Within the tropical and subtropical oceans, tuna forage opportunistically on a wide variety of prey. However, little is known about the trophic ecology of the smallest size classes which play an important role in stock assessments and fisheries management. The foraging behavior of yellowfin tuna, Thunnus albacares (23.5–154.0 cm FL), collected from nearshore Fish Aggregating Devices (FADs) around Oahu was studied using stable isotope and stomach contents analyses. Emphasis was placed on small juveniles. Yellowfin tuna changed their diets significantly between 45 and 50 cm forklength (ca. 1.5 kg). Smallest size classes fed on planktonic organisms inhabiting the shallow mixed layer, primarily larval stomatopod and decapod crustaceans, whereas larger tuna fed on teleosts and adult Oplophorus gracilirostris, a vertically migrating mesopelagic species of shrimp. When interpreting the variation in prey δ
 15N values, we considered both their relative trophic position and δ
 15N values of the nitrogen at the base of the food web. Based on the distinct diet shift of the yellowfin tuna, demonstrated by both isotope and stomach content analyses, we propose a critical mass threshold was reached at about 45 cm FL that enabled sufficient endothermic capability to allow tuna to access prey dwelling in deeper, colder water. These ontogenetic changes in foraging range and commensurate shift in diet of small tunas would affect their vulnerability to fishing pressure.

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A rapid ontogenetic shift in the diet of juvenile yellowfin tuna from Hawaii

The Mobulidae are zooplanktivorous elasmobranchs comprising two recognized species of manta rays (Manta spp.) and nine recognized species of devil rays (Mobula spp.). They are found circumglobally in tropical, subtropical and temperate coastal waters. Although mobulids have been recorded for over 400 years, critical knowledge gaps still compromise the ability to assess the status of these species. On the basis of a review of 263 publications, a comparative synthesis of the biology and ecology of mobulids was conducted to examine their evolution, taxonomy, distribution, population trends, movements and aggregation, reproduction, growth and longevity, feeding, natural mortality and direct and indirect anthropogenic threats. There has been a marked increase in the number of published studies on mobulids since c. 1990, particularly for the genus Manta, although the genus Mobula remains poorly understood. Mobulid species have many common biological characteristics although their ecologies appear to be species-specific, and sometimes region-specific. Movement studies suggest that mobulids are highly mobile and have the potential to rapidly travel large distances. Fishing pressure is the major threat to many mobulid populations, with current levels of exploitation in target fisheries unlikely to be sustainable. Advances in the fields of population genetics, acoustic and satellite tracking, and stable-isotope and fatty-acid analyses will provide new insights into the biology and ecology of these species. Future research should focus on the uncertain taxonomy of mobulid species, the degree of overlap between their large-scale movement and human activities such as fisheries and pollution, and the need for management of inter-jurisdictional fisheries in developing nations to ensure their long-term sustainability. Closer collaboration among researchers worldwide is necessary to ensure standardized sampling and modelling methodologies to underpin global population estimates and status.

Biology, ecology and conservation of the Mobulidae.

The use of fish aggregating devices (FADs) by purse seine fisheries has come under increasing criticism for its potential deleterious impacts on tuna stocks, for high levels of by-catch and threats to the biodiversity of tropical pelagic ecosystems. Here, we review the current state of scientific knowledge of this fishing technique and current management strategies. Our intent is to encourage objective discussion of the topic and highlight areas worthy of future research. We show that catching juvenile tuna around FADs does not necessarily result in overfishing of stocks, although more selective fishing techniques would likely help obtain higher yield. Levels of non-tuna by-catch are comparable to or less than in other commercial tuna fisheries and are primarily comprised of species that are not considered threatened. Accordingly, to minimize impacts on ecosystem balance, there is merit in considering that all species captured in purse seine fisheries (excluding vulnerable species such as turtles and sharks) should be retained, but the consequences of such a measure should be carefully examined before implementation. The take of vulnerable species could be further reduced by introduction of additional mitigation measures, but their potential benefits would be limited without parallel efforts with other gears. Finally, there is no unequivocal empirical evidence that FADs represent an ‘ecological trap’ that inherently disrupts tuna biology although further research should focus on this issue. We encourage RFMOs to expand and improve their FAD management plans. Under appropriate management regimes, FAD fishing could be an ecologically and economically sensible fishing method.

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Is it good or bad to fish with FADs? What are the real impacts of the use of drifting FADs on pelagic marine ecosystems?

Purse seine fishing on fish aggregating devices (FADs) has expanded considerably during the last 15 yr in tropical tuna purse seine fisheries, and FADs currently account for about 70 % of their reported tuna catches. The scientific community has expressed concern over the consequences of this fishing practice in terms of yield per recruit and suspected detrimental effects on FAD-associated tunas. To explore possible detrimental effects, we compared stomach fullness, fish plumpness, growth rate, and migration behaviour between free school and drifting FAD-associated tunas for skipjack tuna Katsuwonus pelamis and yellowfin tuna Thunnus albacares caught in the Atlantic and Indian Oceans. Significant differences in fish plumpness and individual growth rates were found, suggesting that individuals associated with drifting FADs were less healthy than those in free schools. Since stomach fullness indicated that tunas associated with FADs eat less than those in free schools, the difference in growth rate and condition could be the consequence of altered feeding patterns. For each species, significant changes in migratory direction and displacement rates were observed in the presence of drifting FADs. These findings support the hypothesis that FADs act as a super-stimulus, misleading tunas to make inappropriate habitat selection. However, further studies are required to investigate the long-term effect of FADs on the entire life cycle of tunas.

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Drifting fish aggregation devices could act as an ecological trap for tropical tuna species

This paper develops a comparative overview of the development of Fish Aggregating Devices (or fads) by the tropical purse seine fisheries in various ocean areas (Atlantic, Indian, Western and Eastern Pacific Oceans). First, a comparative review of fishery statistics is developed. Recent catches on fads by purse seiners can be estimated at a level of one million tons yearly. This comparison allows to describe the fishing zones and catch trends over time, the species composition and size taken in the various fad fisheries. Skipjack tuna is the dominant species in most fad fisheries, but significant amounts of yellowfin and bigeye are also frequently taken under fads. The paper also presents an overview of various observer data concerning the by-catches of the various fisheries using fads. This comparison allows to estimate the total worldwide yearly by-catches at about 100,000 t, and their species composition in each ocean. The fad technology used in the various oceans by purse seiners is described. The recent use of fads by purse seiners has introduced major uncertainties in most stock assessments, because analyses are hampered by changes in the fishing effort in a fad fishery, by changes in fishing zones and in sizes caught. As a last point, the management of fads presently done or in view by the various tuna bodies are introduced. It appears that the present massive use of fads worldwide is perhaps an unsafe fishing mode, which could produce serious overfishing of many stocks. There is then a consensus that the use of fads needs to be controlled and limited to sustainable biological levels.

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A worldwide review of purse seine fisheries on FADs

We investigated the effects of a purse-seine fishery with drifting fish aggregating devices (FADs) in the South Sherbo area of the Equatorial Atlantic, located between 0–5N and 10–20W. There had been no surface fishing activity in the area until 1975. Since 1991, fishing operations on schools of tuna associated with FADs has become widespread and this offshore area has developed into a major fishing zone. Exploitation rates are high between November and January. The fishery exploits multispecies concentrations of skipjack (71%), bigeye (15%), and yellowfin (14%) tunas of similar size (mode: 46-cm forklength). The use of FADs increased the vulnerability of small tunas and induced changes in fishing patterns. The mean individual weight of skipjack caught has decreased since 1991, due either to overfishing or to a growth change. Data from scientific observers were used to estimate discards and by-catches generated by FAD fishing during 1998. Discards of tunas (including frigate and little tunas) represented 7.6% of the total catch. Other by-catch (dominated by wahoo, billfish, triggerfish, sharks, barracudas, and dolphinfish) represented 2.3%, including 0.4% discarded at sea. Stomach content analysis showed that a mesopelagic species, Vinciguerria nimbaria (Photichthyidae), which during daylight concentrated in the upper layers in dense schools, was the main prey of all small tunas. The South Sherbro area appears to have exceptional environmental conditions. It is suggested that the FAD fishery may have wide-ranging effects on the migration of tunas in general and on the productivity of the skipjack population in particular.

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Exploitation of small tunas by a purse-seine fishery with fish aggregating devices and their feeding ecology in an eastern tropical Atlantic ecosystem

By-catch and tuna discards estimation and characteristics for the various species groups are presented for the European purse seine tuna fishery (France and Spain) for the period 2003-2007. Data were collected through French and Spanish observer programs representing a total of 1,958 observed fishing sets. Total by-catch and tuna discards estimation were derived from a stratification based on fishing mode, seasons (quarters) and spatial area. The estimation relied on raising factors based on major commercial tuna catch expressed in tons per 1000 t of tuna landed. Total tuna discards and by-catch was estimated to be about 9,585 t corresponding to 35.5 t per 1000 t of tuna landed. Tuna discards represents 54 % (19.2 t/1000 t) of the total amount, followed by fishes (33.7 %, 12.0 t/1000 t), sharks (10.1, 3.6 t/1000 t), billfishes (1.5 %, 0.5 t/1000 t), and rays (0.7 %, 0.2 t/1000 t). By-catch species composition, main species length and sex structure, and percentage of utilization are also presented. Total by-catch estimates and ratios are compared with those previously published in the literature. The issue raised by the choice of a consistent extrapolation method is finally discussed.

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By-catch and discards of the european purse seinetuna fishery in the Indian Ocean : estimation and characteristics for the 2003-2007 period

Machine learning for characterizing tropical tuna aggregations under Drifting Fish Aggregating Devices (DFADs) from commercial echosounder buoys data

Abidjan is the biggest fishing harbour of tropical tunas in the Gulf of Guinea (around 100000 Mt per year). Three canneries work on four species (yellowfin, skipjack, bigeye and albacore). Small size tuna, minor tuna and by-catches are refused by the canneries and are sold on the local market fish. It is the only part of the industrial purse seine catches directly sold in Abidjan. Since 1990, fishing operations on schools of tuna associated with artificial drifting FADs has become widespread. Log fishery allow to catch a majority of small tunas, and generates by-catches of various other pelagic species associated to the logs. The landed amounts of local market fish have increased: from 8500 Mt per year estimated between 1987 and 1990,21000 Mt is reached between 1991 and 1993. Since 1994, controls have been carried out by French fishing companies. A sampling scheme allows us to analyse precisely the landings since 1998. From an interdisciplinary approach, ,ve study the socio-economic role of this local fish market, especially the links between the different actors on this market, and we analyse his impact on fishing strategies by identifying the distribution networks (actors, prices, profit) and estimating the turnovers connected to this sector.

Le faux-poisson d'Abidjan et la peche sous DCP derivants dans l'Atlantique tropical Est: circuit de commercialisation et role socio-economique

Summary This paper deals with the use of drifting fish aggregating devices (DFADs) in the Indian Ocean and of the potential interest to manage purse seine fisheries through limiting their number. Potential risks associated with a massive use of DFADs are first discussed. Based on new information on the numbers of DFADs released by the French fleet in the 2003-2014 period, this paper estimates the levels and trends of the total numbers of DFADs deployed and active at-sea. It is estimated that the total number of DFADs numbers has been increasing by about 70% since the early 2000s and that they could reach 10,500-14,500 nowadays. A good knowledge of the total numbers of DFADs is urgently needed to better estimate the fishing effort and capacity of purse seine fisheries. Future limitations in the number of DFADs could be a direct and efficient way to reduce fishing effort exerted by purse seiners and their support vessels. Following a precautionary approach, we suggest that IOTC could consider setting a cap on the number of DFADs drifting at-sea and that threshold reference levels could be based on the year 2013, at least to slow down the trend observed in the overall fishing capacity on DFADs. Such measures should be first carefully analysed by an ad hoc IOTC DFAD multidisciplinary working group to ensure their efficient implementation and to allow an improved sustainability of the concerned fisheries.

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Daniel Gaertner

Papers

Exploitation of small tunas by a purse-seine fishery with fish aggregating devices and their feeding ecology in an eastern tropical Atlantic ecosystem

By-catch and discards of the european purse seinetuna fishery in the Indian Ocean : estimation and characteristics for the 2003-2007 period

Machine learning for characterizing tropical tuna aggregations under Drifting Fish Aggregating Devices (DFADs) from commercial echosounder buoys data

Le faux-poisson d'Abidjan et la peche sous DCP derivants dans l'Atlantique tropical Est: circuit de commercialisation et role socio-economique

Managing tropical tuna purse seine fisheries through limiting the number of drifting fish aggregating devices in the Indian Ocean: food for thought