Quantifying the increase in fishing efficiency due to the use of drifting FADs equipped with echosounders in tropical tuna purse seine fisheries

Abstract

Numerous pelagic species are known to associate with floating objects (FOBs), including tropical tunas. Purse seiners use this behaviour to facilitate the capture of tropical tunas by deploying artificial drifting fish aggregating devices (dFADs). One major recent change has been the integration of echosounders in satellite-tracked GPS buoys attached to FOBs, allowing fishers to remotely estimate fishable biomass. Understanding the effects of this new technology on catch of the three main tuna species (yellowfin tuna, Thunnus albacares; bigeye tuna, Thunnus obesus; and skipjack tuna, Katsuwonus pelamis) is important to accurately correct for this change in catch-per-unit-effort (CPUE) indices used for stock assessments. We analysed catch data from the French purse seine fleet for the period 2010–2017 in the Indian Ocean to assess the impact of this fleet’s switch to echosounder buoys around 2012. Results indicate that echosounders do not increase the probability a set will be succesful, but they have a positive effect on catch per set, with catches on average increasing by ≈2−2.5 tonnes per set (≈10%) when made on the vessel's own dFADs equipped with an echosounder buoy. Increases were due to a decrease in sets below ≈25 tonnes and an increase in those greater than ≈25 tonnes, with a non-linear transition around this threshold. This increase explains the considerable investment of purse seiners in echosounder buoys, but also raises concerns about bias in stock size estimates based on CPUE if we do not correct for this fishing efficiency increase.

Fuel consumption and air emissions in one of the world’s largest commercial fisheries

“…Since the early 2010s, buoys attached to the FADs include acoustic units that provide real-time information on the biomass of tuna occurring in the vicinity of the drifting rafts (Lopez et al, 2014). This has led to increasing fishing efficiency and success and enables further FAD-fishing throughout the year (Wain et al, 2020). The increasing use of FADs has substantially modified the spatial extent and movement patterns of the purse seiners.…”

“…It should be noted however that vessels targeting FAD-associated schools make more sets per day than when fishing on FSCs (Floch et al, 2019). The FUI for purse seiners targeting FADs would then increase through increased number of fishing sets and associated fuel consumption but decrease through increased catch per set enabled by better selection of the FADs (Wain et al, 2020).…”

“…In the Indian Ocean, the catch of the tuna purse seine fishery, composed of about 50 vessels larger than 65 m, increased from 280,000 t in the late 2000s to almost 500,000 t in 2018 (Fiorellato et al, 2019). In particular, the advent and increasing use of echosounder buoys attached to the FADs deployed at sea has greatly increased the efficiency and catchability of purse seiners over the last decade (Lopez et al, 2014;Wain et al, 2020). Furthermore, 20 support vessels assist the purse-seine fishing fleet by maintaining a network of FADs.…”

How do tuna schools associate to dFADs? A study using echo-sounder buoys to identify global patterns

“…As fishermen have noticed this behaviour, they have used both natural and man-made floating objects, or drifting Fish Aggregating Devices (dFADs), as a tool for finding and catching tropical tunas. The use of dFADs in tuna purse-seine fisheries has gradually increased since the 1980s to the present time, where vessels using dFADs now contribute to 36% of the world’s total tropical tuna catch (Davies et al, 2014; Wain et al, 2021; ISSF, 2021). These widespread changes have highlighted the need to better understand the potential ecological effects of dFADs on tuna ecology and the marine environment, in order to ensure adequate management of fish stocks and dFAD usage.…”

Tuna-AI: tuna biomass estimation with Machine Learning models trained on oceanography and echosounder FAD data

“…In tropical tuna purse-seine fisheries, targeting mainly skipjack tuna ( Katsuwonus pelamis ), yellowfin tuna ( Thunnus albacares ) and bigeye tuna ( Thunnus obesus ), these drifting objects, known as drifting Fish Aggregating Devices (dFADs), have become an essential tool for locating tuna-schools and increasing fishing efficiency. Today, more than 55% of tropical tuna caught by industrial purse-seine vessels in the Indian, Atlantic and Pacific oceans is caught using dFADs, accounting for 36% of the world’s total tropical tuna catch (Wain et al, 2021; ISSF, 2021).…”