An evaluation of trout community population change following two years of mechanical removal of non-native brown trout: towards a better understanding of restoration options for imperiled native fishes

Authors: Phaedra Budy, Gary P. Thiede, W. Carl Saunders

Year: 2010 (xlii)

Abstract

The global loss of biodiversity associated with the spread of invasive species has the ability to alter ecosystem function and presents a ubiquitous scientific and conservation challenge. The biotic resistance hypothesis is one hypothesis invoked to explain the invasion success (or failure) of exotic species introduced into new habitat of exotic species suggesting that invasion is a function of the diversity, density, and/or distribution of the community of native species, and their potential for ‘strong’ interactions with the exotic species (e.g., competition, predation). Towards a broader goal of gaining a better understanding of native trout restoration options, we are testing the hypothesis of biotic resistance in the Logan River using a large-scale, manipulative field study including 1-pass removal and monitoring of 7 km of a high-quality tributary stream, formally dominated nearly exclusively by high densities of brown trout (> 4000/ km). Here we report on changes in population abundance, distribution, and structure in response to a approximately 60% removal of non-native brown trout (Salmo trutta) abundance. In 2009, we removed 4,855 brown trout from 7 km of stream using single pass electrofishing followed by more than 5,500 with repeat sampling the following year. Although the abundance of brown trout actually increased after removal the second year, the increase was largely due to a dramatic increase in young of eth year and juvenile recruitment success. The number of large, adult brown trout was significantly lower the second year, and the native trout also responded with the first successful recruitment event observed in over ten years. The large increase in brown trout recruitment from year 1 to year 2 indicates the brown trout population was limited by density dependence. In addition, this response demonstrates this tributary habitat supports trout recruitment in general, with few if any other limiting factors, and thus suggests it should provide excellent spawning and rearing grounds for native cutthroat trout (Oncorhynchus clarkii utah) restoration. The potential role of biotic resistance has important implications for native fish conservation and restoration - it may be adequate to simply shift the balance to the native fish, but not eliminate non-natives, to prevent further invasion and establishment of non-native trout.