Surface habitat modification through industrial tuna fishery practices

Abstract

Natural floating objects (FOBs) have always been a major component of the habitat of pelagic species. Since the 1990s, the number of FOBs in the open ocean has increased greatly as a result of the introduction of fish aggregating devices (FADs) by the industrial tropical tuna purse seine vessels. These changes, and their potential impacts on the species that associate with FOBs, remain poorly understood. Using fisheries observer data, data from satellite-linked tracking buoys attached to FOBs and Lagrangian simulations, this study quantifies the temporal changes in the density and spatial distribution of FOBs due to the use of FADs in the Indian Ocean (IO) between 2006 and 2018. From 2012 to 2018, the entire western IO is impacted, with FADs representing more than 85% of the overall FOBs, natural FOBs less than 10%, and objects originating from pollution 5%. Results also suggest that both FADs and natural FOBs densities are lower in the eastern IO, but this initial investigation highlights the need for further studies. Our study confirms that FADs have greatly modified the density and spatial distribution of FOBs, which highlights the need to investigate potential consequences on the ecology of associated species.

Benefits, concerns, and solutions of fishing for tunas with drifting fish aggregation devices

“…For dFADs to act as ecological traps, various conditions need to be met and tested, first of which is the modification of the habitat by fishers. A number of studies have now shown that dFADs far outnumber natural floating objects over large areas and are regularly found outside of normal fishing areas (e.g., Dupaix et al, 2021; Imzilen et al, 2022; Maufroy et al, 2017), and therefore have clearly modified the network of natural floating objects.…”

“…In all areas except the WCPO, non‐dFAD PS catch is predominantly composed of large, mature yellowfin (Figure 3, Table S3), so an increase in fishing on FSC would probably lead to a net shift in species composition away from skipjack and bigeye tunas and towards yellowfin tunas (P2 in Figure 4). This would be the case even if the catch of skipjack in FSC in the AO and IO, where it has been suggested that intensive dFAD use may be limiting recruitment of skipjack to FSC (Dupaix et al, 2021), rebounds after a dFAD fishing ban to 1990s species compositions (Table 1), before the large increase in dFAD use starting in the early‐ to mid‐2000s (Maufroy et al, 2017). In the WCPO, due to a combination of non‐negligible aFAD catches and high catches of skipjack in FSC, the species composition (by weight) of non‐dFAD PS sets is closer to that of dFAD sets (Table S3), although bigeye tunas are notably absent as adult bigeye are generally inaccessible to purse seine as they feed below the mixed layer during daylight hours in tropical waters (Reygondeau et al, 2012).…”

“…While floating objects can be of natural origin (e.g., driftwood) all recent data indicates that the vast majority (>85%) of floating objects that are fished in the world's oceans are human‐made dFADs (Dupaix et al, 2021; FAO, 2021; Maufroy et al, 2017). These objects therefore represent a new, artificial, additional element to the pelagic ecosystem that must be managed with care to limit or eliminate a number of potential negative environmental impacts.…”

How technical change has boosted Fish Aggregation Device productivity in the Indian Ocean tuna fishery

“…DFADs are essentially arti cial oating (they can also be found fully submerged) structures which can consist of metal frames and/or bamboo rafts of vertical structure with the substructure stretching 50m below the surface that are deployed in the open ocean by shers to target associated schools of tuna [16]. Since their development in the 1990s there has been a "massive" expansion on the usage of DFADs by the industrial PS eet [14] [17]. All types of FOBs are known to attract large quantities of tuna, and the reasons and mechanisms of this associative behaviour still remain unknown to scientists [12][18].…”

Improving sustainable practices in tuna purse seine fish aggregating device (FAD) fisheries worldwide through continued collaboration with fishers

“…In tropical tuna fisheries around 5 million tonnes were caught in 2020, of which 66 percent of the total catch is made by purse seiners with 36 percent deriving from sets on fish aggregating devices (FADs), 27 percent from unassociated sets and 3 percent from dolphin sets, which are only observed in the Eastern Pacific (ISSF, 2022). Both artisanal and industrial tuna fisheries have employed for decades FADs because they attract diverse species of fish, including tunas (Taquet et al, 2007), but their numbers and catches have rapidly increased in recent decades (Fonteneau et al, 2013;Hall and Roman, 2013;Maufroy et al, 2016;Dupaix et al, 2021). Initially these FADs were fixed in space i.e., anchored FADs (aFADs), but with the advent of radio tracking buoys and later GPS-geolocating buoys, enabling fishers to accurately track FAD position, man-made drifting FADs (dFADs) grew in number and efficiency (Lopez et al, 2014;Wain et al, 2021).…”