Fish aggregating devices drift like oceanographic drifters in the near-surface currents of the Atlantic and Indian Oceans

Abstract

Knowledge of ocean surface dynamics is crucial for oceanographic and climate research. The satellite-tracked movements of hundreds of drifters deployed by research and voluntary observing vessels provide high-frequency and high-resolution information on near-surface currents around the globe. Consequently, they constitute a major component of the Global Ocean Observing System (GOOS). However, maintaining this array is costly and in some oceanic regions such as the tropics, spatio-temporal coverage is limited. Here, we demonstrate that the GPS-buoy equipped fish aggregating devices (FADs) used in tropical tuna fisheries to increase fishing success are also capable of providing comparable near-surface current information. We analyzed millions of position data collected between 2008 and 2014 from more than 15,000 FADs and 2,000 drifters, and combined this information with remotely-sensed near-surface current data to demonstrate that the surface velocity components of FADs and drifters are highly correlated in the Atlantic and Indian Oceans. While it was noted that the subsurface structures of FADs did slow them down relative to the drifters, particularly in the Atlantic Ocean, this bias was measurable and could be accounted for in future studies. Our findings show that the physical meteorological and oceanographic data collected by fishermen could provide an invaluable source of information to the GOOS. Furthermore, by forging closer collaborations with the fishing industry and ensuring their contributions to global ocean databases are properly acknowledged, there is significant scope to capture this data more effectively.

Environmental versus operational drivers of drifting FAD beaching in the Western and Central Pacific Ocean

“…Using time-evolving velocity fields from a high resolution ocean circulation model, large numbers of virtual particles with float characteristics that mimic dFADs can be seeded into the simulated ocean to quantify the possible pathways of these floating objects. Such simulations have been used to understand many aspects of ocean circulation and to investigate the connectivity of passively drifting larvae 20 , nutrient flow 21 , marine plastics 22 , non-passive agents such as tuna themselves 23 , as well as dFADs 24 . In the context of dFAD beaching, Lagrangian particle simulation experiments can provide an independent estimate of the broad-scale probability distribution of connectivity between the ocean and beaching events.…”

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

“…In order to keep simulations simple, and considering that the depth of submerged appendages is only available for around 20% of the associated sets in the observer data, an average length of 50 m was assumed for dFAD appendages. However, Imzilen et al (2019) demonstrated a difference in drifting behaviour between dFADs drogued at different depth. In particular, it was found that dFADs in the Atlantic Ocean, up to 80 m deep, drifted slower than drifters used in oceanographic experiments, while those from the Indian Ocean (50-60 m) had similar drift pattern.…”

“…Drogued floating objects such as dFADs are largely driven by surface ocean currents (Imzilen et al 2019). The upper ocean circulation can be separated into (1) near surface Ekman flow in the upper tens of meters, directly generated by surface winds;…”

“…In the absence of trajectory data from large scale and long-term drifter deployments, Lagrangian simulation models of virtual dFADs can be useful to estimate the probable pathways of passively drifting objects through time (van Sebille et al 2018). These models have been used to track ocean debris (Lebreton et al 2012), ocean animals and larvae (Cetina-Heredia et al 2015, Scutt Phillips et al 2018) and even the potential course of dFADs in the Indian and Atlantic Oceans (Imzilen et al 2016, Imzilen et al 2019. Here, we use recent data from purse seine vessels fishing on dFADs and naturally floating logs in the WCPO to seed a Lagrangian, passively-drifting particle simulation model, under differing physical ocean flow scenarios.…”

Can drifting objects drive the movements of a vulnerable pelagic shark?

“…In this study, a novel approach for estimating the proportion of time silky sharks spend associated with floating objects is proposed. Recently, Imzilen et al (2019) showed that FADs drifted identically to oceanographic drifters, i.e. followed the ocean near‐surface currents.…”