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Though fisheries have intensively used drifting fish aggregating devices (DFADs) over the last three decades to facilitate their catch of tropical tunas, assessing the consequences of the presence of DFADs at sea on tuna behaviour and biology is a challenge. The use of DFADs has resulted in a major increase in the number of floating objects, which are spatially heterogeneous at sea. To date, no scientific consensus exists regarding the effects of DFADs on the large-scale movements and behaviour of tuna, mainly due to the difficulty of disentangling the respective roles of DFADs and environmental factors. Some biological indices show that tuna condition is lower when associated to a floating object than in a free-swimming school. It is not possible, however, to elucidate whether this is the cause or result of the association, or if it affects the fitness of individuals in the long term. Further scientific progress would require (i) the collection of time series of indicators to monitor habitat change, individual behaviour, individual fitness and population dynamics, and (ii) experimental studies to identify the underlying behavioural and biological processes involved the associative behaviour. The extent of the modification of the surface habitat by the massive deployment of DFADs and the current uncertainty of the possible long-term consequences on the individual fitness and dynamics of tuna populations argue for the need for increased awareness of this issue by Regional Fisheries Management Organisations regulating tuna fishing.

Quantification of the physical and biological factors that influence the spatial structuring of food webs is central to inform effective resource management. We used baseline‐corrected stable isotope ratios (δ13C and δ15N) of 63 invertebrate and fish to investigate food web structure across a continental shelf gradient—the Celtic Sea Shelf in the Northeast Atlantic Ocean. Hierarchical clustering on δ13C and δ15N showed that the shelf food web is characterized by four trophic levels with trophic groups spread across pelagic and benthic trophic pathways. Four biomass‐weighted isotopic diversity metrics provided indicators on the status of the system, showing a relatively complex food web with high trophic redundancy at intermediate trophic levels suggesting resilience to disturbances. Two sets of statistical models, at the community scale and for each trophic group, identified five distinct trophic assemblages associated with different chlorophyll a concentrations, water depth, and bottom temperature. A cold, vertically mixed‐water assemblage over the outer shelf comprised the largest habitat and most diverse assemblage, highlighting the importance of cold productive conditions in the Celtic Sea. Trophic group model results were used to generate spatial area predictions to compare functioning of groups using isotopic overlap (similarity and nestedness) metrics. Isotopic niche area was larger (spanning two trophic levels) in shallow habitats, but not in habitats underlying high primary production or nutrient‐rich water masses, suggesting stronger benthic‐pelagic trophic coupling in inner shelf habitats. Results suggest that depth and intensity of pelagic production are major drivers of trophic structure and functioning of Celtic Sea communities.

Predictive selectivity modelling is used to study fishing gear selectivity. These models rely on fish morphology parameters and fall-through experiments. Here, we developed a model of the mechanical interaction between a fish and a netting mesh during an escape attempt. In this model, the fish is described with three parameters: the first parameter measures its mechanical ability to penetrate a mesh and the others are body stiffness coefficients accounting for fish body deformation under mesh twine pressure. These parameters were identified from the results of a bench test simulating a fall-through experiment through a single 60-mm Polyethylene (PE) mesh. A rigid circular cone, a rigid elliptical cone and a small number of horse mackerel and haddock, offering a limited length range, were tested. Results were generalised and compared with those obtained on a piece of netting. Within the model's assumptions, it is possible to predict the success of an escape attempt if one knows the three fish parameters and the mesh load in normal and transverse directions. The study showed that escape potential is greater through an actual deformable mesh than through a non-deformable mesh. The potential size of this difference depends on the mesh load and orientation.

Most European fishing fleets will need to drastically reduce their unwanted catches to comply with new rules of the common fisheries policy. A more practical way to avoid increasing on-board sorting time and issues linked to storage capacity is to prevent unwanted catches in the first place. We assessed the selectivity properties of an experimental fishing gear that combined a 100 mm T90 cylinder with 130 meshes in the extension and a 100 mm T90 codend of 33 meshes (experimental gear) compared to a 100 mm diamond mesh extension and codend (control gear) during commercial trips using twin trawls. Analysis of the relative size composition of catches indicated a significantly higher escapement of small fish of several target species (e.g. Lepidorhombus whiffiagonis, Melanogrammus aeglefinus, Raja spp, and Lophius spp) and non-target species (e.g. Capros aper and Gurnards spp) from the T90 experimental trawl compared to the control trawl (n = 49 hauls), resulting in a significant reduction of unwanted catches of Gadidae, Triglidae, and Caproidae. In contrast, non-negligible commercial losses of small grade target gadoid species were observed. Mixed general linear models showed that the proportion of ray, haddock and anglerfish retained per length class decreased with increased tow duration. The T90 experimental gear will perform at a commercial level when targeting monkfish, megrim, rays and large haddock, however fishers are not likely to use this gear when targeting smaller-bodied species such as cephalopods, small haddock, whiting (Merlangius merlangus) and hake (Merluccius merluccius), because the gear is likely to allow large numbers to escape. Selectivity studies often focus on a short list of target species; however, catches of non-target species under quota can be problematic for some fisheries. For example, under the implementation of the Landing Obligation catches of boarfish could choke the French whitefish demersal fisheries in the Celtic sea, as France has no national quota for that species. The device tested constitutes an efficient solution to mitigate catches for such non-target schooling fish.

This is the final report of the European Commission funded research project "DRuMFISH" (service contract n° EASME/EMFF/2014/l.3.2.4/ SI2.721116). The main aim of the project was to develop models and strategies for providing advice for mixed fisheries that account for: (i) fishing mortality ranges consistent with MSY, (ii) all fish caught being landed, and (iii) significant components of the marine fish ecosystem lacking key biological information. In order to meet this aim, DRuMFISH delivered a review of assessment approaches for data-poor stocks, extended mixed fisheries simulation frameworks to include data-poor stocks. The assessment approaches and simulation frameworks were implemented in 7 case studies. These case studies were mixed fisheries in the Baltic Sea, the North Sea, the Celtic Sea, the Bay of Biscay, the Western Mediterranean, the Adriatic Sea, and the Aegean Sea. Within the case studies, 35 data-poor stock assessments were done. These assessments provided exploitation status of data-poor stocks. Different harvest control rules were subsequently tested for their expected yields and stock biomasses from the mixed fisheries in the simulation frameworks. Now that data-poor stocks can be incorporated with in the mixed fisheries simulation frameworks, the design of new management plans can account for data-poor stocks in mixed fisheries.

For the ecosystem approach to fisheries management, understanding population dynamics and ecosystem resilience in response to the landing obligation acted by the reform of the European Common Fishery Policy (CFP) is currently an important avenue of research. This study attempts to assess the impact of the new CFP on marine food webs. Total carbon and nitrogen loss induced by a discard ban were estimated for the ecosystem of the Bay of Biscay based on French At-Sea Observer data and carbon and nitrogen content of discarded fish. Changes in predator diets were estimated by comparing two Bayesian mixing models, one mimicking the current situation and the second the application of the new CFP (removing the discarded species that would be landed from the model). Results illustrate a substantial shift in predation effort toward remaining available prey. The impact of changes in diet on individual and population health and on ecosystem functioning are important issues that remain to be assessed. A sister study could be carry out in another ecoregion, at a finer spatial scale in order to compare impacts and ecosystem responses. European monitoring programs such as DC-MAP and the Marine Strategy Framework Directive (MSFD) would help us to gain an understanding of how the food web would reorganize itself.

Les recherches en halieutique ont pour objectif d’améliorer les connaissances sur le fonctionnement des populations de poissons afin de transférer celle-ci vers des outils de gestion. Ce travail de thèse repose sur un ensemble d’expériences et de modélisations destinées à approfondir notre compréhension générale du comportement associatif de poissons grands pélagiques avec des objets flottants à la surface de l’océan. Notre objectif est de tester si les milliers d’objets flottants artificiels déployés par les pêcheurs (DCP - Dispositifs de Concentration de Poissons) constituent des pièges écologiques pour les thons tropicaux. Pour mener à bien ce travail, nous avons dans le premier chapitre caractérisé le comportement individuel de thons dans un réseau de DCP ancrés. L’analyse de données de marquage acoustique de 96 thons albacores (Thunnus albacares) (30-96 cm) à Hawaii montre que les thons présentent une plasticité comportementale forte face aux DCP qu’ils rencontrent mais également que le temps de résidence sous les DCP diminue avec la taille des individus. Afin de quantifier l’impact de l’augmentation de la densité de DCP il est essentiel de comprendre les mécanismes et les facteurs qui influencent les temps de résidences sous les DCP. Dans le second chapitre, des expériences de choix binaires suggèrent un rôle de la biomasse agrégée dans la formation, la maintenance et la dispersion des agrégations que forment les thons sous les objets flottants. La quantification de la dynamique des arrivées et des départs des poissons aux DCP permettra de valider les hypothèses que nous proposons concernant les mécanismes sociaux sous-jacents d’une part et d’autre part de tester l’influence de la qualité de l’environnement et de la densité de DCP sur les temps de résidences individuels et la distribution spatiale des populations. Dans le troisième chapitre, la comparaison de facteurs de condition de listaos (Katsuwonus pelamis) matures capturés en bancs libres et sous objets flottants dans une zone naturellement riche en objets flottants, et relativement peu impactée par le déploiement de DCP (Le Canal du Mozambique), nous a permis d’établir un point de référence essentiel pour estimer les effets des perturbations actuelles. Plus généralement, les résultats obtenus dans les différents chapitres tendent à conforter l’hypothèse d’un rôle social et non trophique des objets flottants dans l’écologie de thonidés. Les résultats obtenus durant cette thèse amènent à poser un regard nouveau sur l’hypothèse du piège écologique. Notre travail s’est principalement intéressé à un modèle biologique de choix, les thons tropicaux. Cependant le cadre théorique des questions abordées, les outils d’observations et les méthodes d’analyses développées sont assez génériques pour être appliqués aux autres espèces rencontrées sous les objets flottants. Cette recherche s’intègre plus généralement dans les problématiques visant à mieux comprendre les stratégies comportementales et la distribution des populations dans des environnements multi-sites., Doctorat en Sciences agronomiques et ingénierie biologique, info:eu-repo/semantics/nonPublished