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Impact of environmental complexity and stocking density on affective states of rainbow trout (Oncorhynchus mykiss)


Environmental condition, such as environmental complexity or stocking density, can directly or indirectly influence animal emotion and ultimately, affective state. Affective states of animals can be assessed through judgement bias tests, evaluating responses to ambiguous situations. In this study, we aimed to determine whether environmental complexity and stocking density impacted rainbow trout affective state. Rainbow trout (n = 108) were housed in recirculating aquaculture systems under commercial conditions while trained at tank-level to discriminate between a positively reinforced chamber (feed) in one location and a negative chamber (positive punishment; chase by net for 1 s) in the opposing location. Fish from successful tanks (two out of five tanks) were then housed in treatment tanks of either high- or low- environmental complexity at either high (165 fish/m3) or low (69 fish/m3) stocking density. Trained fish were tested for latencies to approach three intermediate, ambiguous chambers. Fish housed in high-density tanks were faster to enter all chambers than those housed in low-density tanks (8.5 s vs. 15.2 s; P = 0.001), with faster entries into the positive (7.4 s vs. 15.2 s; P = 0.02) and near-negative chambers (10.2 s vs. 17.4 s; P = 0.006), suggesting that these fish were more optimistic to receive a feed reward. Tank complexity did not affect test outcomes. No differences between treatments were observed between body weight, length, and plasma cortisol. Overall, rainbow trout are capable of discriminating between cues during a judgement bias test and fish housed in high-density environments respond more optimistically in ambiguous situations compared to fish in low-density environments.

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Data availability

The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.


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Thank you to Wytheville State Fish Hatchery for donating the animals used in this study and the Universities Federation for Animal Welfare for financially supporting the study. Thank you to Jireh Clarington, Alyssa Selhorst, and Margot Breiner (Virginia Tech) for assistance with the project.


Financial support was received from the Universities Federation for Animal Welfare.

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Authors and Affiliations



All authors contributed to the study conception, design, and data collection. Material preparation was performed by MGA, LJ, SAS, and DDK. MGA, LJ, and AMC performed formal data analysis. The original draft was prepared by MGA and LJ. Manuscript review and editing was performed by SAS and DDK. All authors have read and approved of the final manuscript.

Corresponding author

Correspondence to L. Jacobs.

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Conflict of interest

The authors declare no competing interests.

Ethical approval

All procedures in this study were approved by and in accordance with Virginia Tech’s Institutional Animal Care and Use Committee (Protocol No. 20-074).

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Anderson, M.G., Campbell, A.M., Kuhn, D.D. et al. Impact of environmental complexity and stocking density on affective states of rainbow trout (Oncorhynchus mykiss). Anim Cogn (2022).

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  • Animal welfare
  • Rainbow trout
  • Environmental complexity
  • Enrichment
  • Stocking density
  • Affective state