The use of environmental enrichment has been found to be beneficial in fish rearing performance and behavior. This study evaluated the effects of partial overhead cover and rearing tank substrate on the hatchery performance of juvenile rainbow trout Oncorhynchus mykiss. Four treatments were used in circular tanks: open tanks with concrete blocks, partially-covered tanks with blocks, open tanks with no concrete blocks, and partially-covered tanks with no blocks. There was no significant effect of concrete block substrate use on rainbow trout weight gain or feed conversion ratio. However, fish reared with overhead cover had a significant weight gain increase and a significant feed conversion ratio decrease compared to uncovered tanks. There was no significant interaction between cover and substrate. The use of overhead cover is recommended to increase trout rearing performance in circular tanks.
