Adaptive Phenotypic Plasticity and the Successful Invasion of New Habitats by Non-Native Brown Trout

Global translocations of salmonid species represent excellent case studies for examining the patterns and processes of colonization and population establishment. A key insight from these opportunistic experiments is that anadromous populations more often fail to establish compared to non-anadromous populations; a pattern that has implications for both restoration planning and invasion biology.  Here we explore ecological and evolutionary factors that may underlie successful anadromous invasions using the introduction of non-native brown trout (Salmo trutta) to the island of Newfoundland. This case study is particularly informative because, 1) the contemporary patterns of population distribution and phenotypic differentiation are free of any hatchery effects (i.e. there has been no hatchery propagation for over 100 years) and 2) because anadromy by brown trout apparently facilitated, rather than hindered, the successful biological invasion. We report on the results of a three-population reciprocal transplant experiment designed to simultaneously test three questions: i) have populations of brown trout locally adapted to their novel environments within 130 years? ii) are successful colonists a non-random pre-adapted subset of potential founders? and iii) does phenotypic plasticity (an individual’s ability to express different phenotypes) facilitate establishment?

Offspring of wild-born parents from three populations were grown under common environmental conditions until large enough to PIT tag and released in equal proportions into their home stream and two others.  Additionally, we tagged equal numbers of wild-born fish of the same age within each ‘home’ stream as a complimentary approach to investigate local adaptation. Prior to release a suite of phenotypic traits, such as body size and shape, were measured from each individual. Extensive electrofishing surveys were conducted to recapture surviving individuals 2.5 months and 1 year later. Twenty-three percent of the original 1200 fish were recaptured and results indicate evidence of local adaptation, with regards to survival and growth, in at least one of the three streams. Analyses also explore ‘favored-founder’ effects and the role of adaptive phenotypic plasticity on population persistence in novel environments.