Thermal Tolerance Limits and Cardiac Acclimation Potential of Sablefish (Anoplopoma fimbria) Embryos and Yolk-Sac Larvae Incubated at Different Temperatures




Schellenberg, Chrissy

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Average global ocean temperatures and the frequency and intensity of marine heat waves have been increasing over the last century. Temperature plays a critical role in defining the geographical range of the majority of marine species. Some species may respond to ocean warming trends by shifting their latitudinal and depth ranges, while others may be able to cope with changes in temperature through phenotypic plasticity and local adaptations. If a species is unable to shift its distribution or has limited thermal plasticity, it may face severe population declines or local extinction. Therefore, describing thermal tolerance limits is a useful tool for predicting how a given species will respond to ocean warming. Due to its commercial importance, sablefish (Anoplopoma fimbria) is a fish species of particular interest in British Columbia. Sablefish are semi-demersal and spawn along the continental slopes of the Pacific coast from California to Alaska. Their various life history stages occupy different depth strata and thus experience substantially different environments with respect to temperature (as well as salinity, oxygen, etc.). Adult sablefish spawn at depths that exceed 300 m and embryos sink to depths of ~1,000 m after fertilization. Embryos hatch into yolk-sac larvae until they become mobile at the post-yolk-sac larvae stage. The latter migrate to near-surface waters (<3 m) at which temperatures are approximately 12-15°C in the late spring. Heart rate is a temperature-dependent performance measure and has been used to gain insight into the thermal tolerance of many adult fishes. However, few studies have used this approach with the early life stages of fishes such as embryos and yolk-sac larvae (YSL). The purpose of this study was to assess whether sablefish embryos and YSL have the potential for cardiac acclimation by examining changes in their thermal tolerance limits when incubated at temperatures outside of what they experience in a natural setting (~5°C). Cardiac performance was assessed during an acute temperature challenge from 2.0° to 12.0°C in increments of 1.0°C (at a rate of 1°C 40 min-1) for individuals incubated at 3.0°C, 5.0°C (control), and 7.0°C. Embryos were video recorded at each 1.0°C increment and videos were viewed at a later date to determine heart rate at each temperature. This study attempted to use temperature breakpoint analysis, commonly used in studies of adults, on these early life stages to assess cardiac performance. It was hypothesized that sablefish embryos and yolk-sac larvae incubated at warmer temperatures would have a higher thermal tolerance than sablefish embryos and YSL incubated at colder temperatures, as seen in other fish species. There was some degree of thermal compensation of cardiac function with temperature in sablefish embryos and YSL as mean heart rate increased with incubation temperature throughout acute warming. YSL had consistently higher mean heart rate values at any given temperature of the acute temperature challenge when compared to embryos incubated at the same temperature. TAR is the temperature at which the heart first becomes arrhythmic is considered a sub-lethal index because the organism is expected to experience cardiac collapse soon after. TAR was reached for 100% of embryos incubated at 3.0°C at an average temperature of 8.6 ± 1.0°C. In contrast, only 18% and 33% of embryos incubated at 5.0° and 7.0°C exhibited arrhythmia (mean TAR were 9.0 ± 3.0 and 8.5± 1.5°C, respectively). The lower thermal limit for embryos incubated at 7.0°C was likely near 1.0°C, which was determined during preliminary testing. Neither the upper or lower limits were reached for YSL during the acute temperature challenge. No mortalities were observed during any acute temperature challenges. Overall percent mortality throughout the entirety of the experiment could not be determined due to limitations in the experimental setup and reduced staff working on this project due to COVID-19 safety protocols. This study is also the first to investigate whether transporting sablefish embryos from a hatchery to a research facility at different stages of development had an effect on their cardiac performance during acute warming. Embryos were transported in a cooler from the sablefish hatchery on Salt Spring Island to UVic via ferry and vehicular transport. Time of transportation did not significantly change the temperature at which heart rate reaches its maximum or TAR. There was also consistent overlap in mean heart rate ± standard error at each temperature of the acute temperature challenge between these two groups. Therefore, there appears to be no indication that transportation affected the heart rate response of sablefish embryos when incubated at the same temperature. However, future studies may want to confirm this by identifying and comparing other breakpoint temperatures that characterize physiological performance. Determining whether transportation has effects on cardiac performance may be of interest to other researchers who need to transport fish embryos from the field to the laboratory. Heart rate measurements during an acute temperature challenge of sablefish embryos and yolk-sac larvae (YSL) incubated at various temperatures provided initial insight to their overall success in a warming climate. Currently, it is projected that waters at depths of 1,000 m will warm on average by less than a degree by the end of the 21st century. The results of this study suggest that the early life stages of sablefish may not be exposed to critical temperatures in the near future, but future impacts on overall physiological decline remain unknown. The novel data presented here lay the groundwork for future researchers to continue to characterize the thermal tolerances of the early life stages of sablefish, and the likely response of this important species to ocean warming.  



thermal tolerance, cardiac acclimation, acute warming, heart rate, sablefish embryos and yolk-sac larvae