Wild juvenile Atlantic salmon (Salmo salar) offer insights into movement patterns of territorial freshwater fishes in relation to high temperature and proximity to thermal refuges

When exposed to increased water temperatures, salmonids abandon territories and relocate to areas of cool water (thermal refuges). Using juvenile Atlantic salmon (Salmo salar) as a model species, our study, conducted over two seasons (2009 and 2010) with 636 salmon parr tagged by Passive Integrated Transponder (PIT) across 9 km of the Little Southwest Miramichi river, investigated the effects of high temperature stress on their movement. We aimed to determine the extent of their travel to thermal refuges, primary direction, duration within these refuges, and post-event redistribution. In 2009, despite temperatures peaking at 26.1˚C, no significant parr aggregations were noted. However, 2010 experienced notable changes: temperatures exceeding 27.3˚C for four days prompted 32.7% of the parr to form aggregations, with movements significantly greater than in 2009 (p < 0.001), peaking at 7.4 km. Most migrations were localized within specific river reaches. By late fall, 92.3% of parr returned to or near their original tagging locations. These observations underscore the importance of thermal refuges and the adaptive strategies juvenile salmon employ during extreme temperature events.