Our aim was to study the relationship between the sex-determining genes, sox9a and cyp19a, and water temperature in Gadus chalcogrammus. We assessed the sex ratio based on the expression levels of sox9a and cyp19a at different water temperatures (5, 8, 11, and 14 °C) and at different stages of walleye pollock development (embryos, larvae, and juveniles). Next, we used immature walleye pollock to assess sox9a expression in males and cyp19a and vitellogenin (VTG) expression in females at different water temperatures. Males expressed sox9a in the gonadal tissues, while females expressed cyp19a in the gonadal tissues and VTG in the blood plasma. In the first experiment, cyp19a expression was higher at 5 °C and 8 °C, and sox9a expression was higher at 11 and 14 °C. In the second experiment, sox9a expression remained relatively stable, but cyp19a expression decreased with increasing temperature, decreasing significantly after 14 °C. Similar patterns were also observed for VTG expression. These results indicate that lower water temperatures increase cyp19a expression, which increases the female ratio. Higher water temperatures increase sox9a expression, which increases the male ratio. Therefore, this study highlights the potential of the sex-determining genes and the influence of water temperature.

The climate crisis and growing petroleum demand have put the health of aquatic animals in jeopardy. Fish are sensitive to chemical pollutants in aquatic environments, such as polycyclic aromatic hydrocarbons, dioxins, and dibenzofurans. This study investigated the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and β-naphthoflavone (β-NF) exposure on histopathological and immunohistochemical features and expression patterns of cytochrome P450 1 (CYP1) family genes in black rockfish, Sebastes schlegelii. Histopathological alterations in the liver included congested central vein, sinusoidal dilatation, lymphocyte infiltration, and severe vacuolation within hepatocytes. The most prevalent alterations in TCDD-exposed kidneys were glomerular enlargement, narrowing of tubular lumen, melanomacrophage centers (MMCs), and necrosis. Moreover, CYP1A immunostaining was strong in renal tubules following TCDD exposure. All CYP1 family genes (CYP1A, CYP1B, CYP1C1, and CYP1C2) were significantly increased in the gills, liver, and kidney exposed to TCDD. Similarly, a significant increase of CYP1A mRNA expression in the kidney was observed upon exposure to TCDD (30.9-folds) and β-NF (25.5-folds) compared with that of the control group (p < 0.05). TCDD and β-NF exposure exerted more adverse effects on the kidney than the liver, and TCDD had a greater in vivo toxic effect than β-NF. The combined histopathological, immunohistochemical, and molecular alternations may be helpful for diagnosing chemical contaminant exposure in S. schlegelii.

The optimal dietary vitamin C (VC) levels for walleye pollock (Gadus chalcogrammus) remain undefined. This study aimed to assess the effect of dietary VC levels on the growth performance and biochemical parameters of grower walleye pollock and determine the optimal VC level for their diet. Six experimental diets (VC0, VC1, VC3, VC5, VC7, and VC10) with VC levels of 3.24, 21.92, 63.31, 101.42, 145.46, and 202.51 mg kg⁻¹ diet, respectively, were fed to fish (initial mean weight: 173.5 ± 0.31 g) for 8 weeks. At the end of the feeding trial, fish fed the VC7 and VC10 diets exhibited significantly higher growth (final body weight, weight gain, and specific growth rate) and improved feed utilization (feed efficiency and protein efficiency ratio) compared with fish fed the VC0 diet (p < 0.05). The VC3–VC10 diets significantly reduced plasma superoxide dismutase (SOD) activity (p < 0.05). Compared with the VC0 group, fish fed the VC7 and VC10 diets showed significantly elevated growth hormone and insulin-like growth factor-1 levels in plasma (p < 0.05). In conclusion, dietary VC supplementation in walleye pollock improved growth performance and SOD activity. Moreover, broken-line analysis on weight gain indicated that the optimal dietary VC level for grower walleye pollock was approximately 156.42 mg kg⁻¹ diet.

Starry flounder (Platichthys stellatus) is a commercially important cold-water fish. Our aim was to investigate the effects of fluctuating water temperature on flounders after periods of starvation and feeding. Fish were divided into starvation and feeding groups. The water temperature was increased stepwise in experiment 1; more focused variations, based on the results of experiment 1, were studied in experiment 2. At temperatures ≤27 °C, there was no significant difference observed in survival. At 28 °C, mortality increased, survival was lower (21%) in the starvation group than in the feeding group (46%), and weight loss was the highest (15%) in the starvation group. In experiment 2, survival was ≥86%, and there was no significant difference between the starvation/feeding groups. However, when the water temperature was increased to 27 °C after being decreased to 12 °C, weight loss was the highest (11%). Glucose, cortisol, superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH) levels increased with increasing water temperature, and then gradually decreased. Glutamic pyruvic transaminase (GPT)/glutamic oxaloacetic transaminase (GOT) levels showed large variations among individuals. Triglyceride, cholesterol, and protein levels gradually decreased with long-term starvation. Survival was not affected by water temperature drop ≤27 °C after starvation/feeding. These results indicate that 27 °C is the upper limit of tolerable water temperature for the survival of starry flounders. Therefore, aquaculture farms should ensure maintaining water temperatures at ≤27 °C during high-temperature periods.

The walleye pollock Gadus chalcogrammus is an important commercial species in Japan whose larvae and eggs may be negatively affected by the cold water mass from the coastal Oyashio current that is present in the spawning ground of the Japanese Pacific stock of this species. Therefore, we investigated egg and larval specific density, larval mortality, and behavioral response to temperature change during the ontogenetic development of the walleye pollock to understand the effect of this cold surface water mass (<1.5 °C). Egg and larval specific density varied during development but were lower than the corresponding values from the coastal Oyashio waters. Within our study temperature range (0.3–10.0 °C), the number of days to 50% mortality (D₅₀) was high at 3.1 °C. Below and above this temperature, the D₅₀ showed a decreasing trend. Regarding larval response, at 1.5°C and 5.0°C, newly hatched larvae occurred abundantly in the surface layer, irrespective of the surface and rearing temperatures. When these larvae were released into a thermally stratified water column (surface: 1.5 °C, bottom: 5.0 °C), larvae reared at 5.0 °C with the mouth open and yolk sac completely absorbed moved to the lower layers. However, larvae reared at 1.5 °C remained in the surface layer. These results suggest that the cold water mass could negatively affect larval survival and may limit the escape ability of larvae from unfavorable cold conditions.

We aimed to determine the optimal conditions for the mass culture of rotifers, which can be used as feed for cold-water fish species at low temperatures. The growth and specific growth rates (SGRs) of rotifers were assessed considering water temperature, salinity, density, dissolved oxygen (DO) levels, and the amount of Chlorella supplied as feed. The growth of rotifers was higher at 15 °C than at 10 °C and at salinities of ~11–17 psu. Initial inoculation densities of 500 and 700 individuals/mL resulted in the highest rotifer density, and SGR was highest at 100 individuals/mL. DO concentration did not significantly affect the growth and SGRs of rotifers. Enrichment with fatty acids is important to supplement the diet of cold-water fish species. Highly unsaturated fatty acid content increased with enrichment time to 14.04 ± 0.86% at 12 h and 15.58 ± 2.20% at 24 h. Thus, the optimal conditions for rotifer mass culture are a water temperature of 15 °C, salinity of 11–17 psu, initial inoculation density of 300–500 individuals/mL, DO concentration of 8 mg/L or more, and Chlorella supply at 7.5 × 10¹² cells/mL. Therefore, the present study suggests optimal culture conditions of rotifers at low temperatures for breeding cold-water fish species.