A novel method to measure the impact of water quality on judgement bias in wild juvenile fish

Methods to examine judgement bias in free-living animals in situ are required to addressecological, conservation and animal welfare questions. Wild animals make behavioural decisionsbased on complex information, and judgement bias is an experience-induced adjustment in thecognitive appraisal of ambiguous information. Following on from recent research showingjudgement bias in fish, we developed a novel approach to measure population-level judgementbias using the natural tendency of juvenile Murray cod (Maccullochella peelii) to approach a lightsource, but move away from larger, potentially predator fish. Population-level judgement biaswas determined from the number of Murray cod caught in three slightly different light trapscontaining; 1) a light-only (positive stimulus), 2) a predator-model (negative stimulus) and 3) anambiguous-model (ambiguous stimulus). Ten water quality parameters were also recorded ateach site. All combinations of water quality parameters were included in models to examine howwell they explained (i) the presence of Murray cod and (ii) in sites where Murray cod were presentthe population-level judgement bias. The best models were selected using Akaike’s InformationCriterion. We caught 113 Murray cod at 19 out of 33 sites and modelling highlighted theimportance of dissolved oxygen (P=0.02–0.05 in top logistic models) to explain presence/absenceof fish, confirming the threat of low dissolved oxygen for this species. More Murray cod werecaught in light-only (positive stimulus) traps than in predator-model (negative stimulus) traps(P=0.04). Population-level judgement bias was overall negative, indicating a general tendency toavoid the ambiguous-model light trap. The top linear model (AICc=57.71, R2=0.63, P=0.025)indicated that in combination, there was greater avoidance of the ambiguous stimulus (i.e. a morepessimistic response) as salinity (P=0.043) and filterable reactive phosphorous increased(P=0.055) and pH decreased (P=0.013). The above water quality parameters were not nearknown lethal levels, indicating a need to better understand the sub-lethal effects of water parameterson fish behaviour and physiology. Our findings indicate that methods to measurepopulation-level judgement bias can support research on the function of judgement bias and itspossible relation to affect in fish. More generally, the method provides a potentially useful tool tobring together conservation biology and animal welfare disciplines.