Abstract
In an effort to increase purse seine fishing efficiency for tropical tunas, over 30,000 drifting Fish Aggregating Devices (dFADs) are deployed every year by fishers in the Western and Central Pacific Ocean (WCPO). The use of dFADs also impacts ecosystems, in particular through marine pollution and dFAD beaching. This paper presents the first estimate of dFAD beaching events in the WCPO (>1300 in 2016–2017) and their distribution. Lagrangian simulations of virtual dFADs, released subject to contrasting deployment distributions, help us determine the relative importance of operational versus environmental drivers of dFADs drifting to beaching areas. The highest levels of beaching, occurring on Papua New Guinea and Solomon Islands, are likely a result of the prevailing westward oceanic circulation and subsequent local processes driving dFADs towards land. Similarly, high beaching rates in Tuvalu appear to be due to the general circulation of the WCPO. In contrast, beaching in Kiribati Gilbert Islands appear to be more strongly related to dFAD deployment strategy. These findings indicate that reducing beaching events via changes in deployment locations may be difficult. As such, management approaches combining dFAD deployment limits, the use of biodegradable dFADs, recoveries at-sea close to sensitive areas and/or beached dFAD removal should be considered.
Highlights
Over the last few decades there has been a fundamental shift in the way tuna aggregations are tracked and subsequently caught
The cleaned dataset consisted of 22,620 observed drifting Fish Aggregating Devices (dFADs) trajectories (84% of the original dataset) split into 32,665 at-sea drift trajectories separated by periods on-board a vessel (15,455 dFADs had a single, continuous drift track)
The largest number of beaching events were in the exclusive economic zones (EEZs) of Papua New Guinea (483), Solomon Islands (379), Kiribati Gilbert Islands (155) and Tuvalu (117) (Fig. 1)
Summary
Over the last few decades there has been a fundamental shift in the way tuna aggregations are tracked and subsequently caught. The Western and Central Pacific Ocean (WCPO) contains the largest tropical tuna purse seine fishery in the world, with more than 1.8 million tonnes captured in 20174, mostly composed of skipjack and yellowfin tuna, with a smaller catch of small bigeye tuna in dFAD sets This region has the highest number of dFAD deployments in the world, estimated at more than 30,000 per year[1,10]. Areas with much lower or higher numbers of beaching events than expected, given the local dFAD density, are likely to be influenced by local bathymetry or circulation near the coast Understanding these drivers of beaching events is important to effectively manage dFAD purse seine fisheries in the WCPO. Quantifying the connectivity between areas of high beaching and the surrounding ocean in a statistically robust way would require observations from many more dFADs
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