Effects of simple shelters on growth performance and welfare of rainbow trout juveniles

Environmental enrichment is a promising strategy to improve the welfare of fish in captivity. However, the utilization of enrichment in aquaculture is still infrequent, maybe because there is a paucity of knowledge about how its effects depend on factors such as fish species, developmental stage, social environment or the type and extent of enrichment. In this study, we evaluated the effects of physical enrichment on the welfare of rainbow trout juveniles, by exposing them to simple plastic screen shelters. Juveniles of approx. 15 g were introduced to two types of submerged shelters: full screens (Full) or partial screens (Semi), and fish welfare was assessed and compared to a control group (without shelters) by evaluating fish growth and condition, extent of external lesions, and the neuroendocrine responses to acute and repeated stress. During the eleven-week experimental period, the fish in the sheltered units gradually developed a clear shelter-seeking behavior when exposed to external disturbance. Fish growth, condition factor and mortality were not affected by shelter presence. The presence of full shelters had a modest protective effect on fin damage: both pectoral fin- and total fin damage scores were reduced (> 10%) in this group with respect to the control group; the percentage of fish with severe damage in the pectoral fin was reduced in the Full group with respect to the Control (63% vs 82%). Partial shelters had no significant effect on fin damage scores, when compared to the control group. The presence of shelters did not affect the general level of stress upon standardized acute or repeated stressors. However, fish used to the presence of shelters showed a more intense startling response when exposed to stressors that forced them to abandon the shelter protection. Altogether, this study shows potential for shelters to be used as a welfare-promoting strategy in trout farming, but further research is needed to optimize the shelter type and design and the proper timing for its application.