Effects of Predation by Nonnative Smallmouth Bass on Native Salmonid Prey: The Role of Predator and Prey Size

Abstract

The size of predators that consume the most fish and the size of prey fish that are the most vulnerable to predation are important factors to consider when assessing the predation risks to valued prey fish such as Chinook salmon Oncorhynchus tshawytscha in the Pacific Northwest. We found that native salmonids' risk of predation by nonnative smallmouth bass Micropterus dolomieu in the lower Yakima River, Washington, generally decreased with increasing predator and prey size. Among smallmouth bass, those with fork lengths (FLs) ranging from 150 to 199 mm consumed 42.9% of the salmonids consumed. Overall, most of the salmonids were consumed by smallmouth bass smaller than 250 mm (69.6%), and the vast majority were consumed by smallmouth bass smaller than 300 mm (83.6%). Small smallmouth bass were much more abundant than large smallmouth bass, and the proportion of smallmouth bass that contained salmonid prey items in the gut decreased with increasing predator size. Salmonids that were 100 mm or larger were rarely consumed by smallmouth bass. We found that the maximum relative length of salmonids ([prey length/predator length] × 100) consumed by smallmouth bass was 56.6%. In addition, the relative length of salmonid prey decreased with increasing smallmouth bass size. Smallmouth bass generally ate salmonids at lengths that were less than 50% of predator capacity and that averaged 25% of predator length. The introduction of smallmouth bass to the Yakima River appears to have changed the size-based predation risk dynamics in the lower river, which were historically dominated by northern pikeminnow Ptychocheilus oregonensis.