SCTB17 Working Paper

BIO–1

Diet of yellowfin in different areas of the western and central Pacific.

Valérie ALLAIN

Oceanic Programme Secretariat of the Pacific Community Noumea, New Caledonia

July 2004 2 Introduction To develop approaches of 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 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. The area covered by the study is extended and the samples are collected more or less close to the equatorial warm pool and on a large east-west transect. An hypothesis to explain the important tuna production in the warm pool where is low, is that the tuna move extensively to feed at the edge of the warm pool, and particularly in the area in the east. It is then suspected that different food webs support the tuna production in the different areas of the Pacific. To examine this hypothesis, the diet of the yellowfin is analyzed to try to detect differences in the different areas of the western and central Pacific using stomach content data and stable .

Methods Sampling programme, sampling protocol Stomach samples are collected from target () and species by observers from the different national observer programmes in the area. Since the beginning of the programme in January 2001, 49 sampling trips have been done, 39 on longline boats and 10 on purse seine vessels. Sixteen sampling trips were organised in French Polynesia, 11 in New Caledonia, three in Federated States of Micronesia, three in Papua New Guinea, 14 in Solomon Islands and two in Cook Islands. For further details on the sampling programme, the sampling protocols and the stomach examination protocol check SCTB16 - BBRG-6 http://www.spc.int/OceanFish/Html/SCTB/SCTB16/bbrg6.pdf and http://www.spc.int/OceanFish/Html/TEB/EcoSystem/index.htm.

Stomach examination Classical procedure is used to analyze the stomachs: -Fullness coefficient is determined according to a scale from 0 (empty) to 4 (full) (see caption of Figure 4 p.7 for details). If baits are present, they are removed to determine the fullness coefficient. However fullness coefficient is a subjective parameter which determination will vary with the examiner, so another way to evaluate and quantify the stomach fullness is to calculate the stomach content weight – predator weight ratio (= stomach content weight * 100 / predator weight, in %). The weight of the predator is estimated from the measured length and using a length-weight converting factor (in this case for the yellowfin: a=0.00001908 and b=2.9776199, based on the measure of 329 from Regional Observer Data).

-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 6 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.

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,

3 Balistidae, the of Myctophidae, () 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 from the same area. Frequency of occurrence of an item is the number of stomachs where this item is present divided by the number of non-empty stomachs. Percentages of number and weight are the respective number and weight of the taxon studied divided by total number or weight of this taxon of all the samples, by area.

Stable isotope analysis Chemical elements can exist in both stable and unstable (radioactive) forms. Most elements of biological interest (including C, H, O, N, and S) have two or more stable isotopes (atoms of the same atomic number but different atomic weights due to a different number of neutrons), with the lightest of these present in much greater abundance than the others. Among stable isotopes the most useful as biological tracers are the heavy isotopes of carbon and nitrogen. These two elements are found in the earth, the atmosphere, and all living things. Each has a heavy isotope (13C and 15N) with a natural abundance of ~1% or less and a light isotope (l2C and 14N) that makes up all of the remainder, in the case of nitrogen, or virtually all in the case of carbon (carbon also has a radioactive isotope, 14C.). Studies examining stable isotopes at or near natural abundance levels are usually reported as delta, a value given in parts per thousand or per mil (‰). Delta values are not absolute isotope abundances but differences between sample readings and one or another of the widely used natural abundance standards which are considered delta = zero (e.g. air for N, Pee Dee Belemnite for C). For more information on isotopes: http://www.uga.edu/~sisbl/stable.html.

The nitrogen isotopic composition of marine fauna is particularly sensitive to (Fry, 1988). δ15N isotope values are used to estimate the trophic level occupied by the tunas, other predators, their forage prey species, and . It implies to know the δ15N values of the component at the base of the () that has a trophic level of 1 by definition. The trophic level of the other components is then calculated assuming an increase in δ15N of 3.4‰ per trophic level: δ15N of herbivorous = δ15N of phytoplankton + 3.4‰. The same rule applies to δ13C with an increase of 1‰ per trophic level. If the isotope values of the base of the are not known the trophic level cannot be calculated but the isotope values give an idea of the relative positions of the different components in the food web. The carbon isotopic composition of phytoplankton and consumers often reflects the algal sources of production, with high δ13C values associated with rapid growing diatoms characteristic of and blooms (Popp et al., 1998; 1999). δ13C values are used to identify different sources of primary production, and to distinguish rapid production associated with upwelling. The combination of δ15N and δ13C will serve to map different regions of primary and secondary production in the Pacific Ocean. Although there are few preliminary isotope data for fisheries in the equatorial Pacific, we anticipate clear regional signals. We expect high δ15N associated with the eastern Pacific due to denitrification, low δ15N in the western Pacific and gyres associated with nitrogen fixation (Deutsch et al., 2001), and we expect high δ13C associated with large diatoms in upwelling regions, lower δ13C in low areas. This kind of isotope cartography has been successful in the North Pacific and in the Atlantic in studies of POM (Particulate Organic Matter) and zooplankton (Wu et al., 1997; Perry et al., 1999; Waser et al., 2000), and has been recently suggested as an appropriate way to use isotopes in the open ocean (Rost et al., 2002). The isotope analysis is conducted on muscle and liver of individual fish. Samples are analyzed with a mass spectrometer in the stable isotope biogeochemical laboratory at the University of Hawaii. Precision and accuracy of isotopic determinations of laboratory standards has been better than ±0.1‰ over the last three years of operation of the equipment.

4

Characteristics of the samples For this study, 195 have been sampled so far, they come from different areas in the Pacific (Figure 1). Isotope analysis was conducted on 37 of them while the 195 stomachs were examined.

MICRO

PAPUA

NC POLY

Figure 1. Positions of the yellowfin tuna sampled for the study in the western and central Pacific. MICRO: Micronesia includes EEZ from Marshall Islands, Federated States of Micronesia, Nauru, Kiribati (Gilbert Islands) and nearby international waters; NC: New Caledonia EEZ; PAPUA: includes EEZ from Papua New Guinea and Solomon Islands; POLY: Polynesia includes French Polynesia and Cook Islands.

Most of the samples come from New Caledonia (82) and Polynesia (52), 39 were sampled in Micronesia and 22 in Papua New-Guinea – Solomon Islands (Table 1). The fish were caught on longline except for the samples from Micronesia and for some samples from PAPUA that were caught by purse seine (Table 1). Length distribution of the yellowfin varies according to the area and it is mainly linked to the gear used (Figure 2). Samples caught by purse seine in MICRO are between 30 and 60 cm with a mean at 62 cm (Table 1); fish caught on longline are larger: most of them are between 110 and 130 cm (mean = 125 cm) in NC, between 110 and 120 cm (mean = 102 cm) in PAPUA and at 90 cm (mean = 97 cm) in POLY. The percentage of females is close to 50% in the samples of NC and POLY (Table 1). The small number of fish sexed in MICRO (27) and PAPUA (20) could explain the values observed in these areas, 41 and 70% respectively. 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. It could explain the biased sex-ratio in favour of females in PAPUA. Also, in MICRO the fish are small and sex determination can be difficult when the individuals are not yet at maturity (minimal length at sexual maturity for females 59 cm – Block, 2001).

5 According to the gear used the percentage of empty stomachs will also vary with a classical high percentage of empty stomachs in purse seine catch (MICRO) and a low percentage in longline catch (NC, PAPUA, POLY). The difference in the percentage of empty stomachs observed between the 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.

number of mean length length range % females % empty longline purse seine other gear samples (UF cm) (UF cm) (nb of fish sexed) stomachs

MICRO 39 39 62.0 35 - 158 40.7 79.5 (27)

NC 82 82 125.3 85 - 155 51.3 4.9 (78)

PAPUA 22 17 5 102.9 35 - 155 70.0 13.6 (20)

POLY 52 39 13 97.1 49 - 160 51.2 7.7 (41)

Table 1: Characteristics of the samples of yellowfin from the different areas. Other gear: handline, troll line. UF: upper jaw-fork length.

30

25

20 PA PUA MICRO 15 NC POLY 10 Number of individuals

5

0 30 40 50 60 70 80 90 100 110 120 130 140 150 160 Length (UF cm)

Figure 2: Length distribution of the yellowfin sampled in the different areas. UF: upper jaw–fork length, equivalent to SL standard length for yellowfin.

6 Results – Discussion

Presence of baits and fullness coefficient For fish caught on longline, in 65-70% of the cases, bait was not found in the stomachs, whatever the area (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. 17-23% of the yellowfin contained 1 bait and if a noticeable percentage of stomachs with 2 baits (up to 10% for POLY) 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 only concerns a small quantity of fish (5 individuals), but some of them manage to swallow the baits on several hooks without being hooked; up to 6 baits were found in one stomach. No differences were noticed between the areas.

80

70 NC 60 PAPUA 50 POLY

40

30

Percentage of stomachs 20

10

0 0123456 Number of baits

Figure 3: Percentage of stomachs containing baits for fish caught on longline in the different areas.

When considering only the non-empty stomachs (stomach fullness >=1), most of them, for the four areas, contain less than half of the stomach volume (Figure 4). Except the case of MICRO where all the samples were collected on purse seine, there are no major differences between areas; however there are more ½ full and >½ full stomachs in PAPUA. When considering a less subjective parameter, the ‘stomach content weight – predator weight ratio’ for the non-empty stomachs (Table 2), PAPUA samples do not appear different anymore and samples from NC and PAPUA show a similar average value (0.34 and 0.31% respectively) while MICRO is 0.87% and POLY 0.23%. When non-empty, samples from MICRO are more full than in the other areas, but there are only 8 non-empty stomachs, and it is probably related to the fishing method. POLY samples have lower values than in the other areas. The maximum ‘stomach content weight – predator weight ratio’ varies between 1.33 and 3.20% and for most of the non-empty stomach samples, the amount of food found in the stomach represents less than 0.5% of the weight of the predator (Figure 5). This low degree of fullness could mean that they eat small amount of preys at the same time and/or that the digestion process is fast enough to eliminate quickly the food ingested. In the case of fish caught on longline it is also important to note that once they are hooked the fish stop eating but the digestion continues, sometimes for several hours until the fish is landed onboard, and then the stomach content weight is underestimated. It also happen in the case of purse seine when brailing takes a long time.

7

90

80 MICRO NC 70 PAPUA 60 POLY 50

40

30

Percentage of stomachs 20

10

0 01234 Fullness coefficient

Figure 4: Percentage of stomachs according to their fullness coefficient in the different areas. 0: empty stomach, 1: stomach less than half full, 2: stomach half full, 3: stomach more than half full, 4: stomach full.

n Mean Stdev Min Max MICRO 8 0.87 1.11 0.0038 3.20 NC 78 0.34 0.50 0.0003 2.22 PAPUA 19 0.31 0.40 0.0117 1.33 POLY 47 0.23 0.31 0.0014 1.45 Table 2: ‘Stomach content weight - Predator weight ratio’ for non-empty stomach yellowfin in the different areas.

3.5 MICRO NC 3 PAPUA POLY 2.5

2 (%) 1.5

1

0.5

Stomach content weight - predator ratio 0 30 50 70 90 110 130 150 170 Length (UF cm)

Figure 5: ‘Stomach content weight - Predator weight ratio’ (%) according to the length of the predator (yellowfin) in the different areas.

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 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. If not taking into account POLY for which the small number of samples does not allow a proper analysis, no large differences exist between the different areas (Figure 6). Only a small amount of preys are in digestion state 1 (1.9, 7.0, 1.4%): the digestion process starts very quickly once preys are ingested. Most of the preys are in digestion states 2 and 3 with highest percentages in digestion state 3. Taking into account the loss of weight between digestion states 2 and 3, the higher percentages in digestion state 3 suggest that this is the longest state between these 2. Albeit the large loss of biomass between digestion state 3 and 4, there is still a large amount of preys in state 4 indicating that the digestion of the hard parts of the preys is a long process and that they accumulate in the stomach probably over several meals; it is important information to know to calculate daily ration.

70 MICRO 60 NC 50 PA PUA 40 POLY

30

20

10 Percentage of food weight 0 1234 State of digestion

Figure 6: State of digestion of all the preys found in the stomachs, by area. 1: fresh, 2: whole, partially digested, 3: fragmented, advanced digestion, 4: hard part remains.

If considering the three most frequent prey types, fish, mollusc (mainly ) and crustacea, differences in digestion states can be observed between the prey types and the areas (Figure 7). Digestion of the fish is very similar between the different areas, and as it is the main prey in quantity, the process is similar to the one described for all preys mixed (Figure 6). It is also similar for molluscs in PAPUA and POLY; for this prey type in NC, the pattern is slightly different: the amount of mollusc in digestion state 3 is larger suggesting that they are digested more quickly than the fish. The lack of identificartion of mollusc preys, mainly squids does not allow to check if the difference in digestion between areas is related to the species consumed. For crustacea, the pattern is very similar for NC and POLY, with most of the preys in digestion state 2; in PAPUA, crustacea are mainly in digestion states 1 and 2. in general the digestion of the crustacea is slower than for fish and mollusc and it is plausible due to the chitinous carapace. In the case of PAPUA it appears that the largest amount of crustacea are megalopa stages that have very hard shell, while the largest quantities of this prey type are soft amphipods in NC and soft amphipods and Caridae in

9 POLY, explaining why crustacea are digested more quickly in NC and POLY. Digestion process is very similar in the different areas for fish that is the main prey in quantity. Some differences appear between areas for the digestion of molluscs and crustacea and it is related to the prey items and probably to the quantities involved.

100 100 MICRO PAPUA 80 80

60 60

40 40

20 20

0 0 1234 1234 Percentage of weight per prey group prey per of weight Percentage group prey per of weight Percentage

100 100 FISH NC POLY 80 MOLLUSC 80 CRUSTA CEA 60 60

group 40 group 40

20 20

Percentage of weight per prey prey per of weight Percentage 0 prey per of weight Percentage 0 1234 1234 State of digestion State of digestion

Figure 7: 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 Data for MICRO are given for information, but no analysis can be conducted because of the small amount of samples containing prey items (8 stomachs).

Prey groups The most frequent prey found in the stomachs are fish, mollusc, crustacea and groups but not in the same order according to the area (Table 3). In NC, fish and molluscs are found at a high frequency (97 and 90%) while crustacea are only present in 62% of the stomachs and in 17%. In PAPUA crustacea are present in all stomachs, molluscs in 89%, fish in 84% and invertebrate in 21%. In POLY fish is the most frequent prey (90%), followed by mollusc (85%) and crustacea (79%); invertebrate is found in less than 10% of the stomachs. Stones are found in 10% of the stomachs collected in NC and PAPUA; they are volcanic floating stones coming from volcanic islands in the area. In terms of weight, the order of the most important preys is the same for the three areas: fish, mollusc and then crustacea, but the percentages are different. In NC, fish represent 84% of the food ingested, mollusc and crustaces are only 14 and 1% respectively. In PAPUA and POLY differences are smaller between the prey groups and very similar in the two areas: fish is about 59%, mollusc 25% and crustacea 12%.

10 In abundance the pattern is different for the three areas, but in all cases fish is not the first prey. In NC 43% of the prey items are mollusc, crustacea represent 32% and fish only 19%. In PAPUA and POLY crustacea are the most numerous with about 62%, then fish is 22% and mollusc 13% in PAPUA, and mollusc is 20% and fish 16% in POLY.

MICRO NC PAPUA POLY %W %N %F %W %N %F %W %N %F %W %N %F FISH 99.76 21.43 62.50 84.15 19.43 97.44 60.02 21.79 84.21 57.72 16.36 89.58 MOLLUSC 13.66 42.96 89.74 21.23 12.59 89.47 27.09 19.77 85.42 CRUSTACEA 0.18 78.57 37.50 1.20 31.55 61.54 14.20 61.74 100.00 10.26 63.06 79.17 INVERTEBRATE 0.16 3.32 16.67 0.90 1.76 21.05 0.53 0.81 8.33 VEGETAL 0.00 0.07 1.28 0.19 0.12 5.26 MINERAL 0.17 2.61 10.26 0.96 1.94 10.53 RUBBISH 0.00 0.07 1.28 0.09 0.06 5.26 Unrecognizable 0.06 0.00 12.50 0.64 0.00 15.38 2.42 0.00 36.84 4.40 0.00 18.75 Table 3: Frequency of occurrence (%F) and percentage of the total prey numbers (%N) and weights (%W) of the different prey groups in the four areas. 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.

F 100 100 MICRO C % W PAPUA 80 % N 80 C F 60 60

40 40 M number (%N) number 20 20

0 0 Percent of weight (%W) or or (%W) of weight Percent 0 20406080100 0 20 40 60 80 100

F 100 100 NC POLY 80 80 C F 60 60 C 40 40 M

number (%N) number 20 20

0 0 Percent of weight (%W) or 0 20 40 60 80 100 0 20406080100 Frequency of occurence (%F) Frequency of occurence (%F)

Figure 8: Diet importance of the different prey groups: frequency of occurrence vs. percent weight and number.

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. In the three areas the diet appears diversified, different prey groups are consumed at high frequencies and percentages even if the values are different according to the areas (Figure 8). In NC yellowfin have a more fish dominant diet than in PAPUA and POLY where mollusc and crustacea are also important items

11 Prey items Prey richness The number of prey items identified is 58, 44 and 50 respectively for NC, PAPUA and POLY (Table 5). However, diversity of diets in the different areas is underestimated because of the unidentified prey items. It is important to note that 11% of the preys (%N) are unidentified fish, and unidentified squids (Teuthida) are 7% in NC, they are respectively10 and 3% in PAPUA and 5 and 5% in POLY. The higher percentage of unidentified squids in NC is related to more advanced digestion state of this prey type in the area (cf. § State of digestion of the preys p.8). A higher number of non-empty stomachs examined in NC (78) explains easily the higher diversity observed in the area, while only 19 and 47 non-empty stomachs were respectively examined in PAPUA and POLY. When diversity is examined at the level of fish families, the number is similar in the 3 areas (20-21). The number of families identified for cephalopods is low, but the diversity is smaller in PAPUA and POLY (4 and 3 respectively) than in NC (6 families). The low identification level of the crustacea does not allow a comparison of the number of families.

The most frequent and important preys (Table 5 p.13-15) NC: Not taking into account the unidentified items, the most frequent preys are the squids 47%, the amphipod Phronima sp. 41%, the amphipod Hyperiidae 22%, the mollusc gasteropod Carinaria sp. 37%, the fish Alepisaurus sp. 19%, the decapod crustacea megalopa stage 18%, the invertebrate salp 15%, the fish Chiasmodontidae 15% and the Scombridae 14%. In abundance the most numerous items are Carinaria sp. 7%, Phronima sp. 8% and tiny Caridae 15%, but they all represent less than 1% in weight. In weight the items contributing most to the diet in NC are the Scombridae 29%, the skipjack 13%, and the squid Stenoteuthis oualaniensis 5.5%. Most of the preys consumed by YFT in NC, and for which an habitat type could be defined, are epipelagic (15%N, 21%W) and mesopelagic (15%N, 41%W) (Table 4). It is a mixing of epipelagic preys such as skipjack, (Cheilopogon sp.), Bramidae, Phronima sp. and of epipelagic- mesopelagic preys such as Alepisaurus sp., Scombridae, Chiasmodontidae and the squid Stenoteuthis oualaniensis.

PAPUA: The most frequent preys are, the squids 47%, small crustacea such as megalopa stages, , Enoplometopus sp., Caridae, Stomatopoda (mantis shrimp), Amphipoda, Phronima sp., Hyperiidae which are present in 21 to 84% of the stomachs. Juvenile stages of /coastal fish are also frequent: Acanthuridae, Pomacanthidae, Chaetodontidae, Balistidae (31-42%) and Bramidae 37% and Paralepididae 21% are fish frequently found in the samples; the salps are found in 21% of the stomachs. In abundance only small crustacea represent more than 5% of the preys (megalopa 38%, Shrimp 8%). In weight the flying fish Exocoetidae represent 11% but it’s only 2 specimens, other important prey items in weight are squids 9%, the squid Stenoteuthis oualaniensis 7%, the megalopa stage 7%, the Paralepididae and the Acanthuridae 4%. Most of the preys in this area are epipelagic preys (51%N, 38%W, flying fish, skipjack, small crustacea: megalopa stages, Phronima sp.) and a large part is formed by the pelagic juvenile stages of the reef/coastal fish as detailed in the previous paragraph (12%N, 14%W) (Table 4). The Paralepididae constitute most of the Mesopelagic/bathypelagic preys in this area.

POLY: Similarly to PAPUA, squids 48% and small crustacea are the most frequent preys in POLY (megalopa stages, shrimp, Enoplometopus sp., Caridae, Stomatopoda (mantis shrimp), Amphipoda, Phronima sp., Hyperiidae, 19-31%). Frequent fish are Paralepididae 17% and the juveniles of the reef/coastal Balistidae 23% and Acanthuridae 12%. In abundance, only the small Caridae shrimp are numerous 41%, followed by the megalopa stages 6%, the squid 5% and the juveniles of the reef fish Siganidae 4%. In weight the squid represent 17%, Siganidae 11%, Paralepidae. 9% and the Sphyraena sp. 7%, but it is only one large specimen.

12 Although the large majority of the preys could not be classified into an habitat type, it seems the pattern in this area is similar to the one observed in PAPUA with a large portion of the preys being epipelagic (15%N, 14%W, skipjack, small crustacea) and pelagic juvenile stages of the reef/coastal fish (9%N, 22%W, Siganidae, Acanthuridae, Balistidae, Ostraciidae). Paralepididae and Nemichthyidae are the fish species included in the mesopelagic-bathypelagic portion of the diet (3%N, 12%W).

MICRO NC PAPUA POLY Prey type %N %W %N %W %N %W %N %W Type not identified 92.86 36.10 64.76 35.38 35.10 41.94 72.61 52.94 Demersal - (juveniles Pelagic) 3.71 1.41 11.82 13.62 9.45 21.75 Epipelagic 7.14 63.90 14.53 21.31 51.33 37.73 14.99 13.57 Epipelagic / Mesopelagic 14.85 40.59 0.18 0.83 0.04 0.08 Mesopelagic / Bathypelagic 2.15 1.31 1.57 5.88 2.90 11.66 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. Shaded cells: >5%.

13 MICRO NC PAPUA POLY prey items type habitat %F %N %W %F %N %W %F %N %W %F %N %W Fish Anguilliformes Nemichthyidae Nemichthyidae MB O 2.08 0.72 1.60 Stomiiformes Sternoptychidae Sternoptychidae M O 3.85 0.26 0.07 Stomiiformes Sternoptychidae Argyropelecus aculeatus M O 1.28 0.07 0.05 Stomiiformes Sternoptychidae Argyropelecus sp. M O 2.56 0.13 0.07 Stomiiformes Sternoptychidae Sternoptyx sp. M O 1.28 0.13 0.06 2.08 0.04 0.10 Myctophiformes Myctophidae Myctophidae M O 5.26 0.06 0.02 Myctophiformes Myctophidae Myctophum obtusirostre M O 2.08 0.04 0.52 Aulopiformes Paralepididae Paralepididae MB O 2.56 0.20 0.12 21.05 0.72 4.25 16.67 1.70 8.83 Aulopiformes Alepisauridae Alepisaurus sp. PEM O 19.23 1.30 4.81 10.53 0.12 0.68 2.08 0.04 0.08 Beloniformes Exocoetidae Exocoetidae E NO 10.53 0.12 11.01 Beloniformes Exocoetidae Cheilopogon sp. E NO 1.28 0.07 3.94 Clupeiformes E N 5.26 0.06 1.11 Clupeiformes Engraulidae Engraulidae E N 2.56 0.13 0.09 Beryciformes Holocentridae Holocentridae ljP - aD NO 15.79 0.18 0.05 2.08 0.04 0.18 Chaetodontidae Chaetodontidae D N 5.13 0.46 0.02 31.58 0.90 0.47 2.08 0.04 0.06 Perciformes Bramidae Bramidae E O 3.85 0.20 3.47 36.84 0.60 1.20 4.17 0.09 0.07 Perciformes Bramidae Pterycombus petersii E O 1.28 0.07 0.04 Perciformes Elagatis bipinnulatus E NO 12.50 7.14 63.90 Perciformes Pomacanthidae Pomacanthidae D N 2.56 0.46 0.01 42.11 4.28 0.71 4.17 0.09 0.03 Perciformes Chiasmodontidae Chiasmodontidae MB O 15.38 1.30 0.93 5.26 0.06 0.48 6.25 0.34 0.40 Perciformes Acanthuridae Acanthuridae D N 1.28 0.07 0.02 36.84 2.41 3.95 12.50 1.32 1.29 Perciformes Acanthuridae Acanthurus sp. D N 8.33 0.47 1.86 Perciformes Serranidae D N 5.26 0.18 0.12 2.08 0.04 0.01 Perciformes Serranidae D N 1.28 0.07 0.00 Perciformes Siganidae Siganidae D N 4.17 3.83 11.07 Perciformes Priacanthidae Priacanthidae D N 5.26 0.06 0.17 Perciformes Scombrolabracidae Scombrolabrax heterolepis M O 5.26 0.06 0.54 Perciformes Gempylidae Gempylidae P O 1.28 0.07 0.18 5.26 0.06 0.39 6.25 0.13 0.65 Perciformes Gempylidae Neoepinnula orientalis. Be 2.08 0.04 0.21 Perciformes Trichiuridae/Gempylidae jP - aEBe O 2.56 0.13 0.20 Perciformes Scombridae Scombridae P NO 14.10 1.30 28.62 5.26 0.06 0.41 2.08 0.34 0.32 Perciformes Scombridae Katsuwonus pelamis E O 5.13 0.26 12.76 5.26 0.24 2.89 2.08 0.17 1.08 Perciformes Scombridae Thunnus alalunga EM O 5.26 0.06 0.15 Perciformes Echeneidae Remora sp. O 2.08 0.04 0.83 Perciformes Sphyraenidae Sphyraena sp. NO 2.08 0.04 7.05

14 Perciformes Ostracoberycidae Ostracoberyx dorygenys Ba O 10.53 0.48 0.21 Perciformes Ostracoberycidae Ostracoberyx paxtoni Ba O 1.28 0.07 0.00 Perciformes Acropomatidae Howella sp. JE – aM O 1.28 0.07 0.07 Zeiformes Zeidae Zeidae 1.28 0.13 0.01 Tetraodontiformes Balistidae Balistidae DP NO 36.84 1.09 2.10 22.92 0.81 2.61 Tetraodontiformes Balistidae Canthidermis maculatus E NO 1.28 0.07 0.12 Tetraodontiformes Balistidae Balistapus sp. D N 2.08 0.04 0.06 Tetraodontiformes Monacanthidae Monacanthidae D N 5.13 0.39 0.09 5.26 0.18 0.23 2.08 0.04 0.03 Tetraodontiformes Ostraciidae Ostraciidae D N 5.13 0.26 0.20 10.53 0.12 0.11 6.25 0.17 0.09 Tetraodontiformes Ostraciidae Lactoria diaphana jE - aD jO - aN 1.28 0.07 0.54 4.17 0.09 3.13 Tetraodontiformes Tetraodontidae DP N 6.41 0.33 0.06 5.26 0.06 4.73 Tetraodontiformes Tetraodontidae Lagocephalus lagocephalus E O 1.28 0.07 0.06 Tetraodontiformes Diodontidae Diodontidae D N 2.56 0.13 0.03 2.08 0.09 0.18 Tetraodontiformes Diodontidae Diodon sp. jP - aD N 1.28 0.07 0.26 long orange fish 1.28 0.13 0.00 Epipelagic fish E 2.08 0.13 0.28 Unidentified fish 50.00 14.29 35.86 91.03 11.01 27.21 89.47 9.71 24.23 81.25 5.41 15.08 Molluscs/Cephalopoda Teuthida Histioteuthidae Histioteuthidae O 1.28 0.07 0.01 Teuthida Enoploteuthidae Abralia sp. M O 5.26 0.12 0.26 Teuthida Enoploteuthidae Enoploteuthidae M O 5.26 0.06 0.12 Teuthida Ommastrephidae Ommastrephidae P NO 1.28 0.07 0.09 5.26 0.06 1.46 Teuthida Ommastrephidae Hyaloteuthis pelagica P NO 5.13 0.33 0.21 4.17 0.34 0.53 Teuthida Ommastrephidae volatilis P NO 2.08 0.04 0.51 Teuthida Ommastrephidae Stenoteuthis oualaniensis P NO 1.28 0.07 5.50 10.53 0.97 6.58 4.17 0.13 3.80 Teuthida Onychoteuthidae Onychoteuthidae P O 1.28 0.07 0.12 2.08 0.09 1.64 Teuthida Onychoteuthidae Onychoteuthis sp. P O 1.28 0.07 0.04 Teuthida Onychoteuthidae Moroteuthis sp. P O 5.26 0.12 0.76 Teuthida Onychoteuthidae Moroteuthis lonnbergi P O 5.26 0.06 0.87 4.17 0.09 0.71 Teuthida Loliginidae Loliginidae N 1.28 0.07 0.03 Teuthida Teuthida (Unidentified squid) 47.44 6.78 4.56 47.37 2.71 8.75 47.92 4.64 16.94 Sepiida Sepiida 1.28 0.07 0.02 Octopoda Octopoda 1.28 0.07 1.58 6.25 0.13 0.05 Octopoda Argonautidae Argonauta sp. P 6.41 1.04 0.20 10.53 0.18 0.04 Octopoda Octopodidae Octopodidae D 8.97 0.59 0.10 8.33 0.17 0.48 Unidentified cephalopoda 16.67 0.59 0.39 26.32 0.90 1.84 18.75 0.30 1.40 Cephalopod beaks 80.77 25.93 0.30 73.68 5.91 0.21 68.75 13.46 0.95 Molluscs/Gastropoda

15 Atlantidae Atlanta sp. P 1.28 0.07 0.00 6.25 0.13 0.02 Carinariidae Carinaria sp. P 37.18 6.97 0.40 36.84 1.27 0.15 8.33 0.17 0.03 Cavoliniidae Cavolinia sp. P 1.28 0.07 0.00 15.79 0.18 0.04 4.17 0.09 0.01 Unidentified mollusc 10.26 0.13 0.12 Crustacea Amphipoda Amphipoda E 2.56 1.50 0.02 10.53 0.24 0.05 18.75 2.90 1.05 Amphipoda Hyperiidea E 21.79 1.89 0.05 68.42 2.65 0.44 25.00 0.89 0.21 Amphipoda Phronimidae Phronima sp. E 41.03 8.01 0.71 47.37 4.40 2.71 25.00 2.34 1.14 Isopoda Isopoda E 2.08 0.09 0.07 Copepoda Copepoda 2.08 1.28 0.01 Decapoda Megalopa stage E 17.95 2.28 0.05 84.21 38.30 6.72 31.25 6.18 1.04 Decapoda Palinura lE 1.28 0.07 0.00 10.53 0.18 0.16 2.08 0.34 0.14 Decapoda Scyllaridae Scyllaridae 5.26 0.06 0.01 Decapoda Scyllaridae Parribacus sp. lE 1.28 0.07 0.01 5.26 0.12 0.06 Decapoda Paguridae Paguridae 4.17 0.26 0.04 Decapoda Galatheoidea 1.28 0.07 0.00 Decapoda Penaeoidea 2.08 0.09 0.39 Decapoda Shrimp 37.50 78.87 0.18 10.26 0.65 0.03 57.89 7.66 1.33 29.17 3.49 0.56 Decapoda Enoplometopidae Enoplometopus sp. 1.28 0.07 0.00 52.63 4.64 1.66 18.75 1.96 0.53 Decapoda Caridae Caridae 3.85 15.31 0.10 21.05 0.78 0.05 27.08 41.01 4.06 Decapoda Galatheacarididae Galatheacaris abyssalis 2.56 0.39 0.15 Stomatopoda Stomatopoda lE 6.41 0.65 0.03 57.89 2.05 0.76 27.08 1.87 0.53 Stomatopoda Harpiosquillidae Harpiosquillidae lE 1.28 0.07 0.03 Unidentified crustacea 11.54 0.52 0.03 26.32 0.42 0.15 27.08 0.38 0.49 Invertebrate Salp 15.38 3.26 0.15 21.05 1.75 0.89 4.17 0.68 0.41 Unidentified invertebrate 2.56 0.07 0.01 5.26 0.18 0.07 4.17 0.13 0.12 Vegetal 1.28 0.07 0.00 5.26 0.12 0.19 Mineral Stone 10.26 2.61 0.17 10.53 1.93 0.95 Rubbish Rubbish (human product) 1.28 0.07 0.00 5.26 0.06 0.09 Unrecognizable Unrecognizable 12.50 0.00 0.06 15.38 0.00 0.65 36.84 0.00 2.41 20.83 0.00 4.43 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, Ba=bathydemersal = between 1000 and 4000 m depth. Habitat: N=neritic=from the coastline to a line where depth in around 200 m (include shore, coastal, reef, ), 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 Preys ingested vary between less than 10mm and 560mm, but in general the yellowfin tunas in the three areas swallow preys less than 150mm long (Figure 9). The proportion of larger preys is 13% in NC, but is negligible in PAPUA and POLY (4 and 3%). Actually yellowfin tuna sampled in NC are larger than in the two other areas (125, 102 and 97cm of average length respectively) explaining the higher proportion of large preys in this area. The bulk of the preys consumed in the three areas is less than 60mm long (68, 83 and 83% respectively) and in average they measure 68, 42 and 48mm respectively in NC, PAPUA and POLY.

0.25 NC 0.2

0.15

0.1 Proportion of number Proportion 0.05

0 0-10 30-40 60-70 90-100 120-130 150-160 180-190 210-220 240-250 270-280 300-310 330-340 360-370 390-400 420-430 Length range (mm)

0.25 PAPUA

0.2

0.15

0.1 Proportion of number Proportion 0.05

0 0-10 30-40 60-70 90-100 120-130 150-160 180-190 210-220 240-250 270-280 300-310 330-340 360-370 390-400 420-430 Length range (mm)

0.25 POLY

0.2

0.15

0.1 Proportion of number Proportion 0.05

0 0-10 30-40 60-70 90-100 120-130 150-160 180-190 210-220 240-250 270-280 300-310 330-340 360-370 390-400 420-430 Length range (mm)

Figure 9: Length-frequency distribution of the YFT preys in the different areas. Only one prey item was measured in MICRO area, length-frequency could not be displayed for this area.

1718 In the predator size-range studied, no significant trend is observed in the length of preys according to the length of the predator (Figure 10): between 60 and 160cm, in the three areas yellowfins are able to swallow the same sizes of preys. However Figure 9 showed that the larger fish from NC, even if they also eat small preys, they are able to eat larger preys. Preys eaten measure in average 5% of the predator size and preys up to 60% of predator size have been ingested. These very large preys are Nemichthyidae, extremely long filamentous fish. No significant trend is observed in the prey length/predator length ratio vs. predator length confirming that there is no clear relationship between the sizes of the preys and predators in the predator length range examined (Figure 10).

450 0.4 NC y = -0.0003x + 0.087 400 0.35 2 y = 0.1784x + 46.377 R = 0.0037 350 2 R = 0.0013 0.3 300 0.25 250 0.2 200 0.15 150

Prey length (mm) length Prey 0.1 100

50 0.05 Prey lengthPrey / Predator length ratio 0 0 0 50 100 150 200 0 50 100 150 200 Predator length (cm) Predator length (cm)

450 0.45 y = 0.0001x + 0.021 400 PAPUA 0.4 y = 0.4982x - 15.967 R2 = 0.0039 350 R2 = 0.0438 0.35

300 0.3

250 0.25

200 0.2

150 0.15 Prey lengthPrey (mm) 100 0.1

50 0.05 Prey lengthPrey / Predator length ratio 0 0 0 50 100 150 200 0 50 100 150 200 Predator length (cm) Predator length (cm)

600 0.7 Nemichthyidae y = -0.0003x + 0.0829 POLY R2 = 0.0217 Nemichthyidae 500 y = 0.0645x + 41.395 0.6 R2 = 0.0009 0.5 400 0.4 300 0.3 200

Prey length (mm) length Prey 0.2

100 0.1 Prey lengthPrey / Predator length ratio 0 0 0 50 100 150 200 0 50 100 150 200 Predator length (cm) Predator length (cm)

Figure 10: Prey length vs. Predator length and Prey length/Predator length ratio vs. predator length.

1819 Stable isotope analysis A clear isotope difference is observed between the samples of MICRO and NC, samples of PAPUA are between the two (Figure 11). NC yellowfins are characterised by low isotope values (mean δ13C=-16.3‰, δ15N=10.0‰) compared to PAPUA (δ13C=-16.1‰, δ15N=11.4‰) and MICRO (δ13C=-15.4‰, δ15N=13.9‰). NC samples are about one trophic level lower than MICRO samples. Such a difference for individuals of the same species could come from a difference in length of the individuals, smaller usually feeding on smaller preys at lower trophic level. If in our sample there is a large difference of length between samples from NC and MICRO, samples from NC are larger (120 cm) and we would then expect them to be at higher values than the one from MICRO (58 cm). Unfortunately the lack of data on the diet of fish from MICRO does not allow explaining this unexpected result but it is highly improbable that the smaller yellowfin from MICRO feed on one trophic level higher than the fish from NC even if their diet is different. A more plausible explanation is that the isotope values of the components at the base of the food chain are higher in the MICRO area than in NC and this difference at the food base would be detected at the higher trophic levels such as the tuna level. These results highlight the fact that if isotopes can provide information on diet of the predator, particularly on the trophic level, isotopes have to be used in conjunction with diet analysis. Such difference between the yellowfins from the different areas shows however that isotopes are good markers of the geographic origin of the fish and could then be used, along with other markers such as heavy metal and tag data, to infer the origin and the movements of the fish between the different areas.

18 MICRO 16 NC

14 PAPUA

12 d15N

10

8 + 1‰ + 3.4‰

6 -18.00 -17.00 -16.00 -15.00 -14.00 d13C

18 0 16 -2 14 -4 d15N - MICRO -6 12 d15N - NC -8 10 d15N - PAPUA -10 d13C - MICRO d15N 8 d13C d13C - NC -12 6 d13C - PAPUA -14 4 -16 2 -18 0 -20 0 20 40 60 80 100 120 140 160 Length (UF cm)

Figure 11. Carbon and nitrogen stable isotope individual values, mean and standard deviation of the yellowfin tunas in the different areas, and individual values vs. individual length.

1920

Conclusion. The analysis of the stomach content and isotope data showed that the yellowfins have similarities and differences in their feeding strategies in the different areas of the western and central Pacific. For similarities, in all the areas small amount of food is found in the stomachs from which we can infer that they probably only eat small quantities of preys at the same time and/or that digestion is fast enough to eliminate quickly the food ingested (except the hard parts). The main preys are fish, mollusc and crustacea, and diversity in fish families in similar (20 families identified). Preys ingested are less than 60mm in general and represent 5% in average of the size of the predator. Main differences are observed at the prey level. In NC if the diet is diversified, the preferred preys are fish, mainly epipelagic (skipjack, flying fish) and epipelagic / mesopelagic species (Alepisaurus sp., Chiasmodontidae, Scombridae); squids are also important. In PAPUA, along with epipelagic fish (skipjack, flying fish), epipelagic crustacea are also consumed (Phronima sp., megalopa stages) as well as pelagic juvenile stages of reef/coastal fish (Acanthuridae, Pomacanthidae, Chaetodontidae, Balistidae), mesopelagic / bathypelagic Paralepididae and squids. In POLY, pelagic juvenile stages of reef/coastal fish (Siganidae, Balistidae, Ostraciidae), epipelagic fish (skipjack) and crustacea (Amphipoda, Phronima sp., megalopa stages) but also mesopelagic / bathypelagic Paralepididae and Nemichthyidae and squids are consumed. Clear differences are also observed on isotope values, but they are more probably related to the geographic origin of the fish rather than to drastic difference in the diet. From these first results different trophic structure seem to exist in the different areas studied, however, similarities and differences observed need to be confirmed by increasing the number of samples analysed, particularly in the MICRO area for diet studies and in PAPUA and POLY for the isotope data to enhance the complementarity’s of the two techniques. Explanatory parameters such as the island effect, oceanographic features (fronts, thermocline, seamounts) will be included in the analysis. To be included in the ecosystem approach to fisheries, these detailed studies need to be extended to the different components of the ecosystem. The analysis of the diet and isotope values of the other predator and prey species and the inclusion of the data in ecosystem modelling (, Ecosim, IBM, Sepodym) should allow to reach this goal; it will be done at the western and central Pacific level but also at the whole Pacific level (PFRP project “Trophic structure and tuna movement in the cold tongue-warm pool pelagic ecosystem of the equatorial pacific” http://www.soest.hawaii.edu/PFRP/ocean/allain.html, and OFCCP- GLOBEC project http://www.spc.int/oceanfish/html/globec/index.asp ) and comparisons with other oceans are also part of the study project (CLIOTOP – GLOBEC http://www.pml.ac.uk/globec/main.htm) (Allain, 2004 – SCTB17, INFO-ECO-1).

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