Aggregation processes of tuna under drifting fish aggregating devices (dFADs) assessed through fisher's echosounder buoy in the Indian ocean

The aggregative behavior of tuna around floating object is widely exploited by the
industrial purse-seine fishery, which deploy thousands of floating objects each
year in all oceans in order to improve their catches. These fish aggregating devices
(FADs) are generally equipped with echosounder buoys that can collect acoustic
data, conferring to these devices the status of privileged observation platforms
for the fish communities that aggregate. Using a classification model based on
supervised learning algorithms trained on M3I buoy data, we were able to
translate the acoustic data collected along the trajectories of 5748 drifting FADs
newly deployed between 2016 and 2018 in the Indian Ocean into presence or
absence of tuna aggregation. Analysis of the resulting time series indicated that
drifting FADs are colonized by tuna aggregation over an average of 39 days. The
results also revealed, for the first time, that the residence time of a tuna
aggregation around a single DFAD is about 6 days and that DFADs spend on
average 9 days without tuna. Thus, DFADs appear to be occupied by tuna
aggregation about 43 % of their soaking time. We showed that these metrics can
manifest spatial and temporal variations.