Insights from the judgement bias paradigm: social group and tank size do not affect mental state in female guppies.

Optimal holding conditions are key to animal welfare. How stressful husbandry is perceived by the animal can be determined via an assessment of an animal's mental state - where it is positioned on the continuum between optimistic and pessimistic states - and can be measured using the judgement bias paradigm. In this test, individuals are trained to distinguish a rewarded from an unrewarded cue before being presented an ambiguous, intermediate cue. The response time to the ambiguous cue is then indicative of mental state. A shorter latency suggests a more positive (optimistic) mental state and a longer latency a more negative (pessimistic) mental state. Here, the authors used the judgement bias paradigm to assess the impact of standard laboratory housing conditions on the mental states of female guppies (Poecilia reticulata). As it is debated which holding conditions confer optimal welfare, they tested the impact of husbandry on mental state by keeping animals for 3 weeks in small or large social groups in either small or large tanks. They found that the different standard housing conditions used did not lead to differences in mental state. As an unexpected side result, they found that female guppies seem lateral. Their findings of comparable mental state across housing conditions suggest either that guppies perceive the tested conditions as equally stressful or, alternatively, that guppies are relatively resilient to the combination of group and tank sizes tested in this study. The authors conclude that the judgement bias paradigm can be a useful tool to assess fish welfare.