ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on May 26, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(5):1044-1052; doi:10.1093/icesjms/fsm065
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Food composition and feeding habits of little tunny (Euthynnus alletteratus) in continental shelf waters of Côte d'Ivoire (West Africa)
1 Centre de Recherches Océanologiques, BP V 18 Abidjan, Côte d'Ivoire
2 Laboratoire d'Hydrobiologie, UFR Biosciences, Université de Cocody Abidjan, 22 BP 582 Abidjan 22, Côte d'Ivoire
Correspondence to L Bahou: tel: +225 21355014/08402024; fax: +225 21351155; e-mail: lbahoucrothon{at}yahoo.fr
Bahou, L., Koné, T., N'Douba, V., N'Guessan, K. J., Kouamélan, E. P., and Gouli, G. B. 2007. Food composition and feeding habits of little tunny (Euthynnus alletteratus) in continental shelf waters of Côte d'Ivoire (West Africa). – ICES Journal of Marine Science, 64: 1044–1052.The stomach contents of 170 little tunny, Euthynnus alletteratus, sampled between June 2003 and December 2004 were examined. Fish size ranged from 27 to 81 cm fork length, and all fish were caught in gillnets deployed over the continental shelf off Côte d'Ivoire (West Africa). The type and quantity of prey ingested changed seasonally. Outside the major upwelling period the diet was more varied. Overall, fish were the dominant prey of all sizes of little tunny, far exceeding crustaceans, of which shrimps and prawns were commonest but were not found in the stomachs of juveniles (<42 cm FL) or larger adults (
53 cm FL). Little tunny are carnivorous fish that feed opportunistically. A relationship was found between the size of the prey and the size of the predator.
Keywords: diet, little tunny, upwelling, West Africa
Received 30 August 2006; accepted 18 March 2007; advance access publication 26 May 2007.
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Introduction |
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In oceanic ecosystems, tunas and swordfish are considered to be apex predators, occupying generally the highest trophic level (see Jones, 1982). Studies on tuna diet are numerous (for a review, see Stretta, 1988) and reveal the opportunistic feeding behaviour of these fish, which adapt their habits to feed on whatever is available. Most of these studies are conducted on large tuna species caught by purse-seine, longline, or in other fishing gears. In the Gulf of Guinea, skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) are actively fished by tuna purse-seiners and baitboats, and smaller species such as little tunny (Euthynnus alletteratus) or frigate tuna (Auxis thazard) are a bycatch of such fleets. Notwithstanding, large catches of both little tunny and frigate tuna are also made in the small-scale fisheries of West African countries such as Côte d'Ivoire.
Since 1984, a canoe fishery using drifting gillnets with large meshes has developed off Abidjan (Côte d'Ivoire) as an extension of the Ghanaian "nifa nifa" fishery which started in 1974 (Mensah and Doyi, 1994; Bard et al., 2002). This fishery is a multispecies one (Bard and Amon Kothias, 1985; Amon Kothias, 1986), fishers primarily targeting billfish, sharks, and tunas. Bahou (2001) showed that most tuna catches were made during the main upwelling season (MUS) and that the proportions of little tunny (E. alletteratus), frigate, and bullet tunas (Auxis spp.) were important in terms of nominal catch. It is known that upwelling is seasonal along the coast of Côte d'Ivoire during the cooler season between June and October (Varlet, 1958; Morlière, 1970). Full descriptions of upwelling dynamics are given by Morlière (1970), Verstraète (1970), and Morlière and Rébert (1972). This upwelling, like other upwelling in the Gulf of Guinea, cannot be explained by applying the classical, wind-driven Eckman model alone (Chidi and Abé, 2002). These last authors examined current-induced upwelling and remote forcing, two of the popular theories on upwelling dynamics. The cool season is considered by several authors (Binet, 1993; Herbland and Le Loeuff, 1993; Reyssac, 1993) to be crucial in priming the productivity of the ecosystem. Surface water temperatures fall below 25°C, surface water salinity rises, dissolved oxygen levels generally fall, there is a plankton bloom, and most fish spawn then (Mensah and Koranteng, 1988; Koranteng, 1995). Cold water reaching the surface brings nutrients into the euphotic layer, boosting the planktonic foodweb (Binet, 1995). According to Reyssac (1993), the greatest production of phytoplankton occurs each year during the long cool season, although it can sometimes be moderated by short phases of decline. Most zooplankton species (with the exception of the most thermo-tolerant ones) expand during this phase as they take advantage of the blooms of phytoplankton (Binet, 1993).
Despite their relative abundance and importance to the fishery, little is known about the feeding habits of little tunny. According to several authors (Blaber, 1997; Cruz-Escalona et al., 2000; Hajisamae et al., 2003), studies of trophic ecology are useful and fundamental to an understanding of the functional role of fish within their ecosystems. The overall objective of this study was therefore to describe the feeding habits of E. alletteratus, a scombrid that off Côte d'Ivoire matures at 42 cm fork length (FL) (Cayré et al., 1988; ICCAT, 2003). A specific goal was to investigate seasonal variations in prey composition and to determine whether or not there was any relationship between predator length and the length of the main fish prey. Food composition and changes in diet are discussed in relation to season and size class.
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Material and methods |
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Sampling procedure
Fish were collected from a fishery, which operated with canoes powered by 40 hp motors and manned by about six persons. The fishing areas are located at the edge of the continental shelf, which is relatively narrow in the vicinity of Abidjan (Figure 1). Fishers generally operated daily from Tuesday to Saturday, but they stayed ashore when the weather was poor or there was no fuel for the canoes. Fishing was at night and landings were made mainly in the morning at the port. Tuna were caught with gillnets of 25 and 35 mm mesh joined to each other to form a single drifting gillnet up to as much as 2000–2500 m long (see Bard et al., 2002). Each drifting gillnet was set before 19:00 local time, either perpendicular to the coast or to the east-flowing current. A buoy was attached to the top of each gillnet and lead weights hung below the bottom of the net to keep it stretched, and the whole unit was allowed to drift in the current. With the canoe illuminated by an oil lamp or locally-designed small lamp, fishers inspected the net from one end to the other every 30 min, and lifted the net up occasionally to gather up the fish that had been caught and bring them on board. Fishing operations generally ceased by 04:00 local time.
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Samples were taken weekly at Abidjan fishing port between June 2003 and December 2004, as often as tuna occurred in the catches. Fish were taken to the laboratory for processing and stomach content analysis. Little tunny were measured to the nearest centimetre FL, i.e. from the snout tip to the fork of the caudal fin, and weighed to the nearest 0.1 g. They were subdivided into two main groups, juveniles (<42 cm FL) and adults (
42 cm FL), based on the maturity data of Cayré et al. (1988) and as reported by ICCAT (2003) for this species. Variation in diet inside the adult group was investigated by grouping fish in 2 cm size classes: <42 (n = 22); 42–44 (n = 48); 45–47 (n = 50); 48–50 (n = 27); 51–53(n = 10); and >53 cm (n = 13). Fish were also grouped by sampling period for the analysis: (i) during the MUS (June–September) and (ii) out of the MUS (OMUS) (October–May).
Analysis of stomach contents
Fish were dissected and their stomachs removed. Stomach fullness was determined on a scale of 0–4 corresponding to: 0, empty or containing only digestive fluids or accumulated material such as otoliths; 1, filled to one-quarter of stomach volume; 2 half-filled; 3, full stomach, more than three-quarters of the stomach filled, but not reaching the pyloric sphincter; and 4, very full stomach, i.e. a very distended stomach. Prey were sorted and classified into four categories: fish, cephalopods, crustaceans, and gastropods. Prey items were weighed to the nearest 0.01 g. Measurable, undigested prey items were measured to the nearest 1 mm. Stages of digestion of the food items were recorded. As the stomach contents of captured tuna would have been influenced by the digestive process, the initial weight of each stomach content was reconstituted according to the method of Bard (2001). The fresh weights of prey were then calculated, dividing the wet weight of each prey item by a coefficient corresponding to its stage of digestion. The sum of these weights was therefore the total fresh weight of all items found in the stomach.
The taxonomic status of each item was determined using the keys and descriptions of Blache et al. (1970), Schneider (1992), and Nakamura and Parin (1993). Some cephalopods and crustaceans were lumped together because of the difficulty in identifying the digested material.
Data analysis
The diet of E. alletteratus was assessed using percentage frequency of occurrence by number (n), by weight (W), by corrected percentage of occurrence (Fc), and by the index of relative importance (IRI), the last evaluating the importance of single taxa in the diet of the fish (Pinkas et al., 1971). The calculations used the following formulae:
- %n = ni x 100/nt), where ni is the total number of prey i, and nt is the total number of all prey found (Hureau, 1970);
- %W = Wi x 100/Wt), where wi is the total wet weight of prey item i, and Wt is the total wet weight of all prey (Hyslop, 1980);
- %Fc = Fi/
Fi x 100, with Fi = ni/nt, where Fi is the frequency with which prey item i was recorded, ni the number of stomachs containing prey item i, and nt is the total number of full stomachs examined (Rosecchi and Nouaze, 1987; Gray et al., 1997);
- IRIi = (ni + Wi ) x Fci, and expressed as %IRI = (IRI/IRI) x 100.
The intraspecific dietary overlap between size classes was investigated using the Morisita (1959) quantitative index of similarity, as modified by Horn (1966):
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varies from 0, when the feeding regimes are completely distinct, to 1, when the regimes are identical. Above 0.6, the overlap between feeding regimes is considered to be biologically significant.
A Mantel test on two matrices containing the intraspecific overlap data between pairs of predator groups during the MUS and OMUS was used to determine whether or not diets overlapped significantly. The interpretation of the Mantel test is based on: (i) one of two hypotheses, the null hypothesis H0 (the matrices are not correlated) or the alternative hypothesis Ha (the matrices are correlated), and (ii) comparison of computed p-values to the significance level 
(0.05).
Niche breadth for the utilization of food resources was calculated using the Shannon–Wiener index (Krebs, 1989), defined as
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The relationship between predator size and prey size was examined using predator size and dominant prey lengths [standard length (SL) and total length (TL)] by fish prey category. Length of prey per predator size was based only on predator with measurable (i.e. less digested) fish prey in their stomach. Predator–prey size relationships were tested by a Pearson correlation test between prey length and predator size.
Statistical analyses were performed using STATISTICA 7.1 software (Statsoft, Inc.) and XLSTAT 2007.2 software (AddinsoftTM).
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Results |
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Prey of little tunny
Of the 170 stomachs collected, just 4 (2.35%), all out of the MUS, were categorized as level 0, empty or containing only digestive fluids or accumulated material such as otoliths. All 51 stomachs collected during the MUS contained food, and 115 of the 119 (96.64%) collected outside the MUS contained food. The number of empty stomachs was marginally not significant. There was no conclusive trend with size in the proportion of empty stomachs. In all, 23 prey taxa belonging to 15 families were identified: 19 fish species, 1 squid, and 3 crustacean species (Table 1).
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Fish were the main prey mainly due to the presence of two species, Atlantic bigeye (Priacanthus arenatus) and largehead hairtail (Trichiurus lepturus). Fish dominated the diet of little tunny by occurrence (Fc = 82.44%) and dominance (%IRI = 78.88). They were followed by crustaceans (Fc = 11.83% and %IRI = 20.94). Little tunny also preyed heavily on juvenile and larval scombrids. Cephalopods and gastropods contributed little to the diet (Table 1). From the values of %IRI, we conclude that E. alletteratus preyed mainly on fish.
Diet in relation to season and size class
The Spearman rank correlation test on the %IRI contribution of prey items consumed during the MUS and OMUS showed a significant difference in the diet by season (p < 0.05). The diet of little tunny was dominated by few prey species, supported by the low value of the Shannon–Wiener diversity index (H' = 1.22 ± 0.11).
During main upwelling season
Fish dominated the diet and were found in all size classes of predator (Table 2). They contributed 71.95%Fc, followed by crustaceans (%Fc = 18.29) and cephalopods (%Fc = 9.76). The consumption of fish, crustaceans, and cephalopods was particularly important in predators of FL 45–50 cm, the total values of %Fc in those two size classes being higher than those in other size classes (Table 2).
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Crustaceans and cephalopods seemed to be scarce or not consumed by small little tunny (<44 cm FL), and were totally absent from the stomachs of large little tunny (>51 cm FL). Of the crustaceans consumed by the little tunny in the medium size classes, shrimps and prawns dominated (Table 2).
The consumption of juvenile Auxis spp. and other scombrids (such as A. rochei or E. alletteratus and Scomber japonicus), although not marked in any size class, nevertheless suggests that cannibalism does take place.
Out of main upwelling season
Fish dominated the diet by frequency of occurrence (Table 3). At a species level, P. arenatus and T. lepturus were the most abundant prey (25.38 and 21.32%, respectively). Although not very important, crustaceans contributed to the diet of little tunny of >42 cm FL. Shrimps and prawns were taken by fish of 42–53 cm FL (Table 3).
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Fish, crustaceans, and cephalopods were recorded together only in little tunny of size classes 45–47, 51–53, and >53 cm, with relatively high percentage frequency of occurrence. During the OMUS, there was little consumption of scombrids by little tunny. As few gastropods occurred in juvenile stomachs only, they are considered to have been incidental prey.
Predator–prey relationships
Of the 1402 P. arenatus consumed by 60 tuna, 642 measurable (less digested) ones were selected to analyse the relationship between predator length and prey length. The lower limit of prey length remained static (probably reflecting availability), but the upper limit increased with size of little tunny. Prey length increased as little tunny grew, and there was a significant positive linear relationship between prey SL and predator FL (r2 = 0.2013; p < 0.05; Figure 2).
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Of the 417 T. lepturus consumed by 33 little tunny, the measurable 190 were selected to investigate the relationship between predator length and prey length. Overall prey length ranged from 20 to 44.3 cm. T. lepturus length was positively correlated to length of little tunny (r2 = 0.1419; p < 0.05; Figure 3).
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Dietary overlap
As the main pattern to emerge was a difference in diet composition between MUS and OMUS samples of little tunny, we examined the intraspecific dietary overlap separately for the two seasons. The Morisita and Horn index computed by pairs of predator size classes showed overlapping regimes between size classes related to season (Table 4).
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Few overlapping regimes were observed during the MUS. There was no overlap between the diets of fish <53 and >53 cm FL, nor did the diet of juveniles overlap with that of adult fish. However, OMUS, the incidence of overlapping regimes increased. The feeding regime of juveniles overlapped with that of some adult little tunny. No perfect overlap existed between size classes caught during the MUS and OMUS.
The Mantel test also failed to show significant overlap between diets during and OMUS. Figure 4 shows no correlation between the two matrices (Table 5), and the Mantel r-statistic [r(AB)] and corresponding two-tailed p-value for a significance level () of 0.05 were 0.155 and 0.604, respectively. We accept the null hypothesis H0 and reject the alternative hypothesis Ha, because the p-value computed is superior to the significance level ( = 0.05). The risk of rejecting the null hypothesis H0 whereas it is actually true is 60.4%. Figure 5 shows the distribution from which the p-value was obtained.
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Discussion |
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Little tunny in the continental shelf waters of Côte d'Ivoire clearly feed primarily on fish. The fish consumed are either demersal [e.g. Atlantic bigeye, flying gurnard (Dactylopterus volitans)], benthopelagic [e.g. largehead hairtail, promethean escolar (Promethichthys prometheus)], or pelagic [e.g. chub mackerel, S. japonicus; little tunny; mirrowing flyingfish (Hirundichthys speculiger)]. Additionally, the dominance of Atlantic bigeye and largehead hairtail, which are reported to be active at night (Schneider, 1992; Nakamura and Parin, 1993) points to nocturnal feeding behaviour by little tunny. This assertion contradicts the observations of other authors (Bullis, 1967; Wichlund, 1968; Chur, 1977), who stated that little tunny fed in daylight.
Trophic specialization could be associated with less dietary overlap during the MUS. Given the few overlapping trophic regimes during the MUS and the relatively more frequent overlap out of the MUS, we infer that food resource partitioning is more widespread within little tunny size classes during the OMUS than during the MUS. A possible reason for this could be the abundance of prey during enrichment of the environment in the MUS, as noted by several authors (Binet, 1993; Herbland and Le Loeuff, 1993; Reyssac, 1993), resulting in high prey density that could restrict foraging to the most profitable prey types. Winemiller (1989) cited several authors who observed that many predatory fish exhibit changes in relative body proportions and other anatomical traits during growth associated with greater feeding specialization. Any anatomical change that is possibly associated with trophic specialization in little tunny would probably reduce the efficiency of shrimp and prawn feeding and increase the efficiency to capture fish. Cayré (1984) made a similar observation for skipjack (K. pelamis) in the Atlantic Ocean. In tuna, one anatomical trait that could preclude prey rejection is the comb-like structure located on the gills. In the current study, shrimps and prawns were not eaten by juveniles (<42 cm FL) or by larger adults (>53 cm FL), possibly through this comb-like anatomical structure not functioning adequately to retain these taxa.
The feeding behaviour of little tunny seems to depend on the most readily available prey. Clearly, opportunistic behaviour plays a role in feeding, because smaller fish are able to capture relatively large P. arenatus or T. lepturus, and little tunny tend to capture these prey regardless of their own size. In any event, predation on T. lepturus and P. arenatus undoubtedly coincided with increased abundance of these prey in the environment over the study period. However, the analysis of stomach contents did not indicate any clear selection for fish prey by different predator size classes, because all little tunny examined had taken similar prey. Clearly, trophic ontogeny in little tunny proceeds as a continuum of dietary change rather than by distinct segregation of food resources between size classes. In other words, as little tunny grow they tend to change their diet. These changes may be simply attributable to evolution in feeding habits with increasing size of predator. Three factors are likely involved in producing size-related patterns of feeding over size classes: (i) juvenile little tunny are constrained by their small size to take relatively small fish as prey; (ii) only following a probable period of initial growth can little tunny of size ranging between 42 and 53 cm FL switch to feeding on both crustaceans and fish, and (iii) when >53 cm FL, adult little tunny tend to prefer consuming fish to crustaceans or cephalopods.
The low value of the Shannon–Wiener diversity index recorded in the present study indicates that the diet of little tunny is dominated by relatively few items, so little tunny can be considered specialist feeders. They feed on fish, cephalopods, and crustaceans, which makes them carnivorous. Previous studies (Postel, 1954) in the tropical Atlantic Ocean have reported cannibalism by little tunny. In the present study cannibalism was marked during the MUS when the quantity of scombrids consumed was relatively greatest.
The occurrence of sardinellas in the stomachs of little tunny was less important during the MUS than OMUS. The relatively high frequency of occurrence of the round sardinella (Sardinella aurita) out of the MUS conflicts with the general belief that this species is associated with upwelling areas (Schneider, 1992), and hence supposed to occur mainly during upwelling. Possible reasons were, as Bard and Hervé (1995) concluded for the upwelling ecosystems of Ghana and Côte d'Ivoire for skipjack and yellowfin tuna: (i) predation by tuna on these sardinellas does not seem systematic and therefore would be moderate, and (ii) sardinellas stay in upwelling waters where they are not easily available to tuna.
In summary, little tunny feeding behaviour is based on heavy consumption of fish, mainly T. lepturus and P. arenatus, and moderate consumption of shrimps and prawns. Though their small size constrained them to exploiting relatively small prey, juveniles preyed mainly on fish, as did adult little tunny with which their feeding regime overlapped outside the main upwelling season. Little tunny respond to seasonal changes in food availability, which reflects the species' opportunistic behaviour and trophic adaptability, allowing them to take advantage of the most readily available prey in the environment at any time. They tend to rely mainly on the abundant fish as prey, although they will occasionally exploit squid and crustaceans to enhance their diet.
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Acknowledgements |
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This paper was derived from a doctoral thesis submitted to the "Laboratoire d'Hydrobiologie, UFR Biosciences" of the University of Cocody, Abidjan. The first author thanks the EU for financial support via a research grant to the Centre de Recherches Océanologiques (CRO), and the CRO and the French Institute for Research Development for allowing him to work on their sites and for placing infrastructure at his disposal. All authors also thank the anonymous reviewers for constructive remarks and suggestions.
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