Effect of environmental enrichment on the stress response of juvenile black rockfish Sebastes schlegelii

Abstract

The stress response is one of the most important aspects of fish welfare in aquaculture. However, relatively few studies focusing on the effect of environmental enrichment on the fish stress response have been conducted, and their limited results were considerably contradictory. The present study aimed to investigate whether the enrichment type and amount had significant effects on the basal stress level and physiological and behavioral responses to acute stress of juvenile black rockfish Sebastes schlegelii. Fish were reared for eight weeks in environments with two enrichment types and three enrichment amounts (i.e., no environmental enrichment control (C), low-amount plant enrichment (PL), medium-amount plant enrichment (PM), high-amount plant enrichment (PH), low-amount structure enrichment (SL), medium-amount structure enrichment (SM), and high-amount structure enrichment (SH)), and subsequently, they were successively subjected to two common acute stressors (i.e., air exposure and confinement) and sampled at 0, 0.5, 1, 3 and 6 h after stress. In general, the plant enrichment groups experienced significantly higher basal stress (indicated by cortisol level and opercular beat rate) than the structure enrichment groups. Meanwhile, no enrichment control group and low-amount enrichment groups experienced significantly higher basal stress than the other two enrichment amount groups. After being subjected to acute stress, the peak cortisol levels of the high-amount enrichment groups were significantly higher than that of the other three enrichment amount groups. The PM, PH and SL fish recovered to basal stress levels at 1 h, the C and PL fish recovered at 3 h, and the SM and SH fish recovered at 6 h after the stress. Moreover, their basal cortisol levels showed strong correlations with peak cortisol levels and recovery time from stress. The correlation between cortisol level and opercular beat rate was also strong. Taken together, these results provide the first evidence to show that enrichment type and amount had significant effects on the fish stress response and might have important applications in fish husbandry and welfare. Based on these results, we suggest that it may be optimal for fish basal stress level and development of an adaptive capacity to provide a medium-amount (approximately 50% floor space coverage) of mixed enrichment in aquaculture.