Richard G. Manzon

Anthropogenic activities can result in an unstable thermal environment which can alter key stages of fish development. Our study aimed to determine how incubation temperature influences the heat shock protein response and thermal tolerance in Yellow Perch, embryos and post-hatch fish. Yellow Perch are freshwater fish of economic, cultural, and recreational value. Embryos were incubated at 12, 15, and 18 ℃ and exposed to an acute 2 h thermal stress of + 3, 6, or 9 ℃ at key developmental stages (O1-O4, and 30 and 60 dph). Transcript abundance of key heat shock proteins was quantified and critical thermal maximum (CTmax) was determined for post-hatch fish. The magnitude and nature of the heat shock protein transcript abundance changed throughout development, with incubation temperature, and in response to both handling and thermal stress. Embryos at 12 and 18 ℃ were more sensitive to handling stress and significantly upregulated levels of hsp70-1, 70-2, and 47 with handling irrespective independent of thermal stress. By comparison, embryos at 15 ℃ did not increase hsp transcript levels with handling alone but they more readily increased hsp transcript abundance in response to thermal stress. Significant increased were detected between the + 6 or 9 ℃ treatments and the handling control in embryos incubated at 15 ⁰C. CTmax in post-hatch larvae (O4) from embryos incubated at 12 °C was higher than both 15 and 18 ⁰C groups. However, by 60 days post-hatch, there was no difference between 12 and 15 ⁰C incubations groups with both being significantly higher than Yellow Perch incubated at 18 ⁰C. Our research shows that early thermal environments can have a persistent influence on the nature of the heat shock protein response and its plasticity as well as whole animal thermal tolerance in fish 60 days post-hatch.

Temperate waters, such as the Great Lakes, are predicted to increase in temperature by 1°C every decade. Many poikilothermic fish thermoregulate behaviourally, moving to more suitable thermal environments. Embryos are incapable of locomotion and may be exposed to non-optimal temperatures during development. Previous work has suggested that temperature increases during embryogenesis can alter growth rates in fish. However, less is known of whether these early-life exposures to elevated temperatures can impart alterations to the phenotypic plasticity of performance traits, particularly in temperate species. We hypothesized that increased embryonic incubation temperature would diminish the larval performance of yellow perch (Perca flavescens), a critical cultural and ecological species of fish. We reared yellow perch embryos at 12, 15 or 18°C until hatching; after hatching, the temperature was raised to a common garden 18°C, their preferred post-hatch temperature. We assessed exploratory behaviour, metabolism (oxygen consumption) and cardiac performance throughout early development. At hatch, 12°C fish exhibited the greatest swimming activity, with 18°C fish consuming the least oxygen and possibly experiencing mitochondrial dysfunction. Cardiac development was more advanced at hatch in 18°C fish. Yet, warmer incubated fish had diminished movement and increased oxygen consumption at 20 days post-hatch, demonstrating long-term disruptions of increased temperature in the embryonic environment. Overall, elevations in rearing temperature may cause metabolic dysfunction and behavioural alterations, potentially impacting the survival of yellow perch.

Temperate waters, such as the Great Lakes, are predicted to increase by 1°C every decade. Poikilothermic fish thermoregulate behaviourally, moving to more suitable thermal environments. Embryos are incapable of locomotion and may be exposed to non-optimal temperatures during development. Increased temperature alters the normal development of the yellow perch ( Perca flavescens), however, whether altered incubation temperature influences the development of metabolism and function in these fish remains unknown. We hypothesized that increased embryonic incubation temperature would disturb cardiac and metabolic function and the behaviour of yellow perch larvae. We reared yellow perch embryos at 12°C, 15°C, or 18°C until hatching; after hatching, the temperature was raised to a common garden 18°C, their preferred post-hatch temperature. We assessed exploratory behaviour, metabolism (oxygen consumption), and cardiac performance throughout early development. At hatch, 12°C fish exhibited the greatest swimming activity, with 18°C fish consuming the least oxygen and possibly experiencing mitochondrial dysfunction. Cardiac development was more advanced at hatch in 18°C fish. Yet, warmer incubated fish had diminished movement and increased oxygen consumption at 20 days post-hatch, demonstrating long-term disruptions of increased temperature in the embryonic environment. Overall, elevations in rearing temperature may cause metabolic dysfunction and behavioural alterations, potentially impacting the survival of yellow perch.

Anthropogenic impacts can lead to increased temperatures in freshwater environments through thermal effluent and climate change. Thermal preference of aquatic organisms can be modulated by abiotic and biotic factors including environmental temperature. Whether increased temperature during embryogenesis can lead to long-term alterations in thermal preference has not been explicitly tested in native freshwater species. Lake (Coregonus clupeaformis) and round (Prosopium cylindraceum) whitefish were incubated at natural and elevated temperatures until hatching, following which, all groups were moved to common garden conditions (15°C) during the post-hatching stage. Temperature preference was determined at 8 months (Lake whitefish only) and 12 months of age (both species) using a shuttle box system. Round whitefish preferred a cooler temperature when incubated at 2 and 6°C compared with 0.5°C. Lake whitefish had similar temperature preferences regardless of age, weight and incubation temperature. These results reveal that temperature preference in freshwater fish can be programmed during early development, and that round whitefish may be more sensitive to incubation temperature. This study highlights the effects that small increases in temperature caused by anthropogenic impacts may have on cold-adapted freshwater fish.

Thermal preferenda are largely defined by optimal growth temperature for a species and describe the range of temperatures an organism will occupy when given a choice. Assays for thermal preferenda require at least 24 hours, which includes a long acclimation to the tank, limits throughput and thus impacts replication in the study. Three different behavioral assay experimental designs were tested to determine the effect of tank acclimation and trial length (12:12, 0:12, 2:2; hours of tank acclimation: behavioral trial) on the temperature preference of juvenile lake whitefish, using a shuttle box system. Average temperature preferences for the 12:12, 0:12, and 2:2 experimental designs were 16.10 ± 1.07 °C, 16.02 ± 1.56 °C, 16.12 ± 1.59°C respectively, with no significant differences between the experimental designs (p= 0.9337). Ultimately, length of acclimation time and trial length had no significant impact, suggesting that all designs were equally useful for studies of temperature preference.

Lake whitefish (Coregonus clupeaformis) embryos incubated at low temperatures have a longer incubation period and hatch at a significantly greater size than those incubated at warmer temperatures. We examined hatch timing and morphological characteristics for whitefish embryos reared under different constant and varying temperatures to determine whether the thermal dependence of hatching size reflects differences in their development stage. Our results show that lake whitefish embryos hatch at different temperature-dependent developmental stages, and this is the dominant factor affecting size-at-hatch. The term “heterograde” is proposed for the thermal dependence of hatching stage to differentiate it from hatching that occurs at a fixed developmental stage. A method to quantify this effect is given using a ratio that describes the difference in relative development at hatching between different viable constant incubation temperatures. Heterograde hatching is proposed as a possible mechanism to synchronize the timing of hatch to the break-up of winter ice cover despite variability in the date of spawning and in the onset of spring break-up.