While structural complexity has been used extensively during hatchery rearing in an attempt to improve poststocking survival, the potential impact of this complexity on hatchery rearing performance has largely been ignored. This study examined the growth and condition of juvenile Rainbow Trout Oncorhynchus mykiss reared in nearly fully covered circular tanks containing nine vertically oriented round aluminum rods (structurally complex) compared with that of trout reared in almost fully covered tanks with no rods (control) at two water velocities (2.0 body length (BL)/s and 2.75 BL/s). After 51 days, total tank weight gain was significantly higher in the structurally complex tanks than in the control tanks, regardless of water velocity. The feed conversion ratio was also significantly better in the structurally complex tanks [mean (SD) 1.25 (0.10)] than in the control tanks [1.66 (0.41)]. There were no significant differences in hepatosomatic index, splenosomatic index, and viscerosomatic index between the fish from the structurally complex and the control tanks. Total tank weight gain was significantly higher and the feed conversion ratio significantly lower in the lower water velocity tanks than in the higher water velocity tanks. These results indicate that vertically oriented aluminum rods can be used to increase structural complexity and improve Rainbow Trout rearing performance in circular rearing tanks, with minimal effects on regular tank cleaning.

This study evaluated the effectiveness of two different environmental enrichment structures on the growth and condition of juvenile Rainbow Trout Oncorhynchus mykiss. Fish were reared in covered tanks with vertically suspended linear or spherical structures. Spherical structures were comprised of seven colored plastic balls that were vertically stacked and suspended on a string from the overhead tank covers, while linear structures consisted of vertically oriented aluminum rods. In the tanks with either of the environmental enrichment treatments, weight gain and feed conversion ratio were significantly improved relative to control tanks. Weight gain and feed conversion ratio were also significantly improved with the use of spherical structures compared to the use of linear structures. To provide substantial benefits during the rearing of Rainbow Trout in circular tanks, the use of vertically suspended spherical structures is recommended.

Environmental enrichment is the addition of substrate or structural complexity to typically sterile culture environments, but the effects of enrichment on fish rearing performance are relatively unknown. This 127‐d study examined the growth and condition of juvenile Rainbow Trout Oncorhynchus mykiss reared in near‐fully covered circular tanks (1.8 m diameter) with three novel environmental treatments: (1) eight vertically suspended nylon strings with seven colored (6.4 cm diameter) plastic spheres (high density), (2) five vertically suspended strings with seven colored plastic spheres (low density), and (3) a mixture of five vertically suspended colored plastic spheres and nine vertically suspended aluminum rods. Near‐fully covered tanks without enrichment were used as a control. Weight gain, feed conversion ratio, individual fish length, and fish weight were significantly improved in environmentally enriched tanks compared with those features in the unenriched control. Among all of the treatments, the tanks of fish with high‐density spheres had significantly higher ending tank weights (i.e., total weight of fish in tank) and weight gain, both of which were nearly 65% and 75% greater, respectively, than in the control tanks. However, high density spheres also interfered with the self‐cleaning nature of the circular tanks. No significant differences in splenosomatic indices, viscerosomatic indices, hepatosomatic indices, and relative fin lengths were observed among the three enrichment treatments. Based on the results of this study, the use of either low‐density spheres or a mix of rods and spheres is recommended during the rearing of juvenile Rainbow Trout in circular tanks.

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.

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