Size Related Variability in the Summer Diet of the Blackfin Tuna

Home , Blackfin tuna

Journal of Applied Ichthyology J. Appl. Ichthyol. 25 (2009), 669–675 Received: July 28, 2008 2009 The Authors Accepted: March 12, 2009 Journal compilation 2009 Blackwell Verlag, Berlin doi: 10.1111/j.1439-0426.2009.01327.x ISSN 0175–8659

Size related variability in the summer diet of the blackﬁn tuna (Thunnus atlanticus Lesson, 1831) from Tobago, the Lesser Antilles By M. Headley1,2, H. A. Oxenford2, M. S. Peterson3 and P. Fanning4

1Caribbean Regional Fisheries Mechanism Secretariat, Kingstown, St Vincent and the Grenadines; 2Centre for Resource Management and Environmental Studies, University of the West Indies, Cave Hill Campus, Bridgetown, St Michael, Barbados; 3Department of Coastal Sciences, The University of Southern Mississippi, Ocean Springs, MS, USA; 4Food and Agriculture Organisation, Subregional Oﬃce in the Caribbean, Christ Church, Barbados

Summary longevity greater than 5 years and perhaps up to 8 years Blackfin tuna (Thunnus atlanticus) is a small epipelagic oceanic (Collette and Nauen, 1983; Neilson et al., 1994; Collette, 2002). species known only from the western Atlantic. In Tobago, the The diet of blackfin tuna is broadly described in Collette Lesser Antilles, blackfin tuna is caught by the artisanal fishery. (2002), but the diet of this species from the Lesser Antilles is The diet of this species was examined during the summer of unknown. As such, the primary goal of this study was to 2004 for fish landed at the Charlotteville fish market in determine the summer diet of blackfin tuna landed in Tobago, Tobago. T. atlanticus ranged from 32 to 91 cm FL (0.7– with specific objectives being to examine diet relative to gender 12.4 kg). Overall numerical abundance of prey items com- and body size. prised fish (48%), crustaceans (46%) and cephalopods (6%). Prey species included small pelagics such as anchovies (ranked Methods as most important prey overall), juveniles of larger pelagics such as jacks, juveniles of fish found in coral reef communities Blackfin tuna were sampled daily at the fish market in as adults, e.g. squirrelfishes, and some mesopelagic species. Charlotteville, Tobago (Fig. 1) from 31 July to 17 September The importance of major diet categories differed significantly 2004. On landing, all fish were brought to the market; blackfin with predator size, with fishes becoming more important and tuna were randomly selected, wet weighed whole on a Salter crustaceans less important with increasing size of the blackfin 20 kg hanging scale to the nearest 0.1 kg, measured for fork tuna. This study has improved our scant knowledge of the length (FL) to the nearest 1.0 cm with a flexible tape and blackfin tuna diet in the Lesser Antilles. 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- Introduction cera were subsequently removed by the fish vendors and Blackfin tuna (Thunnus atlanticus) is a warmwater species fishers, and bagged together with their identifying flagging tape known only from the western Atlantic off MarthaÕs Vineyard, labels. Stomachs were then wet weighed to the nearest 1.0 g on Massachusetts in the north, throughout FAO Area 31, south a field balance and Ôstate of fullnessÕ was assessed visually at the to Rio de Janeiro, Brazil. The limiting factor in its distribution fish market as 0 = empty, 1 = ¼ full, 2 = ½ full, 3 = ¾ full appears to be the 20C isotherm (Collette, 2002; Froese and and 4 = full. Stomachs were then placed into individually Pauly, 2008). Blackfin tuna is seasonally important to artisanal labelled Ziploc bags containing saturated brine (made from (small-scale commercial) and sport fisheries throughout the rock salt), stored in covered buckets, and examined within Lesser Antilles (Mahon, 1990, 1993; Doray et al., 2002), and in 1 day after collection. other locations in the wider Caribbean including Cuba (Carles Prey items were identified to the lowest possible taxon using and Valle, 1989; Carles Martin, 2002), the Dominican Barnes (1987), Brusca and Brusca (1990), Carpenter (2002), Republic (ICCAT, 2006), the Bahamas and Florida (Mahon, Froese and Pauly (2004), and Wood and Day (2004). The 1993; Collette, 2002), Venezuela (ICCAT, 2006), Bermuda condition of the prey items was recorded as 1 = fresh (almost (Luckhurst et al., 2001), and Brazil (Freire et al., 2005). The completely intact), 2 = bleached (had most of the skin but reported exploitation of T. atlanticus has increased consider- was discoloured), 3 = skinned (all skin was missing and only ably from around 600 tonnes in the 1950s to 2000 t by the late flesh remained), 4 = broken (broken pieces that belonged to 1970s, to 4000 t by the early 2000s, with a peak of 5258 t in one individual), and 5 = fragments and bones. Whole prey 2002 (FishStat Plus; http://www.fao.org). items were counted and measured (fish in FL, cephalopods in Despite the obvious importance of this species, very few mantle length and crustaceans from the eyes to telson) studies have specifically examined blackfin tuna, with a dearth individually to the nearest 0.1 cm. Unidentifiable prey items of information on the biology of small tunas in general were recorded as such. Prey items were individually weighed (ICCAT, 2004). Aspects of T. atlanticus biology are summa- (except crustaceans) and the total weight for each taxon group rized in Idyll and de Sylva (1963), Carles (1971), Collette and present in any stomach was recorded to the nearest 1.0 g. Nauen (1983), Collette (2002) and Freire et al. (2005). Blackfin When it was not possible to obtain length measurements or tuna are considered one of the small tunas, reaching a counts due to advanced digestion, the fragments and bones maximum size of around 100 cm fork length and having a were simply weighed and recorded as unidentified.

U.S. Copyright Clearance Centre Code Statement: 0175–8659/2009/2506–0669$15.00/0 670 M. Headley et al.

16°35'N occurrence in the diet. We used three predator size groups (small, medium and large), coinciding with observed cohorts in Guadeloupe the predator catch (see below). Separate SpearmanÕs rank correlations were performed to examine further associations

between predator size and the number of ﬁshes and of s

Dominica crustaceans consumed.

e l

l Results i Martinique

t A total of 184 blackﬁn tuna (64 females, 109 males and 11 of

undetermined gender) were caught by the artisanal ﬁshery in n Charlotteville, Tobago between 31 July and 17 September 2004

A and examined for stomach contents. Mean size was 66 cm FL

(range 32–91 cm) and 6.4 kg (range 0.7–12.4 kg). Three size r

e cohorts were considered: small (0.75–3.25 kg), medium (3.5– St Vincent s 9.75 kg) and large (10–12.5 kg) predators for the purposes of Barbados s this study (Fig. 2). e No empty stomachs were observed; the number of prey L items per stomach varied from 1 to 36, and 81.6% of the blackﬁn tuna had stomachs that were half full or more (Fig. 3). Grenada The condition of the majority of the prey items was fresh, bleached or skinned, which made identiﬁcation relatively easy;

Charlotteville very few prey were in the more advanced stages of digestion (broken, fragments and bones). TOBAGO All Ôstomach fullnessÕ categories were represented in each time period (Fig. 3a), and although the ¼ full category appeared to be slightly more frequent in the afternoon, there Trinidad was no significant difference between morning and afternoon fish in stomach fullness (chi-square 2 · 4 contingency test: v2 = 0.81, df = 3, P = 0.72). Likewise, although females appeared to have slightly fewer full stomachs and more ¼ full 9°28'N stomachs than males (Fig. 3b), there was no significant 63°24'W 58°57'W difference between the sexes in stomach fullness (chi-square Fig. 1. Map of Lesser Antilles showing Tobago and the study site, 2 · 4 contingency test: v2 = 2.00, df = 3; P = 0.58). Charlotteville

Importance of prey items in the diet was assessed using Comparison of diet among predator size classes percent numerical abundance, percent frequency, percent total The dietary importance of each major prey category, based on weight, and the index of relative importance (IRI: Corte´ s, frequency of occurrence in stomachs, varied significantly 1997; Oxenford and Hunte, 1999). The relationship is among predator length classes (Kruskal–Wallis tests: for fish 2 IRIi =(%Ni +%Wi) · %Fi, where %Ni is the percent of prey v = 21.356, n = 43, 108, 33, df = 2, P < 0.001; for 2 food item i in number, %Wi is percent of food item i by crustaceans v = 28.223, n = 43, 108, 33, df = 2, weight, and %Fi is the percent frequency of occurrence of the P < 0.001). The median number of fishes consumed by food item i in all stomachs. IRI values are also presented on a blackfin tuna increased with predator size, whilst the median percentage basis (%IRI: Corte´ s, 1997). number of crustaceans decreased (Fig. 4). The correlation Blackfin tuna stomach fullness between time of day between predator length and prey number is highly significant collected, and between males and females were compared with chi-square contingency tests. Diet data were initially examined using frequency of occurrence of prey items by family, but 12 Small subsequently pooled into three major prey categories: fishes, Medium crustaceans and cephalopods; prey items belonging to most 10 Large families occurred too infrequently across tunas to justify an 8 analysis at the family level (23 of the 27 prey families identified occurred in < 10% of tunas). Predator gender and size were 6 considered as factors influencing diet in preliminary analyses, Frequency but gender was omitted in the final analyses because there was 4 no evidence that it played a role. To examine predator size 2 effect on diet, we performed a Kruskal–Wallis test with the predator size group as a factor and the frequency of prey items 0 012345678910111213 as the dependent variable. Because of the inherent differences Weight (kg) in prey size among the major prey item categories, we Fig. 2. Size-frequency histogram for 184 blackfin tuna caught off conducted a separate analysis for each category, i.e. one for Tobago and examined for stomach content. Size shown as wet weight fishes and one for crustaceans; cephalopods were not included (kg) and designated predator size groups. Small (0.75–3.25 kg), in these and other subsequent analyses due to their very low medium (3.5–9.75 kg) and large (> 10 kg) tuna indicated by shading Summer diet of blackfin tuna from the Lesser Antilles 671

50 16 (a) Morning (a) Afternoon 40 14

12 30 10 20 8 Percent frequency 10 6

0 4

1/4 full 1/2 full 3/4 full Full Number of fish prey State of stomach 2

0 (b) 50 Females –2 Males 40 Small Medium Large N = 43 108 33 30 Predator size group

20 (b) 40

Percent frequency 10 30 0 1/4 full 1/2 full 3/4 full Full State of stomach 20 Fig. 3. Comparison of stomach fullness (a) blackfin tuna caught in the morning (n = 133) and afternoon (n = 51), and (b) between female (n = 64) and male (n = 109) blackfin tuna caught off Tobago 10

0 for both groups, the relationship being positive for fish prey Number of crustacean prey and negative for crustacean prey (SpearmanÕs rank correlation: for fish prey rs = 0.29, n = 184, P < 0.001; for crustaceans –10 rs = )0.383, n = 184, P < 0.001). Small Medium Large N = 43 108 33 Predator size group Overall diet variability Fig. 4. Box and whisker plots showing median number of prey items A total of 1 753 prey items comprising 35 fish species from 19 consumed by small (0.75–3.25 kg), medium (3.5–9.75 kg) and large families, four crustacean orders, and three cephalopod orders (> 10 kg) blackfin tuna caught off Tobago, separately for (a) fishes were quantified and represented at least seven ecological and (b) crustaceans. Upper and lower whiskers represent largest and smallest values not considered outliers, respectively; upper and lower categories and life stages (Table 1). The fish species consumed shaded box ends represent the 75th and 25th percentile. Horizontal line included small epipelagics, juveniles of large epipelagics, is the 50th percentile (median). Symbols represent outliers (o = 1.5 benthopelagics and coral reef species, and mesopelagics. The box-length and * = > 3 box-lengths distance from box) small pelagic fish species included anchovies (Engraulidae; mainly Anchoa cayorum), flyingfishes (Exocoetidae), halfbeaks (Hemiramphidae), herrings (Clupeidae), an oceanic pufferfish; was dominant. Prey from the class Cephalopoda included Lagocephalus lagocephalus (Tetraodontidae), and butterfishes squid (Loliginidae), octopus (Octopodidae) and bobtail squid (Stromateidae). The diet also comprised juveniles of larger (Sepiolidae). None of the squid were identified to species but pelagic species such as jacks (Carangidae), sailfish (Istiophor- four octopuses and the bobtail squid were identified, although idae) and bluefishes (Pomatomidae), and the pelagic juvenile none were dominant in the diet. phases of species generally found in coral reef communities as Fishes were the most important group of prey regardless of adults such as squirrelfishes (Holocentridae), triggerfishes the criteria used to judge importance, being responsible for (Balistidae) and filefishes (Monacanthidae). Some deep ben- 48.6% of all prey items by number, 85.6% by weight and thopelagic and mesopelagic fish, many of which are known to 97.8% by frequency of occurrence (Table 1, Fig. 5). The most undertake diurnal migrations to the surface, were also found in commonly eaten fish species was the key anchovy, A. cayorum, the stomachs. These included slimeheads (Trachichthyidae), which accounted for 50.6% by numerical abundance of all hairtails (Trichiuridae), barracudinas (Paralepididae), lanternf- fishes eaten, 43.6% by weight and was found in 46.2% of the ishes (Myctophidae) and snake mackerels (Gempylidae). stomachs examined (Table 1). Crustaceans ranked second, Crustaceans were also important in the diet, and cephalo- representing 45.8% of all prey items by number, 6.4% by pods to a lesser extent. The crustaceans consumed included the weight and 44% by frequency of occurrence (Table 1, Fig. 5). pelagic larvae of the order Decapoda, Isopoda and the family Cephalopods ranked third of all prey items with 5.6% by Portunidae. Crustaceans from the Mysidacea and Euphausi- number, 8% by weight, and 13.6% by frequency (Table 1, acea families were also consumed, but no crustacean group Fig. 5). 672 M. Headley et al.

Table 1 Summary of prey consumed by blackﬁn tuna caught oﬀ Tobago, July–September 2004

Habitat ⁄ No. of Combined Frequency of Group category Family ⁄ order Prey item prey prey WT occurrence

Fishes Pelagic Carangidae Caranx crysos (Mitchill) 6 196 3 Decapterus macarellus (Cuvier) 13 402 3 Decapterus punctatus (Cuvier) 15 1 051 7 Seriola dumerili (Risso) 14 377 4 Seriola rivoliana (Valenciennes) 28 804 8 Seriola zonata (Mitchill) 10 338 7 Clupeidae Etrumeus teres (DeKay) 2 116 1 Harengula clupeola (Cuvier) 62 1 622 18 Harengula humeralis (Cuvier) 21 533 5 Istiophoridae Makaira nigricans (Lace´ pe` de) 2 102 1 Pomatomidae Pomatomus saltatrix (Linnaeus) 4 192 4 Belonidae Platybelone argalus argalus (Lesueur) 4 245 4 Engraulidae Anchoa cayorum (Fowler) 431 11 026 85 Anchoa colonensis (Hildebrand) 2 36 1 Anchoa lyolepis (Evermann & Marsh) 14 312 5 Exocoetidae Exocoetus volitans (Linnaeus) 16 607 6 Exocoetus sp. 2 51 1 Hirundichthys affinis (Gu¨ nther) 12 517 6 Hemiramphidae Oxyporhamphus micropterus similis (Bruun) 2 90 2 Hemiramphus balao (Lesueur) 3 148 3 Mugilidae Mugil curema (Valenciennes) 4 247 4 Tetraodontidae Lagocephalus lagocephalus (Linnaeus) 7 327 5 Stromateidae Peprilus paru (Linnaeus) 2 65 1 Coral reef Balistidaea Balistes capriscus (Gmelin) 7 348 7 Balistes vetula (Linnaeus) 2 106 2 Xanthichthys ringens (Linnaeus) 6 286 6 Monacanthidaea Cantherhines pullus (Ranzani) 6 183 2 Monacanthus ciliatus (Mitchill) 16 636 9 Holocentridaea Sargocentron vexillarium (Poey) 6 218 4 Neoniphon marianus (Cuvier) 3 130 3 Benthopelagic Trichiuridaea Lepidopus altifrons (Parin & Collette) 6 322 4 Trachichthyidae Unidentified 2 108 2 Mesopelagic Paralepididae Unidentified 6 220 5 Myctophidae Unidentified 10 246 5 Gempylidae Nealotus tripes (Johnson) 4 191 4 Unidentified Unidentified Fish 102 2 916 46 Subtotal 852 25 314 180 Crustaceans Pelagic Decapoda Portunidae 110 339 22 Unidentified 57 156 5 Stenopodidea 119 370 9 Isopoda Isopoda 100 309 10 Euphausiacea Euphausiacea 93 229 9 Mysidacea Mysidacea 194 487 17 Crustaceans Unidentified 130 – 15 Subtotal 803 1 890 81 Cephalopods Pelagic Octopodidae Unidentified 5 95 1 Octopus burryi (Voss) 7 155 1 Octopus defilippi (Verany) 5 78 2 Pteroctopus schmidti (Joubin) 3 38 1 Scaeurgus unicirrhus (Chiaie) 6 173 2 Sepiolidae Semirossia tenera (Verrill) 2 26 1 Loliginidae Unidentified 70 1 798 18 Subtotal 98 2 363 25 Total 1 753 29 567 184 aFishes represent juvenile phases that inhabit the epipelagic zone. WT = wet weight (g).

Major prey groups to weight. The Portunids occurred most frequently among the Major prey groups were selected based on an IRI of ‡ 24 crustaceans consumed and Exocoetidae were minor in impor- (Table 2). Twelve major prey categories accounted for 78.2% tance by numerical abundance but important by weight. of all prey in number, 78.1% by weight and occurred in 91.8% The size distribution of all prey items consumed ranged from of all stomachs examined. The Engraulidae (mainly A. cayo- 1 to 16 cm and was clearly tri-modal (Fig. 6), with the three rum) was clearly the most important with a %IRI value of prey size groups being 0–4 cm, 5–10 cm, and 11–16 cm. The 77.1, which was much larger than the second ranking prey length distributions for the twelve major prey groups are group, Carangidae, with a %IRI value of 6.6 (Table 2). The shown in Fig. 7. The smallest size group (0–4 cm) comprised next most influential prey groups had %IRI values ranging Portunidae, Stenopodidea, Isopoda, Mysidacea and Euphausi- from 2 to 7, with all others representing < 1 (Table 2). Mysids acea; the medium size group (5–10 cm) comprised Loliginidae, were the second most important prey group with respect to Engraulidae and juvenile fishes, and the largest group (11– numerical abundance, but were fairly insignificant with regard 16 cm) comprised juveniles from Carangidae, Balistidae and Summer diet of blackfin tuna from the Lesser Antilles 673

100 400

350 90 Numerical abundance Weight 300 80 Frequency 250 70 200 60

Frequency 150 50 100

40 50

Percent frequency 30 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 20 Prey length (cm) Fig. 6. Length-frequency distribution of all prey items in stomachs of 10 184 blackfin tuna. 0 Fishes Crustaceans Cephalopods Our data are also consistent with the qualitative descriptions Fig. 5. Comparison of the relative importance of the three major prey of diet for blackfin and other small tunas such as bullet tuna groups consumed by blackfin tuna off Tobago, shown separately by Auxis rochei (Risso), little tunny Euthynnus alletteratus numerical abundance, weight and frequency of occurrence (Rafinesque) and skipjack tuna E. pelamis (Linnaeus) given in Collette (2002), indicating that small tunas in general feed Monacanthidae, and adults from Clupeidae and Exocoetidae. on a wide variety of small fishes, especially clupeids, squid and Prey from the two small size groups were consumed more pelagic crustaceans. frequently than prey from the third size group. Prey items The presence of mesopelagic and deep benthopelagic fishes measuring lengths of 2, 7 and 8 cm were also the most in the stomachs of blackfin tuna suggests that T. atlanticus also frequently consumed. feed in deep water and ⁄ or that some feeding occurs at night when mesopelagic micronekton migrate toward the surface. It is commonly believed that tunas feed only during the day, but Discussion sonic tracking experiments show that some tunas also feed at The presented results from the area off Tobago in the Lesser dusk (Bard et al., 1998). Taquet et al. (2000) noted that Antilles indicate that T. atlanticus feed constantly through the blackfin are found to 400 m depths, suggesting that they could day, eating both epipelagic and deeper water species. However, be foraging in deep water. Finally, the high diversity of prey the most common prey in the stomachs was epipelagic, which consumed, apparent foraging in different habitats, and the indicates that blackfin tuna feed mainly in surface waters. Our localized nature of the diet appears to be characteristic of findings on the dominant prey groups corroborate in general tropical migratory pelagic species (e.g. Scott and Tibbo, 1968; with those of Guevara (1984), who noted that blackfin tuna Abitia-Cardenas et al., 1997; Oxenford and Hunte, 1999; (38–69 cm FL) consumed fishes (83.0% volume – mainly Sierra et al., 2001). This euryphagous feeding strategy is likely Balistidae, Ostraciidae, Carangidae, and Grammicolepidae), to be an adaptation for coping with low biomass of forage molluscs (12.1% – mainly squid and octopus) and crustaceans species in a tropical ocean (Oxenford and Hunte, 1999; Sierra (4.9% – mainly Squilla and Lysosquilla stomatopods) off et al., 2001). Cuba, but that the key species and the relative ranking of As T. atlanticus off Tobago increase in size, there appears to crustaceans and cephalopods differ. Likewise, other quantita- be a shift in the diet. Small blackfin tuna consume large tive studies of the diet of blackfin tuna from different areas off numbers of crustaceans and few fish, whilst the bigger Cuba also report that fishes are the most important prey group T. atlanticus consume more fish and fewer crustaceans. This by volume, followed by cephalopods and then crustaceans may reflect a difference in foraging strategy or may be a result (Sierra et al., 2001). of predator size. Larger blackfin tuna would be able to swim

Table 2 Relative dietary importance of main Habitat prey family ⁄ orders of blackﬁn tuna Category Prey Group N N% W W% F F% IRI %IRI caught oﬀ Tobago Pelagic Engraulidae 447 25.5 11 374 38.47 91 49.46 3 163.62 77.1 Carangidae 86 4.91 3 168 10.71 32 17.39 271.66 6.62 Clupeidae 85 4.85 2 271 7.68 24 13.04 163.43 3.98 Mysidacea 194 11.07 487 1.65 17 9.24 117.46 2.86 Loliginidae 70 3.99 1 798 6.08 18 9.78 98.55 2.40 Portunidae 110 6.27 339 1.15 22 11.96 88.74 2.16 Exocoetidae 30 1.71 1 175 3.97 13 7.07 40.17 0.98 Stenopodidea 119 6.79 370 1.25 9 4.89 39.32 0.96 Isopoda 100 5.7 309 1.05 10 5.43 36.68 0.89 Euphausiacea 93 5.31 229 0.77 9 4.89 29.74 0.72 Balistidae 15 0.86 740 2.5 15 8.15 27.38 0.67 Monacanthidae 22 1.25 819 2.77 11 5.98 24.06 0.59 Totals 1 371 78.21 23 079 78.05 169 91.85 – 99.93 674 M. Headley et al.

Acknowledgements This paper is based on a MSc Research Paper submitted to the University of the West Indies by the senior author. We thank E. Caesar of the Department of Marine Resources and Fisheries, Tobago House of Assembly, for providing logistical support, and the fishers and fish vendors of Charlotteville, Tobago for allowing access to their fish. Funding was provided by the FAO LAPE project (FI:GCP ⁄ RLA ⁄ 140 ⁄ JPN) and by CERMES, UWI. We acknowledge statistical assistance from H. Valles and thank N. J. Brown-Peterson and two anonymous reviewers for comments on an earlier version of the paper.

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