., 7 July 2000).

Diet and Distribution

In Lake Washington, juvenile chinook are distributed along shorelines (Fresh, pers. comm., 18 November 1999). Sampling of both the limnetic and littoral zones of Lake Washington has shown that from early February through late May, young-of-the-year chinook occupy the littoral zone exclusively (Warner and Fresh 1999). They feed primarily on aquatic insects (chironomid pupae) (Fresh, pers. comm., 18 November 2000) and terrestrial insects (Wydoski and Whitney 1979; Tabor and Chan 1996). Rondorf et al. (1990) found that in a Columbia River reservoir, the diet of juvenile chinook salmon consisted primarily of zooplankton and terrestrial insects; in free-flowing river sections the diet consisted mostly of aquatic insects. Juvenile chinook adapt to local prey abundance by modifying their selection of prey items (Rondorf et al. 1990).

Chinook juveniles, predominantly large individuals, begin appearing in limnetic sampling gear in late May and June in Lake Washington (Fresh, pers. comm., 9 September 1999). Increasing use of the limnetic zone may be an ontogenetic response, a response to increasing temperatures in the littoral zone, or merely represent the capture of outmigrating smolts (Fresh, pers. comm., 9 September 1999). Hamilton et al. (1970) observed an ontogenetic shift with increasing fish body size from littoral foraging on aquatic and terrestrial insects, to limnetic foraging on zooplankton, by coho salmon in a reservoir (see below).

Reimers (1971) found that in the Sixes River, chinook fry (~40 mm) occupied shallow water and were closely associated with shoreline features such as logs and debris, but larger fish (~55 mm) occupied the entire channel. Others have noted a similar transition from an affinity for shoreline structure and cover for newly emerged chinook fry, to a progressively offshore distribution as fish size increases (Lister and Genoe 1970; Weitkamp and Campbell 1980; Roper et al. 1994). In the lower Willamette River, Ward et al. (1994) reported that young-of-the-year chinook caught in vertical gill nets were a mean distance of 31 meters from shore. However, all of these studies were in rivers or riverine portions of estuaries, where current velocity and channel width influenced habitat selection. Even though the fish in Reimers' (1971) study were utilizing the entire estuary, they were still occupying habitat that did not exceed 5 meters in depth.

Temperature

The distribution and residence time of juvenile chinook in Lake Washington may be influenced by temperature. Bjornn and Reiser (1991) reported the preferred temperature for chinook as 12 to 14° C, and temperatures from 23 to 25° C could be lethal and were actively avoided. Offshore temperatures at a depth of 1 meter in Lake Washington typically exceed 15° C from mid-May through mid-October, and exceed 20° C from mid-July into September, with maxima generally near 25° C in early August (http://dnr.metrokc.gov/wlr/waterres/lakes/wash.htm). Nearshore summer surface-temperatures may exceed those offshore. Despite high temperatures, chinook can be found in the littoral zone during summer. During 1984, chinook were captured throughout the summer in littoral gill nets (3-10 m deep) and in offshore vertical gill nets from mid-July to mid-August at depths of 12 to 18 meters, which corresponded with the thermocline (Beauchamp, Univ. of Washington, unpubl. data). Chinook smolts (and adults) are often found near the surface in water above 18° C (Warner, pers. comm., 7 July 2000).

Behavioral thermoregulation by salmonids is common. Roper et al. (1994) suggested that high temperature (20° C) in the lower reaches of Jackson Creek, Oregon, caused the emigration of age-0+ chinook. Biro (1998) reported that some young-of-year brook trout (Salvelinus fontinalis) in an Ontario lake remained in the littoral zone during periods of high water temperature (23-27° C), but they held and defended positions at cold groundwater seeps. Given the propensity for littoral foraging by juvenile chinook, and that summer temperatures in the littoral zone may be undesirable or potentially lethal, chinook may either leave Lake Washington in mid-summer, limit the timing of their activity, or limit their distribution to cooler areas of the lake. The energetic consequences of such avoidance behavior could reduce the growth rate of juvenile chinook, potentially reducing marine survival. Smolt size has been found to positively correlate with marine survival (Quinn and Peterson 1996). Observations of juvenile chinook in the littoral zone of Lake Washington decline in early to mid-July; this phenomenon could possibly be a behavioral response to high temperatures, but could also be due to smolts leaving the lake (Fresh, pers. comm. 18 November 1999). An avoidance of the littoral zone by chinook during summer would segregate them from shore-based sampling efforts.

Both returning adult and juvenile chinook in Lake Sammamish and Lake Union must also contend with anoxic conditions in the hypolimnion from July through October (http://dnr.metrokc.gov/wlr/waterres/lakes/thermo.htm). High temperatures in the epilimnion restrict chinook to depths below 5 to 10 meters, while anoxic conditions below depths of 15 to 20 meters prevent chinook use, thus concentrating them in the relatively narrow (5-10 m) metalimnion (http://dnr.metrokc.gov/wlr/waterres/lakes/ thermo.htm). These physical restrictions of chinook distribution limit juvenile foraging opportunities, and expose juvenile fish to predators occupying habitat in the metalimnion. In addition, these physical conditions are a stress to holding adults that could cause pre-spawning mortality and reduced egg survival for those adults that survive to spawn.