Vertically-Suspended Environmental Enrichment Improves Growth of Landlocked Fall Chinook Salmon during Initial Hatchery Rearing

Environmental enrichment has been shown to improve juvenile salmonid growth during hatchery rearing. This study examined the effects of vertical-ly-suspended environmental enrichment starting seven days after feed train-ing and continuing for the next 32 days of landlocked fall Chinook salmon (Oncorhynchus tshawytscha) rearing. At the end of this experiment, final tank weights, gain, percent gain, and feed conversion ratio were all significantly improved in the tanks of salmon with environmental enrichment compared to barren tanks, with a nearly 25% improvement in feed conversion ratio. Individual salmon in tanks with environmental enrichment were significantly longer and weighed significantly more than salmon from tanks without enrichment. Specific growth rate was also significantly improved with the use of enrichment. The use of vertically-suspended environmental enrichment to improve growth and rearing efficiency is recommended during the initial rearing of landlocked fall Chinook salmon.

[1]- [7]. However, materials located at the bottom of the tank to mimic natural habitats have negative consequences for the hydraulic self-cleaning of circular tanks, leading to decreased water quality, increased labor requirements, and increased disease potential [8]- [13].
In order to add enrichment, but still maintain circular tank hydraulic self-cleaning, vertically-suspended environmental enrichment was developed.
Although there have been numerous studies examining vertically-suspended environmental enrichment during the hatchery rearing of salmonids in circular tanks, these studies have primarily used juvenile or larger fish. No studies have examined the use of such enrichment beginning with initial feeding. Thus, the objective of this study was to evaluate the use of vertically-suspended environmental enrichment during the initial feeding and rearing of Chinook salmon.
These fish had been feed trained for seven days prior to the start of the experiment. The study lasted 32 days, ending on February 5, 2019.
Two treatments (n = 5) were used. Corrugated plastic covered the top of all tanks, with only a small area open for feed delivery, as described by Walker et al. [23]. Control tanks had no structure present, while the other tanks had a vertically-suspended array of four aluminum angles. The aluminum angles (each side 2.5-cm wide × 57.15-cm long) were suspended through the corrugated plastic covers as described by Krebs et al. [19]. The angles were arranged so that the angled portion faced into the direction of water flow and did not inhibit the hydraulic self-cleaning of circular tanks [19].
The salmon were fed BioVita Starter (Bio-Oregon, Longview, Washington, USA) every 15-min during daylight hours using automatic feeders. Feeding rates were determined by the hatchery constant method [24], with an expected feed conversion ratio of 1.1 and a projected growth rate of 0.08 cm·d −1 , which was a rate slightly above satiation.

Results
Final tank weight, gain, percent gain, and feed conversion ratio were all significantly improved in the vertically-suspended environmentally enriched tanks compared to control tanks, with a nearly 25% improvement in feed conversion ratio (Table 1). Individual salmon in tanks with environmental enrichment were significantly longer and weighed significantly more than salmon from tanks without enrichment (Table 2). Specific growth rate was also significantly improved with the use of enrichment. Salmon were on average 1.5 mm longer and 0.1 g heavier in enriched tanks compared to un-enriched control tanks.

Discussion
The results of this study clearly indicate that vertically-suspended environmental enrichment is beneficial during the initial rearing of Chinook salmon. These results are consistent with those observed using larger juvenile Chinook salmon by Rosburg et al. [16]. It is unknown how vertically-suspended enrichment is improving growth in hatchery-reared fish. Kientz et al. [15] hypothesized the benefits from vertically-suspended environmental enrichment were due to the creation of lower water velocity microhabitats that allowed the fish to decrease energy expenditures when not feeding [26]. The alterations in circular tank flow dynamics from vertically-suspended structure reported by Moine et al. [27] and Muggli et al. [28] support this hypothesis. The improved feed conversion ratios observed in this study provide further evidence of bioenergetic improvements with vertically-suspended enrichment. Because the salmon in the barren tanks were subjected to continual velocities, in comparison to the salmon in the enriched tanks containing locations with lower velocities, the fish in the unenriched tanks may have experienced exercise fatigue due to higher feed conversion ratios [25].
Other investigations not using suspended structures have also reported changes in overall water velocities can influence trout growth [29] [30] [31] [32].
As in previous studies utilizing vertically-suspended environmental enrichment, no negative effect on the self-cleaning nature of circular tanks was observed [19] [21] [22]. As long as adequate incoming water flows are maintained, no extra maintenance or cleaning is needed with the small array of angles as used in this study.

Conclusion
In conclusion, this study expands the window of expected benefits from vertically-suspended environmental enrichment to include the period of initial feed-