Diet of mahi-mahi, wahoo and lancetfish in the western and central Pacific. Valérie ALLAIN

Transcription

1 SCTB16 Working Paper BBRG-6 Diet of mahi-mahi, wahoo and lancetfish in the western and central Pacific. Valérie ALLAIN Oceanic Fisheries Programme Secretariat of the Pacific Community Noumea, New Caledonia June 23

2 Introduction To develop ecological approaches of fisheries management it is important to take into account species interactions and underlying ecosystem dynamics. Assessing the impact of fisheries and environmental effects on the ecosystem implies a good comprehension of this system. Predation induces an important mortality in the ecosystem that is often higher than fishery mortality, and determining trophic interactions between species is a major step towards a better understanding and modeling of the ecosystem dynamic. A large sampling programme has been implemented in the western and central Pacific to collect samples and determine the diet of the top predators of the pelagic ecosystem. In this report, the number of samples already collected is reported and a first analysis of the diets of wahoo, dolphinfish and lancetfish, three important bycatch, is done. 2 Methods Sampling programme Stomach samples are collected from target fishes (tunas) and bycatch species by observers from the different national observer programmes in the area (Cook Islands, Federated States of Micronesia, Fiji, Kiribati, New Caledonia, Papua New Guinea, French Polynesia and Solomon Islands). Since the beginning of the programme in January 21, 23 sampling trips have been done, 19 on longline boats and four on purse seine vessels (Figure 1). Nine sampling trips were organised in French Polynesia, seven in New Caledonia, three in Federated States of Micronesia, two in Papua New Guinea and Cook Islands. 14 W 16 W 18 E 16 E 14 E 12 E Northern Marianas Hawaii 2 N Guam Marshall Islands Palau Federated States of Micronesia Australia Papua New Guinea Nauru Solomon Is New Caledonia Vanuatu Kiribati Tuvalu Wallis & Futuna Fiji Phoenix (KI) Tokelau Samoa Cook Islands Am Samoa Tonga Niue Line Islands (KI) French Polynesia Pitcairn N 2 S New Zealand PURSE SEINE LONGLINE 4 S Figure 1. Sampling trips done in the western and central Pacific.

3 3 Two protocols were designed for longline and purse-seine. They are available at Observers are asked to sample two fish of each species caught per set and to collect the stomachs as well as data on species, length, position date and time. All the species are to be considered: target species (tunas) but also bycatch and discard species. Stomachs are frozen, stored at the harbour and sent to SPC for analysis. Stomach examination Classical procedure is used to analyze the stomachs: -Fullness coefficient is determined according to a scale from (empty) to 4 (full) (see caption of Figure 2 p.7 for details). If baits are present, they are removed to determine the fullness coefficient. -Preys are sorted by species or group, identified at the lowest taxonomic level, a digestion state is attributed (from 1 to 4, see details in caption of Figure 4 p.8), development state is determined when possible (larvae, juvenile, adult), they are counted, weighted and measured. The number of baits, the presence of parasites, the number of cephalopod beaks, gladius and otoliths are recorded. Species were determined using - Smith, M.M. and Heemstra, P.C. Eds Smiths sea fishes. Springer-Verlag. Berlin. 147p. (Fish). - Carpenter, K.E. and Niem, V.H. Eds. 21. FAO species identification guide for fishery purposes. The living marine resources of the Western Central Pacific. FAO. Rome. 6 vol. 4218p. (Fish, Molluscs, Crustacea). - Tree of life Web project. (Cephalopods). Taxonomic classification used follow data provided by Integrated Taxonomic Information System It is important to note that some species are underestimated due to a high digestion rate and/or because of a lack of specific structure that could help for identification. The typical jaws of Tetraodontiformes, Alepisaurus, Gempylidae/Trichiuridae, the dorsal spine of Monacanthidae, Balistidae, the photophores of Myctophidae, Ommastrephidae (squids) facilitate identification of these species, even when they are in an advanced state of digestion. Data on habitat and type in Table 5 (p.15) are extracted from Fishbase ( and FAO species identification guide. For data analysis, frequency of occurrence (%F), percentage of number (%N) and percentage of weight (%W) were calculated by taxon, cumulating all the data. Frequency of occurrence of an item is the number of stomachs where this items is present divided by the number of non-empty stomachs. Percentages of number and weight are the respective number and weight of the taxonstudied divided by total number or weight of this taxon of all the samples, by predator species.

4 4 Results Discussion Species sampled and percentage of empty stomachs Since the beginning of the sampling programme, 47 different species or groups of species have been collected: billfish, sharks and rays, tunas and other species, but only 18 species have more than 1 samples (Table 1). They represent a total of 1 stomachs examined of which 37.8% were empty. If considering the percentage of empty stomachs by gear, it appears that 15.4% only of the 62 specimens collected on longline had empty stomachs when this percentage reach 71.6% for the 398 fish caught by the purse seine. While, in general, species caught by the two gears are different, few common species were collected from LL and PS, but in this case, the percentage of empty stomachs is always higher for the purse seine (blue marlin, bigeye, skipjack, yellowfin, dolphinfish) except for the silky shark. It appears that all the silky sharks from PS were caught during FAD sets and their stomachs contained small preys typical of the aggregating devices (Kyphosus sp.). The fact that the PS silky shark show a low percentage of empty stomach could then be explained by a non-typical feeding behavior due to the presence of the FAD. Among the species caught by the longline, only two of them, with more than 1 samples, present a percentage of empty stomachs higher than 5%: blue shark and escolar. Sharks are known to regurgitate when caught on longline, and they are also believed to eat large amount of food followed by long periods of starvation, it is not surprising then to have a high percentage of empty stomachs. Biology and behaviour of escolar is nearly unknown and no satisfactory explanation could be found to explain the high value of empty stomachs for this species, however regurgitation is a possibility. The bias in the percentage of empty stomachs observed between the fishing gears is easily explained: fish caught on the longline are always in an active feeding phase, while schooling fish caught with the purse seine are not necessarily in such a phase. Purse seining around FADs could however modify this tendency, at least for some of the predators. The most numerous species collected are skipjack, yellowfin, bigeye, dolphinfish, lancetfish, albacore, wahoo and rainbow runner (more than 4 samples).

5 Species Number of stomachs Longline Purse seine Total % empty stomachs Number of stomachs % empty stomachs Number of stomachs 5 % empty stomachs BILLFISH Broadbill swordfish (Xiphias gladius) Black marlin (Makaira indica) Indo-pacific blue marlin (Makaira mazara) Indo-pacific sailfish (Istiophorus platypterus) Short-billed spearfish (Tetrapturus angustirostris) Striped marlin (Tetrapturus audax) SHARKS and RAYS Blue shark (Prionace glauca) Hammerhead shark (Sphyrna spp.) Unidentified shark (Elasmobranchii) Short-finned mako shark (Isurus oxyrinchus) Oceanic white tip (Carcharhinus longimanus) Long-fin mako shark (Isurus paucus) Bigeye thresher shark (Alopias superciliosus) Silky shark (Carcharhinus falciformis) Pelagic sting-ray (Dasyatis violacea) Unidentified manta ray (Mobulidae) TUNA Bigeye tuna (Thunnus obesus) Skipjack tuna (Katsuwonus pelamis) Yellowfin tuna (Thunnus albacares) Albacore (Thunnus alalunga) Frigate tune (Auxis thazard) OTHER SPECIES Dolphinfish (Coryphanea hippurus) Moonfish (Lampris guttatus) Longnose lancetfish (Alepisaurus ferox) Rainbow runner (Elagatis bipinnulata) Wahoo (Acanthocybium solandri) Black mackerel (Scombrolabrax heterolepis) Snake mackerel and escolar (Gempylidae) Escolar (Lepidocybium flavobrunneum) Roudi escolar (Promethichthys prometheus) Snake mackerel (Gempylus serpens) Oilfish (Ruvettus pretiosus) Unidentified barracuda (Sphyraena spp.) Blackfin barracuda (Sphyraena qenie) Great barracuda (Sphyraena barracuda) Dealfish (Desmodema polystictum) Filefish unicorn leatherjacket (Aluterus monoferos) Crestfish unicornfish (Lophotus capellei) Oceanic triggerfish (Balistidae) Longfin batfish (Platax teira) Mackerel scad (Decapturus macarellus) Triple tail (Lobotes surinamensis) Sickle pomfret (Taractichthys steindachneri) Big scaled pomfret (Taractichthys longipinnis) Golden trevally (Gnathanodon speciosus) Drummer blue chub (Kyphosus cinerascens) Chiasmodontidae Total Table 1: Number of analysed stomachs and percentage of empty stomachs by species and by gear. Shaded cells represent species with more than 1 samples.

6 6 Wahoo, dolphinfish and lancetfish Wahoo, dolphinfish and lancetfish are 3 common bycatch of the longline fishery. Wahoo and dolphinfish have an important commercial value while lancetfish are discarded at sea. A first analysis of their feeding strategies will be presented in this study. Considering the small amount of samples, 43, 65 and 57 respectively, the data cannot be stratified by time, area, gear, sex or length range, but will provide a broad overview of the diet of these species. Characteristics of the samples Most of the samples come from New Caledonia and French Polynesia EEZ, but not exclusively, and they were mainly caught by longline (Table 2). The three predators have similar percentages of empty stomachs and their size range is comparable, approximately between 5 and 17 cm. The five lancetfish under 5 cm were sampled in the stomachs of larger lancetfish. The percentage of females is high for the three species. It is probably an artefact of sex determination, at least for wahoo and lancetfish for which only 37 and 36 fish were sexed (62 for dolphinfish). Observers are asked not to record the sex if they are not sure of the determination and, as, by the presence of eggs, females are more easily recognized, sex-ratio is probably biased in favour of females. number of samples % empty stomachs Wahoo (Acanthocybium solandri) Dolphinfish (Coryphanea hippurus) gear 4 LL 3 PS 53 LL 12 PS area 28 NC 2 NR 1 FP 2 CK 1 PNG 42 NC 4 FSM 7 FP 4 CK 8 NR Longnose lancetfish (Alepisaurus ferox) LL 53 NC 4 FP length range (UF cm) % females (5 specimens < 5 cm) 66.6 Table 2: Characteristics of the samples of wahoo, dolphinfish and lancetfish. LL: longline, PS: purse seine, NC: New Caledonia, FP: French Polynesia, FSM: Federated States of Micronesia, CK: Cook Islands, NR: Nauru, PNG: Papua New Guinea, UF: upper jaw-fork length. Fullness coefficient and presence of baits Respective percentages of empty stomachs for wahoo, dolphinfish and lancetfish are 18.6, 21.5 and 19.2%. For the remaining stomachs, most of them, for the three species, contain less than half of the stomach volume (Figure 2). 4.6% of wahoo have half-full stomachs, 13.8 and 15.6% respectively for dolphinfish and lancetfish. Only dolphinfish and lancetfish have stomachs more than half-full or full (cumulated percentage: 6.1 and 5.2% respectively). This low degree of fullness could mean that they eat small amount of preys at the same time or that the digestion process is fast enough to eliminate quickly the large amounts of food ingested.

7 7 Percentage of stomachs Wahoo Dolphinfish Lancetfish Fullness coefficient Figure 2: Percentage of stomachs according to their fullness coefficient. : empty stomach, 1: stomach less than half full, 2: stomach half full, 3: stomach more than half full, 4: stomach full. For fish caught on longline, in more than 6% of the cases, bait was not found in the stomachs of dolphinfish and lancetfish, while this percentage is only of 35% for wahoo (Figure 3). Absence of the bait can be explained by the fact that if the bait is not very well fixed on the hook, it can probably easily be removed when the predators try to swallow it. Another explanation is that once the predator is hooked, the bait is probably regurgitated when the fish fights to escape. This phenomenon seems less common for the wahoo. For this species 37% of the fish are found with one bait in the stomach, only 21 and 28% for the two other species. If a noticeable percentage of stomachs with 2 baits (up to 1% for the wahoo) can partially be explained by the fact that, sometimes, two baits are fixed on the same hook, this explanation cannot be invoked when 3 or more baits are found in the same stomach. It seems that wahoo and in a lesser extent dolphinfish developed the capability of swallowing the baits without being hooked. Up to 9 baits were found in one stomach of wahoo, and it can be supposed that, in areas where many longlines are deployed, some species could follow the line and use the baits as a food resource, with however a high risk to be caught. Percentage of stomachs Wahoo Dolphinfish Lancetfish Number of baits Figure 3: Percentage of stomachs containing baits for fish caught on longline.

8 8 State of digestion of the preys If feeding is supposed more or less continuous, when considering a sample representative of the population, the distribution of the percentage of preys in the different states of digestion can give an indication on the dynamic of the digestion process. State 1 of digestion can be supposed relatively short as once the skin of the fish/mollusc is removed, it is already considered in state 2. States 2 and 3 can be supposed more or less of the same length, they are two states of disaggregation of soft parts of the preys, while state 4 could be a longer one as hard parts such as skeletons, cephalopod beaks and gladius are considered in this state. It is also important to remind that if there is little weight difference between a prey in state 1 and 2, there is an important loss of biomass between states 2, 3 and 4. Taking into account these hypotheses, the most represented states could be considered as the longest in term of time bearing in mind the loss of biomass as the digestion progress. Only 3% of the preys are in digestion state 1 for wahoo (Figure 4), and considering the above assumptions, this let suppose that the digestion process starts very quickly once preys are ingested. Most of the preys are in state 2 (6%) while only 22% are in state 3; it is difficult to state if this high difference is due to the loss of weight between the 2 states or if the state 2 is longer. Nearly 15% of the weight of the preys is in state 4. In the case of the dolphinfish, the distribution of the weights of preys by digestion state is very regular. Albeit the loss of biomass characterizing state 3, this is the state with the highest percentage (37%). This accumulation could suggest that states 1 and 2 are relatively short compared to state 3. Contrary to the two other species, most of the preys are in state 1 (53%) for the lancetfish. This state probably lasts for a long time. Fewer preys are observed in state 2 (42%) and state 3 (5%), and less than.5% is in state 4. The loss of biomass between the different states could not explain this distribution. As already observed by different authors, it is supposed that lancetfish preys are digested in the intestine rather than in the stomach. This allows a higher identification rate of the preys compared to other fish where preys are highly digested and then more difficult to identify. Percentage of food weight Wahoo Dolphinfish Lancetfish State of digestion Figure 4: State of digestion of all the preys found in the stomachs, by predator. 1: fresh, 2: whole, partially digested, 3: fragmented, advanced digestion, 4: hard part remains. If considering the two most frequent prey types, fish and mollusc (mainly cephalopods), differences in digestion states can be observed (Figure 5). For wahoo and dolphinfish results are similar: molluscs are in more advanced digestion state than fish. A faster digestion of the mollusc compared to the fish can then be supposed. An important difference between these two species is the very high percentage of molluscs in state 4 in dolphinfish compared to wahoo. Dolphinfish seems to accumulate for longer time the hard parts of the molluscs (beaks and gladius); the elimination process seems slower or less frequent. In the case of lancetfish, there is no noticeable difference of digestion between fish and molluscs. However digestion does not seem to take place in the stomach

9 and it would be necessary to examine the intestine to detect any digestion differences between preys. 9 Percentage of weight per prey type Fish Wahoo Mollusc Percentage of weight per prey type Dolphinfish Percentage of weight per prey type Lancetfish State of digestion Figure 5: Repartition of quantities of preys by state of digestion for the three predators. 1: fresh, 2: whole, partially digested, 3: fragmented, advanced digestion, 4: hard part remains. Description of the diet Prey groups The most frequent prey found in wahoo and dolphinfish stomachs is the fish group while it is the mollusc group for lancetfish (Table 3). For wahoo two other prey have frequencies higher than 1%: mollusc and crustacea, there is only one for dolphinfish: mollusc and 3 for lancetfish: fish, crustacea and invertebrate. In term of number of preys the tendency already observed in frequencies is the same. In term of biomass, fish represent 88% of the weight of preys ingested by the wahoo and mollusc 11%. Fish compose nearly all the prey weight of dolphinfish with a percentage of 98%. The most frequent preys of the lancetfish represent similar weights with 47% of the weight for both mollusc and fish. Crustacea that was an important prey in frequency and abundance represent less than 2% of the weight of the preys.

10 Wahoo Dolphinfish Lancetfish prey group %F %N %W %F %N %W %F %N %W Fish Mollusc Crustacea Invertebrate Vegetal Mineral Rubbish Unrecognizable Table 3: Frequency of occurrence (%F) and percentage of the total prey numbers (%N) and weights (%W) of the different prey groups. Invertebrate: all invertebrates except molluscs and crustacea, i.e. mainly worms, salp (see Table 5 caption for details). Vegetal: floating algae or pieces of wood. Mineral: floating volcanic stones. Rubbish: human products such as plastic pieces. Unrecognizable: not identifiable prey items. Distribution of prey groups according to their frequency of occurrence in the stomachs and the percentage of weight or number they represent is a way to determine the feeding strategy of the predator, that is, to determine if it is a specialist or a generalist feeder. The three graphs show three different feeding strategies (Figure 6). 1 Percent Weight or Number 1 %W 8 Wahoo %N fish crustacea mollusc Frequency of occurence Percent Weight or Number 1 Dolphinfish fish mollusc Frequency of occurence Percent Weight or Number 1 Lancetfish fish mollusc 2 invertebrate crustacea Frequency of occurence Figure 6: Diet importance of the different prey groups: frequency of occurrence vs. percent weight and number.

11 11 Dolphinfish is an example of very specialised predator for which fish is nearly the only prey consumed. Wahoo has a slightly more diverse diet with mollusc representing a more important part. At last, diet of the lancetfish shows the more diverse diet with two main preys consumed in similar quantity and two less frequent preys that however represent little biomass. Prey items Prey richness The number of prey items identified is 24, 42 and 63 respectively for wahoo, dolphinfish and lancetfish (Table 5). It is however important to note that 24% of the preys (%N) are unidentified fish, and unidentified squids (Teuthida) are 22% in wahoo diet, 46% are unidentified fish in dolphinfish diet while they only represent 6% of prey items for lancetfish (Teuthida are respectively 1% and less than 1%). The high percentages of unidentified items are linked to more advanced digestion state of the preys found in wahoo and dolphinfish stomachs (cf. State of digestion of the preys p.8). Hence diversity of diets of wahoo and dolphinfish are probably underestimated compared to lancetfish. Diversity of preys is the highest for lancetfish that consumes a large variety of cephalopods (9 families) but also other invertebrates and many fish families (21). Albeit an underestimation of prey diversity, dolphinfish appears to consume a large number of different fish families (21), but few cephalopods (3 families). For wahoo, underestimation of prey diversity due to advanced digestion state of the preys probably explains why the numbers of fish (15) and cephalopod (1) families are so much lower. The most frequent and important preys (Table 5 p.13-15) Wahoo: Not taking into account the unidentified fish, the most frequent preys are squids 4% Chiasmodontidae 23%, Alepisaurus 14%, Gempylidae, Scombridae and shrimps (11%). In term of number, the most ingested items are squids 22%, Siganidae 1%, Chiasmodontidae 7% and Decapoda 6% of which megalopa larvae account for 4%. In weight, Lagocephalus represents 49%, but it can be considered as an artifact as this is due to a unique large fish consumed by one of the wahoos. However Tetraodontidae still compose 5% of the weight of the preys (not considering Lagocephalus), Scombridae 7%, squids 7%, Alepisaurus 5% and Chiasmodontidae 4%. Wahoo mainly feeds on pelagic and epipelagic preys, Scombridae, Tetraodontidae that represent 19.6% of the preys in number but 68.34% in weight (Table 4). The demersal preys also represent a large number of items (16.8%) but their weight is only 3%. Indeed this prey type gather reef and lagoon fish such as Acanthuridae, Monacanthidae and Siganidae which juveniles are, for some of them, known to be pelagic, for the others the young pelagic phase is suspected. Alepisaurus is the only Epipelagic/Pelagic/Mesopelagic prey consumed by wahoo and if in number it is only 2%, it represents 4.8% of the weight. Mesopelagic/Bathypelagic preys, mainly represented by Chiasmodontidae for wahoo is not negligeable, it accounts for 7.6% of the number of preys and 4.6% in weight. Dolphinfish: The most frequent preys of dolphinfish are Tetraodontidae 22%, Exocoetidae, Balistidae and Diodontidae with 16% and Monacanthidae 12%. In number Balistidae, Tetraodontidae and Diodontidae are the families with the highest percentages with respectively 11, 7 and 5%. However it is important to remind that 46% of the preys are unidentified fish (34% in weight). In weight Exocoetidae compose 26% of the preys, Balistidae 1%, Tetraodontidae 1% with Lagocephalus 5%, and Carangidae and Diodontidae 6%. Most of the preys are Epipelagic/Pelagic: Balistidae, Tetraodontidae and Exocoetidae mainly (29%N, 54%W). Another important category of preys (16%N, 8%W) are pelagic juveniles of reeflagoon fish such as Diodontidae, Monacanthidae, Chaetodontidae, Holocentridae, Acanthuridae and Siganidae. Epipelagic/Pelagic/Mesopelagic fish are anecdotical in number and weight, and

12 12 represented by Alepisaurus, but also juveniles of Molidae. Mesopelagic / Bathypelagic are only a minor type of preys for dolphinfish. Lancetfish: The most frequent prey, by far, is Carinaria sp. 72%, a pelagic Gastropoda of 3-4 cm long with a thin shell. Alepisaurus is also a frequent prey 33%, as well as small crustacea: Hyperiidea and Phronima 24%. Octopodidae and Onychoteuthidae are present in 17 and 13% of the stomachs while Polychaeta and Sternoptyx occur at a frequency of 11%. In number, Carinaria is also the most numerous prey 47% and Amphipoda represents 9%. Otherwise, all other numerous preys occur at low percentages. Lancetfish shows a high level of cannibalism as Alepisaurus represents the highest percentage of prey weight with 26%. Carinaria, albeit its small size, but because it is ingested in large number, represents 18% in weight. The cephalopods Onychoteuthidae occur at a rate of 13%, of which Moroteuthis equals 5%. Lactoria diaphana, a boxfish, represents 7% of the prey weight. Lancetfish most important preys are Epipelagic/Pelagic that represent 64% in number and 42% in weight. A large number of preys are included in this category, but the main ones are Carinaria, Amphipoda, Onychoteuthidae, Argonauta. However the other prey categories also represent a large part of the diet in weight. Epipelagic/Pelagic/Mesopelagic, only represented by Alepisaurus, is 26%. Demersal preys, mainly Lactoria but also Octopodidae, account for 14%, and Mesopelagic/Bathypelagic preys are 11% in weight with Sternoptychidae, Chiasmodontidae and the octopus Amphitretidae. Wahoo Dolphinfish Lancetfish Prey type %N %W %N %W %N %W Demersal - (juveniles Pelagic) Epipelagic / Pelagic Epipelagic / Pelagic / Mesopelagic Mesopelagic / Bathypelagic Table 4: Percentages of number (%N) and weight (%W) cumulated by prey type for the three predators. See Table 5 caption for definitions of prey types. Sums of %N and %W per column is less than 1% as the lack of identification of some of the preys doesn t allow determining their prey type. Shaded cells: >5%.

13 Wahoo Dolphinfish Lancetfish prey items type habitat %F %N %W %F %N %W %F %N %W Fish Anguilliformes Leptocephalus P O Stomiiformes Sternoptychidae Argyropelecus aculeatus M O Stomiiformes Sternoptychidae Sternoptyx sp. M O Aulopiformes Paralepididae Paralepididae MB O Aulopiformes Paralepididae Lestidium atlanticum je - am O Aulopiformes Alepisauridae Alepisaurus sp. EPM O Lophiiformes Lophiiformes Beloniformes Exocoetidae Exocoetidae E NO Beloniformes Exocoetidae Oxyporhamphus sp. E NO Lampriformes Trachipteridae Trachipterus trachypterus M O Beryciformes Beryciformes Beryciformes Anoplogasteridae Anoplogaster cornuta jemb - amb O Beryciformes Holocentridae Holocentridae lp - ad NO Scorpaeniformes Scorpaenidae Pterois sp. lp - ad NO Scorpaeniformes Sebastidae Sebastidae lp - aepmb Scorpaeniformes Triglidae Triglidae D N Scorpaeniformes Dactylopteridae Dactylopteridae jp - ad jo - an Perciformes Kyphosidae Kyphosus sp. D N Perciformes Chaetodontidae Chaetodontidae D N Perciformes Bramidae Bramidae E O Perciformes Bramidae Pterycombus petersii E O Perciformes Bramidae Taractichthys sp. E O Perciformes Carangidae Carangidae DP NO Perciformes Carangidae Atropus atropos P N Perciformes Pomacentridae Amphiprion sp. D N Perciformes Chiasmodontidae Chiasmodontidae MB O Perciformes Acanthuridae Acanthuridae D N Perciformes Acanthuridae Naso sp. D N Perciformes Siganidae Siganidae D N Perciformes Scombrolabracidae Scombrolabrax heterolepis M O Perciformes Gempylidae Gempylidae P O Perciformes Gempylidae Rexea sp. Be O Perciformes Trichiuridae Trichiuridae jp - aebe O Perciformes Trichiuridae/Gempylidae jp - aebe O Perciformes Scombridae Scombridae P NO

14 Perciformes Scombridae Katsuwonus pelamis E O Perciformes Xiphiidae Xiphias gladius P NO Perciformes Zoarcidae Zoarcidae D NO Tetraodontiformes Balistidae Balistidae DP NO Tetraodontiformes Balistidae Canthidermis maculatus E NO Tetraodontiformes Balistidae Xenobalistes sp. P NO Tetraodontiformes Monacanthidae Monacanthidae D N Tetraodontiformes Monacanthidae Cantherhines sp. jp - adp N Tetraodontiformes Ostraciidae Ostracion sp. D N Tetraodontiformes Ostraciidae Lactoria diaphana je - ad jo - an Tetraodontiformes Tetraodontidae Tetraodontidae DP N Tetraodontiformes Tetraodontidae Lagocephalus lagocephalus E O Tetraodontiformes Diodontidae Diodontidae D N Tetraodontiformes Diodontidae Diodon sp. jp - ad N Tetraodontiformes Diodontidae Cyclichthys spilostylus jp - ad N Tetraodontiformes Molidae Molidae EM O long orange fish Mesopelagic fish M O rond orange Ag fish Unidentified fish Molluscs/Cephalopoda Teuthida Cranchiidae Cranchiidae P O Teuthida Enoploteuthidae Abralia sp. M O Teuthida Joubiniteuthidae Joubiniteuthis portieri P NO Teuthida Ommastrephidae Ommastrephidae P NO Teuthida Ommastrephidae Hyaloteuthis pelagica P NO Teuthida Ommastrephidae Ornithoteuthis volatilis P NO Teuthida Onychoteuthidae Onychoteuthidae P O Teuthida Onychoteuthidae Onychoteuthis sp. P O Teuthida Onychoteuthidae Moroteuthis lonnbergi P O Teuthida Oegopsina Teuthida Teuthida (Unidentified squid) Sepiida Sepiolidae Sepiolina nipponensis D Sepiida Sepiolidae Eupryma tasmanica D Sepiida Sepiida Octopoda Amphitretidae Amphitretidae M Octopoda Argonautidae Argonauta sp. P Octopoda Octopodidae Octopodidae D

15 Octopoda Octopodidae Octopus sp. D Unidentified cephalopoda Molluscs/Gastropoda Atlantidae Atlanta sp. P Carinariidae Carinaria sp. P Cavoliniidae Cavolinia sp. P Thecosomata P Unidentified mollusc Crustacea Amphipoda E Hyperiidea E Phronimidae Phronima sp. E Isopoda E Megalopa stage E Galatheidae Munida E Shrimp Stomatopoda E Unidentified crustacea Invertebrate Nemata (Phylum) Platyhelminthes (phylum) Polychaeta (Annelida) Salp Unidentified invertebrate Vegetal Algae Wood Mineral Stone Rubbish Rubbish (human product) Unrecognizable Unrecognizable Table 5: Frequency of occurrence (%F), percentages of number (%N) and weight (%W) of the different prey items for the three predators. Type: D=demersal=bottom dwelling fish or up to few 1's m above the bottom, P=pelagic=fish living without relation with bottom, Be=benthopelagic=fish with demersal and 'pelagic' habits (up to few 1's m above bottom), E=epipelagic=pelagic fish between the sea-surface and 2 m depth, M=mesopelagic=pelagic fish between 2 and 1 m depth, B=bathypelagic = pelagic fish between 1 and 4 m depth. Habitat: N=neritic=from the coastline to a line where depth in around 2 m (include shore, coastal, coral reef, continental shelf), O=oceanic = areas where bottom is deeper than 2 m. l=larvae, j=juvenile, a=adult. Shaded cells: >1% for %F, >5% for %N and %W.

16 Size-distribution of the preys Wahoo don t swallow preys less than 1mm, and preys in the size range -1mm only represent.46% for dolphinfish and 1.81% for lancetfish (Figure 7). However, it is important to note that in the case of wahoo, the smallest predator examined was 72cm long while they were respectively 44 and 15.5cm for the two other species. The lack of small prey in wahoo diet is then probably a bias linked to the length of the predators sampled. The biggest prey swallowed is 39mm for wahoo, 3mm for dolphinfish and 335mm for lancetfish, approximately the same size for the three predators. The mode of the prey length-frequency distribution is 3-4mm for wahoo and dolphinfish with respective percentages of 12.3 and 14.8%. This mode is smaller for lancetfish: 1-2mm with a percentage of 22.3%. For the three predators, most of the preys are less than 12mm: 87., 85.2 and 91.% respectively, and the average size of the preys is 75.1, 73.2 and 53.6mm. In the -12mm prey-size range, the shape of the length-frequency distribution is not the same for the three species. In the case of wahoo, distribution is even (3-1mm>1%) while is it skewed towards smaller sizes for dolphinfish (3-8mm>1%) and this phenomenon is accentuated for lancetfish (1-4mm>1%). If the three predators studied swallow the same size-range of preys, lancetfish ingests smaller preys than wahoo and dolphinfish Wahoo % number Length of the preys (mm) Dolphinfish % number Length of preys (mm) Lancetfish % number Length of preys (mm) Figure 7: Length-frequency distribution of the preys for the three predators.

17 In the predator size-range studied, no tendency is observed in the length of preys according to the length of wahoo (Figure 8): between 1 and 17cm, wahoo are able to swallow the same sizes of preys. Preys eaten measure in average 6% of the predator size and preys up to 3% of predator size have been ingested. The prey length/predator length ratio tends to decrease with increasing predator length (Figure 8). In the case of dolphinfish, size of the preys seems to increase with the size of the predator. There is little data for predators under 75cm and more samples would be necessary to confirm that they only eat preys under 1mm. Fish bigger than 75cm eat large preys of 2mm and more. Preys ingested represent, in average, 6.5% of the length of the predator and the maximum value observed was 25%. A slight decreasing trend is observed in prey length/predator length ratio with increasing predator length. The size-range of lancetfish sampled is large: from 2 to 16 cm and an increasing size of the preys is observed with the size of the predator. Small preys are consumed at all predator size but large preys (>1mm) appears in the stomachs of fish larger than 7cm. However one large fish (25mm) has been observed in the stomach of a 164mm lancetfish (prey length/predator length ratio=1.52), it is a long thin leptocephalus larvae. Not taking. into account this exception, preys ingested are in average 8% of the predator length and largest preys are 4% of predator length. The prey length/predator length ratio tends to decrease with increasing predator size Prey length (mm) y =.1238x R 2 = Predator length (cm) Wahoo Ratio Prey length/pred. length y = -3E-5x R 2 = Predator length (mm) Wahoo Prey lengh (mm) y =.4943x R 2 = Predator length (cm) Dolphinfish Ratio Prey length/pred. length y = -2E-5x R 2 = Predator length (mm) Dolphinfish Prey length (mm) y =.3276x R 2 = Predator length (cm) Lancetfish Ratio Prey Length/Pred. length y = -7E-5x R 2 = Predator length (mm) Lancetfish Figure 8: Prey length vs. Predator length and Prey length/predator length ratio vs. predator length.

18 Summary Wahoo With a percentage of empty stomachs less than 2% and small amount of food observed in stomachs, it can be supposed that wahoo feeds frequently in small quantities or that digestion is a fast process. It is noticeable that wahoo developed the capability of eating baits from the longlines, it is not rare to find more than 2 baits in the same stomach. Digestion probably starts very quickly once the preys are swallowed and then disaggregation of the preys takes more time. Cephalopds seem to be digested quicker than fish. Wahoo is not a strict piscivorous predator even if fish is the main prey in frequency, number and weight. Molluscs are also an important part of the diet as well as crustacea that however accounts for only a small quantity in term of weight. Underestimation of prey diversity due to advanced digestion state of the preys probably explains the low number of fish and cephalopod families identified: 15 and 1 respectively. This epipelagic predator primarily feeds on pelagic and epipelagic fish such as Scombridae and Tetraodontidae, but it also dives to catch deeper preys such as lancetfish and Chiasmodontidae. Pelagic juveniles of reef-lagoon fish are numerous in the diet even if they represent a little weight. Shrimps and squids are also an important part of the diet. Most of the preys ingested by wahoo are between 3 and 1mm and measure in average 6% of predator length. Dolphinfish Like wahoo, dolphinfish probably eats small quantities of food at the same time or digests fast, 2% of the stomachs being empty and most of the specimens having stomachs half-full or less than halffull. Dolphinfish also developed the ability to eat several baits from the longline, although it is less frequent than in the case of the wahoo. Digestion doesn t seem to start as fast as for wahoo and preys accumulate in an advanced state of digestion. Molluscs seem to be digested more quickly than fish and mollusc hard parts are supposed to accumulate for long periods of time before being eliminated. Dolphinfish is a strict piscivorous predator and, in term of weight, other preys only constitute 2.3% (mainly mollusc). Although 46% of the fish are unidentified, 21 families were determined. Dolphinfish only feeds on epipelagic preys: Tetraodontidae, Balistidae, Exocoetidae (flying fish) and pelagic juveniles of reef-lagoon fish such as Diodontidae, Monacanthidae, Chaetodontidae, Acanthuridae. Most of the preys measure between 3 and 8mm, that is 6.5% of the predator length in average. Lancetfish Similar percentage of empty stomachs (19%) and fullness coefficient indicate, as for wahoo and dolphinfish, that lancetfish probably eats small amount of preys at the same time or that the digestion if fast. If 2 or 3 baits are sometimes found in the stomachs, lancetfish doesn t feed on longline baits as wahoo and dolphinfish do. Lancetfish is supposed to digest in the intestine rather than in the stomach; preys in advanced digestion state are not observed. This species has a diverse diet. Mollusc and fish are found in similar quantities and crustacea and invertebrate are also present but in smaller quantities. Twenty-one fish families and nine cephalopod families were identified. Lancetfish prey mainly on epipelagic/pelagic organisms such as the small gastropoda Carinaria sp., amphipods, the squids Onychoteuthidae and the pelagic octopus Argonauta sp.. The presence of the non-vertically migrant fish Sternoptyx sp. shows that the predator can dive in mesopelagic depths to feed on Sternoptychidae and Chiasmodontidae. Cannibalism is an important part of lancetfish diet. Most of its preys measure between 1 and 4mm, and if they are smaller than preys found in wahoo and dolphinfish, they represent a larger percentage of the predator length: 8%.

19 19 2 Conclusion. The three predators studied caught on the longlines show different feeding strategies: dolphinfish is a surface piscivorous predator, wahoo also consumes small amounts of mesopelagic preys and if mainly piscivorous, it diversifies its diet eating small quantities of cephalopods and shrimps; lancetfish feeds at the surface and in deeper waters on fish and molluscs but also on small quantities of crustacea and invertebrates. Hence if part of their diet overlap they cannot be considered as strict competitors. Trophic interactions are important data for a better understanding of the ecosystem dynamic, and results of this large study on predator diet will be used in the modelling of the western and central Pacific pelagic ecosystem. Data acquired in the area will allow improving the preliminary Ecopath model already developed (cf. BBRG-5. Godinot, O. & Allain, V. 23. A preliminary Ecopath model of the warm pool pelagic ecosystem.). Diet studies are complemented by an isotope analysis (δ 13 C and δ 15 N) of the same predators that should provide data on the trophic levels of the different species. This trophic study and Ecopath modelling are also part of a Pacific-wide PFRP project entitled Trophic structure and tuna movement in the cold tongue-warm pool pelagic ecosystem of the equatorial pacific. In this project, it is suggested that tuna productivity in the western and central Pacific Ocean is tied to upwelling along the equator in the central and eastern Pacific. The project proposes to test this hypothesis by combining diet analysis, stable isotopic compositions, food-web modeling, and stable isotope markers to trace tuna movements and trophic-level variation in the equatorial Pacific. The main objectives of the study are i) to define the trophic structure of the pelagic ecosystems in the western, central and eastern parts of the tropical Pacific Ocean, ii) to establish an isotope-derived (upwelling-related) biogeography of the pelagic tropical Pacific ecosystems, and iii) to characterize large-scale tuna movements related to upwelling regions along the equator. Results of this study should help define ecosystem linkages leading to tuna production and the effect of climate variability on the systems. Principal investigators are Valerie Allain (SPC), Robert Olson (IATTC), Felipe Galván-Magaña (CICIMAR Mexico), Brian Popp (University of Hawaii), Brian Fry (Louisiana State University). More details on the study can be obtained at and