Size related variability in the summer diet of the blackfin tuna (Thunnus atlanticus Lesson, 1831) from Tobago, the Lesser Antilles By M. Headley 1,2 , H. A. Oxenford 2 , M. S. Peterson 3 and P. Fanning 4 1 Caribbean Regional Fisheries Mechanism Secretariat, Kingstown, St Vincent and the Grenadines; 2 Centre for Resource Management and Environmental Studies, University of the West Indies, Cave Hill Campus, Bridgetown, St Michael, Barbados; 3 Department of Coastal Sciences, The University of Southern Mississippi, Ocean Springs, MS, USA; 4 Food and Agriculture Organisation, Subregional Office in the Caribbean, Christ Church, Barbados Summary Blackfin tuna (Thunnus atlanticus) is a small epipelagic oceanic species known only from the western Atlantic. In Tobago, the Lesser Antilles, blackfin tuna is caught by the artisanal fishery. The diet of this species was examined during the summer of 2004 for fish landed at the Charlotteville fish market in Tobago. T. atlanticus ranged from 32 to 91 cm FL (0.7– 12.4 kg). Overall numerical abundance of prey items com- prised fish (48%), crustaceans (46%) and cephalopods (6%). Prey species included small pelagics such as anchovies (ranked as most important prey overall), juveniles of larger pelagics such as jacks, juveniles of fish found in coral reef communities as adults, e.g. squirrelfishes, and some mesopelagic species. The importance of major diet categories differed significantly with predator size, with fishes becoming more important and crustaceans less important with increasing size of the blackfin tuna. This study has improved our scant knowledge of the blackfin tuna diet in the Lesser Antilles. Introduction Blackfin tuna (Thunnus atlanticus) is a warmwater species known only from the western Atlantic off MarthaÕs Vineyard, Massachusetts in the north, throughout FAO Area 31, south to Rio de Janeiro, Brazil. The limiting factor in its distribution appears to be the 20°C isotherm (Collette, 2002; Froese and Pauly, 2008). Blackfin tuna is seasonally important to artisanal (small-scale commercial) and sport fisheries throughout the Lesser Antilles (Mahon, 1990, 1993; Doray et al., 2002), and in other locations in the wider Caribbean including Cuba (Carles and Valle, 1989; Carles Martin, 2002), the Dominican Republic (ICCAT, 2006), the Bahamas and Florida (Mahon, 1993; Collette, 2002), Venezuela (ICCAT, 2006), Bermuda (Luckhurst et al., 2001), and Brazil (Freire et al., 2005). The reported exploitation of T. atlanticus has increased consider- ably from around 600 tonnes in the 1950s to 2000 t by the late 1970s, to 4000 t by the early 2000s, with a peak of 5258 t in 2002 (FishStat Plus; http://www.fao.org). Despite the obvious importance of this species, very few studies have specifically examined blackfin tuna, with a dearth of information on the biology of small tunas in general (ICCAT, 2004). Aspects of T. atlanticus biology are summa- rized in Idyll and de Sylva (1963), Carles (1971), Collette and Nauen (1983), Collette (2002) and Freire et al. (2005). Blackfin tuna are considered one of the small tunas, reaching a maximum size of around 100 cm fork length and having a longevity greater than 5 years and perhaps up to 8 years (Collette and Nauen, 1983; Neilson et al., 1994; Collette, 2002). The diet of blackfin tuna is broadly described in Collette (2002), but the diet of this species from the Lesser Antilles is unknown. As such, the primary goal of this study was to determine the summer diet of blackfin tuna landed in Tobago, with specific objectives being to examine diet relative to gender and body size. Methods Blackfin tuna were sampled daily at the fish market in Charlotteville, Tobago (Fig. 1) from 31 July to 17 September 2004. On landing, all fish were brought to the market; blackfin tuna were randomly selected, wet weighed whole on a Salter 20 kg hanging scale to the nearest 0.1 kg, measured for fork length (FL) to the nearest 1.0 cm with a flexible tape and tagged with flagging tape to allow identification at gutting. For each fish, the time of capture (morning = 8.00–12.00 hours, or afternoon = 14.00–18.00 hours) was also noted. The vis- cera were subsequently removed by the fish vendors and fishers, and bagged together with their identifying flagging tape labels. Stomachs were then wet weighed to the nearest 1.0 g on a field balance and Ôstate of fullnessÕ was assessed visually at the fish market as 0 = empty, 1 = ¼ full, 2 = ½ full, 3 = ¾ full and 4 = full. Stomachs were then placed into individually labelled Ziploc bags containing saturated brine (made from rock salt), stored in covered buckets, and examined within 1 day after collection. Prey items were identified to the lowest possible taxon using Barnes (1987), Brusca and Brusca (1990), Carpenter (2002), Froese and Pauly (2004), and Wood and Day (2004). The condition of the prey items was recorded as 1 = fresh (almost completely intact), 2 = bleached (had most of the skin but was discoloured), 3 = skinned (all skin was missing and only flesh remained), 4 = broken (broken pieces that belonged to one individual), and 5 = fragments and bones. Whole prey items were counted and measured (fish in FL, cephalopods in mantle length and crustaceans from the eyes to telson) individually to the nearest 0.1 cm. Unidentifiable prey items were recorded as such. Prey items were individually weighed (except crustaceans) and the total weight for each taxon group present in any stomach was recorded to the nearest 1.0 g. When it was not possible to obtain length measurements or counts due to advanced digestion, the fragments and bones were simply weighed and recorded as unidentified. J. Appl. Ichthyol. 25 (2009), 669–675 Ó 2009 The Authors Journal compilation Ó 2009 Blackwell Verlag, Berlin ISSN 0175–8659 Received: July 28, 2008 Accepted: March 12, 2009 doi: 10.1111/j.1439-0426.2009.01327.x U.S. Copyright Clearance Centre Code Statement: 0175–8659/2009/2506–0669$15.00/0 Applied Ichthyology Journal of