Effects of different levels of environmental enrichment on the sheltering behaviors, brain development and cortisol levels of black rockfish Sebastes schlegelii

Environmental enrichment is a promising way to increase the fitness and survival of fish released for stock enhancement and biological conservation programs. However, the effects of enrichment in different studies on the fish behavior, physiology and neural development were considerably different. The present study aimed to investigate whether enrichment levels had significant effects on the shelter-seeking behavior, risk-taking behavior, brain compartments size, growth performance, stress level and immune function of black rockfish Sebastes schlegelii. Larval fish were reared for four (post-larval fish) and seven (juvenile fish) weeks in environments with three enrichment levels (i.e., no environmental enrichment items (C), few environmental enrichment items (EF) and many environmental enrichment items (EM)); subsequently, the behavioral and physiological parameters were determined. In general, juvenile fish reared with few enrichment items had significantly better specific growth rate and final body weight, but higher whole-body cortisol levels than EM and C fish. Meanwhile, the muscle acid phosphatase (ACP), alkaline phosphatase (AKP) and total superoxide dismutase (T-SOD) activities of EM juveniles were better than those of either C or EF fish generally. Enrichment did not affect the two types of sheltering behaviors or the relative cerebellum volumes during the post-larval stage, while the conditions changed during the juvenile stage. The proportion to seek shelter successfully of EF juveniles was significantly higher compared to that of other treatments, and few enrichment items also reduced maladaptive risk-taking behavior, i.e., EF juveniles took a longer time to leave the shelters. The relative cerebellum area of EF juveniles was significantly larger than that of either C or EM fish, while no significant differences were detected in other sub-brain areas. These results indicate that enrichment levels indeed had significant effects on the fish behavioral phenotype, brain plasticity and physiological condition, and in our experimental design, the fish reared with few enrichment items showed more natural sheltering behaviors and brain phenotypes and EF treatment could be beneficial for the fitness of released fish, however, more adaptive behaviors (e.g., foraging behavior, competitive ability) and field survival rate need to be investigated to validate our conclusions.