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

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 50 m 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 50 m, 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

Biodegradable drifting fish aggregating devices: Current status and future prospects

The structure, materials and designs of drifting Fish Aggregating Devices (dFADs) have generally remained rudimentary and relatively unchanged since they first came into use in the 1980 s. However, more recently, dFADs have been increasing in dimensions and the prevailing use of plastic components. Abandoned, lost or discarded dFADs can therefore contribute to the global marine litter problem. Transitioning to biodegradable and non-toxic materials that have a faster rate of decomposition, and are free of toxins and heavy metals, relative to synthetic materials, has been prescribed as an important part of the solution to reducing marine pollution from industrial tuna fisheries that rely on dFADs. This review of the current state of dFADs considers aspects related to the use of biodegradable materials in their construction, including; regulations related to dFAD materials, trials of biodegradable designs and materials and future alternatives. During the last decade, regulatory measures at tuna Regional Fishery Management Organizations (tRFMOs) have gradually moved towards the clear recommendation to use biodegradable materials in dFAD construction together with other measures limiting the number of active dFADs and the use of netting materials. However, to provide operational guidance, more clarity is needed, starting with a standardised definition of biodegradable dFADs among tRFMOs. Research involving dFAD natural and synthetic materials is required, along with improved data collection for monitoring the transition of dFAD materials against specified standards for biodegradable dFADs. In addition, alternative and complementary actions need to be explored to contribute to minimising adverse effects of dFADs on the environment. Acknowledging the current difficulties for the implementation of fully biodegradable dFADs in tuna fisheries, a stepwise process towards the implementation of commercially viable biodegradable dFADs should be considered.Peer reviewe

Interactions spatio-temporelles entre requins baleines, cétacés et pêche thonière tropicale à la senne dans une perspective de conservation, dans les océans Atlantique et Indien

In the frame of the Ecosystem Approach to Fishery (EAF) management, impact of the tropical tuna purse-seine fishery on targeted and incidentally captured species should be investigated. They may include megafauna species, such as sharks, rays, cetaceans, turtles or sea birds, which often are emblematic and vulnerable species. Tropical tuna purse-seiners actively search, at the surface of the sea, for clues indicating the presence of tuna schools (e.g. birds, floating objects, whales, dolphins or whale sharks). In the eastern Atlantic and western Indian Oceans, the main two fishing modes are sets on free swimming tuna schools and schools associated to natural or artificial floating objects, thereafter called Fish Aggregating Device (FAD). However dedicated studies on fishing sets associated to whale sharks and cetaceans are still lacking. The aim of this thesis is therefore, using logbook and scientific onboard observer data, to investigate the spatio-temporal co-occurrences and/or interactions between whale sharks, cetaceans and the tuna purse-seine fishery within an ecosystem conservation perspective. This work underlines that the megafauna/ fishery co-occurrence occurs in specific spatio-temporal strata: i) Gabon to Angola (April–September), ii) the Mozambique Channel (June–September), and iii) East of Seychelles (April–September). As baleen whales and whale sharks are filter feeders, the co-occurrence with the purse-seine fishery was mostly linked to highly productive environments (i.e. using proxys including chlorophyll-a concentration). In addition fishing sets involving megafauna were relatively high before 2000 (up to 20% of the sets), but are nowadays less frequent (AC3 and 1.5% of the sets associated to baleen whales and whale sharks). The purse-seine fishery appears to have a relatively low impact on megafauna species with mortality rates of 1.4% for whale sharks and 5.6% for cetaceans. Whale shark satellite tagging also confirms these results on the longer term, but the low sample size precludes any final conclusion. While dolphins are present in fishing areas, very few interactions with the fishery was detected, which highlights the striking difference with the eastern Pacific Ocean where half the sets are associated to dolphin herds. In addition, the diversity of targeted and bycatch species captured under whale shark and baleen whale sets was also investigated. Whale sharks are principally associated to skipjack and yellowfin (of various sizes) tunas and baleen whales mostly to large yellowfin tuna. In addition, bycatch species associated to these two megafauna groups was relatively low and dominated by the silky shark, and bycatch diversity was close to the one found for free swimming tuna schools. Finally, real and/or simulated encircled megafauna conservation measures or fishing effort management measures (especially FAD moratoria including larger ones) were investigated. The first ones were found to have limited consequences on tuna catch and bycatch. Conversely FAD moratoria had limited impacts on the number of megafauna associated fishing sets, due to the fact that the main spatio-temporal strata of megafauna and FAD sets differ. However larger and longer moratoria could be beneficial for juvenile tuna and some bycatch species. Overall, this thesis has lead to increase the knowledge on megafauna/ fishery interactions, essential in the general framework of setting up an EAF in the tropical tune purse-seine fishery.Dans le cadre de l’Approche Ecosystémique des Pêches (AEP), il est nécessaire d’évaluer l’impact de la pêche thonière tropicale à la senne sur les espèces ciblées et sur les espèces accessoires. Parmi ces dernières, les espèces de la mégafaune, telles que des requins, raies, cétacés, tortues, oiseaux marins, sont souvent emblématiques et vulnérables. Les thoniers senneurs tropicaux recherchent activement à la surface de l’eau tout indice de la présence de bancs de thon (e.g. oiseaux, objets flottants, baleines, dauphins ou requins baleines). Dans l’est de l’océan Atlantique et l’ouest de l’océan Indien, les deux modes de pêches principaux sont les captures de thons en bancs libres et celles sur bancs associés à un objet flottant, naturel ou artificiel, rassemblés ici sur sous le terme de dispositif de concentration de poisson (DCP). Les calées associées aux requins baleines et aux cétacés sont toutefois peu étudiées. L’objectif de cette thèse est donc d’analyser les co-occurrences et/ou interactions spatio-temporelles entre requins baleines, cétacés et pêche thonière à la senne, dans une perspective de conservation de l’écosystème. Ce travail, basé sur des données de livres de bord et d’observateurs scientifiques embarqués, a montré que la co-occurrence pêche thonière/ mégafaune se localise dans des strates spatio-temporelles relativement précises: i) du Gabon à l’Angola (avril–septembre), ii) dans le Canal du Mozambique (juin–septembre) et iii) à l’est des Seychelles (avril–septembre). Les baleines et requins baleines étant planctivores, la co-occurrence avec la pêche à la senne est principalement liée à une forte productivité primaire (appréhendée à travers des proxys tels que la concentration en chlorophylle-a). De plus, les calées sur ces deux groupes étaient assez élevées avant 2000 (jusqu’à 20% des calées), et qu’elles sont moins fréquentes aujourd’hui (AC3 et 1,5% des calées associées à des baleines et des requins baleines, respectivement). L’impact de la pêche à la senne sur ces espèces semble relativement faible au regard du taux de mortalité apparente de 1,4% pour les requins baleines et 5,6% pour les cétacés. Des marquages satellites réalisés sur les requins baleines, confirment ces observations sur le plus long terme, mais la taille de l’échantillon limite la formulation de conclusions définitives. Concernant les dauphins, bien que présents sur les zones de pêche, ils interagissent très peu avec celle-ci, soulignant ainsi une différence majeure avec l’océan Pacifique est où ce mode de pêche est majoritaire. La diversité spécifique des espèces cible et accessoire associées aux calées sur mégafaune a également été étudiée. Les requins baleines sont associés aux listaos et à l’albacore (dans une large gamme de tailles), alors que les baleines le sont principalement à de gros albacores. De plus, la capture accessoire associée à ces deux groupes de mégafaune est relativement faible et dominée par le requin soyeux et la diversité spécifique est proche de celle trouvée sous les bancs libres de thons. Enfin les effets de mesures de conservation vis-à-vis de la mégafaune encerclée ou de mesures de gestion de l’effort de pêche, notamment les moratoires sur DCP réelles ou simulées (e.g. moratoires élargis), ont été analysés. Les premières ont eu un effet limité en terme de captures cibles et accessoires, alors que les moratoires impactent peu le nombre de calées associés à la mégafaune, ceci en raison du décalage spatio-temporel des co-occurrences. Cependant des moratoires élargis pourraient être bénéfiques pour les thons juvéniles et certaines espèces associées. Par l’analyse quantitative des interactions entre la pêche thonière à la senne et la mégafaune, cette thèse apporte des connaissances essentielles sur les espèces étudiées dans le cadre de la mise en place d’une AEP, applicable à la pêche thonière tropicale

Forecasted consequences of simulated FAD moratoria in the Atlantic and Indian Oceans on catches and bycatches

Given the increasingly extensive use of drifting fish aggregation devices (FADs) by the purse-seine fisheries targeting tropical tunas, fishing effort restrictions have been introduced to manage tropical tuna stocks. However, these measures are focused on the protection of juvenile tunas and do not take account of the potential impact on bycatch or associated megafauna (whales and whale sharks). An iterative "fishing-day" Monte Carlo simulation model was developed to investigate the consequences on tropical tunas and bycatch of introducing extensive area 6-month moratoria on FAD activities. The model allowed for variability in a range of plausible values of the parameters characterizing the fishing operations conducted by European purse-seiners in the eastern tropical Atlantic and western Indian Oceans for the period 2005-2014. Monte Carlo simulations, using probabilities based on these fishery data, were carried out for the French and Spanish fishing fleets separately to account for differences in fishing strategies. The models predicted a decrease in FAD sets and an increase in free school sets. As a consequence, the catch of small tuna (= 10 kg) increased, leading to an overall increase in tuna catch of 100-200 tons/year/vessel in the Atlantic Ocean, and a decrease of 400-1500 tons/year/vessel in the Indian Ocean. The bycatch decreased in the Indian Ocean, while in the Atlantic Ocean billfishes, turtles and chondrichthyans bycatch increased slightly and other bony fishes decreased. Because fishing practices were modified, whale and whale shark associated sets increased slightly in the Indian Ocean

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

Drifting fish aggregating devices (dFADs) are human-made floating objects widely used by tropical tuna purse seine (PS) fisheries to increase catch of target species. However, dFAD use has several negative impacts, including increased potential for overfishing, higher juvenile tuna catch, higher bycatch compared to other PS fishing modes, ghost-fishing, and generation of marine litter. Based on these impacts, some stakeholders, especially environmental non-governmental organizations and other competing fishing industries, suggest that dFADs should be completely banned. We list the pros and cons of dFAD fishing; address how to improve current management; and suggest solutions for the sustainability of dFAD fishing in the long term. A dFAD ban would lead to major changes in the availability and sourcing of tuna for human consumption and decrease the licensing revenue received by many developing states. Most importantly, we argue that tools exist today to manage for, reduce or eliminate most of the negative impacts of dFADs (e.g., bans on discards, limits on active dFADs, biodegradable non-entangling constructions, time-area deployment closures, recovery programs, and full data transparency, among others). Management decisions based on sound scientific reasoning are needed to address the legitimate concerns surrounding dFAD use and ensure the sustainability of both pelagic and coastal ecosystems and tropical tuna PS fisheries

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

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

Catch and bycatch captured by tropical tuna purse-seine fishery in whale and whale shark associated sets: comparison with free school and FAD sets

International audienc

Catch and bycatch captured by tropical tuna purse-seine fishery in whale and whale shark associated sets: comparison with free school and FAD sets

International audienc

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

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