Brook Trout habitat selection and movements in fragmented streams at high temperatures

Abstract

Objective: Stream temperatures are increasing in freshwater systems as the climate changes. When summer temperatures approach -thermal limits, fish temporarily occupy coolwater refuges. We tested whether selection for cool locations was influenced by mean temperatures within sites (sites were 0.7-2.5 km long), conditional on other habitat variables (depth and cover), and restricted by the spacing of instream barriers that might limit movement and opportunities to select habitat. Methods: We studied eastern Brook Trout Salvelinus fontinalis, whose distribution is temperature-limited and shrinking with climate change. We selected four sites in Rhode Island, United States, because they varied in mean summer temperature and the distance between barriers. Using radiotelemetry, we tracked 60 trout from June to August 2021, and habitat selection was quantified by comparing nine habitat variables at used (n = 321) and available (n = 677) locations. Results: Used locations were similar in temperature to those available (mean = 18.1°C and 21.2°C) within the two cooler sites, whereas trout in the two warmer sites selected cooler locations (used = 19.1°C and 20.5°C, available = 24.0°C and 24.4°C). Trout generally selected deeper locations, but selection was weaker at the two warm sites, suggesting a trade-off with temperature selection. Selection for proximity to cover (mostly woody debris and undercut banks) was strong at all sites, suggesting that this is prioritized. Fifteen trout emigrated 1-8 km downstream into areas that were cooler, or similar, in temperature to their original site. Seven emigrants traversed downstream barriers while relocating to cooler locations, so their ability to do so was uncompromised by barriers. Conclusions: Trout thus displayed nuanced responses to summer warming that were conditional on mean temperatures within sites. Restoration activities to reduce water temperatures and improve river connectivity should improve fishes' ability to locate coolwater refuges and should be designed so that other habitat requirements are not compromised.