Analysis of Asymmetric Interactions between Nonnative Brown Trout, Salmo trutta, and Native Brook Char, Salvenlinus fontinalis, as a Mechanism of Habitat Fragmentation in the Gunpowder River Watershed of Central Maryland
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Brook char, Salvelinus fontinalis, is the only salmonid that is native to the Mid-Atlantic and South Eastern United States. Within the state of Maryland, it is estimated that brook char have been expatriated from over 60% of the species historic range (Heft et al. 2006). Similar population declines have been observed throughout the Central and Southern Appalachian regions (Argent et al. 2018; Kanno et al. 2016). Brook char require cold water temperatures and near pristine water conditions for wild populations to persist. Recent studies have shown that brook char populations are negatively affected by the presence of the non-native salmonids brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss (Hitt et al. 2016; Ohlund et al. 2008; Malmros 2006). Within the Gunpowder the watershed of Central Maryland, wild populations of brook char and brown trout exist together in the same sections of habitat. The objective of this study was to determine if interference competition from non-native brown trout is having a significant negative impact on brook char populations within the Gunpowder River watershed. Fish assemblage data were collected by the Maryland Biological Stream Survey (MBSS) and the Maryland Department of Natural Resources Freshwater Fisheries by backpack electrofishing between the years of 1983 and 2017. These data were used to calculate population densities for brook char and brown trout. Fish population densities were compared to the archived MBSS physical and biological habitat data using the unpaired t-test to analyze variance between fish densities and physical habitat variables. Regression analysis was used to determine if significant trends in population densities and distribution had occurred over time. It was found the brown trout are moving progressively upstream into brook char habitat and are contributing to reduced brook char densities and increased population fragmentation and isolation.