Thermal performance curves for aerobic scope and specific dynamic action in a sexually dimorphic piscivore: implications for a warming climate.

Digestion can make up a substantial proportion of animal energy budgets, yet our understanding of how it varies with sex, body mass, and ration size is limited. A warming climate may have consequences on animal growth and feeding dynamics that will differentially impact individuals in their ability to efficiently acquire and assimilate meals. Many species, such as walleye (Sander vitreus), exhibit sexual size dimorphism (SSD), whereby one sex is larger than the other, suggesting sex-differences in energy acquisition and/or expenditure. Here we present the first thorough estimates of specific dynamic action (SDA) in adult walleye using intermittent-flow respirometry. We fed male (n=14) and female (n=9) walleye two ration sizes; 2% and 4% of individual body weight, over a range of temperatures from 2 - 20°C. SDA was shorter in duration and reached higher peak rates of oxygen consumption with increasing temperatures. Peak SDA increased with ration size and decreased with body mass. The proportion of digestible energy lost to SDA (i.e., the SDA coefficient) was consistent at 6% and was unrelated to temperature, body mass, sex, or ration size. Our findings suggest that sex has a negligible role in shaping SDA, nor is SDA a contributor to SSD for this species. Standard and maximum metabolic rates were similar between sexes but maximum metabolic rate decreased drastically with body mass. Large fish, which are important for population growth due to reproductive hyperallometry, may therefore face a bioenergetic disadvantage and struggle most to perform optimally in future, warmer waters.

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 (CTmax) 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 CTmax. 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.

CTmax in brook trout (Salvelinus fontinalis) embryos shows an acclimation response to developmental temperatures but is more variable than in later life stages.

Critical thermal maximum (CTmax ) is widely used to measure upper thermal tolerance in fish but is rarely examined in embryos. Upper thermal limits generally depend on an individual's thermal history, which molds plasticity. We examined how thermal acclimation affects thermal tolerance of brook trout (Salvelinus fontinalis) embryos using a novel method to assess CTmax in embryos incubated under three thermal regimes. Warm acclimation was associated with an increase in embryonic upper thermal tolerance. However, CTmax variability was markedly higher than is typical for juvenile or adult salmonids.

Short-term acclimation dynamics in a coldwater fish.

Critical thermal maximum (CTmax) is widely used for measuring thermal tolerance but the strong effect of acclimation on CTmax is a likely source of variation within and among studies/species that makes comparisons more difficult. There have been surprisingly few studies focused on quantifying how quickly acclimation occurs or that combine temperature and duration effects. We studied the effects of absolute temperature difference and duration of acclimation on CTmax of brook trout (Salvelinus fontinalis), a well-studied species in the thermal biology literature, under laboratory conditions to determine how each of the two factors and their combined effects influence critical thermal maximum. Using an ecologically-relevant range of temperatures and testing CTmax multiple times between one and 30 days, we found that both temperature and duration of acclimation had strong effects on CTmax. As predicted, fish that were exposed to warmer temperatures longer had increased CTmax, but full acclimation (i.e., a plateau in CTmax) did not occur by day 30. Therefore, our study provides useful context for thermal biologists by demonstrating that the CTmax of fish can continue to acclimate to a new temperature for at least 30 days. We recommend that this be considered in future studies measuring thermal tolerance that intend to have their organisms fully acclimated to a given temperature. Our results also support using detailed thermal acclimation information to reduce uncertainty caused by local or seasonal acclimation effects and to improve the use of CTmax data for fundamental research and conservation planning.