The challenge of assessing the effects of drifting fish aggregating devices on the behaviour and biology of tropical tuna

Though fisheries have intensively used drifting fish aggregating devices (DFADs) over the last three decades to facilitate their catch of tropical tunas, assessing the consequences of the presence of DFADs at sea on tuna behaviour and biology is a challenge. The use of DFADs has resulted in a major increase in the number of floating objects, which are spatially heterogeneous at sea. To date, no converging scientific results exist regarding the indirect effects of DFADs (unrelated with fishing mortality) on the large-scale movements and behaviour of tuna, mainly due to the difficulty of disentangling the respective roles of DFADs and environmental factors. Some biological indices show that tuna condition is lower when associated to a floating object than in a free-swimming school. It is not possible, however, to elucidate whether this is the cause or result of the association, or if it affects the fitness of individuals in the long term. Further scientific progress would require (i) the collection of time series of indicators to monitor habitat change, individual behaviour, individual fitness and population dynamics, and (ii) experimental studies to identify the underlying behavioural and biological processes involved in associative behaviour. The extent of the modification of the surface habitat by the massive deployment of DFADs and the current uncertainty of the possible long-term consequences on the individual fitness and dynamics of tuna populations argue for the need for increased awareness of this issue by Regional Fisheries Management Organisations regulating tuna fishing