Shorezone Structures and Salmonid Predators

Many of the predators that juvenile chinook are exposed to are active, cruising hunters (i.e., other salmonids, many piscivorous birds, northern pikeminnow). Smallmouth and largemouth bass generally utilize ambush or habituation foraging strategies (Hobson 1979). Fayram and Sibley (2000) determined that smallmouth bass in Lake Washington occupied littoral home ranges that radiated 100 to 200 meters from the focal point and generally did not extend below 8-meter depths. Because of this propensity for ambush foraging and shoreline orientation, bass are expected to benefit from artificial structures placed in the littoral zone. Yellow perch utilize "non-structural" areas (Paxton and Stevenson 1979). Recent evidence of the role of cutthroat trout as the major predator of juvenile salmonids in Lake Washington (Lake Washington Sockeye Studies Interim Workshop 2000), raises the question of whether cutthroat trout might also benefit from shorezone development, regardless of their foraging method. Shoreline development could potentially increase the rate of predation on juvenile chinook by several principal means: 1) reducing prey refuge habitat by modifying the structure of the shoreline (critical in all predator-prey interactions, but especially critical for prey of mobile predators such as cutthroat trout); 2) providing concealment structures for ambush predators such as bass and sculpin; 3) creating enough structure to reduce bass home range sizes; 4) providing artificial lighting that allows for around-the-clock foraging by predators; 5) potentially increasing migration routes for smolts and rearing fry, thus increasing exposure to predators; and 6) potentially increasing the bass population by increasing the amount of potential spawning habitat.

Bass are generalist piscivores, eating salmonids when their distributions overlap. In a study of the impacts of introduced fish in lakes of the northeastern United States, Whittier and Kincaid (1999) observed that native brook trout populations had been nearly extirpated, or had to be maintained by stocking, in lakes where smallmouth bass had been introduced. Salmonids were a greater proportion of the diet of largemouth bass than of smallmouth bass in Lake Sammamish (Pflug 1981). Tabor et al. (2000) observed the opposite in the Lake Washington Ship Canal. Pflug (1981) proposed that bass exploitation of the seasonal abundance of outmigrating salmonids was responsible for the unusually high growth rate of bass in Lake Sammamish. An analogous situation exists in coastal Massachusetts lakes where exploitation of anadromous herring contributes to the production of "trophy" bass (Yako et al. 2000).

Much of the bass predation on salmonids in the Lake Washington system corresponds with the out-migration of smolts in the spring and summer (Stein 1970; Pflug 1981; Pflug and Pauley 1984; Fayram and Sibley 2000; Tabor et al. 2000). This phenomenon has also been observed in the Columbia River (Gray and Rondorf 1986; Vigg et al. 1991; Poe et al. 1991; Zimmerman 1999). In the mid-Columbia River, ocean-type chinook fry were the only identified salmonids found in smallmouth bass stomachs by Tabor et al. (1993). The Columbia River studies indicated that salmonids were only seasonally abundant in bass diets, and that other fish species, crayfish, and other invertebrates provided the bulk of bass prey items. The Lake Washington Ship Canal may be an exception due to the tight bottleneck that it imposes on outmigrating salmon smolts. Preliminary evidence from a study by Tabor et al. (2000) indicates large populations of both largemouth and smallmouth bass in the Ship Canal coinciding with the outmigration of salmon smolts. Analysis of stomach contents indicated that age-0+ chinook were the predominant salmonid prey item, constituting approximately 50 percent of the diet of smallmouth bass; preliminary consumption rates for April - July 1999 were 0.3 smolts/stomach for smallmouth bass and 0.1 smolts/stomach for largemouth bass (Tabor et al. 2000).As discussed above, patterns of bass predation in the Lake Washington Ship Canal are similar to those in the Columbia River. Preliminary analysis of Ship Canal smallmouth bass stomach-contents from May through July 1999 found that salmonids represented approximately 60 percent of the diet of bass from 200 to 249 mm, and 50 percent of the diet in bass 250 mm and larger (Tabor et al. 2000). The large numbers of bass in the Ship Canal, and their high rates of consumption of salmon smolts (primarily chinook but also coho and sockeye), pose a substantial threat to chinook salmon migrating from the Lake Washington system; however, actual losses due to predation and the proportion of the smolt population these losses represent have not been computed yet (Tabor et al. 2000). Consumption estimates from limited preliminary sampling in 1997 by the Muckleshoot Indian Tribe indicate that as many as 100,000 chinook smolts could have been consumed in a 90-day migration period (Warner, pers. comm., 7 July 2000).

The distributional overlap of chinook with bass in the Lake Washington system is more prolonged than in the mid-Columbia River due to a temperature regime that favors bass. Water temperatures in Lake Washington typically exceed 10 C by mid-April, and 15 C by June ( Thus, the peak of chinook outmigration from Lake Washington (June) corresponds with increasing bass activity and metabolic demands. Furthermore, the outmigration of chinook juveniles continues into late August, prolonging the distributional overlap of chinook and bass. Predation rates in the Ship Canal have likely increased in response to climate change and its effects on predator metabolism. Increasing water temperatures in Lake Union from 1973 to 1996 have produced an estimated increase in predation rates of 18 percent, 16 percent, 13 percent and 9 percent for smallmouth bass, rainbow trout, northern pikeminnow, and largemouth bass, respectively (Stock et al. 2000). Thus, with projected increases in global temperatures over time, predation rates on juvenile salmonids will likely continue to increase even if predator populations and other habitat variables remain constant.