Tuna aggregation dynamics at Drifting Fish Aggregating Devices: a view through the eyes of commercial echosounder buoys

This study addresses novel questions on the dynamics of tuna aggregations around floating objects, using echosounder buoys data collected throughout the drifts of newly deployed Drifting Fish Aggregating Devices (DFADs) in the Atlantic Ocean (AO) and Indian Ocean (IO). Time series of presence/absence of tunas were obtained by supervised classification of acoustic data. To avoid biases related to the variability in individual DFAD soak times, a new approach was developed to estimate the average colonization time of new DFADs by tuna aggregations. We showed that tunas colonize DFADs after an average of 16days in the AO, and 40days in the IO. Moreover, the analysis indicated that the time span during which tuna aggregations occupy DFADs is driven by a time-independent process with short- and long-term residence modes. On average, DFADs were continuously occupied by tuna aggregations for 6 and 9days in the IO and AO, respectively. The time between two consecutive aggregations at the same DFAD averaged 9days in the IO and 5days in the AO. Throughout their soak time after being colonized, DFADs remained occupied for a larger proportion of time in the AO (63%) than in the IO (45%).