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Home , Anoplogaster cornuta, Crestfish, Cyclichthys, Gempylidae, Lancetfish, Rainbow runner

SCTB16 Working Paper

BBRG-6

Diet of mahi-mahi, and in the western and central Pacific.

Valérie ALLAIN

Oceanic Programme Secretariat of the Pacific Community Noumea, New Caledonia

June 2003 2 Introduction To develop ecological approaches of fisheries management it is important to take into account interactions and underlying ecosystem dynamics. Assessing the impact of fisheries and environmental effects on the ecosystem implies a good comprehension of this system. induces an important mortality in the ecosystem that is often higher than 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 , is done.

Methods Sampling programme Stomach samples are collected from target () and bycatch species by observers from the different national observer programmes in the area (Cook Islands, Federated States of , Fiji, Kiribati, New Caledonia, Papua New Guinea, French Polynesia and Solomon Islands). Since the beginning of the programme in January 2001, 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.

180°E 160°W 140°W 120°E 160°E 140°E

20°N Northern Hawaii Marianas

Guam Marshall Islands

Federated States of Micronesia Palau

0°N Kiribati Papua New Guinea Nauru Phoenix (KI) Line Islands (KI) Tuvalu Tokelau Cook Solomon Is Wallis & Samoa Islands Futuna Am Samoa Vanuatu Fiji Niue 20°S Tonga French Polynesia New Australia Caledonia Pitcairn

40°S PURSE SEINE New Zealand LONGLINE

Figure 1. Sampling trips done in the western and central Pacific.

3 Two protocols were designed for longline and purse-seine. They are available at http://www.spc.int/OceanFish/Html/TEB/EcoSystem/foodisotope.htm. Observers are asked to sample two 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 from 0 (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 beaks, gladius and are recorded. Species were determined using - Smith, M.M. and Heemstra, P.C. Eds. 1986. Smiths’ fishes. Springer-Verlag. Berlin. 1047p. (Fish). - Carpenter, K.E. and Niem, V.H. Eds. 2001. 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. http://tolweb.org/tree?group=Cephalopoda&contgroup=Mollusca (). Taxonomic classification used follow data provided by Integrated Taxonomic Information System http://www.itis.usda.gov/. 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 , Alepisaurus, /Trichiuridae, the dorsal spine of Monacanthidae, Balistidae, the photophores of Myctophidae, Ommastrephidae () 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 (http://www.fishbase.org/home.htm) 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 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: , and rays, tunas and other species, but only 18 species have more than 10 samples (Table 1). They represent a total of 1000 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 602 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 , bigeye, skipjack, yellowfin, dolphinfish) except for the silky . 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 10 samples, present a percentage of empty stomachs higher than 50%: blue shark and . 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. 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, , wahoo and rainbow runner (more than 40 samples).

5

Longline Purse seine Total Number of % empty Number of % empty Number of % empty Species stomachs stomachs stomachs stomachs stomachs stomachs BILLFISH Broadbill swordfish (Xiphias gladius) 11 18.2 11 18.2 Black marlin (Makaira indica) 4 0.0 4 0.0 Indo-pacific blue marlin (Makaira mazara) 10 10.0 6 83.3 16 37.5 Indo-pacific sailfish (Istiophorus platypterus) 8 0.0 2 100.0 10 20.0 Short-billed spearfish (Tetrapturus angustirostris) 6 0.0 6 0.0 Striped marlin (Tetrapturus audax) 13 0.0 13 0.0 SHARKS and RAYS Blue shark (Prionace glauca) 26 69.2 26 69.2 Hammerhead shark (Sphyrna spp.) 2 0.0 2 0.0 Unidentified shark (Elasmobranchii) 1 100.0 1 100.0 Short-finned mako shark (Isurus oxyrinchus) 9 44.4 9 44.4 Oceanic white tip (Carcharhinus longimanus) 3 100.0 3 100.0 Long-fin mako shark (Isurus paucus) 3 0.0 3 0.0 shark (Alopias superciliosus) 1 100.0 1 100.0 Silky shark (Carcharhinus falciformis) 4 50.0 7 14.3 11 27.3 Pelagic sting-ray (Dasyatis violacea) 8 12.5 8 12.5 Unidentified manta ray (Mobulidae) 3 0.0 3 0.0 Bigeye tuna (Thunnus obesus) 71 4.2 22 86.4 93 23.6 Skipjack tuna (Katsuwonus pelamis) 35 8.5 214 74.3 249 65.0 (Thunnus albacares) 105 3.8 44 77.3 149 25.5 Albacore (Thunnus alalunga) 54 3.7 54 3.7 Frigate tune (Auxis thazard) 10 100.0 10 100.0 OTHER SPECIES Dolphinfish (Coryphanea hippurus) 53 11.3 12 66.6 65 21.5 Moonfish (Lampris guttatus) 32 9.4 32 9.4 Longnose lancetfish () 57 19.2 57 19.2 Rainbow runner (Elagatis bipinnulata) 40 47.5 40 47.5 Wahoo (Acanthocybium solandri) 40 12.5 3 100.0 43 18.6 Black (Scombrolabrax heterolepis) 3 66.6 3 66.6 and escolar (Gempylidae) 2 100.0 2 100.0 Escolar (Lepidocybium flavobrunneum) 15 66.6 15 66.6 Roudi escolar ( prometheus) 3 66.6 3 66.6 Snake mackerel (Gempylus serpens) 2 100.0 2 100.0 (Ruvettus pretiosus) 1 100.0 1 100.0 Unidentified (Sphyraena spp.) 2 50.0 3 66.6 5 60.0 Blackfin barracuda (Sphyraena qenie) 1 100.0 1 100.0 Great barracuda (Sphyraena barracuda) 12 16.6 12 16.6 Dealfish (Desmodema polystictum) 1 0.0 1 0.0 Filefish unicorn leatherjacket (Aluterus monoferos) 5 80.0 5 80.0 Crestfish unicornfish (Lophotus capellei) 1 0.0 1 0.0 Oceanic triggerfish (Balistidae) 7 85.7 7 85.7 Longfin batfish (Platax teira) 4 75.0 4 75.0 (Decapturus macarellus) 8 87.5 8 87.5 Triple tail (Lobotes surinamensis) 1 0.0 1 0.0 Sickle pomfret (Taractichthys steindachneri) 1 0.0 1 0.0 Big scaled pomfret (Taractichthys longipinnis) 1 100.0 1 100.0 Golden trevally (Gnathanodon speciosus) 6 33.3 6 33.3 Drummer blue chub (Kyphosus cinerascens) 1 100.0 1 100.0 Chiasmodontidae 1 0.0 1 0.0 Total 602 15.4 398 71.6 1000 37.8 Table 1: Number of analysed stomachs and percentage of empty stomachs by species and by gear. Shaded cells represent species with more than 10 samples.

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 50 and 170 cm. The five lancetfish under 50 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 , females are more easily recognized, sex-ratio is probably biased in favour of females.

number of % empty length range gear area % females samples stomachs (UF cm) 28 NC 2 NR 40 LL Wahoo (Acanthocybium solandri) 43 18.6 10 FP 2 CK 72 - 174 70.2 3 PS 1 PNG 42 NC 4 FSM 53 LL Dolphinfish (Coryphanea hippurus) 65 21.5 7 FP 4 CK 44 - 152 66.1 12 PS 8 NR 15.5 - 164 Longnose lancetfish (Alepisaurus ferox) 57 19.2 57 LL 53 NC 4 FP 66.6 (5 specimens < 50 cm) 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

90 Wahoo 80 Dolphinfish 70 Lancetfish 60 50 40 30 20 Percentage of stomachs of Percentage 10 0 01234 Fullness coefficient

Figure 2: Percentage of stomachs according to their fullness coefficient. 0: 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 60% 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 10% 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.

80 Wahoo 70 Dolphinfish 60 Lancetfish 50

40

30

20 Percentage of stomachs Percentage 10

0 0123456789 Number of baits

Figure 3: Percentage of stomachs containing baits for fish caught on longline.

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 (60%) 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 0.5% 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.

70 Wahoo 60 Dolphinfish Lancetfish 50

40

30

20

Percentage of food weight food of Percentage 10

0 1234 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.

70 Wahoo 60 Fish 50 Mollus c 40 30 type 20 10 0

Percentage of weight per prey Percentage of weight 1234

70 Dolphinfish 60 50 40 30 type 20 10 0

Percentage of weight per prey Percentage of weight 1234

70 Lancetfish 60 50 40 30

prey type 20 10

Percentage of weight per 0 1234 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 10%: 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.

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Wahoo Dolphinfish Lancetfish prey group %F %N %W %F %N %W %F %N %W Fish 94.29 60.80 87.87 96.08 92.62 97.70 71.74 24.20 47.16 Mollusc 48.57 23.20 11.02 17.65 3.28 1.61 91.30 56.64 46.81 Crustacea 17.14 12.00 0.17 1.96 1.37 0.03 45.65 10.08 1.79 Invertebrate 1.96 0.82 0.01 26.09 8.24 2.92 Vegetal 3.92 0.82 0.06 Mineral 5.71 3.20 0.88 1.96 0.27 0.05 2.17 0.17 1.16 Rubbish 3.92 0.55 0.51 6.52 0.50 0.08 Unrecognizable 8.57 0.80 0.05 5.88 0.27 0.04 8.70 0.17 0.08 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).

100 %W Wahoo %N 80 fish 60

40 crustacea 20 mollus c 0

Percent WeightPercent or Number 0 20 40 60 80 100 Frequency of occurence

100 Dolphinfish fish 80

60

40

20 mollusc 0

Percent Weight or Number 0 20406080100 Frequency of occurence

100 Lancetfish 80

60 fish 40 mollusc

20 invertebrate crustacea 0

Percent Weight or Number or Weight Percent 0 20406080100 Frequency of occurence

Figure 6: Diet importance of the different prey groups: frequency of occurrence vs. percent weight and number.

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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 40% Chiasmodontidae 23%, Alepisaurus 14%, Gempylidae, and (11%). In term of number, the most ingested items are squids 22%, Siganidae 10%, 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 . However 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 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 10%, Tetraodontidae 10% with Lagocephalus 5%, and 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 reef- lagoon fish such as Diodontidae, Monacanthidae, Chaetodontidae, Holocentridae, Acanthuridae and Siganidae. Epipelagic/Pelagic/Mesopelagic fish are anecdotical in number and weight, and

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 Amphitretidae.

Wahoo Dolphinfish Lancetfish Prey type %N %W %N %W %N %W Demersal - (juveniles Pelagic) 16.8 3.02 16.12 8.05 7.56 13.93 Epipelagic / Pelagic 19.6 68.34 28.69 54.33 64.2 41.85 Epipelagic / Pelagic / Mesopelagic 2.00 4.83 0.82 0.35 3.19 25.93 Mesopelagic / Bathypelagic 7.6 4.59 1.09 0.51 5.21 10.58 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 100% 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 6.52 0.67 0.27 Stomiiformes Sternoptychidae Argyropelecus aculeatus M O 8.70 0.84 2.50 Stomiiformes Sternoptychidae Sternoptyx sp. M O 10.87 1.68 2.22 Paralepididae Paralepididae MB O 2.860.40 0.30 1.96 0.27 0.27 8.70 1.18 0.60 Aulopiformes Paralepididae Lestidium atlanticum jE - aM O 2.17 0.17 0.34 Aulopiformes Alepisauridae Alepisaurus sp. EPM O 14.29 2.00 4.83 3.92 0.55 0.31 32.61 3.19 25.93 Lophiiformes Lophiiformes 2.17 0.17 0.01 Beloniformes Exocoetidae Exocoetidae E NO 15.69 3.28 21.16 Beloniformes Exocoetidae Oxyporhamphus sp. E NO 3.92 1.37 4.65 Trachipteridae Trachipterus trachypterus M O 2.17 0.17 0.47 Beryciformes Beryciformes 4.35 0.67 0.50 Beryciformes Anoplogasteridae Anoplogaster cornuta jEMB - aMB O 2.17 0.34 0.26 Beryciformes Holocentridae Holocentridae lP - aD NO 5.88 1.37 0.34 2.17 1.34 0.17 Scorpaeniformes Scorpaenidae Pterois sp. lP - aD NO 4.35 0.34 0.27 Scorpaeniformes Sebastidae Sebastidae lP - aEPMB 2.86 0.40 0.06 Scorpaeniformes Triglidae Triglidae D N 2.17 0.17 0.05 Scorpaeniformes Dactylopteridae Dactylopteridae jP - aD jO - aN 1.96 0.27 0.04 2.17 0.34 0.19 Kyphosidae Kyphosus sp. D N 1.96 0.27 0.07 Perciformes Chaetodontidae Chaetodontidae D N 2.860.40 0.01 5.88 1.37 0.11 6.52 0.67 0.05 Perciformes Bramidae Bramidae E O 1.96 0.27 0.04 4.35 0.34 0.22 Perciformes Bramidae Pterycombus petersii E O 2.86 0.40 0.51 Perciformes Bramidae Taractichthys sp. E O 2.17 0.17 0.09 Perciformes Carangidae Carangidae DP NO 3.92 1.09 4.41 Perciformes Carangidae Atropus atropos P N 1.96 0.27 2.10 Perciformes Pomacentridae Amphiprion sp. D N 2.17 0.17 0.03 Perciformes Chiasmodontidae Chiasmodontidae MB O 22.86 7.20 4.29 1.96 0.55 0.17 2.17 0.17 1.06 Perciformes Acanthuridae Acanthuridae D N 8.574.00 1.74 3.92 1.09 0.26 4.35 0.50 0.12 Perciformes Acanthuridae Naso sp. D N 2.17 0.34 0.16 Perciformes Siganidae Siganidae D N 2.86 10.80 0.62 1.96 1.91 0.43 Perciformes Scombrolabracidae Scombrolabrax heterolepis M O 2.17 0.17 0.89 Perciformes Gempylidae Gempylidae P O 11.43 1.60 2.62 4.35 0.34 0.61 Perciformes Gempylidae sp. Be O 4.35 0.67 0.65 Perciformes Trichiuridae Trichiuridae jP - aEBe O 1.96 0.55 0.39 Perciformes Trichiuridae/Gempylidae jP - aEBe O 5.71 0.80 1.66 Perciformes Scombridae Scombridae P NO 11.43 1.60 5.23 1.96 0.27 0.47

14 Perciformes Scombridae Katsuwonus pelamis E O 2.86 1.20 1.36 3.92 0.82 0.40 Perciformes Xiphiidae Xiphias gladius P NO 2.86 0.80 0.02 1.96 0.27 0.01 Perciformes Zoarcidae Zoarcidae D NO 2.17 0.17 0.03 Tetraodontiformes Balistidae Balistidae DP NO 8.57 1.60 0.23 15.69 8.20 3.24 2.17 0.17 0.19 Tetraodontiformes Balistidae Canthidermis maculatus E NO 7.84 1.91 3.48 Tetraodontiformes Balistidae Xenobalistes sp. P NO 1.96 0.55 3.67 Tetraodontiformes Monacanthidae Monacanthidae D N 8.57 1.20 0.41 11.76 3.55 0.43 4.35 0.34 0.06 Tetraodontiformes Monacanthidae Cantherhines sp. jP - aDP N 1.96 0.27 0.29 Tetraodontiformes Ostraciidae Ostracion sp. D N 3.92 0.55 0.13 Tetraodontiformes Ostraciidae Lactoria diaphana jE - aD jO - aN 4.35 0.34 7.34 Tetraodontiformes Tetraodontidae Tetraodontidae DP N 8.57 1.60 5.39 21.57 5.19 4.58 6.52 0.50 0.08 Tetraodontiformes Tetraodontidae Lagocephalus lagocephalus E O 2.860.40 48.81 5.88 2.19 5.23 4.35 0.34 0.35 Tetraodontiformes Diodontidae Diodontidae D N 15.69 3.28 1.34 Tetraodontiformes Diodontidae sp. jP - aD N 2.86 0.40 0.24 5.88 1.64 2.88 Tetraodontiformes Diodontidae Cyclichthys spilostylus jP - aD N 1.96 0.55 2.02 Tetraodontiformes Molidae Molidae EM O 1.96 0.27 0.03 long orange fish 9.80 2.19 0.16 8.70 1.68 0.31 Mesopelagic fish M O 1.96 0.27 0.08 rond orange Ag fish 2.17 0.17 0.02 Unidentified fish 68.57 24.00 10.82 86.27 46.17 34.35 28.26 5.71 1.08 Molluscs/Cephalopoda Teuthida Cranchiidae Cranchiidae P O 2.17 0.17 0.15 Teuthida Enoploteuthidae Abralia sp. M O 2.17 0.17 0.20 Teuthida Joubiniteuthidae Joubiniteuthis portieri P NO 2.17 0.17 0.11 Teuthida Ommastrephidae Ommastrephidae P NO 2.17 0.17 0.56 Teuthida Ommastrephidae Hyaloteuthis pelagica P NO 1.96 0.55 0.13 2.17 0.17 0.17 Teuthida Ommastrephidae Ornithoteuthis volatilis P NO 2.17 0.17 0.97 Teuthida Onychoteuthidae Onychoteuthidae P O 2.17 0.17 3.60 Teuthida Onychoteuthidae Onychoteuthis sp. P O 13.04 1.68 4.09 Teuthida Onychoteuthidae Moroteuthis lonnbergi P O 4.35 0.50 5.44 Teuthida Oegopsina 4.35 0.34 0.54 Teuthida Teuthida (Unidentified ) 40.00 22.00 7.26 9.80 1.37 1.37 6.52 0.67 0.67 Sepiida Sepiolidae Sepiolina nipponensis D 4.35 0.34 0.43 Sepiida Sepiolidae Eupryma tasmanica D 2.17 0.17 0.08 Sepiida Sepiida 2.17 0.17 0.03 Octopoda Amphitretidae Amphitretidae M 2.17 0.34 2.03 Octopoda Argonautidae Argonauta sp. P 5.71 0.80 2.42 1.96 0.27 0.05 8.70 1.18 4.32 Octopoda Octopodidae Octopodidae D 1.96 0.27 0.01 17.39 2.18 4.85

15 Octopoda Octopodidae Octopus sp. D 2.17 0.17 0.11 Unidentified cephalopoda 2.86 0.40 0.05 3.92 0.27 0.01 2.17 0.17 0.01 Molluscs/Gastropoda Atlantidae Atlanta sp. P 2.17 0.17 0.01 Carinariidae Carinaria sp. P 71.74 46.89 18.32 Cavoliniidae Cavolinia sp. P 1.96 0.27 0.00 2.17 0.17 0.01 Thecosomata P 2.17 0.17 0.03 Unidentified mollusc 3.92 0.27 0.22 2.17 0.17 0.03 Crustacea Amphipoda E 2.86 1.20 0.02 4.35 0.84 0.09 Hyperiidea E 8.57 3.20 0.05 1.96 0.27 0.01 23.91 5.38 0.93 Phronimidae Phronima sp. E 23.91 2.69 0.58 Isopoda E 2.17 0.17 0.01 Megalopa stage E 5.71 4.40 0.01 1.96 0.55 0.01 Galatheidae Munida E 2.17 0.17 0.02 11.43 1.60 0.07 1.96 0.27 0.00 Stomatopoda E 1.96 0.27 0.02 Unidentified crustacea 5.71 1.60 0.01 6.52 0.84 0.16 Invertebrate Nemata (Phylum) 1.96 0.82 0.01 6.52 1.18 0.20 Platyhelminthes (phylum) 4.35 0.34 0.04 Polychaeta (Annelida) 10.87 1.85 0.56 Salp 4.35 0.50 0.07 Unidentified invertebrate 4.35 4.37 2.04 Vegetal Algae 1.96 0.27 0.01 Wood 1.96 0.55 0.05 Mineral Stone 5.71 3.20 0.90 1.96 0.27 0.05 2.17 0.17 0.08 Rubbish Rubbish (human product) 3.92 0.55 0.50 6.52 0.50 1.28 Unrecognizable Unrecognizable 8.57 0.80 0.05 3.92 0.27 0.03 8.70 0.17 0.08 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 10's m above the bottom, P=pelagic=fish living without relation with bottom, Be=benthopelagic=fish with demersal and 'pelagic' habits (up to few 100's m above bottom), E=epipelagic= between the sea-surface and 200 m depth, M=mesopelagic=pelagic fish between 200 and 1000 m depth, B=bathypelagic = pelagic fish between 1000 and 4000 m depth. Habitat: N=neritic=from the coastline to a line where depth in around 200 m (include shore, coastal, , continental shelf), O=oceanic = areas where bottom is deeper than 200 m. l=larvae, j=juvenile, a=adult. Shaded cells: >10% for %F, >5% for %N and %W.

1716 Size-distribution of the preys Wahoo don’t swallow preys less than 10mm, and preys in the size range 0-10mm only represent 0.46% 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 390mm for wahoo, 300mm for dolphinfish and 335mm for lancetfish, approximately the same size for the three predators. The mode of the prey length-frequency distribution is 30-40mm for wahoo and dolphinfish with respective percentages of 12.3 and 14.8%. This mode is smaller for lancetfish: 10-20mm with a percentage of 22.3%. For the three predators, most of the preys are less than 120mm: 87.0, 85.2 and 91.0% respectively, and the average size of the preys is 75.1, 73.2 and 53.6mm. In the 0-120mm 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 (30-100mm>10%) while is it skewed towards smaller sizes for dolphinfish (30-80mm>10%) and this phenomenon is accentuated for lancetfish (10- 40mm>10%). If the three predators studied swallow the same size-range of preys, lancetfish ingests smaller preys than wahoo and dolphinfish.

25 Wahoo 20

15

10 % number % 5

0

0 0 0 0 0 0 0 0 3 60 90 2 8 1 4 0 3 1 150 1 2 2 270 3 3 360 390 Length of the preys (mm)

25 Dolphinfish 20

15

10 % number 5

0

0 0 0 0 0 0 3 60 90 5 7 6 9 120 1 180 210 240 2 300 330 3 3 Length of preys (mm)

25 Lancetfish 20

15

10 % number 5

0

0 0 0 0 0 0 0 0 3 60 90 2 8 1 4 0 3 1 150 1 2 2 270 3 3 360 390 Length of preys (mm)

Figure 7: Length-frequency distribution of the preys for the three predators.

1718 In the predator size-range studied, no tendency is observed in the length of preys according to the length of wahoo (Figure 8): between 100 and 170cm, wahoo are able to swallow the same sizes of preys. Preys eaten measure in average 6% of the predator size and preys up to 30% 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 100mm. Fish bigger than 75cm eat large preys of 200mm 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 20 to 160 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 (>100mm) appears in the stomachs of fish larger than 70cm. However one large fish (250mm) 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 40% of predator length. The prey length/predator length ratio tends to decrease with increasing predator size.

400 0.40 Wahoo Wahoo

300 0.30

200 0.20 y = 0.1238x + 58.564 y = -3E-05x + 0.1036 R2 = 0.0016 R2 = 0.0191 100 0.10 Prey length (mm)

0 0.00 Ratio Prey length/Pred. length length/Pred. Prey Ratio 0 50 100 150 200 0 500 1000 1500 2000 Predator length (cm ) Predator length (m m )

400 0.40 Dolphinfish y = -2E-05x + 0.0897 Dolphinfish R2 = 0.0068 300 0.30

200 0.20 y = 0.4943x + 16.402 R2 = 0.0256 100 0.10 Prey lengh (mm) lengh Prey

0 0.00 Ratio Prey length/Pred. length length/Pred. Prey Ratio 0 50 100 150 200 0 500 1000 1500 2000 Predator length (cm ) Predator length (m m )

400 1.6 y = 0.3276x + 22.242 Lancetfish Lancetfish R2 = 0.0885 300 1.2 y = -7E-05x + 0.1454 R2 = 0.0797 200 0.8

100 0.4 Prey lengthPrey (mm)

0 0 Ratio Prey Length/Pred. length Length/Pred. Prey Ratio 0 50 100 150 200 0 500 1000 1500 2000 Predator length (cm ) Predator length (mm)

Figure 8: Prey length vs. Predator length and Prey length/Predator length ratio vs. predator length.

1819 Summary

Wahoo With a percentage of empty stomachs less than 20% 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 30 and 100mm 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, 20% of the stomachs being empty and most of the specimens having stomachs half-full or less than half- full. 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 () and pelagic juveniles of reef-lagoon fish such as Diodontidae, Monacanthidae, Chaetodontidae, Acanthuridae. Most of the preys measure between 30 and 80mm, 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 10 and 40mm, and if they are smaller than preys found in wahoo and dolphinfish, they represent a larger percentage of the predator length: 8%.

1920 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. 2003. A preliminary Ecopath model of the warm pool pelagic ecosystem.). Diet studies are complemented by an isotope analysis (δ13C and δ15N) 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 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 http://www.spc.int/OceanFish/Html/TEB/EcoSystem/foodisotope.htm and http://www.soest.hawaii.edu/PFRP/ocean/allain.html