Aggregation process of drifting fish aggregating devices (DFADs) in the Western Indian Ocean: Who arrives first, tuna or non-tuna species?

Abstract

Floating objects drifting in the surface of tropical waters, also known as drifting fish aggregating devices (DFADs), attract hundreds of marine species, including tuna and non-tuna species. Industrial tropical purse seiners have been increasingly deploying artificial man-made DFADs equipped with satellite linked echo-sounder buoys, which provide fishers with information on the accurate geo-location of the object and rough estimates of the biomass aggregated underneath, to facilitate the catch of tuna. Although several hypotheses are under consideration to explain the aggregation and retention processes of pelagic species around DFADs, the reasons driving this associative behavior are uncertain. This study uses information from 962 echo-sounder buoys attached to virgin (i.e. newly deployed) DFADs deployed in the Western Indian Ocean between 2012 and 2015 by the Spanish fleet (42,322 days observations) to determine the first detection day of tuna and non-tuna species at DFAD and to model the aggregation processes of both species group using Generalize Additive Mixed Models. Moreover, different seasons, areas and depths of the DFAD underwater structure were considered in the analysis to account for potential spatio-temporal and structure differences. Results show that tuna species arrive at DFADs before non-tuna species (13.5±8.4 and 21.7±15.1 days, respectively), and provide evidence of the significant relationship between DFAD depth and detection time for tuna, suggesting faster tuna colonization in deeper objects. For non-tuna species, this relationship appeared to be not significant. The study also reveals both seasonal and spatial differences in the aggregation patterns for different species groups, suggesting that tuna and non-tuna species may have different aggregative behaviors depending on the spatio-temporal dynamic of DFADs. This work will contribute to the understanding of the fine and mesoscale ecology and behavior of target and non-target species around DFADs and will assist managers on the sustainability of exploited resources, helping to design spatio-temporal conservation management measures for tuna and non-tuna species.

Risks to large marine protected areas posed by drifting fish aggregation devices

“…One possible benefit of FADs (dFADs and anchored fish aggregating devices) for MPAs is their potential as scientific research platforms (Moreno et al 2016). A series of scientific dFADs equipped with echosounders could provide estimates of colonization rates (Orue et al 2019) and much needed and cost-effective fisheriesindependent indices of abundance data (Moreno et al 2016;Santiago et al 2017). Such data could be invaluable in improving understanding of the spatial and temporal distributions of tuna and other pelagic predators within large MPAs.…”

Towards acoustic discrimination of tropical tuna associated with Fish Aggregating Devices

“…This result could be useful to improve abundance estimation in cases where the only source of information is the data from vertical echo-sounders (and there is a lack of information about the volume of the aggregation). The potential of the mean depth to improve abundance estimates could be tested in the algorithms used by the echo-sounder buoys used to track DFADs [50, 51].…”

“…Although tuna species could also be found at shallower depths and some by-catch species can have a partially overlapping vertical distribution during some hours of the day, studies using tag sensors have shown that tuna species generally remain deeper than non-target species [53,54]. The few studies that have assessed the presence of tuna at DFADs using echo-sounders have not considered data above 25 m to avoid targeting non-tuna species [51, 55]. More effort is needed to study the natural behavior of tuna and other species at DFADs by region, using tags with pressure (depth) sensors would allow a better understanding of the vertical distribution of species at DFADs under different environmental conditions and thus allowing a more accurate vertical segregation of tuna and non-tuna species.…”

“…Furthermore, a combined and simultaneous use of acoustic biomass by species and remote sensing oceanographic parameters, could serve to better understand the variation of the abundance, residence time and species composition of aggregations at DFADs related to the environment. Recent research efforts using echo-sounder buoy´s biomass estimations lack data on species composition, which hinders both a better estimate of biomass underneath DFADs and also species-specific studies [51, 66, 67]. Filling these knowledge gaps would allow designing sound science-based conservation measures, as a sustainable number of DFADs at sea and effective spatial and temporal closures.…”

Regional connectivity and spatial densities of drifting fish aggregating devices, simulated from fishing events in the Western and Central Pacific Ocean

“…This appendage structure reduces drift speed and appears to increase the attractive power of the device, although the mechanisms driving the aggregation of tunas and other pelagic species around floating objects is not fully understood (Bromhead et al 2003). Generally, dFADs are left to drift for 3-4 weeks before any fishing takes place around them, a period during which they begin to colonise and aggregate both target and non-target species (Moreno et al 2007, Orue et al 2019. Then dFADs may be set on several times over the proceeding months, with an average lifetime of 6 months in the WCPO (Escalle et al 2018a).…”