Thermal stressors during embryo incubation have limited ontogenic carryover effects in brook trout

Winter climate is changing rapidly in northern latitudes, and these temperature events have effects on salmonid thermal biology. Stressors during winter egg incubation could reduce hatching success and physiological performance of fall-spawning fishes. Here we quantified the potential for ontogenic carryover effects from embryonic thermal stress in multiple wild and hatchery-origin populations of brook trout (Salvelinus fontinalis), a temperate ectotherm native to northeastern North America. Fertilized eggs from four populations were incubated over the winter in the laboratory in four differing thermal regimes: ambient stream-fed water, chronic warming (+2 °C), ambient with a mid-winter cold-shock, and short-term warming late during embryogenesis (to stimulate an early spring). We examined body size and upper thermal tolerance at the embryonic, fry (10 weeks post-hatch and 27-30 weeks post-hatch) and gravid adult (age 2+) life stages (overall N = 1482). In a separate experiment, we exposed developing embryos to acute seven-day heat stress events immediately following fertilization and at the eyed-egg stage, and then assessed upper thermal tolerance (CT_max) 37 weeks post-hatch. In all cases, fish were raised in common garden conditions after hatch (i.e., same temperatures). Our thermal treatments during incubation had effects that varied by life stage, with incubation temperature and life stage both affecting body size and thermal tolerance. Embryos incubated in warmer treatment groups had higher thermal tolerance; there was no effect of the mid-winter melt event on embryo CT_max. Ten weeks after hatch, fry from the ambient and cold-shock treatment groups had higher and less variable thermal tolerance than did the warmer treatment groups. At 27-30 post-hatch and beyond, differences in thermal tolerance among treatment groups were negligible. Collectively, our study suggests that brook trout only exhibit short-term carryover effects from thermal stressors during embryo incubation, with no lasting effects on phenotype beyond the first few months after hatch.