A Proximate Cause of Brook Trout Loss Due to Climate Change: Biotic Interactions with Gill Lice and Brown Trout

Climate change may be the ultimate cause of Brook Trout (and Brown Trout) population decline or extirpation, but proximate causes may involve factors other than intolerance to high temperatures. We hypothesize that species interactions between Brook Trout, Brown Trout, and gill lice in the context of changing environmental conditions can lead to Brook Trout declines and possibly extirpation. Gill lice Salmincola edwardsii were first documented in Ash Creek, Wisconsin, USA in 2010 and became epizootic in 2012 coincident with changes in environmental conditions consistent with climate change. Gill lice infection prevalence was 95% and intensity averaged 5 and 23 (maximum 16 and 97) parasites per age 0 and age 1 or older trout; Brown Trout were not infected. Stock-recruitment data indicated a 77% decline in Brook Trout (but not Brown Trout) recruitment in 2012 compared to 2007-2011. Community dominance shifted to Brown Trout, and Brook Trout are at risk of extirpation. We present additional data from other sympatric trout populations in Wisconsin streams in which Brook Trout have declined, approaching extirpation, following recent epizootics of gill lice, which support the hypothesis that biotic interactions under stressful environmental and ultimately climatic conditions can be a proximate cause of Brook Trout loss.