Smallmouth Bass Predation on Hatchery and Wild Salmonids in the Yakima River, Washington

Abstract

Predation on hatchery and wild salmonids by nonnative smallmouth bass Micropterus dolomieu was examined in the lower Yakima River, Washington, in March through June of 1998 to 2001. Smallmouth bass were sampled weekly using a drift-boat electrofisher in two sections of the river. Average abundance estimates of smallmouth bass 150 mm fork length or longer for the 4 years sampled ranged from 3,347 in late March to 19,438 in early June. Abundance estimates from 1998 to 2001 all showed a similar trend of increasing abundance throughout the spring. Salmonids were identified in the guts of smallmouth bass throughout the sampling period and were most prevalent during the month of May. Ocean-type Chinook salmon Oncorhynchus tshawytscha constituted 47% of all the fish species identified in the gut samples. Smallmouth bass were estimated to have consumed an average of 200,405 salmonids yearly from March 22 to June 16 in 1998–2001; of these, only 3,176 were yearling salmonids (primarily spring Chinook salmon). Smallmouth bass predation on all yearling salmonids never exceeded 0.6% of the annual production of hatchery and wild fish combined. We estimated that as much as 85% of the ocean-type Chinook salmon consumed by smallmouth bass in a given year were of natural origin. Estimated smallmouth bass consumption of hatchery ocean-type Chinook salmon has only comprised up to 4% of production in a single year. Our estimates of consumption on ocean-type Chinook salmon are likely to be underestimates because we did not sample throughout the entire rearing and emigration period of these fish. Our results indicate that smallmouth bass can have negative impacts on ocean-type Chinook salmon, particularly those that are naturally produced, which are generally smaller and available longer than hatchery fish.