Effects of Introduced Fishes on Wild Juvenile Coho Salmon Using Three Shallow Western Washington Lakes

Pacific salmon declines have been blamed on hydropower, overfishing, ocean conditions, and land-use practices; however, less is known about introduced fish impacts. Most of the hundreds of lakes and ponds in the Pacific Northwest contain introduced fish and many of these water bodies are also important for salmon production, especially coho salmon. Over two years, we examined predation impacts of ten common introduced fishes (brown bullhead catfish Ameiurus nebulosus, black crappie Pomoxis nigromaculatus, bluegill Lepomis macrochirus, golden shiner Notemigonus crysoleucas, green sunfish Lepomis cyanellus, largemouth bass Micropterus salmoides, pumpkinseed Lepomis gibbosus, rainbow trout Oncorhynchus mykiss, warmouth Lepomis gulosus, and yellow perch Perca flavescens) and two native fishes (cutthroat trout Oncorhynchus clarki, and prickly sculpin Cottus asper) on wild juvenile coho salmon Oncorhynchus kisutch in three shallow western Washington lakes, all located in different watersheds. Of these species, largemouth bass were responsible for an average of 98% of the predation on coho salmon in all lakes, but total impact to each run varied among lakes and years. Very few coho salmon were eaten by black crappie, brown bullhead catfish, cutthroat trout, prickly sculpin, and yellow perch, while other species were not observed to eat coho salmon. Juvenile coho salmon growth in all lakes was higher than in nearby streams. Therefore, food competition between coho salmon and introduced fishes in lakes was probably not limiting coho salmon populations. Largemouth bass are widespread, present in 85% of lowland warmwater public-access lakes of Washington (n=421). Future research would help identify impact of largemouth bass predation across the region, and prioritize lakes where impacts are most severe. Nevertheless, attempts to transplant or increase largemouth bass numbers in lakes important to coho salmon would be counterproductive to coho salmon enhancement efforts.