This study provides a historical overview of the use of drifting fish aggregating devices (DFADs) in purse seine fisheries since the early 1990s, using global tuna fisheries datasets from the four tuna Regional Fisheries Management Organizations (RFMOs). Tropical tuna purse seine fisheries typically target large yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) tunas on free-swimming schools and skipjack (Katsuwonus pelamis) and juveniles of yellowfin and bigeye associated with drifting objects. DFADs have enabled global skipjack catches to markedly increase, and have also introduced major scientific issues for all tuna-RFMOs. In particular, they have strongly modified the fishing strategies of purse seiners that fish on a combination of free-swimming and DFAD-associated schools. Consequently, the cumulated search time traditionally used to quantify nominal fishing effort to assess the status of tuna stocks is inconsistent and cannot be used to derive time series of abundance indices from catch-per-unit of-effort (CPUE). In addition, the lack of information available on the construction, deployment, and use of DFADs has prevented effective monitoring of the fishing pressure over the last two decades exerted by purse seine fleets using this fishing mode. Juveniles of tropical tunas represent a substantial proportion of purse seine catch on DFADs in the three oceans, which has raised particular concern for some bigeye stocks that have been subject to overfishing in the past. Catches of juvenile tunas by DFAD fishing may also result in a decrease in recruitment for fisheries that target adult tunas such as longliners. In addition, some demographic parameters of tunas and other species associated with DFADs may be affected by the resultant habitat modification arising from the widespread deployment of DFADs. Evidence in the literature and provided by the ratio-estimator method suggest that fishing DFAD-associated schools may result in about 100 000 t of bycatch and discards annually. In addition, there is further potential for ghost fishing related mortality of sensitive species such as marine turtles and pelagic sharks. In this context and following a precautionary approach, we finally discuss the increasing need for all tuna-RFMOs to reduce, or at least monitor and control, the use of DFADs to mitigate their adverse effects not only on yellowfin and bigeye stocks but also on open-ocean ecosystems.

This paper is based on recently recovered daily catch and effort data from the Algarve traps (south of Portugal) during the 1898−1900 period. These 18 traps targeted the two migrations of bluefin tuna, entering and leaving the Mediterranean Sea, each year during a period between mid April and the end of August. Their bluefin catches were significant, corresponding to average yearly catches of 61 000 bluefin, i.e., approximately 7900 t. Migration dates and durations towards and away from the Mediterranean Sea were estimated based on the daily catch per unit effort (CPUE) by the traps. Migration patterns were very stable during these three years. The entry migration took place over an average of 42 days, and the return lasted 53 days. Daily catches show large waves of tunas passing during the two migratory phases. It can be hypothesised that each of these waves of tuna spent about 52 days in the Mediterranean Sea, and that each group was stable and consistent during their arrival and return migrations. Migration trajectories along the Algarve coast can be estimated from the daily CPUEs of the different traps. They appear to be different between the arrival and return migrations. The daily data per trap show that most traps caught entering bluefin at quite low CPUE, whereas only a few traps were efficient for catching the returning bluefin, but with much larger catches and CPUEs. The analysis of catch and CPUEs during the entering and leaving phases allowed us to conclude that the catchability and fishing mortality of the bluefin stock in the Algarve trap fishery was much larger during the return migration. Additional daily data from other traps, both historical and contemporary, should be collected and analyzed in order to provide more comprehensive results on the patterns and changes in migration, stock structure and stock exploitation rate.

We analysed the patterns of variation that characterize 33 catch time series of large pelagic fishes exploited by the Japanese and Taiwanese longline fisheries in the Indian Ocean from 1968 to 2003. We selected four species, the yellowfin (Thunnus albacares), the bigeye (T. obesus), the albacore (T. alalunga), and the swordfish (Xiphias gladius) and aggregated data into five biogeographic provinces of Longhurst (2001). We carried out wavelet analyses, an efficient method to study non-stationary time series, in order to get the time-scale patterns of each signals. We then compared and grouped the different wavelet spectra using a multivariate analysis to identify the factors (species, province or fleet) that may influence their clustering. We also investigated the associations between catch time series and a large-scale climatic index, the Dipole Mode Index (DMI), using cross wavelet analyses. Our results evidenced that the geographical province is more important than the species level when analyzing the 33 catch time series in the tropical Indian Ocean. The DMI further impacted the variability of tuna and swordfish catch time series at several periodic bands and at different temporal locations, and we demonstrated that the geographic locations modulated its impact. We discussed the consistency of time series fluctuations that reflect embedded information and complex interactions between biological processes, fishing strategies and environmental variability at different scales.

The paper analyzes the daily catch, fishing effort and fish size data of the purse seine fleet fishing in the western Indian Ocean in February 2005, when a major concentration of tuna occurred and was heavily exploited by this surface fishery. This tuna concentration event occurred over a period of just 12 days, in an area of about 3500 square nautical miles located to the west of the Seychelles. This small stratum produced a total catch of 22 000 t, corresponding to 6.5% of the total fishing mortality of all adult yellowfin (Thunnus albacares) in the entire Indian Ocean in 2005. Sets were made mainly on free schools and the catch mainly composed of large yellowfin tuna. The average CPUE and the average catch per set were very large, 65 t and 85 t per fishing day, respectively. This “event” took place in a precise area where a high concentration of chlorophyll had been localized 18 days before. The subsequent concentration of tuna schools probably arose due to the high densities of their prey feeding on this large phytoplankton biomass. This phytoplankton bloom was observed at the edge of an anticyclonic eddy, but its origin and its high density cannot be fully explained by available environmental data. The adult yellowfin were probably at a reproductive stage and actively feeding on the local food chain generated by the phytoplankton bloom. Such an event is extreme, but typical of tuna purse seine fisheries where fleets often search for such tuna patches. These events play an important role in tuna fisheries due to the increase in fishing effort and fishing efficiency of purses seiners, both of which increase their impact on the resource. The fine scale study of such events and their improved integration into tuna stock assessments is recommended.

Yellowfin tuna (Thunnus albacares) are known to preferentially occupy the surface mixed layer above the thermocline and it has been suggested that they are physiologically restricted to water temperatures no more than 8 °C colder than surface waters. However, we here report for dive data acquired from a large yellowfin tuna which demonstrate for the first time that this species is indeed capable of making prolonged dives into deep cold waters. A yellowfin tuna (134 cm fork length) caught near an anchored fish aggregating device (FAD) in the Seychelles (Western Indian Ocean) was equipped with an internally implanted archival tag and released. The fish was recaptured 98 days later. As predicted for this species, this fish spent 85% of its time shallower than 75 m (maximum thermocline depth experienced by the fish) but, over the course of the track, it performed three deep dives to 578 m, 982 m and 1160 m. Minimum ambient water temperatures recorded at these depths were 8.6 °C, 7.4 °C and 5.8 °C respectively and varied by up to 23.3 °C from surface temperatures. The fish spent 8.3% of its time in waters more than 8 °C colder than the surface layer and daily experienced a wide range of sea temperatures (mode at 15-16 °C) and of temperatures of the gut cavity (mode at 6 °C). The reason for these dives can not be known. These depths and temperatures significantly exceed those reported in the literature so far and clearly demonstrate that this species has the physiological and behavioral ability to penetrate deep cold sections of the ocean.

Even when fishing effort data are not available in a developing fishery, it is often still possible to develop a simple fishery indicator to obtain information about the status of the stock. Using catch data only, Grainger and Garcia, (FAO Fish. Tech. Pap. 359, 1996) showed that the changes over time in the relative rate of catch increase (RRCI) can be used to detect when a stock reaches its over-fishing level, i.e., the year when RRCI falls to zero. The method presupposes that fishing effort increased steadily over the period concerned. We propose a generalization of this method that consists in obtaining a crude estimate of the maximum sustainable yield (MSY) by plotting the trend in catches against the smoothed RRCI. The yellowfin tuna (Thunnus albacares) fishery in the Eastern Atlantic was used to show the bold relationship between MSY estimates obtained from standard equilibrium production models and from this method. Given that it is very difficult to estimate effective fishing effort for skipjack tuna (Katsuwonus pelamis), we show how this simple fishery indicator can be used to obtain proxies of MSY for skipjack fisheries located in the Eastern Atlantic Ocean and in the Indian Ocean.

This paper compares the species composition of free swimming schools and schools associated with fish aggregating devices (or FADs) observed by scientists in the eastern equatorial Atlantic during the early eighties and late nineties. This comparison shows that in free swimming schools, big changes in the species composition have occurred. The main change is a rarefaction of mixed species free schools (skipjack and small yellowfin or bigeye). This change is probably a real biological one, and possibly a consequence of the large numbers of FADs seeded in the area since 1990. Nowadays, most small tuna living in the equatorial area appear to be concentrated under these drifting FADs instead of in free schools. Further study is recommended in order to evaluate the validity and interpretation of this result and to examine its implications.

A new fishing method has been developed during the last twelve years by the tuna baitboats fishing seasonaly (from July to December) in the area of Senegal and Mauritania (West Africa, 12 to 21° N). Three species of tunas, yellowfin, skipjack and bigeye tunas are caught in similar proportions in this fishery. This method has produced a dramatic increase in the baitboat catch per unit of effort (CPUE). The paper describes this new fishing method where the boat is permanently running associated with a very large tuna school, night and day, during several months. A team of two baitboats exploits each school, operating one after the other. The biological and environmental characteristics of this associated-school fishing are analyzed from detailed log book data. The daily positions of each school and the catches by species and sizes are analyzed for each boat and school. The dynamics of the association between the tunas and the baitboat and the dynamics of this exploitation (i.e. input and output of tunas in each school) are discussed in relation to the peculiarities of the local environment (mainly sea surface temperature and bathymetry). The reasons for the increased CPUE in this associated-school fishing are analyzed. Further researchesshould be conducted in order to better understand how fish behaviour relates to this fishing mode and in order to estimate the potential local yield of the fishery, if increased fishing effort is exerted in the future.

Une méthode d'ajustement de l'analyse des cohortes, combinant calcul en mode inverse et calcul en mode direct, est présentée ; elle permet d'analyser l'évolution des puissances de pêche par flottille et par âge, à partir de la matrice des capturabilités nominales. La méthode est appliquée aux deux principales flottilles exploitant l'albacore dans l'Atlantique Est, entre 1980 et 1990 : les senneurs franco-ivoiro-sénégalais (FIS) et les senneurs espagnols. A l'aide d'un modèle linéaire généralisé, les puissances de pêche globales sont décomposées en deux termes : puissances locales et efficiences globales. Les évolutions tendancielles des trois grandeurs, puissances globales, puissances locales et efficiences globales, par catégorie d'âge ou en moyennes, sont analysées en calculant des taux d'accroissement inter-annuels sur des intervalles pluriannuels. Les changements de tactiques de pêche sont étudiés au travers des corrélations entre puissances de pêche résiduelles appliquées à des groupes d'âge différents.

Les résultats obtenus sont peu sensibles aux paramètres introduits lors de l'ajustement de l'analyse des cohortes. Sur l'ensemble de la décennie, un accroissement des puissances de pêche globales moyennes est mis en évidence, avec un taux estimé à 17 et 9 % par an pour chacune des flottilles FIS et espagnole respectivement. Cet accroissement est lié à une forte augmentation des puissances de pêche globales appliquées aux adultes, alors que celles appliquées aux juvéniles semblent en diminution. Les changements de l'efficience de pêche globale jouent un rôle essentiel dans le cas de la flottille FIS. Trois schémas type d'évolution, traduisant des stratégies de pêche différentes entre flottilles et entre périodes, sont définis ; l'impact des changements de tactique de pêche est souligné. Les raisons susceptibles d'expliquer les variations des puissances de pêche sont discutées, ainsi, plus généralement, que les potentialités et limites de la méthode d'analyse utilisée.

Le modèle composite, communément admis pour décrire les deux stances de croissance de l'albacore dans l'Atlantique Est, est ré-examiné. La valeur du paramètre t0 est corrigée. L'équation de croissance est exprimée en fonction d'un âge conventionnel. Les limites de ce modèle sont soulignées. Différents modèles continus sont ensuite ajustés sur les données de Capisano et Fonteneau (1991), qui estiment l'âge par décomposition polymodale des fréquences de tailles des captures réalisées par les senneurs, durant la période 1983–1988. On montre que l'ajustement des modèles à 3 ou 4 paramètres (Gompertz, Richards et Von Bertalanffy généralisé) est peu satisfaisant. On propose un modèle à 5 paramètres, combinant une fonction linéaire et une loi de Von Bertalanffy généralisée. La validité et l'ajustement de ce modèle sont analysés. Il permet de décrire correctement la croissance dans l'intervalle 40 à 150 cm de longueur à la fourche et 0,25 à 4,25 ans. Afin de disposer d'une équation unique, couvrant l'ensemble de la phase exploitée jusqu'à 170 cm de longueur et 6,0 ans, le modèle est ré-ajusté à des données simulées à partir des modèles précédents. La croissance au sein de la phase exploitée s'exprime alors comme suit :

L(t) = 37,8 + 8,93×t + (137,0 - 8,93×t) × [l - exp(-0,808×t)]7,49

La fiabilité de cette équation est discutée, en relation notamment avec l'existence probable d'un dimorphisme sexuel de croissance. Des hypothèses sont développées pour expliquer l'existence d'une stance de croissance juvénile lente, suivie d'une accélération de croissance au voisinage de 47 cm, puis d'un point d'inflexion au voisinage 90 cm. En particulier, la succession des 2 stances de croissance semble liée aux migrations de l'espèce, à l'intérieur de l'Atlantique Est et vers l'Atlantique Ouest.

Il est bien connu des pêcheurs que les monts sous-marins, nombreux dans l'Atlantique tropical est, concentrent parfois de fortes biomasses de thons. Cette association thons et monts sous-marins n'a paradoxalement fait l'objet d'aucun programme de recherches. Le présent article fait un bilan des captures de thons réalisées par les flottilles thonières internationales sur les monts sous-marins connus dans l'Atlantique tropical est durant les années récentes (période 1980 à 1987). La prise annuelle de thons sur certains monts sous-marins peut atteindre plusieurs milliers de tonnes. Les statistiques de pêche thonière montrent que certains monts sous-marins sont riches en thons, alors que d'autres semblent pauvres en permanence, bien qu'ils soient situés dans des zones prospectées par les pêcheurs. Les particularités biologiques de ce type de pêche sont analysées : les captures sont en général plurispécifiques; albacores (Thunnus albacares), listaos (Katsuwonus pelamis) et patudos (Parathunnus obesus) sont pêchés en proportions voisines avec des individus de tailles relativement petites en comparaison des zones voisines. Des hypothèses sont développées sur les causes et sur la dynamique de l'agrégation des thons autour de ces hauts-fonds en utilisant les statistiques de pêche journalières et les résultats de marquages classiques et par marques émettrices réalisés sur un mont sous-marin. Les perspectives de recherches sur l'association sous-marins et thons et sur l'exploitation rationnelle de ces micro-structures sont discutées. Les positions et les profondeurs des monts sous-marins étant actuellement facilement repérées grâce aux satellites à radar altimétrique, la localisation systématique de ces structures est possible et revêt pour les pêcheurs un intérêt potentiel évident pour réduire les temps de prospections des thoniers.