Assessing the drift of fish aggregating devices in the tropical Pacific Ocean

Abstract

The tropical Pacific Ocean is characterized by its dominant zonal flow, strong climate dependence on the El Niño-Southern Oscillation (ENSO) and abundant tuna stocks. Tuna fisheries in the West and Central Pacific Ocean accounted for 55% of the world-wide tuna catch in 2019 and are one of the main sources of income in many Pacific island nations. One of the dominant fishing methods in this region relies on the use of drifting fish aggregating devices (dFADs): rafts with long underwater appendages (on average 50m deep) that aggregate fish. Although currents such as the North Equatorial Countercurrent (NECC) and South Equatorial Current (SEC) in the tropical Pacific Ocean vary strongly with ENSO, little is known about the impact of this variability in flow on dFAD dispersion. In this study, virtual Lagrangian particles are tracked for the period 2006 to 2021 over the domain in a 3D hydrodynamic model and are advected in simulations with only surface flow, as well as simulations using a depth-averaged horizontal flow over the upper 50m, representing virtual dFADs. Zonal displacements, eddy-like behaviour and ENSO variability are then studied for both types of virtual particles. It was found that virtual particles advected by surface flow only are displaced up to 35% farther than virtual dFADs subjected to a depth-averaged flow, but no other major differences were found in dispersion patterns. The strongest correlations between ENSO and virtual dFAD dispersion for the assessed variables were found in the West Pacific Ocean, with Pearson correlation coefficients of up to 0.59 for virtual dFAD displacement. Connections between ENSO and eddy-like behaviour were found in the western part of the SEC, indicating more circulation and meandering during El Niño. These findings may be useful for improving sustainable deployment strategies during ENSO events and understanding the ocean processes driving the distribution of dFADs.