Effect of water temperature on walleye pollock (Gadus chalcogrammus) embryos, larvae and juveniles: Survival, HSP70 expression, and physiological responses

Abstract

This study investigated the survival rates, physiological responses, and the induction of heat shock protein 70 (HSP70) expression in size-specific walleye pollock (Gadus chalcogrammus) at different water temperatures. Four groups consisting of walleye pollocks in various lifecycle stages—embryos, larvae, small juveniles, and large juveniles—were used. Embryos had the highest hatching rate at 5 °C, which decreased as the water temperature increased. Larval survival rate was the highest at 5 °C. The median lethal temperatures (LT50) for small and large juveniles were 19.1 and 18.9 °C for 7 d, respectively. HSP70 was cloned using the pET21b + vector and expressed in Escherichia coli to analyze HSP70 expression in walleye pollock. Polyclonal antibodies were obtained from BALB/c mice after purification on a His-Trap HP column and used for protein analysis using enzyme-linked immunosorbent assay. HSP70 mRNA and proteins were expressed in similar patterns at both 5 °C and 8 °C, with a drastic increase in HSP70 expression induction at ≥11 °C. The gill tissues of small and large juveniles had the highest HSP70 mRNA and protein levels. Physiological responses of large juveniles' plasma showed that glutamic pyruvic transaminase, glutamic oxaloacetic transaminase, and glucose levels were highest at 14 °C, whereas total cholesterol and triglyceride levels were lowest. Plasma superoxide dismutase and cortisol levels were highest at 14 °C, whereas growth hormone and insulin-like growth factor-1 levels were highest at 8 °C. These results suggest that elevated water temperatures increased stress in the early lifecycle stages (embryos and larvae) of walleye pollock. HSP70 and stress-related indicator levels also increased with an increase in water temperature. Subsequently, the optimal water temperature for walleye pollock growth was determined to be below 8 °C.