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.

The surgical incision required to insert acoustic transmitters into fish peritoneal cavities is typically closed with sutures, staples, or cyanoacrylate adhesive. This study evaluated a novel tag insertion technique that did not use any method of wound closure. Twenty-six adult rainbow trout (Oncorhynchus mykiss) and 26 adult brown trout (Salmo trutta) received a small, 6 mm abdominal incision. A dummy acoustic transmitter tag was inserted into half (N = 13) of the fish in each species. Neither group received any wound closure; no sutures, staples, nor adhesives were used. Tag retention, wound healing, and fish survival were monitored weekly for 13 weeks postsurgery. No fish died over the course of the trial. Tag retention was 100% and 85% in the tagged rainbow trout and brown trout, respectively. The wounds in the untagged rainbow trout were all completely healed by six weeks postsurgery, while wounds of the tagged rainbow trout were completely healed at nine weeks post-surgery. Similarly, wounds in the untagged and tagged brown trout groups were completely healed by the fifth and eighth weeks respectively. This study documents the effectiveness of using a small incision with no wound closure methods during insertion of acoustic tags into rainbow trout and brown trout peritoneal cavities.

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.