Eighty-seven two-hour acoustic surveys (radius 0.8 nautical mile, vertical range 0–500 m) around 17 fish aggregating devices (FADs) were conducted in French Polynesia between December 1995 and February 1997. Associated tuna densities were calculated using two different techniques: echo counting when the fish had sufficient distances from each other and echo integration when the fish swam close together (in schools). No acoustic detection of tuna was observed during 27 of the 87 surveys, representing 81 % of all the nocturnal surveys and 15 % of the diurnal ones. The 60 other surveys showed three different classes of aggregations: (1) ‘deep scattered fish’, observed 45 times, (2) ‘intermediate scattered fish’, observed 16 times, and (3) ‘shallow schooling fish’, observed 16 times. Sometimes aggregations of different classes were observed beneath the same FAD. The size of the fish inside the aggregations (determined from target strength values), the distance between the individuals, and the depth of the fish all decreased from ‘deep scattered fish’ to ‘shallow schooling fish’ (100–300 m for ‘deep scattered fish’, 50–150 m for ‘intermediate scattered fish’, and above the depth of 50 m for ‘shallow schooling fish’). Fish densities also varied according to the class of aggregations: 7.3, 26, and 801 fish per km3 on average for ‘deep scattered fish’, ‘intermediate scattered fish’, and ‘shallow schooling fish’, respectively. The highest densities were observed during daytime, while night-time observations indicated a variety of situations, from the absence of individuals to large amounts of fish.

This article describes tracking experiments conducted on eleven yellowfin tuna using ultrasonic transmitters in French Polynesia between 1985 and 1997. Nine fish were caught near Fish Aggregating Devices (FADs) while the other two were tracked in coastal areas without FADs. The fish showed different patterns of horizontal movements: tight associations with FADs lasting several days, foraging movements confirmed by simultaneous acoustic observations of prey-sized fauna, movements parallel to the shore, and traveling between FADs. This intra- and inter-individual variety of behaviour might depend on the local environment (prey), and on individual biological differences. The influence of FADs, coastlines, and prey on tuna movements is discussed. The lack of information about the surrounding environment, the internal state of the fish and the recent history of the fish usually prevent scientists from adequately interpreting the observed movements. Ideas for future research to studying tuna behaviour near FADs are discussed.

A fuzzy logic model of tuna behaviour near Fish Aggregating Devices (FADs) was developed to reproduce individual differences in horizontal movements observed from ultrasonic telemetry experiments. In this model, the behaviour of an individual is based on its surrounding environment (FADs and prey) and on its internal state (stomach fullness), which depends on its recent past actions. Internal sensors are used to determine the motivation of the fish, combined with external sensors, this determines its movements. Sensory information and motivation are modeled using fuzzy sets. A FAD attracts an individual when it is located within the FAD’s range of influence. The time spent near a FAD depends on the feeding motivation of the fish and on its surrounding environment. If the fish is not hungry, it stays near the FAD. Otherwise, the fish has to forage in order to eat, and might therefore leave the FAD if no prey is available in its vicinity. By varying the environmental conditions near FADs, the model reproduces the different horizontal movement patterns observed for tunas. The model is then extended to allow multiple individuals to co-exist, each individual modeled through the above behavioural model, without any direct or indirect interactions between them. This way, we study the effects of individual behaviour on tuna aggregation near FADs. We find that the model predicts the temporal dynamics of aggregation around FADs exhibited by tunas. By examining the effects of several FAD network models on the aggregation, we also estimate optimal spatial arrangements of FADs.

In Hawaii, a variety of small- and medium-scale pelagic fisheries target fishing effort on a network of coastal moored FADs, natural inshore tuna aggregation points, offshore seamounts and offshore weather monitoring buoys. Large-scale longline vessels also operate in the Hawaii exclusive economic zone (EEZ) and beyond. These circumstances provide an ideal setting for tag-and-release experiments designed to elucidate the movement patterns, residence times, exchange rates and vulnerability of bigeye tuna (Thunnus obesus) and yellowfin tuna (Thunnus albacares) within the Hawaiian EEZ. Preliminary recapture data indicate that FADs, island reef ledges and seamounts exert an overwhelming influence on the catchability of tuna. Recapture rates from these locations vastly outweigh tag returns from open water areas. As of August 31, 1999, a total of l5387 bigeye and, yellowfin tuna ranging in size from 29 to 133 cm fork length (FL) and from 26 to143 cm FL respectively (mean 59.8 ± 14.1 cm; 58.4 ± 17.3 cm) have been tagged and released throughout the Hawaii EEZ. Recapture rates for both species have been similar with an overall recapture rate of l0.3 %. The location of tag releases reflects the importance of associative behavior and schooling to the vulnerability of tuna; seamounts and FADs accounted for 72.4 % and 23.5 % of all tag releases. Within the main Hawaiian Island group (excluding the offshore seamounts and buoys), 83.1 % of all recaptures have been made on anchored FADs and 11.9 % of recaptures have come from ledges or tuna aggregation areas close to the islands where bigeye and yellowfin tuna become vulnerable to hook and line gear. As these studies continue, additional and longer-term recaptures will provide increasingly detailed information on the movement patterns and vulnerability of bigeye and yellowfin tuna as they grow, move and recruit to different fisheries.

Yellowfin and bigeye tunas (Thunnus albacores and Thunnus obesus) were tagged and released between August 1995 and December 1997 at Cross Seamount and NOAA weather buoys about 200 Nmi south of Honolulu. The release and recapture data were stratified into five sites, and a bulk transfer model was used to estimate natural mortality, fishing mortality and transfer rates between the five sites. Bigeye are much more persistent at Cross seamount and less vulnerable to the fishery than yellowfin. Fishing accounts for about 5 % of the total mortality of both bigeye and yellowfin at Cross Seamount. Yellowfin are a major component of catches at inshore FADs in Hawaii. The rate of immigration from Cross Seamount to the inshore FADs is very low for both species. The fishing mortality at Cross seamount is substantial but is not adversely impacting the populations either at Cross or the inshore FADs.

Since 1991, fishing operations on tuna schools associated with drifting Fish Aggregating Devices (FADs) have become widespread in the purse seine fishery in the Gulf of Guinea. In the offshore South Sherbro area (0–5° N, 10–20° W), FAD-associated catches represent about 75 % of the total catch. This FAD fishery exploits concentrations of skipjack mixed with a smaller amount of bigeye and yellowfin tuna of similar size (46 cm), and some large yellowfin. Catches on unassociated tuna schools are mainly composed of large yellowfin in breeding phase and skipjack. Here we studied tuna diet in relation with the aggregation mode (FAD-associated or unassociated tuna schools), species, and size. The stomach contents of around 800 fish were analysed. Numerous empty stomachs were found, especially in fish caught under FADs. Diets were similar for all small-size tuna sharing the same aggregation type. Small tuna mainly feed on Vinciguerria nimbaria (Photichthyidae), a mesopelagic fish of the micronekton, whereas large tuna mainly feed on Scombridae, mixed with Cubiceps pauciradiatus (Nomeidae) when they were caught in unassociated schools. The feeding habits of tuna are discussed with emphasis on the behavior of V. nimbaria. Estimations of the daily ration of similarly sized tuna with the same aggregation mode were very close. The low estimated rations for small, FAD-associated tuna show that logs do not have a trophic function, but rather are a refuge. In contrast, FADs seem to influence the diet of large tuna because of the Scombridae prey that probably is associated to the FAD.

Depuis une quinzaine d’années le développement des pêcheries artisanales des pays insulaires est basé sur la pêche des grands pélagiques à partir de l’introduction d’une innovation majeure, à savoir l’immersion de dispositifs de concentration de poissons (DCP ancrés). Face à cet engouement récent et aux résultats très mitigés, voire aux échecs, souvent observés, l’article propose une démarche méthodologique d’élaboration d’un cadre d’évaluation permettant un classement de la diversité et complexité des transformations induites par l’introduction des DCP et une estimation de la durabilité de leur intégration. La notion de durabilité intègre ici des objectifs à la fois écologiques quant à la conservation de la ressource et socioéconomiques en termes de maîtrise et d’acceptation d’une innovation devant favoriser l’évolution du système. Cette analyse est effectuée en croisant plusieurs points de vue disciplinaires – biologique, géographique, économique et sociologique – et en comparant différentes expériences relevant de contextes et de communautés de pêcheurs variés. Le cadre d’évaluation se compose d’une série de 21 indicateurs de nature biologique, technique, économique et sociale. Une illustration est proposée à partir de trois exemples représentatifs de la diversité des situations : La Réunion, les Comores et l’archipel du Vanuatu. Sur la base d’une évaluation aux dires d’experts, chacun de ces critères fait l’objet d’une notation selon une échelle de 0 à 5 en fonction du caractère inexistant, incertain, notable ou pérenne des effets observés. Enfin dans un souci de d’opérationnalité et d’aide à la décision, une présentation graphique de synthèse est élaborée pour faciliter les comparaisons et identifier les points critiques.

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.

This paper examines the fishing of blackfin tuna (Thunnus atlanticus) around Fish Aggregating Devices (FADs) in Martinique (French West Indies). It is based on the compared analysis of catches from monthly experimental fishing surveys and sampling of commercial landings. The data collected in these two different ways allowed comparison of blackfin tuna length frequencies. A large part of the commercial landings were made up of young immature tuna with a fork length of less than 40 cm, whereas the experimental longline catches were mainly made up of fish with a fork length ranging between 55 and 75 cm. We give evidence that these discrepancies were mostly due to the fishing technique used. Indeed, contrarily to the experimental surveys, Martinican fishermen only fish during daytime and usually on the surface by trolling. This way, fishermen do not have access to the largest fish, which are found deeper. Our results suggested that a new fishing technique such as vertical longlining, could improve commercial catches of big blackfin tuna under FADs.