Landscape factors that shape a slow and persistent aquatic invasion: brown trout in Newfoundland 1883-2010

Aim We investigated watershed-scale abiotic environmental factors associated with population establishment of one of the 'world's 100 worst alien invaders' on a temperate Atlantic island. Within the context of the conservation implications, we aimed to quantify (1) the early history and demographics (numbers and origins) of human-mediated brown trout (Salmo trutta) introductions, (2) the current distribution of established populations, and (3) the watershed-scale environmental factors that may resist or facilitate trout establishment. Location Island of Newfoundland, Canada. Methods We combined field sampling with historical and contemporary records from literature to assemble a presence-absence and physical habitat database for 312 watersheds on Newfoundland. Probability of watershed establishment was modelled with general additive ANCOVA models to control for nonlinear effects of propagule pressure (i.e. the distance to and number of invasion foci within a biologically relevant range) and model performance based on AIC. Results Between 1883 and 1906, 16 watersheds were introduced with brown trout from the Howietoun Hatchery, near Stirling, Scotland. Since that time, populations have established in 51 additional watersheds at an estimated rate of spread of 4 km per year. We did not detect any obvious abiotic barriers to resist trout establishment, but showed that for a given amount of propagule pressure that relatively large and productive watersheds were most likely to be established. Main conclusions Brown trout have successfully invaded and established populations in watersheds of Newfoundland and are currently slowly expanding on the island. Populations are more likely to establish in relatively large and productive watersheds, thereby supporting predictions of island biogeography theory. However, we suggest that all watersheds in Newfoundland are potentially susceptible to successful brown trout invasion and that abiotic factors alone are unlikely to act sufficiently as barriers to population establishment.