Mechanistic modelling reveals tuna physiological condition is not a driver of floating object association.

Assessing the proximate and ultimate causes of animal behaviour is crucial for understanding the impact of human activities on wild species. However, for animals that are difficult to observe in the wild, such as pelagic fish species, identifying the causes and consequences of their behaviour can be particularly challenging. Here, we show the effectiveness of mechanistic modelling, combined with correlative approaches on empirical data, to determine the causation and consequences of behaviour. A behavioural model is developed to investigate the causal relationship between the associative behaviour of tropical tunas with floating objects and their physiological condition. Comparison with empirical data on tuna condition measured in the Western Indian Ocean suggests the rejection of the hypothesis that the physiological condition of tuna is a proximate cause of their associative behaviour. This study contributes to the assessment of the impact on tuna physiology of habitat changes induced by the massive use of floating objects introduced by tuna fishers.