Modeling tuna behaviour near floating objects: from individuals to aggregations

A fuzzy logic model of tuna behaviour near Fish Aggregating Devices (FADs) was developed to reproduce individual differences in horizontal movements o bserved from ultrasonic telemetry experiments. In this model, the behaviour of an individual is based on its surrounding environment (FADs and prey) a nd on its internal state (stomach fullness), which depends on its recent pa st actions. Internal sensors are used to determine the motivation of the fi sh, combined with external sensors, this determines its movements. Sensory information and motivation are modeled using fuzzy sets. A FAD attracts an individual when it is located within the FAD's range of influence. The time spent near a FAD depends on the feeding motivation of the fish and on its surrounding environment. If the fish is not hungry, it stays near the FAD. Otherwise, the fish has to forage in order to eat, and might therefore leav e the FAD if no prey is available in its vicinity. By varying the environme ntal conditions near FADs, the model reproduces the different horizontal mo vement patterns observed for tunas. The model is then extended to allow mul tiple individuals to co-exist, each individual modeled through the above be havioural model, without any direct or indirect interactions between them. This way, we study the effects of individual behaviour on tuna aggregation near FADs. We find that the model predicts the temporal dynamics of aggrega tion around FADs exhibited by tunas. By examining the effects of several FA D network models on the aggregation, we also estimate optimal spatial arran gements of FADs. (C) 2000 Ifremer/CNRS/LNRA/IRD/Cemagref/Edition scientifiq ues et medicales Elsevier SAS.