Impacts of signal crayfish on stream fishes

Invasive species cause biodiversity and economic loss globally. Invasive crayfish have a wide range of effects as a result of their high densities and biomass, feeding at multiple trophic levels, aggressive competition for shelter and ecosystem engineering. In Britain, the invasive signal crayfish Pacifastacus leniusculus has displaced the native white-clawed crayfish Austropotamobius pallipes from much of its former range and occupied previously crayfishfree habitats. Signal crayfish could affect fish populations by preying upon eggs, juvenile and adult fish and competing for food and shelter. This thesis assesses the extent to which signal crayfish can affect densities of the commercially important brown trout Salmo trutta, and a species of benthic fish of conservation concern, the European bullhead Cottus gobio, in upland streams. This thesis also investigates the extent to which signal crayfish predation could affect salmonid egg survival through a combination of controlled field and laboratory experiments. Electric fishing was used to estimate fish densities from tributaries of the upper River Tees, north east England. Model selection was then used to determine which factors most affected bullhead and 0+ (less than one year old) brown trout densities. Signal crayfish density was negatively related to both bullhead density and the density of 0+ brown trout. Substrate composition and flow variables and were also selected as predictors of 0+ brown trout and bullhead densities. Small (10-16.2 mm carapace length (CL)) crayfish and fine material apparently reduced buried sea trout S. trutta egg survival in controlled field experiments. However, in laboratory studies, only larger crayfish were found to significantly reduce the survival of salmon S. salar eggs, and this reduction only affected eggs on the surface of the gravel and not buried eggs. These results suggest that signal crayfish pose a threat to both salmonid and bullhead populations, and that, where possible, the sites chosen for reintroduction or habitat enhancement for salmonids and sculpins should be free of invasive crayfish. Evidence for signal crayfish predation upon buried salmonid eggs remains equivocal, although this study provides definite evidence that large signal crayfish will prey upon exposed Atlantic salmon eggs. Both egg predation, and other mechanisms by which signal crayfish may affect salmonid and bullhead populations, are worthy of further investigation. Structural equation models should be used to further investigate relationships between the densities of a range of fish and crayfish species.