Light and temperature are necessary conditions for migratory fish. The assessment of fish physiology and behavior is important for identifying fish welfare, but also for the assessment of the optimal setting of recirculating aquaculture systems (RASs). This study aimed to explore the interactive effect of photoperiod and temperature on steelhead trout culture. Four treatments were set up with specific settings were as follows: a LP-LT group treated with 16L:8D and 12°C, a LP-HT group treated with 16L:8D and 16°C, a SP-LT group treated with 12L:12D and 12°C, and a SP-HT group treated with 12L:12D and 16°C. Growth performance, behavioral and physiological parameters were measured. Two indexes, locomotor activity and social interaction were used for behavioral analysis, and the results were applied to interpret the behavioral responses to the photoperiod and temperature stimulation in juveniles. The growth performances were significantly lower in treatments LP-LT and SP-LT. The treatment LP-HT had significantly higher growth performance than the other treatments, but no significant differences were noted in survival rate and coefficient of variation. The results of fish behavior indicated that the movement of juveniles should be primarily monitored at high temperatures or long photoperiods, and the state parameters should be primarily monitored at low temperatures or short photoperiods. The results of the physiological parameters showed that the recovery time from stress varied among different treatments. After 60 days of the experiment, superoxide dismutase and alanine aminotransferase dropped back to their initial level. The results of Na+-K+-ATPase showed that although the combined effect of photoperiod and temperature could advance the time of smoltification, it may result in poorer salt tolerance. Our findings underscore the importance of the interaction of photoperiod and temperature on steelhead trout culture. The outcome could provide guidance for the development of effective aquaculture systems.

Recent studies have found that high temperatures cause oxidative stress and even mass mortality in Pacific oysters (Crassostrea gigas). The role of γ-aminobutyric acid (GABA) in improving antioxidative defense in aquatic animals is increasingly of interest. In the present study, the oxidative stress of Pacific oysters to high-temperature stress was examined, and the regulation of GABA on the antioxidative defense was further investigated. Following 6 h of exposure to 28 °C seawater, a significant increase in the mRNA expression levels of nuclear factor-E2-related factor 2 (Nrf2), superoxide dismutase (SOD), and catalase (CAT), as well as the activities of SOD and CAT, was observed in the gill, compared to those at 0 h. An increase of glutamate decarboxylase (GAD), GABA receptor (GABA_AR-α and GABA_BR-B) mRNA levels, and GABA contents were also detected after 28 °C exposure compared to those at 0 h. Furthermore, the activities and mRNA expression levels of SOD and CAT were significantly upregulated after GABA treatment, while decreased after either GAD inhibitor or GABA receptor inhibitor treatment under high-temperature stress. Meanwhile, the enhanced effects of GABA on antioxidant enzyme activities were reduced when Nrf2 was inhibited by ML385, accompanied by an increase in MDA content. After high-temperature stress, compared with the GABA treatment group, the activities and mRNA expression levels of SOD and CAT were significantly upregulated by GSK-3β inhibitor treatment. Meanwhile, the elevation of antioxidant enzyme activities by GABA was attenuated by the AKT inhibitor treatment. Collectively, GABA first activated GABA receptors under high-temperature stress and then increased the activities of SOD and CAT and reduced MDA content by AKT/GSK-3β and Nrf2 pathways to protect the oysters against oxidative damage upon stress. The present results offer new insights for understanding the regulation mechanisms of antioxidative defense by the neuroendocrine system in molluscs.

The Neptunea cumingii (N. cumingii) fishing industry has long relied on expensive and perishable skate (Raja porosa) meat as bait. The unknown chemical attraction mechanism has hindered the development of artificial alternatives. This study employed untargeted metabolomics to analyze the chemical composition of skate meat and combined quantitative behavioral analysis to identify four key attractant compounds. These compounds were taurine, glutamate (Glu), inositol, and lactate. A standardized behavioral assessment system was established using the three parameters of response time, displacement distance, and movement velocity. This system enabled precise quantification of attraction efficacy. Concentration-gradient experiments determined the optimal concentration for all four compounds as 0.1 M. Taurine exhibited the strongest single-compound activity. It reduced response time by 50% and increased displacement distance by 164.5%. The mixture of four compounds at 0.1 M produced significant synergistic effects. The mixture achieved a comprehensive score of 93.6. This score approached that of natural skate meat at 94.8. All behavioral parameters improved by over 69% compared to the best single compound. These findings reveal the key attractant components in skate meat. They provide a scientific basis for developing efficient and stable artificial attractants. This research holds substantial value for promoting sustainable development of the N. cumingii fishing industry.

High temperatures induce oxidative stress and the production of a large amount of malondialdehyde (MDA) in the Pacific oyster Crassostrea gigas, and they can even lead to mass mortality. Aldehyde dehydrogenase (ALDH) degrades MDA and is attracting increasing attention for its role in enhancing antioxidant defense capacity. This study identified 14 ALDH family members in the oyster genome. Among them, CgALDH6A1 harbored a conserved ALDH_F6_MMSDH domain (known to catalyze the oxidation of aliphatic and aromatic aldehydes) and was likely involved in the high-temperature stress response through the detoxification of accumulated toxic aldehydes. In the gills, CgALDH6A1 had significantly higher mRNA expression than other tissues, with a significant increase at 12 h under 28 °C high-temperature stress. During the outdoor aquaculture period, the mRNA transcripts of CgALDH6A1 in the gills exhibited a significant increase from June to October. After the expression of CgALDH6A1 was inhibited by RNAi, the MDA content in the gills increased significantly (1.31-fold, p < 0.01), while the activities of superoxide dismutase (SOD) (0.93-fold, p < 0.05) and catalase (CAT) (0.45-fold, p < 0.001) and total antioxidant capacity (T-AOC) (0.54-fold, p < 0.01) decreased significantly under high-temperature stress. Meanwhile, the gill tissue was observed to be disorganized with obvious filament swelling. After the oysters were treated with CgALDH6A1 agonist (Alda-1), the MDA content (0.59-fold, p < 0.001) in the gills decreased significantly, while the activities of SOD (1.33-fold, p < 0.001), CAT (1.81-fold, p < 0.001), and T-AOC (1.79-fold, p < 0.01) all increased significantly 48 h after high-temperature stress. However, no obvious morphological changes were observed in the gills. These results demonstrate that CgALDH6A1 plays a key role in regulating the oxidative stress response by degrading MDA under high-temperature stress and plays a cooperative role with the antioxidant system in alleviating oxidative stress under high-temperature stress.