Disentangling effects of habitat on salmonid abundance in thermal refuges while accounting for imperfect detection

Abstract

Thermal refuges provide critical habitat for cold‐water‐dependent salmonids in marginal riverscapes during extreme heat events globally. Studies of thermal refuge use by salmonids are complicated by short‐term changes in weather conditions during heat waves, intermittent fish use, and the use of different analytical endpoints. We passively surveyed cold‐water‐dependent brown, rainbow, and brook trout (hereafter “trout”) thermoregulating in several thermal refuges along the Housatonic River, Connecticut, USA, during a two‐week heatwave using underwater action cameras (UACs). Using these data, we assessed the influence of various short‐term riverscape conditions on the abundance of trout within thermal refuges using a zero‐inflated Poisson, time‐stratified N‐mixture model in a Bayesian framework that accounts for imperfect detection. We detected at least one trout per sampling occasion, and estimated abundances summed across all thermal refuges ranged from 63.99 (95% credible interval [CRI] = 55.0–79.0) to 182.04 (167.0–199.0). We estimated higher trout abundances in larger thermal refuges located in low‐gradient river sections, and refuges that were deeper and cooler than the Housatonic River during the sampling occasion (all β estimates >0.0 and did not overlap zero). We also found evidence that less variable and cooler temperature regimes in the nearby Housatonic River led to higher estimated trout abundances within thermal refuges (all β estimates <0.0 and did not overlap zero). Last, we found that trout detection probabilities within thermal refuges were moderate (range = 0.521–0.634) when using UAC surveys and that the video image brightness negatively influenced detection (αbright = −0.193, 95% CRI = −0.330 to −0.058). To aid in trout population persistence in dynamic, marginal riverscapes as the climate changes, focusing on modifiable refuge attributes (e.g., depth, cold‐water plume area), and strategically targeting refuges in riverscape segments beneficial to trout may be tactics that could enhance conservation strategies globally.