An Evaluation of Vertically Suspended Environmental Enrichment Structures during Rainbow Trout Rearing

Abstract

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.

Vertically-Suspended Environmental Enrichment Alters the Velocity Profiles of Circular Fish Rearing Tanks

“…The increase in fish growth and rearing efficiencies using arrays of suspended objects in circular tanks is well documented [27] [28] [29] [30] [31]. These fish rearing benefits are likely at least partially because of the dramatic in-tank velocity changes due to the suspended environmental enrichment.…”

“…These vertically-suspended structures were shown to significantly improve rainbow trout (Oncorhynchus mykiss) rearing performance [27]. Subsequent investigations evaluating various types of vertically-suspended structures, including strings of spheres, plastic pipes, and aluminum angles, have also indicated positive effects during the rearing of multiple salmonid species [28] [29] [30] [31].…”

Use of Suspended Plastic Conduit Arrays during Brown Trout and Rainbow Trout Rearing in Circular Tanks

“…Thus, most hatcheries typically keep rearing units that are devoid of internal structures. Kientz and Barnes (2016), Kientz et al (2018), and Crank et al (2019) described novel enrichment techniques consisting of arrays of vertical metal rods or colored balls suspended from overhead tank covers. When added to small, circular rearing tanks, these arrays did not interfere with the self-cleaning nature of the tanks or increase hatchery labor requirements compared to sterile rearing tanks.…”

“…When added to small, circular rearing tanks, these arrays did not interfere with the self-cleaning nature of the tanks or increase hatchery labor requirements compared to sterile rearing tanks. The arrays were also shown to improve growth rates and feed conversion ratios (FCRs) of Rainbow Trout Oncorhynchus mykiss Kientz et al 2018;Crank et al 2019).…”

“…Previous studies evaluating vertically suspended structure have been conducted only with Rainbow Trout and only in relatively small, 1.8-m-diameter circular tanks Kientz et al 2018;Crank et al 2019). Therefore, the present study had two objectives.…”