Environmental enrichment is the modification of otherwise sterile hatchery rearing units to provide structural complexity. We investigated the use of an array of suspended plastic conduit sections as enrichment in large circular tanks during two experiments. Brown Trout Salmo trutta and Rainbow Trout Oncorhynchus mykiss were reared for 126 and 61 d, respectively, in covered, 3.63‐m‐diameter circular tanks that were either void of any structure or enriched with a suspended array of twenty 0.94‐m‐long sections of plastic conduit. Total tank weight gain and feed conversion ratio were significantly improved for both Brown Trout and Rainbow Trout reared with suspended conduit as enrichment compared to unenriched tanks. Enrichment did not significantly affect individual fish length, weight, or condition factor in either experiment, likely because of small sample sizes. The suspended array did not interfere with tank hydraulic self‐cleaning. Based on the results of this study, the use of vertically suspended enrichment structures in circular tanks is recommended to improve trout rearing efficiencies.

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This study evaluated the effects of two water velocities and three types of vertically-suspended environmental enrichment during the rearing of juvenile rainbow trout (Oncorhynchus mykiss; mean ± SE, initial weight 4 ± 1 g and total length 71 ± 2 mm) in 1.8-m diameter circular tanks. The 2 × 3 experimental design used two velocities (12.2 cm•s −1 and 30.5 cm•s −1) and three structural treatments (aluminum rod array, plastic spheres, or no structure). After 116 days, the fish reared without any structure (control) at 30.5 cm•s −1 had significantly lower total tank weight, gain, percent gain, feed conversion ratio, and specific growth rate compared to the other treatments. Similarly, the fish reared with a velocity of 30.5 cm•s −1 without structure were significantly shorter and lighter than those in the other treatments. There was no significant difference in condition factor and relative fin lengths among the treatments. These results indicate that vertically-suspended environmental enrichment may be impacting fish rearing performance by altering tank water velocities.

A combination of two forms of environmental enrichment (in-tank structure and exercise) was evaluated during the hatchery rearing of juvenile rainbow trout Oncorhynchus mykiss. The study used four treatments: 1) neither exercise nor structure, 2) exercise without structure, 3) structure without exercise, and 4) both exercise and structure. Velocities in the unexercised tanks were a constant 12.2 cm•s −1. Velocities in the exercised tanks alternated weekly with one week at 12.2 cm•s −1 followed by a week at 30.5 cm•s −1. Structural enrichment consisted of an array of vertically-suspended aluminum angles. The use of either environmental enrichment technique significantly improved final tank weight, gain, percent gain, and specific growth rate. Feed conversion ratio was significantly and positively influenced only by structure, but fish on the exercise routine were overfed. There were no significant interactions between exercise and structure, indicating that each form of environmental enrichment operates independently. The results of this study indicate fish rearing performance can be improved with the addition of either vertically-suspended aluminum angles or an exercise routine, but the combination of the two techniques may not be needed to improve rainbow trout growth.

Environmental enrichment is the incorporation of materials into hatchery tanks to simulate a more natural rearing environment. This study investigated the use of a vertically-suspended plastic conduit array as enrichment in 3.63 m circular tanks during Atlantic salmon (Salmo salar) rearing. After 168 days, feed conversion ratio was significantly improved for salmon reared in tanks with enrichment compared to the barren control tanks. Final tank weights and weight gain were not significantly different, although the difference in mean gain of 182.0 kg in the enriched tanks compared to 162.9 kg in the control tanks approached significance. There were no significant differences in individual fish length, weight, or condition factor between the treatments, likely because of large variances in body size due to sexual dimorphism and small sample sizes. The suspended array did not interfere with the hydraulic self-cleaning of the circular tanks. The results of this study support the use of vertically-suspended structures to improve the hatchery rearing performance of Atlantic salmon in circular tanks.