Brown trout crowding out native brook trout
Native brook trout, Salvelinus fontinalis, populations could be at risk as a result of the introduction of Brown trout, Salmo trutta, according to a new U.S. Geological Survey study. Both species are valuable sport fish that coexist in many parts of the world due to stocking introductions.
USGS researchers found that, in New York State, direct interactions between the two species, such as competition for food, have minor effects on diminishing brook trout populations compared to human-caused habitat disturbances. However, repeated, disproportionate stocking of brown trout in brook trout habitats could drastically decrease brook trout numbers.
"There is great potential for brown trout stocking to reduce native brook trout populations," said James McKenna, USGS scientist and lead author of the study. "But brown trout aren't necessarily causing the current brook trout declines, and managers may be able to develop sustainable scenarios to support both fisheries."
The USGS study found that human-induced degradation (from dams and roads, among other causes) of the habitats of both species could affect the populations of either. However, because brook trout do better in forested watersheds, whereas brown trout can thrive in more agricultural environments, degraded watersheds and/or the elimination of forests may affect brook more than brown trout.
McKenna's team used artificial neural network models, regression, principal components analysis, and simulation to evaluate the effects of Brown Trout, environmental conditions, and stocking on the distribution of Brook Trout in the center of their native range. They found evidence for the decline of Brook Trout in the presence of Brown Trout across many watersheds; 22% of sampled reaches where both species were expected to occur contained only Brown Trout. However, a model of the direct relationship between Brook Trout and Brown Trout abundance explained less than 1% of data variation. Ordination showed extensive overlaps of Brook and Brown Trout habitat conditions, with only small components of the hypervolume (multidimensional space) being distinctive.
Simulation results indicated that direct interactions and habitat conditions were relatively minor factors compared with the effects of repeated stocking of Brown Trout into Brook Trout habitat. Intensive annual stocking of Brown Trout could eliminate resident Brook Trout in less than a decade. Ecological differences, harvest behavior, and other habitat changes can exacerbate Brook Trout losses. Custom stocking scenarios with Brown Trout introductions at relatively low proportions of resident Brook Trout populations may be able to sustain healthy populations of both species within their present range.
Read more at USGS.
Fisherman image via shutterstock.