Effects of environmental enrichment on the welfare of juvenile black rockfish Sebastes schlegelii: Growth, behavior and physiology

Abstract

Environmental enrichment is a promising way to enhance fish welfare in aquaculture. However, the observed effects of enrichment on fish growth, behavior and physiology vary widely among studies, and few studies have focused on the quantification of enrichment. The present study aimed to investigate whether enrichment type and level significantly affect the growth performance, aggressive behavior, cortisol level and brain monoaminergic activities of juveniles of the black rockfish Sebastes schlegelii. Juveniles were reared for eight weeks under different combinations of enrichment type and enrichment level: no environmental enrichment (C), low-level plant enrichment (PL), medium-level plant enrichment (PM), high-level plant enrichment (PH), low-level structure enrichment (SL), medium-level structure enrichment (SM) and high-level structure enrichment (SH). Subsequently, behavioral and physiological parameters were determined. In general, the growth performance, feed intake and food conversion efficiency of C, PM, PH and SL fish were relatively higher than those of fish from the other treatments. Low-level plant treatment produced the highest levels of aggressive behavior, cortisol and brain serotonergic system activity, whereas the medium- and high-level structure treatments yielded the lowest levels of these stress-related behavioral and physiological indicators. C fish had significantly higher stress levels than PM, PH and SL fish. No significant difference among treatments was observed in the condition factor, coefficient of weight variation, locomotor activity, growth hormone level or brain dopaminergic system activity. Taken together, these results provide the first evidence to show that the type and level of environmental enrichment have interaction effects on fish growth performance, behavioral phenotype (especially aggressive behavior) and stress-related physiological processes. Since the control fish had significantly higher stress levels than the fish exposed to medium- and high-level plant enrichment and low-level structure enrichment, we suggest that enriching approximately 50% basal area coverage with objects might be optimal for enhancing fish welfare. Moreover, a possible conceptual model is presented to interpret the effects of enrichment on fish welfare.