IMIS

Brown trout (Salmo trutta) were introduced to the Falkland Islands on several occasions during the 1940's and 1950's, mostly for recreational fishing. Since, there has been a marked decline in the native freshwater fish fauna, which consists of only three species of galaxiid fishes, endemic to the Southern Hemisphere (zebra trout Aplochiton zebra, Aplochiton taeniatus, and the Falklands minnow Galaxias maculatus; Vanhaecke et al. 2012). Given the threats to the long-term conservation of the native galaxiids, detailed knowledge about the life history, movement ecology of brown trout and their overlap and interactions with the native species is urgently needed. However, at present virtually nothing is known about the extent and seasonality of brown trout movements in the Falkland Islands, or how these can impact on native galaxiids. Two presence/absence surveys, conducted 10 and 20 years ago respectively, provided valuable synoptic snapshots of the distribution of brown trout and galaxiids in the Falklands. McDowall et al. (2001) used electrofishing to survey the islands in 1999 and showed that brown trout had invaded most rivers with the exception of those in the southwest region of West Falkland (Lafonia). Elsewhere, brown trout showed a high degree of spatial segregation with zebra trout in particular, which may indicate competitive displacement and/or predation of zebra trout by brown trout, and may thus explain the decline of Aplochiton sp. Since the 1950s. At the time of the survey, most zebra trout were concentrated within small refuge areas uninvaded by brown trout. However, the tendency of brown trout to display anadromy in the Falklands, makes it likely that they could eventually colonize all the accessible rivers. The risk of native galaxiid extinction is therefore high. With this in mind, our study aims to answer two main questions about the movement ecology of brown trout in the Falklands: (1) what is the current distribution, abundance and population structure of brown trout? And (2) what are the impacts of brown trout on native galaxiids? These questions will be answered by employing a variety of techniques including environmental DNA to non-destructively assess the distribution and abundance of brown trout and native galaxiids in various watershed, in conjunction with electrofishing, which will allow us to estimate population (or river stock) abundance as well as assess the health and reproductive status of individuals and stocks. Electrofishing will also provide us with an opportunity to capture individual brown trout for acoustic tagging from different watersheds, thus allowing us to meet our first aim by tracking individual movements and dispersal patterns, among watersheds and between East and West Falklands. We will also take fin samples from individuals for genotyping, thus obtaining data on the genetic diversity and gene flow between watersheds of brown trout and native species. Stable isotope analysis will also be conducted on fin samples from brown trout and native galaxiids to determine the levels of trophic overlap and predation between species.