Fish Production in Streams With and Without Natural Broan Trout Populations

Brown trout (Salmo trutta) have been introduced into the waterways throughout the United States, including Pennsylvania, since the 1800’s. They may have limited interactions with native fish species in regions where they do not reproduce successfully, but where they do concerns have arisen regarding the impact they have on native species. Are brown trout having a negative effect on the native fish? If so, could they be outcompeting the natives to the point of localized extirpation? This project compared fish communities, densities, biomass and production in two similar stretches of stream in the White Clay Creek, one known to hold brown trout (the East Branch) and the other without (the Middle Branch). Fish in each branch were collected in June and October 2009 using backpack electrofishing equipment to determine species composition and abundance, population densities and community diversity. Fish were aged using scales and further examination revealed biomass and production of the species present. The fish communities within the branches were stable between June and October (Jaccards index = 0.75 for both branches), but differed between branches (Jaccards index = 0.64 in June and 0.58 in October). The stream without brown trout showed much lower density, biomass, and production of most species, which went counter to our hypothesis that fish would show lower levels of these factors in the presence of brown trout. Only the common shiner and longnose dace showed effects in each of these categories while favoring the branch without brown trout over the branch with brown trout. Although the results do show a possible negative correlation between these species and the brown trout, our study design did not allow us to rule out other factors. Also, the fewer number of fish in the Middle Branch as compared to the East Branch leads us to believe that something may be wreaking havoc with the natural balance of this section (e.g. land-use changes, environmental stressors or climatic factors). Continued research regarding brown trout interactions on the East Branch and stream quality of the Middle Branch is highly recommended.