Improving knowledge on the ecology of tuna species represents a key research priority to ensure the sustainable exploitation of their populations. Currently, a better understanding of the associative behaviour of tropical tunas with floating objects has become a major challenge, given the massive increase in the use of drifting Fish Aggregating Devices (DFADs) over the last few decades.
Introduced in the early 1990s in the tropical tuna purse-seine fishery, DFADs are man-made floating objects specifically designed and deployed to facilitate the capture of tunas by exploiting their natural tendency to gather under objects floating at sea.
Nowadays, all deployed DFADs are equipped with satellite-linked echosounder buoys that allow a remote monitoring of fish aggregations associated with the DFAD by fishers. In addition to the interest that these buoys represent for fishers, the considerable amount of data generated by these devices is of substantial interest for the study of the ecology and behaviour of the pelagic fish communities that associate with floating objects.
The
latest Editor's Choice article investigates the associative dynamics of tuna aggregations with floating objects, using acoustic data collected by around 9400 commercial echosounder buoys on newly deployed DFADs in the Atlantic and Indian oceans. The processing of these data with a dedicated automated classification approach provided time series of presence or absence of tuna aggregations under DFADs, which allowed the assessment of various metrics, related to the association of tuna aggregations with DFADs.
Associative dynamics were found to significantly differ between oceans, with notably much longer DFAD vacancy periods (absence of tunas) in the Indian Ocean than in the Atlantic Ocean. The analysis has also indicated that DFAD occupancy by tuna aggregations is determined by a time-independent process, comprising two main modes of residence: a long-term mode in the range of two or three weeks and a short-term mode with a duration comparable to the average residence times estimated at the level of tuna individuals by previous electronic tagging studies.
These new findings are key for the development of new methods for assessing the abundance of tropical tuna species that exploit their associative behaviour with floating objects.
