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

Abstract

Affective states of animals can be assessed through judgement bias tests, evaluating responses to ambiguous situations. In this study, 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 1s) in the opposing location. Fish from successful tanks (2 out of 5 tanks) were then housed in treatment tanks of either high or low environmental complexity at either high (165 sh/m 3 ) or low (69 sh/m 3 ) stocking density. Trained sh 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 (8.5s vs. 15.2s; P = 0.001), with faster entries into the positive (7.4s vs. 15.2s; P = 0.02) and near-negative chambers (10.2s vs. 17.4s; P = 0.006), suggesting that these sh 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 sh housed in high-density environments respond more optimistically in ambiguous situations compared to sh in low-density environments.

Fish Welfare in Public Aquariums and Zoological Collections

“…The working foundation of the model is that there are four separate domains (i.e., behavior, environmental, nutrition, and health) that culminate into a fifth domain (i.e., mental state) to provide an overall assessment of animal welfare. Therefore, animals that experience a net negative balance might have an affective state that includes anxiety, fear, panic, boredom, and depression, while animals that experience a net positive balance might feel contentment, pleasure, interest, confidence, and a sense of control [ 85 , 88 ].…”