Brown trout in the Falkland Islands: invasion ecology, population structure and genetic diversity

Biological invasions are important causes of biodiversity loss, particularly in remote islands. Non-native salmonids, such as brown trout (Salmo trutta), have been widely introduced throughout the Southern Hemisphere, impacting endangered native fauna, particularly galaxiid fishes, through predation and competition. However, due to their importance for sport fishing and aquaculture, they are often protected, and any attempts to curtail their impacts are generally met with limited support, which poses a conservation conundrum. The best prospect of protecting native galaxiids is to predict where and how salmonids might disperse. This thesis aims to answer three main questions about brown trout in the Falkland Islands in order to provide resource managers with information to facilitate conservation planning to minimize the impacts of brown trout on native galaxiids. (1) determine the distribution of invasive brown trout and native zebra trout, Aplochiton zebra and Aplochiton taeniatus. (2) estimate patterns of movement and (3) assess the population structuring and estimate levels of gene flow between different rivers and populations of brown trout in the Falklands. To meet these aims, I used state-of-the-art methods, including SNP genotyping, stable isotope analysis, acoustic tagging, and environmental DNA (eDNA) analysis. The results of this thesis suggest that establishment success (calculated as the proportion of historical introductions where brown trout became established) was ~88% and that brown trout are continuing to spread from their original sites of introduction. The native Aplochiton species have disappeared from most rivers invaded by brown trout. Four genetically distinct clusters of brown trout were identified, with high levels of gene flow indicating widespread dispersal of brown trout across the Falkland Islands. Without strong containment, brown trout are predicted to invaded nearly all suitable freshwater habitats in the Falklands within the next ~70 years, which might put native galaxiids at a high risk of extinction.