Do drifting and anchored Fish Aggregating Devices (FADs) similarly influence tuna feeding habits? A case study from the western Indian Ocean

doi:10.1016/j.fishres.2010.11.011

Fisheries Research

Volume 107, Issues 1–3, January 2011, Pages 283-290

https://doi.org/10.1016/j.fishres.2010.11.011 Get rights and content

Abstract

Anchored and drifting Fish Aggregating Devices (FADs) are intensively used in tropical tuna fisheries. In both small-scale and industrial fisheries, skipjack (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) are the main targets. The increasing development of this fishing practice by industrial purse seiners has raised the question of the impact of FADs on tuna communities, as they might act as an ecological trap. This study investigated the feeding habits of skipjack and yellowfin tuna associated with anchored and drifting FADs in the western Indian Ocean. The diet of 352 tunas was analysed taking into account the type of FAD, ontogenetic variations, and the resources richness of the area. Poor-food and rich-food areas were defined according to the abundance of stomatopod Natosquilla investigatoris, the main prey of tunas, on the fishing sites. Diet composition was expressed through functional groups of prey. Significant dietary differences were found between both FAD types, as well as an effect of individual size. Around anchored FADs tuna preyed on diverse assemblages of coastal fish and crustacean larvae and juveniles, whereas a low diversity of epipelagic prey dominated the tuna diet associated with drifting FAD. Compared to anchored FADs, the frequency of empty stomachs was significantly higher and the stomach content mass significantly lower among skipjack and small yellowfin tunas caught around drifting FADs. This was magnified in poor-food areas, where drifting FADs often evolved, suggesting that these FADs could negatively impact the growth of skipjack and small yellowfin tuna. Larger yellowfin tuna exhibited differences in their dietary habits between anchored and drifting FADs, and between poor-food and rich-food areas. However, drifting FADs did not impact them as strongly as juveniles of yellowfin or skipjack tunas. Our study gives new highlights on possible detrimental effects of FAD on tunas, and this has to be considered in future sustainable management strategies of tuna fisheries.

Introduction

Fish naturally associate with floating objects in almost all oceans in the world (Fréon and Dagorn, 2000, Castro et al., 2002). This aggregating behaviour is used in small-scale and industrial tuna fisheries so as to concentrate fish around man-made Fish Aggregating Devices (FADs), and then so increase the catches. Among fish, tropical tunas such as skipjack (Katsuwonus pelamis) and yellowfin tunas (Thunnus albacares) frequently associate with floating objects at the surface of the oceans (Gooding and Magnuson, 1967, Fontenau et al., 2000). As a consequence, since the early 1990's, drifting FADs are used in the open ocean and anchored FADs in inshore local tropical fisheries; these devices play an important role in all tropical and sub-tropical tuna fisheries nowadays (Fonteneau, 2000, IATTC, 2002). In the western Indian Ocean, anchored FADs are used in small-scale fisheries, and sets around drifting FADs are a common practice for the industrial purse seine fishery (Tessier et al., 2000).

The reason why fish aggregate so frequently with FADs at the surface of the ocean is still poorly understood. Six main hypotheses have been stated: sheltering, seamark in the ocean, meeting point, resting, feeding, and indication of area of high production (Gooding and Magnuson, 1967, Hunter and Mitchell, 1967, Dagorn et al., 2000, Fréon and Dagorn, 2000, Castro et al., 2002). In addition, several authors suggested that tunas might be trapped around man-made FADs (Marsac et al., 2000), which could lead to an inappropriate habitat selection, and have detrimental effects on their short-term health (Hallier and Gaertner, 2008). According to the ecological trap hypothesis, man-made FADs could drift to non-productive areas and then reduce the feeding activities of associated tunas, which would negatively impact the dynamics of the populations. In the actual context of overfishing and rapid depletion of predatory fish communities worldwide (Myers and Worm, 2003), it is crucial to improve our understanding of the impacts of fishing activities, including the deployment of thousands of FADs in the oceans, on fish communities.

Studies on the diet of fish aggregated around FADs remain scarce. Food habits of FAD-associated tunas have been mainly investigated for yellowfin tunas (T. albacares) in the Pacific Ocean (Brock, 1985, Barut, 1988, Lehodey, 1990, Buckley and Miller, 1994, Grubbs et al., 2002, Graham et al., 2006), and in the Atlantic Ocean (Ménard et al., 2000a, Ménard et al., 2000b). Results differ between studies of the different regions and this is mostly due to the opportunistic feeding behaviour of tunas (Ménard et al., 2006). Then additional studies have to be conducted to take into account regional specificities in stock management. Brock (1985) showed that FAD-associated tunas in Hawaii were less well-fed than their non-FAD relatives that can feed on deep-water shrimps to compensate for the decrease of usual prey. Recently Graham et al. (2006) showed that FAD-associated juvenile yellowfin tuna fed on planktonic organisms inhabiting the shallow mixed layer, primarily stomatopod larvae and decapod crustaceans, whereas larger individuals targeted teleosts and vertically migrating mesopelagic species of shrimps. In French Polynesia FAD-associated yellowfin tuna preyed mostly on reef fish and stomatopod larvae (Lehodey, 1990). In the western Indian Ocean, previous studies have shown that free-swimming schools of surface tunas preyed mainly on epipelagic fish and pelagic crustaceans (Bashmakov et al., 1992, Roger, 1994a, Potier et al., 2004). To our knowledge, no study has been conducted on the feeding habits of tunas found in the vicinity of FADs.

In this study, we analysed the food habits and diet of yellowfin and skipjack tunas caught associated with anchored and drifting FADs in the western Indian Ocean. We aim at providing new insights on the impact of FAD on the feeding behaviour of tunas. For this purpose specific and ontogenetic-related differences in the diet composition by taxonomic and functional groups of prey were compared in tunas caught around drifting and anchored FADs. To highlight the impact of FADs, diet of tunas in free-swimming schools was used as reference of natural and undisturbed conditions.

Section snippets

Study area and sample collection

The study was carried out in the tropical western Indian Ocean, off the Seychelles archipelago, in the northern Mozambique Channel, and off Reunion Island (Fig. 1). Between 2001 and 2006, 243 stomachs of yellowfin tuna (T. albacares) and skipjack tuna (K. pelamis) caught in drifting FADs were collected onboard purse seiners operating in the western Indian Ocean. In October 2002, to the north of the Seychelles, stomachs were collected in free-swimming schools (Potier et al., 2004) and around

Results

We analysed the stomach contents of 175 skipjack and 177 yellowfin tuna caught around FADs. Skipjack and yellowfin tuna caught associated with anchored FADs ranged from 41 to 96 cm (median 69 cm) and from 49 to 170 cm (median 70 cm) in fork length, respectively (Table 1, Fig. 2). Around drifting FADs, skipjack ranged from 31 to 74 cm (median 48 cm) and yellowfin tuna from 35 to 160 cm (median 58 cm). The size of the individuals collected associated with anchored FAD was significantly larger than with

Discussion

In the western Indian Ocean, the feeding habits of tunas differ between individuals associated with anchored and drifting FADs. This difference is primarily related to the assemblages of prey available in their foraging habitats. On the one hand, in the vicinity of anchored FADs, a high diversity of coastal fauna is present and dominated by larval and juveniles stages of crustaceans and reef-fish. These results are similar to what Graham et al. (2006) found in Hawaii around near-shore FADs of

Conclusion

Anchored and drifting FADs do not act similarly on tunas. Off Reunion, where coastal fauna is abundant, anchored FADs attract tunas, but do not impact individuals negatively. Although this result is in accordance with studies conducted in the Pacific, further investigations in the western Indian Ocean should be conducted like in the Comoros and the Seychelles to confirm our observations. For drifting FADs, the situation is more complex and the impact of FADs seems to be location-dependent,

Acknowledgements

Reunion Island Regional Council, IRD, and the BIOPS project funded by La Fondation pour la Recherche sur la Biodiversité (no. CD-AOOI-07-013) supported this study. We thank all fishermen and crew members who helped collecting the data at sea. Laurent Dagorn comments on an early version of the paper were appreciated. Marie-Laure Mangata Ramsamy helped with the English.

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Do drifting and anchored Fish Aggregating Devices (FADs) similarly influence tuna feeding habits? A case study from the western Indian Ocean

Abstract

Introduction

Section snippets

Study area and sample collection

Results

Discussion

Conclusion

Acknowledgements

Aquat. Living Resour.

Aquat. Living Resour.

Fish. Res.

Association of fauna with floating objects in the eastern Pacific Ocean

Food and feeding habits of yellowfin tuna Thunnus albacares (Bonnaterre, 1788) caught by handline around payao in the Moro Gulf. IPTP IPTP/88/WP/18

Diet composition of tunas caught with long lines and purse seines in the western Indian Ocean

Tuna food habits related to the micronekton distribution in French Polynesia

Mar. Biol.

Tuna: Physiology, Ecology, and Evolution

Preliminary study of the feeding habits of pelagic fish around Hawaiian fish aggregation devices or can fish aggregation devices enhance local fisheries productivity

Bull. Mar. Sci.

A review of the impact of fish aggregating devices (FADs) on tuna fisheries

Feeding habits of yellowfin tuna associated with fish aggregation devices in American Samoa

Bull. Mar. Sci.

A general theory on fish aggregation to floating objects: an alternative to the meeting point hypothesis

Rev. Fish. Biol. Fish.

Comparative foraging ecology of a tropical seabird community of the Seychelles, western Indian Ocean

Mar. Ecol. Prog. Ser.

Resource partitioning within a tropical seabird community: new information from stable isotopes

Mar. Ecol. Prog. Ser.

A handbook for the identification of cephalopod beaks

Atlas of tropical tuna fisheries: world catches and environment

Review of fish associative behaviour: toward a generalisation of the meeting point hypothesis

Rev. Fish Biol. Fish.

Ecological significance of a drifting object to pelagic fishes

Pac. Sci.

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

Mar. Biol.

Comparative trophic ecology of yellowfin and bigeye tuna associated with natural and man-made aggregation sites in Hawaiian waters