Differences in embryo survivorship among populations of brook trout ( Salvelinus fontinalis ) under variable winter thermal regimes

The vulnerability of fish populations to climate change is driven primarily by impacts on their most vulnerable life stages. The earliest life stages may, in many cases, be more thermally sensitive than juvenile and adult fish, as developing embryos cannot behaviourally thermoregulate. We quantified the survivorship of brook trout ( Salvelinus fontinalis ) embryos under a range of winter thermal regimes. In the laboratory, embryos from four populations with varying amounts of native or hatchery-introgressed ancestry were reared under four simulated winter thermal regimes relevant to the central portion of the species’ range: ambient stream temperatures, ambient + 2 °C, ambient with a simulated winter snowmelt event, and ambient followed by an early rise to spring temperatures. Embryo survival (hatching success) differed significantly among populations across all thermal regimes, with the pure native population having the lowest survival. No consistent differences in survivorship across thermal regimes were detected, although native populations exhibited more among-family variability in response to thermal stress. Our study does not align closely with others that found reduced embryo survival under thermal stress, but concurs with observations of substantial intraspecific variation in thermal performance in coldwater fishes. Our results suggest that brook trout embryos may be somewhat resilient in the face of warmer and more variable incubation conditions, but it is likely that their resiliency varies among populations. Future experiments that include more extreme and variable conditions to clarify environmental limits of early life stages of brook trout would be useful for conservation planning.