Composition of piscine prey in the diet of large pelagic fish in the eastern tropical Pacific Ocean

FISHERIES SCIENCE 2001; 67: 1063–1074

INTRODUCTION

Many studies have investigated the stomach con-tents of large pelagic fish.1 The diets of tuna havebeen especially well studied, owing to the com-mercial value of these fish.2–4 The prey of thelancetfish Alepisaurus ferox has also been investi-gated, because midwater organisms are oftenretrieved in good condition from stomachs of thisspecies.5,6 Several studies have compared largepelagic fish to understand their feeding habits. Forexample, one report compared food items betweenskipjack and yellowfin tunas caught off the coast of

West Africa.7 Matthews and coworkers comparedthe stomach contents and examined the detailedfood habits in four species of tuna (Thunnusalbacares, Thunnus alalunga, Thunnus obesus, andThunnus thynnus) and lancetfish (A. ferox) takenfrom the western Atlantic Ocean.8 Smale9 com-pared the stomach contents of six species ofteleosts, collected off the coast of South Africa.Tsuchiya et al.10 investigated cephalopods eaten by14 species of pelagic fish in the eastern equatorialPacific Ocean and discussed differences in theirfeeding habits. However, little information on fishcomposition in the stomachs of pelagic fish isavailable for the eastern equatorial Pacific,although small fish are the principal food items forlarge pelagic fish.1

The present study examined the fish composi-tion in the stomach contents of yellowfin tuna T.albacares, bigeye tuna T. obesus, swordfish Xiphiasgladius, striped marlin Tetrapturus audax, dol-phinfish Coryphaena hippurus, lancetfish A. ferox,

Original Article

Composition of piscine prey in the diet of large pelagicfish in the eastern tropical Pacific Ocean

M MOTEKI,1,a,* M ARAI,1 K TSUCHIYA2 AND H OKAMOTO3

1Laboratory of Ichthyology and 2Laboratory of Invertebrate Zoology, Tokyo University of Fisheries,Minato, Tokyo 108-8477 and 3National Research Institute of Far Seas Fisheries, Shimizu,Shizuoka 424-8633, Japan

ABSTRACT: We examined the stomach contents of seven pelagic fish species collected by long-line in the eastern tropical Pacific Ocean: yellowfin tuna Thunnus albacares, bigeye tuna T. obesus,swordfish Xiphias gladius, striped marlin Tetrapturus audax, dolphinfish Coryphaena hippurus,lancetfish Alepisaurus ferox, and pelagic thresher Alopias pelagicus. Fifty fish species from 31 fam-ilies were identified in 222 stomachs examined. The choice of prey fish was very similar betweenbigeye tuna and swordfish, yellowfin tuna and swordfish, yellowfin tuna and dolphinfish, and big eyetuna and pelagic thresher. In contrast, similarity was low between striped marlin and lancetfish, dol-phinfish and pelagic thresher, and lancetfish and pelagic thresher. According to the habitat depthrange of prey fish, the seven predator species were divided into three groups: (i) wide range feedersfrom surface to midwater (yellowfin tuna and striped marlin); (ii) epipelagic feeders (dolphinfish); and(iii) midwater (mesopelagic) feeders (bigeye tuna, swordfish, lancetfish, and pelagic thresher). Themost important prey fish belonged to the following families: Sternoptychidae, Phosichthyidae, Para-lepididae, Omosudidae, Myctophidae, Exocoetidae, Hemiramphidae, Bramidae, Gempylidae, andScombridae. Although pelagic fish predators extensively use these prey families, different dominantfamilies and feeding depths of each predator are considered to reduce trophic competition amongpelagic fish in the eastern equatorial Pacific.

KEY WORDS: dolphinfish, eastern Pacific, food habits, lancetfish, pelagic thresher, stripedmarlin, swordfish, tunas.

*Corresponding author: Tel: 81-8452-4-2933. Fax: 81-8452-4-3449. Email: [email protected]

aPresent address: Research Institute of Marine Bioresources,Fukuyama University, Innoshima, Hiroshima 722-2101, Japan.

Received 16 October 2000. Accepted 27 March 2001.

lies. Numbers of fish examined varied from 19(lancetfish) to 42 (bigeye tuna). The size range offish examined also varied; the smallest specieswere dolphinfish (35–129 cm standard length; SL)and the largest were striped marlin (116–194 cmSL). Numbers and sizes of fish examined are shownin Table 1.

Stomachs were removed on board and immedi-ately fixed in 10% formalin. Following the identifi-cation of food fish to the lowest possible taxonomiclevel, the number of individuals and the frequencyof occurrence of each taxon were recorded. Allstomach contents recovered were deposited in theMuseum of the Tokyo University of Fisheries. Iden-tification was based on whole animal remains andsome skeletal elements. Pianka’s a-index11 basedon the frequency of occurrence at the familial levelwas used as an index indicating similarity of foodcomposition.

and pelagic thresher Alopias pelagicus. All fish were collected by longline in the eastern tropicalPacific Ocean. We provide detailed information onfood habits of the pelagic thresher for the first time,and we discuss feeding depths and similarities offood composition among large, top-level pelagicfish predators.

MATERIALS AND METHODS

The pelagic fish examined in this study were col-lected during surveys on the potential resources oftunas and billfishes undertaken by the NationalResearch Institute of Far Seas Fisheries. Surveyswere carried out from 1994 to 1997 in the easterntropical Pacific Ocean (Fig. 1). Fish were collectedby longline at 74 stations. We examined a total of222 fish representing seven species and six fami-

1064 FISHERIES SCIENCE M Moteki et al.

Fig. 1 Map showing samplingstations occupied during fourcruises. (�) 1994 cruise (RV Kaihatsu-maru); (�) 1995 cruise(RV Shoyo-maru); (�) 1996 cruise(RV Shoyo-maru); (�) 1997 cruise(RV Shoyo-maru).

Table 1 Number (upper) and standard length (lower; cm) of seven species of pelagic fish examined for stomach contents

Date Yellowfin Bigeye Swordfish Striped Dolphinfish Lancetfish Pelagic tuna tuna marlin thresher

1994 22 31 19 – – – –4 June–28 July 94–157 70–158 65–142

(116.2) (113.2) (81.7)1995 3 2 3 9 1 – 1014 May–2 July 113–151 75–116 66–146 136–184 90.8 116–153

(135.0) (95.6) (92.7) (166.5) (134.7)1996 1 2 – 6 4 2 331 May–14 July 137 110–129 164–183 35–80 76–135 86–160

(119.5) (166.7) (51.5) (105.3) (127.6)1997 4 7 3 33 33 17 729 June–6 Sept. 109–169 84–172 75–132 116–194 53–129 43–142 77–161

(141.2) (144.8) (99.0) (172.9) (84.2) (111.3) (110.3)Total no. specimens 30 42 25 48 38 19 20

Numerals in parentheses indicate the mean values of standard length. –, No fish were caught or examined.

RESULTS

Fish from the stomachs of the seven species oflarge pelagic fish were identified to at least 50species representing 31 families (Table 2). Thenumber of individuals and the frequency of oc-currence at the familial level are summarized inTable 3. Similarity indices of food compositionamong the seven species of pelagic fish are shownin Table 4.

Yellowfin tuna

Yellowfin tuna preyed on a large variety of species.A total of 204 prey fish found were identified to atleast 22 species in 18 families. Based on numbersof individuals consumed, sternoptychids Sternop-tyx obscura dominated (61.3%), followed by exo-coetids and bramids (4.4 and 3.9%, respectively). Infrequency of occurrence, however, exocoetids andbramids dominated with 23.3 and 20.0%, respec-tively, whereas sternoptychids only occurred 3.3%.Yellowfin tuna showed high similarity indices tothe other pelagic fish (0.45–0.59) except lancetfishand pelagic thresher (0.15–0.17). Similarity valueswere especially high for dolphinfish and swordfish(0.59 and 0.55, respectively).

Bigeye tuna

Bigeye tuna stomachs contained 324 fish, identi-fied to at least 18 species in 12 families. Sternopty-chids were the most numerous, at 33.6% (101 outof 109 fish were S. obscura), followed by gempylidsat 18.5% (51 out of 60 fish were Diplospinus multi-striatus), paralepidids (11.4%), and myctophids(8.3%). Gempylids and paralepidids occurred themost frequently, 38.1 and 33.3%, respectively.Sternoptychids ranked third in frequency (23.8%).Although bramids and omosudids only repre-sented 3.4 and 3.1% in numbers, they occurred fre-quently, 23.0 and 21.1%, respectively. High valuesof similarity in food fish composition were seenbetween bigeye tuna and all other predators,except dolphinfish. Swordfish and bigeye tunaexhibited the highest similarity index in this study(0.61).

Swordfish

The swordfish examined in this study consumed214 fish that were identified to at least 18 speciesin 13 families. Myctophids were the most abundantfamily (75.8%). Of 160 myctophids found, 104 were

Notoscopelus sp. Bramids and sternoptychids werethe second and third highest numbered individu-als (5.7 and 4.3%, respectively). However, bramidsand myctophids each appeared in 40.0% of sword-fish stomachs. Sternoptychids also had a fairly highfrequency of occurrence (20.0%). Swordfish andboth tuna species (0.55–0.61) had a high similarityindex, while swordfish showed low similarity todolphinfish (0.09).

Striped marlin

A total of 347 fish of 17 species in 12 families werefound in the stomachs of striped marlin. Based onnumbers of individuals found, bramids (Bramaspp.) were dominant in the stomach contents(60.5%), and scombrids had the second highestvalue (11.0%). Fairly high numbers of stromateoidfish, including nomeids and ariommatids, wereconsumed. The combined total for these two fam-ilies was 14.4%. Scombrids had the highest frequency of occurrence, being found in 54.2% ofthe striped marlin examined. Bramids appearedsecond most frequently (20.0%), followed bycoryphaenids (13.3%), and gempylids andtetraodontids (Lagocephalus sp.) (10.4%). The sim-ilarity index between striped marlin and yellowfintuna was fairly high (0.45), but was very low com-pared to lancetfish (0.03).

Dolphinfish

The stomachs of dolphinfish examined contained73 fish, identified to at least 11 species in 10 fami-lies. The most dominant families in terms ofnumbers were exocoetids and hemiramphids (50.0and 14.9%, respectively), and the frequency ofoccurrence was also the highest in these families(52.6 and 21.1%, respectively). Cannibalism wasobserved in dolphinfish, and five individuals ofCoryphaena spp. occurred in 10.5% of the stom-achs examined. Coryphaenids and syngnathidshad the third highest frequency value. Dolphinfishshowed a high similarity value only to yellowfintuna (0.59), whereas low values were observed rel-ative to all others (0.03–0.17).

Lancetfish

Lancetfish only preyed on a small variety of fish. Atotal of 319 individuals were recovered from stom-achs, and identified to at least eight species in eightfamilies. Sternoptychids were most dominant in

Piscine prey of large pelagic fish FISHERIES SCIENCE 1065


Table 2 Fishes eaten by seven species of large pelagic fish

Species Yellowfin tuna Bigeye tuna SwordfishNo. No. % FO (%) No. No. % FO (%) No. No. % FO (%)

GONOSTOMATIDAEGonostoma spp. 17 5.2 2.4 1 0.5 4.0Gonostomatidae spp. 3 1.4 4.0

STERNOPTYCHIDAEArgyropelechus sp. 2 0.6 4.8Polypnus sp. 1 0.3 2.4Sternoptyx obscura 125 61.3 3.3 101 31.2 16.7 3 1.4 8.0Sternoptyx spp. 5 1.5 9.5 6 2.8 12.0

PHOSICHTHYIDAEPolymetme sp.Vinciguerria spp. 5 2.5 3.3

SCOPELARCHIDAEScopelarchus guentheri 6 2.8 4.0Scopelarchus sp.

PARALEPIDIDAELestidium atlanticum 1 0.3 2.4Lestidium prolixumLestidium spp.Lestrolepis spp.Paralepis atlanticaSudis spp. 3 1.5 3.3 12 3.7 11.9Paralepididae spp. 24 7.4 21.4 1 0.5 4.0

OMOSUDIDAEOmosudis lowei 10 3.1 21.4

ALEPISAURIDAEAlepisaurus ferox 6 1.9 14.3 1 0.5 4.0

MYCTOPHIDAEDiaphus spp. 6 1.9 2.4 9 4.2 8.0Lampanyctus spp.Myctophum spp. 12 3.7 4.8 13 6.1 12.0Notoscopelus spp. 104 48.6 12.0Myctophidae spp. 6 2.9 6.7 9 2.8 9.5 34 15.9 16.0

TRACHIPTERIDAETrachipterus sp. 3 1.5 6.7 1 0.5 4.0

EXOCOETIDAECypselurus spp.Exocoetus spp. 9 4.4 23.3Exocoetidae spp.

HEMIRAMPHIDAEOxyporamphus micropterus 1 0.3 2.4Oxyporamphus sp.

ANOPLOGASTRIDAEAnoplogaster cornuta 2 1.0 6.7 1 0.3 2.4 1 0.5 4.0

DIRETMIDAEDiretmoides parini 1 0.5 4.0Diretmoides sp. 6 1.9 9.5

BERYCIDAEBeryx sp. 1 0.5 3.3

SYNGNATHIDAEHippocampus kudaHippocampus spp. 4 2.0 6.7

PRIACANTHIDAECookeolus japonicus 1 0.5 3.3

ECHENEIDIDAERemora sp. 1 0.5 3.3Echeneidae sp. 2 1.0 3.3


Striped marlin Dolphinfish Lancetfish Pelagic thresherNo. No. % FO (%) No. No. % FO (%) No. No. % FO (%) No. No. % FO (%)

13 6.6 10

2 2.7 5.3 308 96.6 68.4

49 25 52 2.7 5.3 1 0.3 5.3

1 0.5 5

17 8.7 251 0.5 5

26 13.3 102 1 52 1 5

54 27.6 35

3 0.9 6.3 1 1.4 2.6 1 0.3 5.3

1 0.3 5.3

1 1.4 2.6

1 1.4 2.6 2 0.6 10.5

1 0.3 2.1 7 9.6 13.21 0.3 2.1 19 26.0 21.12 0.6 4.2 11 15.1 26.3

1 1.4 2.610 13.7 18.4

3 4.1 7.92 2.7 2.6

1 1.4 2.6

Pelagic thresher shark

Shark stomachs contained 196 fish that were identified to at least 11 species in eight families.Paralepidids of at least four species comprised52.0% of the total number of fish consumed bypelagic thresher and appeared with the highest fre-

both number and frequency of occurrence (96.6and 68.4%, respectively). Only one or two indi-viduals represented the other species found.Lancetfish showed fairly high similarity values tobigeye tuna and swordfish (0.41 and 0.38, respec-tively) and low values to striped marlin and pelagicthresher (0.03 and 0.01, respectively).


Table 2 Continued


CORYPHAENIDAECoryphaena hippurus 2 1.0 6.7 5 1.5 2.4 1 0.5 4.0Coryphaena equiselisCoryphaena spp. 2 0.6 2.4 2 0.9 4.0

CARANGIDAENaucrates ductor

BRAMIDAEBrama dussumieri 1 0.3 2.4Brama orcini 7 3.4 13.3 2 0.6 4.8 12 5.6 40.0Brama spp.Pterycombus petersi 1 0.5 3.3 1 0.3 2.4Taractes rubescens 5 1.5 11.9Taractes spp. 2 0.6 4.8

CHIASMODONTIDAEPseudoscopelus sp. 6 2.9 13.3 1 0.3 2.4 1 0.5 4.0

SCOMBROLABRACIDAEScombrolabrax heterolepis 1 0.3 2.4

GEMPYLIDAEDiplospinus multistriatus 4 2.0 13.3 51 15.7 26.2 2 0.9 8.0Nealotus tripes 1 0.5 4.0Gempylidae spp. 9 2.8 9.5 2 0.9 4.0

SCOMBRIDAEAuxis rochei 3 1.5 6.7Auxis thazard 1 0.5 3.3Auxis spp.Thunnus sp.Scombridae spp.

ISTIOPHORIDAEIstiophoridae sp.

NOMEIDAECubiceps capensisNomeidae spp. 1 0.3 2.4

ARIOMMATIDAEAriomma sp.

BALISTIDAECathidermis macurata

OSTRACIIDAELactoria diaphana 2 1.0 3.3Lactoria spp. 1 0.5 3.3

TETRAODONTIDAELagocephalus sp.

DIODONTIDAEDiodon sp. 1 0.5 3.3

Unidentified 14 6.9 26.7 29 9 28.6 6 2.8 20.0TOTAL 204 324 211

Number of individuals, number percentage and frequency of occurrence (FO) by predators are shown.

quency (65.0%). Although phosichthyids (Poly-metme sp.) had the second highest number(25.0%), the frequency of occurrence was low(5.0%). Pelagic thresher showed a high value ofsimilarity index only to bigeye tuna (0.55), and wasnot similar to dolphinfish or lancetfish (0.02 and0.01, respectively).

DISCUSSION

Prey fish were classified into three categories basedon Matthews et al.8 The frequency of occurrence isshown for each family in Fig. 2. Categories weredefined according to their position in the watercolumn as follows: (i) Sargassum-associates:



1 0.3 2.14 1.2 4.2 3 4.1 5.33 0.9 6.3 2 2.7 5.3

1 0.3 2.1

55 15.9 10.44 1.2 6.3 1 0.5 5.0

151 43.5 12.5

1 0.3 5.3 6 3.1 5

7 2.0 10.4

24 6.9 33.3 1 0.5 52 0.6 4.26 1.7 8.31 0.3 2.15 1.4 8.3 1 1.4 2.6

1 0.3 5.3

1 0.5 523 6.6 6.3 1 1.4 2.6 1 0.5 5

27 7.8 2.1

1 0.3 2.1

1 0.3 2.1 2 0.6 10.52 0.6 2.1

14 4.0 14.6

8 2.3 10.4 6 8.2 7.9 2 0.6 10.5 21 10.7 20347 74 319 196

families for each pelagic fish are summarized inTable 5.

Yellowfin tuna foraged widely for species associ-ated with Sargassum and those living in midwater.Eighteen families of fish, the highest number inthis study, were found in 30 stomachs of yellowfin

strictly surface (neustonic) organisms associatedwith drifting Sargassum in the upper 5 m of water;(ii) near surface: found within the upper 20 m; and(iii) midwater: both vertical migrators and thosewhich remain at depths. The main feeding depthestimated in this study and the main prey fish


Table 3 Families of fish eaten by seven species of large pelagic fish


Gonostomatidae 17 5.2 2.4 4 1.9 4.0Sternoptychidae 125 61.3 3.3 109 33.6 23.8 9 4.3 20.0Phosichthyidae 5 2.5 3.3Scopelarchidae 6 2.8 4.0Paralepididae 3 1.5 3.3 37 11.4 33.3 1 0.5 4.0Omosudidae 10 3.1 21.1Alepisauridae 6 1.9 11.9 1 0.5 4.0Myctophidae 6 2.9 6.7 27 8.3 14.3 160 75.8 40.0Trachipteridae 3 1.5 6.7 1 0.5 4.0Exocoetidae 9 4.4 23.3Hemiramphidae 1 0.3 2.4Anoplogastridae 2 1.0 6.7 1 0.3 2.4 1 0.5 4.0Diretmidae 6 1.9 9.5 1 0.5 4.0Berycidae 1 0.5 3.3Syngnathidae 4 2.0 6.7Priacanthidae 1 0.5 3.3Echeneididae 3 1.5 6.7Coryphaenidae 2 1.0 6.7 7 2.2 4.8 3 1.4 8.0CarangidaeBramidae 8 3.9 20.0 11 3.4 23.0 12 5.7 40.0Chiasmodontidae 6 2.9 13.3 1 0.3 2.4 1 0.5 4.0Scombrolabracidae 1 0.3 2.4Gempylidae 4 2.0 13.3 60 18.5 38.1 5 2.4 8.0Scombridae 4 2.0 6.7IstiopholidaeNomeidae 1 0.3 2.4AriommatidaeBalistidaeOstraciidae 3 1.5 6.7TetraodontidaeDiodontidae 1 0.5 3.3Unidentified 14 6.9 26.7 29 9.0 28.6 6 2.8 20.0Total 204 324 211

Number of individuals, number percentage and frequency of occurrence (FO) by predators are shown.

Table 4 Similarity indices (Pianka’s a-indices) among seven large pelagic fish based on frequency of occurrence ofprey fish at the familial level

A B C D E F G

A. Yellowfin tuna – 0.473 0.549 0.451 0.590 0.166 0.154B. Bigeye tuna – 0.608 0.239 0.075 0.409 0.551C. Swordfish – 0.262 0.085 0.382 0.134D. Striped marlin – 0.174 0.030 0.116E. Dolphinfish – 0.106 0.016F. Lancetfish – 0.011G. Pelagic thresher –

tuna. Yellowfin tuna had high similarity indices inprey fish composition compared to other pelagicfish, except lancetfish and pelagic thresher (0.02and 0.01, respectively). Particularly high similari-ties were noted to swordfish and dolphinfish,which are typical mesopelagic and epipelagicfeeders, respectively, indicating that yellowfin tunafeed mainly in midwater and the surface layer.

Main fish exploited by bigeye tuna belong to themidwater group, which represented 12 out of 14prey families in this study and 93.1% of identifiedindividuals. Bigeye tuna showed a high similarityto swordfish and pelagic thresher, both of whichare midwater feeders. Extensive exploitation ofsternoptychid, myctophid, and bramid fish wasshared with swordfish, and paralepidid exploita-tion was similar to that of pelagic thresher. Overlapof prey fish utilization between yellowfin and

bigeye tuna was moderate in terms of the similar-ity index (0.47), with seven families exploited byboth tuna. With regard to feeding depth, yellowfintuna foraged widely for a variety from Sargassum-associates to midwater fish, whereas bigeye tunapreyed mainly on midwater fish. Exocoetids,ostraciids, and syngnathids were present in thestomachs of yellowfin tuna, but not in the stom-achs of bigeye tuna. Several families, including thebramids, gempylids, myctophids, omosudids, andparalepidids, are midwater fish that undertakediurnal vertical migrations. These families weremore frequently present in bigeye tuna than in yel-lowfin tuna. A similar tendency was observed incephalopod prey.10 Differences in stomach con-tents between yellowfin and bigeye tunas may beattributed to behavioral differences, such asfeeding time. Yellowfin tuna feed mainly during the



13 6.6 10.02 2.7 5.3 308 96.6 68.42 2.7 5.3 1 0.3 5.3 49 25.0 5.0

1 0.5 5.0102 52.0 65.0

3 0.9 6.3 1 1.4 2.6 1 0.3 5.31 1.4 2.6 1 0.3 5.31 1.4 2.6 2 0.6 10.5

4 1.2 6.3 37 50.0 52.611 14.9 21.1

5 6.8 10.51 1.4 2.6

8 2.3 13.3 5 6.8 10.51 0.3 2.1

210 60.5 20.0 1 0.5 5.0

7 2.0 10.4 1 0.3 5.3 6 3.1 5.038 11.0 54.2 1 1.4 2.6 1 0.5 5.0

1 0.3 5.323 6.6 6.3 1 1.4 2.6 2 1.0 10.027 7.8 2.1

1 0.3 2.13 0.9 4.2 2 0.6 10.5

14 4.0 10.4

8 2.3 10.4 6 8.1 7.9 2 0.6 10.5 21 10.7 20.0347 74 319 196

Striped marlin have been reported to competefor food with other pelagic predators.15 In thepresent study, we observed little competitionbetween marlin and other pelagic predators.Striped marlin foraged for a wide diversity of fishfrom Sargassum-associates to midwater species.Sargassum-associates were represented by six families, which was the largest number in thisstudy and comparable to yellowfin tuna. Based onnumbers of individuals, however, near-surface andmidwater fish accounted for 90.9% of prey fishidentified. Among the pelagic predators examined,only striped marlin preyed heavily on scombrids(66.8%), which are a near-surface group of fish. Wespeculated that striped marlin reduce competitionwith other pelagic fish by extensively pursuingscombrids, which they are able to catch due totheir fast swimming ability.

Dolphinfish are apparently surface feeders, for-aging mainly for Sargassum-associates (85.5% innumber of prey fish identified). Similarity in foodfish between dolphinfish and all other predatorsexcept yellowfin tuna was low. Dolphinfish preyedmainly on exocoetids and hemiramphids, for acombined total of 65.8%. In our study, dolphinfishwere considered to be in competition with yel-lowfin tuna for feeding on epipelagic fish, such as

daytime and bigeye tuna feed at night.3,12 Further-more, it is well documented that bigeye tuna are distributed deeper in the water column thanyellowfin tuna.3,13,14

Swordfish preyed mainly on sternoptychid,myctophid, and bramid fish in midwater. Fish families appearing in the stomach contents ofbigeye tuna and swordfish overlapped extensively.Eleven families were commonly consumed, repre-senting 78.6% of the total number of fish families in the stomachs of bigeye tuna and 84.6% in sword-fish. The only families found in the stomachs of bigeye tuna and not in swordfish were the omosu-dids and scombrolabracids, while gonostomatids,scopelarchids, and trachipterids were only found inswordfish. Overlaps in the diets of swordfish andyellowfin tuna were observed, with nine familiesbeing commonly consumed by both species.Although both species commonly consumedbramids, the fish family principally eaten by yel-lowfin tuna was the Sternoptychidae, whereas myc-tophids were only eaten by swordfish. In the samewaters, swordfish feed on large-sized cephalopods,such as diamondback squid Thysanoteuthisrhombus, flying squid Sthenoteuthis oualaniensis,and pelagic octopus Tremoctopus violaceuswhereas tunas feed on smaller cephalopods.10


Fig. 2 Frequency of occurrence of the families of prey fish occurring in the stomachs of seven species of pelagic fishpredators. Families are separated into three categories: Sargassum-associates, near-surface and midwater.

exocoetids, coryphaenids, and syngnathids.According to Manooch et al.,16 the stomach con-tents of dolphinfish along the south-eastern andGulf coast of the United States consisted princi-pally of balistids rather than exocoetids. Differ-ences in these two studies are probably explainedby differences between study areas. The amount ofdrifting Sargassum is likely more limited in theopen ocean than in coastal waters.17 Manooch etal.16 found Sargassum debris in 48.6% of dolphin-fish stomachs examined, whereas we found nonein this study.

We observed that lancetfish foraged mainly formidwater sternoptychids, which closely resemblesthe diet composition observed in the central equa-torial Pacific.6 Although the food composition oflancetfish differs geographically, sternoptychidsare important food fish in the open ocean (sum-marized in Moteki et al.6). Yellowfin tuna, bigeyetuna, and lancetfish competed for sternoptychidprey. On the other hand, the neustonic Sargassum-associate tetraodontiform fish were also found inyellowfin tuna and lancetfish stomachs. Moteki etal.6 showed that lancetfish prey heavily on bothgelatinous midwater octopus (Japetella diaphana)and surface-living octopus (Argonauta spp.). Thus,lancetfish appear to feed primarily on slow-swimming, less muscular, and/or gelatinousspecies in a wide range of the water column.

Pelagic threshers preyed mainly on small mid-water fish, such as paralepidids, phosichthyids,and gempylids, whereas Sargassum-associated fish were not found in the stomach contents of threshers. Although there was a fairly high similar-ity to bigeye tuna, very little competition betweenpelagic thresher and other pelagic fish was noted.Squids (found in 66% of stomachs) and scombrids(27%) were the most common food items of the bigeye thresher (Alopias superciliosus) in thewestern North Atlantic Ocean,18 whereas in thepresent study, neither squids nor scombrids werefound in the stomachs of the pelagic thresher. Littleis known on the food habits of thresher sharks(family Alopiidae, composed of three species),although it is known that they are able to use theirextremely long upper caudal lobe to kill prey.18–21

Detailed studies on stomach contents are requiredto clarify the differences in food preferencesamong thresher sharks.

The most important prey fish (i.e. more than20% in number of individuals or frequency ofoccurrence) consumed by pelagic predators in this study belonged to the following families:Sternoptychidae, Phosichthyidae, Paralepididae,Omosudidae, Myctophidae, Exocoetidae, Hemi-ramphidae, Bramidae, Gempylidae, and Scombri-dae. Of these 10 families, the bramids and


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obesus) in the Gulf of Guinea and its place in the trophicsystem of the pelagic zone. J. Ichthyol. 1974; 14: 765–775.

5. Okutani T, Tsukada S. Squids eaten by lancetfish and tunasin the tropical Indo-Pacific Ocean. J. Tokyo Univ. Fish. 1988;75: 1–44.

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7. Dragovich A, Potthoff T. Comparative study of food of skip-jack and yellowfin tunas off the coast of West Africa. Fish.Bull. (Wash. D.C.) 1972; 70: 1087–1110.

8. Matthews FD, Damkaer DM, Knapp LW, Colette BB. Food ofthe western north Atlantic tunas (Thunnus) and lancet-fishes (Alepisaurus). NOAA Tech. Rep. NMFS SSRF 1977; 706:1–19.

9. Smale MJ. The feeding habits of six pelagic and predatoryteleosts in eastern Cape coastal waters (South Africa). J.Zool. London, B1 1986; 341: 357–409.

10. Tsuchiya K, Okamoto H, Uozumi Y. Cephalopods eaten bypelagic fishes in the tropical east Pacific, with special refer-ence to the feeding habitat of pelagic fish. La Mer 1998; 36:57–66.

11. Pianka ER. The structure of lizard communities. Ann. Rev.Ecol. Syst. 1973; 4: 53–74.

12. Kobayashi H, Yamaguchi Y. Feeding ecology and hookingtendency of tunas and marlins in the eastern equatorialPacific. Nippon Suisan Gakkaishi 1971; 37: 83–89.

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16. Manooch CS, Mason DL, Nelson RL. Food of gastrointesti-nal parasites of dolphin Coryphaena hippurus collectedalong the southeastern and Gulf coasts of the United States.Nippon Suisan Gakkaishi 1984; 50: 1511–1525.

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sternoptychids were extensively exploited by fourout of the seven large pelagic fish (Table 5). In addi-tion, exocoetids and paralepidids dominated theprey species in two of the predators. These familiesare reported to be important prey items for pelagicfish in other studies.6,8,15 Although pelagic fishpredators extensively use these prey families, dif-ferent dominant families and feeding depths ofeach predator are considered to reduce trophiccompetition among pelagic fish in the easternequatorial Pacific (Table 5).

In the present study, we incorporated data froma wide-ranging area. However, the composition ofstomach contents often varies with location andseason.6,9,22 Thus, to verify niche overlap in terms offood competition, it is necessary to compare stom-achs taken from a small area within a short timeperiod. Further, the size ranges of the predatorsexamined were fairly wide, and the food habits ofpelagic fish vary with size.7,9,23 It is necessary toknow the detailed food habits of each species inorder to assess competition among large pelagicfish.

ACKNOWLEDGMENTS

We are grateful to Dr Eric Hochberg, Santa BarbaraMuseum of Natural History, and Dr Akihiko Yatsu,National Research Institute of Fisheries Science,for their critical reading of the manuscript. We arealso deeply indebted to the captains and crews ofRV Kaihatsu-Maru and RV Shoyo-Maru for collect-ing the pelagic fish predators. Our thanks also goto Drs Kiyoshi Fujita and Hiroshi Kohno, Labora-tory of Ichthyology, Tokyo University of Fisheries(TUF), for their invaluable advice during this study.We sincerely thank Miss Hiromi Kudo, The Univer-sity of Tokyo, and Mr Niimi Ogata (TUF) for collecting important literature.

REFERENCES

1. Smale MJ. Cephalopods as prey. IV. Fishes. In: Clarke MR(ed). The Role of Cephalopods in the World’s Oceans. Philo-sophical Transaction of the Royal Society of London, B 1996;351: 1067–1081.

2. King JE, Ikehara II. Comparative study of food of bigeye tunaand yellowfin tuna in the central Pacific. Fish. Bull. (Wash.D.C.) 1956; 57: 61–85.

3. Watanabe H. On the difference of the stomach contents of the yellowfin and bigeye tunas from the western equatorial Pacific. Rep. Nankai Reg. Fish. Res. Lab. 1958; 7:72–81.

4. Borodulina OD. The feeding of the bigeye tuna (Thunnus


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Composition of piscine prey in the diet of large pelagic fish in the eastern tropical Pacific Ocean