*THE USE OF A GROUP OF MACRO PLANKTONICORGANISMS (EUPHAUSIID CRUSTACEANS) IN THE STUDY

OF WARM \X'ATE~ PELAQIC FOOD WEBS

,CLAUDE rtOGER"

O. R. S. T. O. M. Oceanoqraphie, 24, rue Bayard75008, Paris, France

Peraglc food webs of the tropical Pacific have been studiedthrough a two-step. strategy; Firstly, a group of macroplanktonicanimals, i.e:,. euphaustids 'v/as selected' because of its abundancearid' its utilisation by a large range cif' predators. This groupwas studied as thoroughly .aa.possible with respect to itspartici­pation in the, trophic chain' and ,its vertical distribution. Se­condly, this group was used as a 'tracer' in the stomach contentsof fishes. The knowledge thus gained was used tozietermlne

'predator-prey relationship",

From the point of "iew'of 'the availability of'euphausiidsto predators, there are two main types 'of euph ausiids Le.; de­pending upon whether they remain during the day in watersshallower than 300-400 rn (e.g. St.ulocheiron sp.) or they inhabitthis layer only at night; otherwise live indeepr waters duringthe day (e.g. Euiitiausia sp.).

In the stomach contents of fishes taken from the stamach~of longlme tunas, StYlOCh~iron euphausiids were largely round.Thus, these fishes appear 'to be principally .eptpelagtc and dayfeeders. On the contrary; both SlYloCheironand EUPhausia spp,oecur in the stomachs of vertically migrating fishes.

A study of the feeding rhythm of euphausiids showed thatStulocheiron sp. are typically day feeders; whereas Eutrnausia sp.feed either continuously or mainly at night.

These results led to the following conclusions:

1. At least two types of relatively independent food websexist: the first one is entirely limited to the first 300 m and canbe exemplified as: phytoplankton -> small zooplankton -> sty­locheiron euphausiids -+ epipelagic fishes -+ longline tunas. Thesecond food chain can be represented as: phytoplankton ->

small zooplankton __ Euphausia sp. -> migrating fishes -> deepwa ter fishes. '

The second food web is directed towards the enrichment ofthe deep layers, and feeds at night on the first food web. Thepoint is that the latter is not feeding at that time. Thus, thereseems to exist an 'energy valve' which allows the transfer ofenergy from the first food web to the second. A reverse flow,however, does not seem to occur. In other words, a combinationof feeding rhythms and vertical migrations accelerates thedownward energy transfer and slackens the upward one.

3. Th e study of vertical distributions, stomach contents andfeeding rhythms appears to be a reliable index of the determina­tion of food web.

*Invited paper

Pelagic Food Webs

INTRODUCTION

Several different approaches have been employed for the study of marine. zooplankton and the type of investigations whIch was to be undertaken heredepended on many parameters. "such as:

1) the type of facilities available (ships, sampling gear, laboratory rearing,physlological measures, chemlcal analysis etc).

11) environmental factors (to be llnked with the local pollution problemsor to be connected wIth the fishery of a particular regton).

ill) the time available for providing an answer (months, years) .

From all these conditions including some others, the general trends obtain­ed In the study are as follows :

A group of oceanographers based In New Caledonia, in the southwesterntropical Pacific Ocean, de cided to undertake a long-term study on developingan understanding of the pelaglc community.

In these regions of the PacIfic, there is no continental shelf, that is, thedepth increases sharply just outside the coral reef surrounding the islands. Thusthere Is nocommerclal fishery based 'on trawling. There is, how.ever, a smaliscale fishery insIde the lagoon, of local interest. The only important resourcesaround the Islands are tunas and it is hoped that fishing of small coastal tunassu ch as the skipjack tuna (Euthynnus pelamis) will probably develop soon. Atpresent, mostly large longline tunas are caught. These are albacore (Thunnusatatunoas and yellowfin (Thunnus albacares) tunas, fished by long llnes operatedfrom the Japanese vessels.

The research group focussed its attention on the study of ,pelagic commu­nitIes which ultimately get llnked wIth the tuna resources. This Involved therelationship between the entire pelagic community from zooplankton to tuna.

The implications of the proj ect were not clear from the beginning, andhence, for several years, a part of the time at sea was devoted to large scaleexploratory cruises in this llttle known part of the Pacific Ocean. The area understudy extended from the equator to southe rn tropic, and fr om the Society Islandsin the Central ·Pacific to the Cor al Sea. Later on, more restricted areas wereselected for detailed investigations , in the Equatorial Cen t ral Pacific and arou ndNew Cal edonia.

MATERIAL AND METHODS

Sampling procedures wer e first t ested for a long time because the typeof approach envisaged largely depended upon the rellability and effect iveness offi eld sampling.

The followin g three main devices were used as a routine to sa mple thedifferent levels of the community:

1) an ordinary conical plankton net , one m in diamet er and 0.33 mm meshwidt h was used to coll ect zoopl ankton.

Ii) a 3.2 m l saa cs-Kidd Midwater Trawl was used to coll ect mi cronektonicanimals, m ainly fish and crust aceans (Gr andperrin and Michel, 1970).

ill) a Japan ese long line, app roxima te ly 20 km long (65 baskets x 6 hooks= 390 hooks) baited with Col olabis sa i r a , was used for tuna fishin gbetween 30 and 200 m depth.

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In addition to these, a few other devices wer e used occasionally to getthe data on some specific problems, namely:

a) an opening-closing OMORI "larval" 'n et 160 cm diameter (Omori, 1965)was used to get a precise picture of the vertical distributions of zooplank­tonic and micronektonic species.

b) a pelagic trawl with an opening head of approximately 40 m2 (wh ichis 5 times larger than the 3.2 m Isaacs-Kidd mid water trawl) was triedto catch larger fish to flll in the gap existing between micronektonicfish 5-10 cm long caught by the Isaacs-Kidd trawl and tuna. But herethe results were very disappointing.

c) a vertical long line was used to check the vertical distribution of tunadown to 600 m deep.

On the whole, approximately 2000 samples from the Isaacs-Kidd and asimilar number from the plankton net were collected along with a few hundredtunas (approximately 400 in number).

It has been generally accepted that the link between one community andthe other is largely through trophic chain. Thus, the two points of special interestwere:

A. examination of stomach contents and identification of prey organismsin the stomachs and also of the feeding rhythm of the predator.

B. vertical distributions of the species to know whether trophic relationshipbetween the two species is possible.

These two points demanded a very complete sampling programme forzooplankton and micronekton, both with reference to time of the day (that ishauls had to be made at all times of the day) and with reference to depth.Most cruises included a series of tows ranging successively from the surface to1200 m depth.

The collected material was very large and sorting of each group provedto be very time-consuming. Thus, several devices were develop ed to speed up thetrea tment of samples in the laboratory. These comprise of :

a) a large plastic cylinder 160 cm high and 20 cm in diameter (inside) whichhad an ascending current of water. This separated the animals accordingto their densities (Michel and Grandperrin, 1971) .

b) a seri es of filters consisting of grilles of equidistant glass rods, so as tosort out the organisms into several size groups (Roger and Wauthy,19(8) .

Su bsequen t ly, sorting by hand was n ecessary to separate groups or species.However, the pr eliminary treatments noted abov e spe ed ed up the procedure.

In each sample, sorting was conducted up to group (fish , euph a usiids .copepods etc.). Then, the identification wa s don e up to sp ecies in 4 groupsnamely, micron ektonic fish , eupha usiids , arnphipods and copepods. The analysisof the stomach con t en ts was made in fishes and euphausiids.

In tunas, all stomach con t en ts were analysed and when ever pos sible,identification of prey was done up to the lev el of species for fish, euphausiids andamphipods.

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RESULTS AND DISCUSSION

The main results r elated to zooplan kton and micronekton have beenpublished elsewhere (Legand et ai., 1972 ; Roger. 1973a-f, 1974a and b, 1975; Rogerand Grandperrtn, 1976).

Euphausllds on an average formed 10'% of the catch in Isaacs-Kidd Trawlin terms of" biomass. If micronektonic fish are not consider ed in the same cate­gory, this percentage goes up to 20. Although In the tropical Pacific ' about 38species of -eup h a usttds have been reported earlier, only 15 to 20 were presentIn any given sample. It was therefore, easy to sort them out from the other­com ponen ts . The euphauslid species showed differences in their vertical distribu­tion and feeding rhythm. Different species of the same genus showed roughly thesame type of behaviour.

In the samples obtained from plankton net and midwater trawl, two maingenera formed the bulk of the . catch of euphausirds. There were :

i) Eupruiusia with approximately 50% in numbers (main species : E. diome­aae and E. tenera along the equator and E. brevis and E. mutica in the­

. tropical parts).il) stylocheiron with approximately '30% in numbers (main species: S.

abbreviatum and S. carinatum).

The third genus in abundance was Nemaioscelis, The genus Thysanopodawas also present' in the -samples of the Isaacs-Kidd trawl which was found to be­specially well-suited to catch the large-sized specimens of this genus.

The vertical distribution and daily vertical migration of the two maingenera, Euptuiusia and stutocneiron. was as fol1ows :

Euptuius ia sp. ("E'" in Fig. 1) were typically migrating animals. They were­found in the upper 200 m at night, but they got congregated between 400 and600 m during the day.

Stylocheiron sp, ("S " in Fig. 1), on the other hand, were essential1y non­migrating forms which stayed mainly around 200-300 m depth both during theday and at night.

Analysis of the stomach contents of the different species also showed aclear picture. In vertically-migrating Eupnausia, most of the species feed through­out the day and night; but feeding at night, i.e., when they inhabit the richupper layers, was more intense than that during the day in the deeper layers.It has been suggested that this apparent feeding during the day was probablya bias because the dig estive processes and pas sage of food through the stomachget slowed down during the day because temperature is very low in those deeplayers and probably stomach contents during the day represent remains of thefood consumed at night. Probably this will not be th e right explan at ion becauseon e of the species namely, Eupiuiusia tenera h ad a very hi gh percentage ofempty stomachs during the day. This is probably because that this sp ecies issmall-sized and it do es not find in its deep daytime blotope the tiny prey whichwould be of suitable size for it to consume. This in dica tes that dig estion is notstopped as a result of low temperature in the deep daytime habitat. Thus, thevalues obs erv ed for fullness of stomachs in the other sp eci es during the daytim eare likely to be du e to an actual f eeding activity during the day. P erhaps. the­most im port an t thing to rem ember h er e is that ver t ical1y-migrating E uphaus iaspecies feed mainly at n ight in the upper lay ers .

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Claude Roger

The fullness of stomachs over the 24 hr period for the non-migratingStulocheiron species which inhabit permanently the 200-300 m layer indicatesthat all of them are strictly day-feeders. Nearly all stomachs collected duringthe night were found to be empty. The sharp decrease in the fullness of stomachsfound at the end of the day in ' some of the species also gives an estimate of. theapproximate duration of the food .r et aln ed in the stomach. From the time when80% of the stomachs are full and that when all are empty, the time differenceis less than 3 hr (in the case of S. affine). Thus, it can be inferred that theduration for the food to remain in the stomach is about 2 hr.

An inverse relationship was found between the magnitude of daily verticalmigration and the intensity of feeding rhythm. The vertically migrating Euptuiu­sia feed both during the day and at night. Similarly, vertically migrating Thysa­nopoda species are also typically continuous feeders but the non-migrating sty­loctieiron show a well defined feeding rhythm (Roger, 1975). The feeding rhythmassociated in this way with vertical migration has not been clearly demonstratede arli er , and hence these observations are of special interest.

The next link in the community is with mi cronektonic fish which feedon euphausilds and other planktonic animals. The micronektontc fish caught inthe plankton nets and midwater trawls either live in deeper waters ("DF" inFig. 1) or migrate vertically (HMF" in Fig. 1) with the deep-scattering layer.Notable fam1l1es of these fishes are Myctophidae and Gonostomatidae. Theycome in the upper 300 m layer only at night and during the day they live atdepths' greater than ' 4.00 m. Th·ese . fishes are most abundant at 600 m depthduring the day.

An analysis of stomach contents of these vertically-migrating fish showedthat their food largely consists of crustaceans such as copepods, amphipods ande uph a usiids. The larger fishes of this group are ichthyophagous. They eat almostthroughout the day and night. Euphausitds found in their stomachs belong toEuptuiusia sp. and stylocheiron sp . almost in equal numbers.

Tunas (HT" in Fig. 1) form the upper-most link in the com m un ity andthese fishes live mainly above the 450 m zone. Their stomach contents mainlyinclude micronektonic fish and cephalopods. The last group const it ut es themain gap in our studies as these animals, because of their good swimmingpower, avoid nets. Thus, very little is known about their abundance, verticaldistribution and feeding habits.

The interesting point noticed in the food of tunas is that micronektonicfish found in their stomachs are almost always very different from those caughtby the plankton nets and midwater trawl. As noted earlier, the fish caught bythe n ets were either from the deep water or the vertically-migrating forms suchas Myctophidae and Gonostomatidae. The tuna stomachs largely containedepipelagic fish (HEF" in Fig . 1) such as Gempylidae and Bramidae, which wererarely caugh t by the nets because they are rast swimmers. Thus, in the tunastomachs no vertically-migrating fish (Myctophidae and Gonostomatidae) occur­ed . These formed the bulk of fish catches by the nets during the night from thesame depths where tuna occurred. This probably indicates that the tunas (alba­core and yellowfin) do not feed during the night. A further evidence of thiscan be obtained from the fact that attempts to catch the tuna by long linesduring the night were always unsuccessful.

Since tunas do not fe ed during the night they are not able to make useof the migrating fauna for their fo od and these an im als form a very large

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Pelagic Food Webs

biomass. The food organisms 'of tuna thus get limited to epipelag!c fishes ' whichstay at depths shallower than 300-400 m during the day.

A further analysis of the stomach contents of the epipelagic fishes foundin the stomachs of tunas showed that they contain three times more Stulocneiron:euphausiids than Euiituiusia species. As noted earlier, these fishes are not caughtby the nets because they are fast swimmers and hence it is difficult to concludeat what time these are eaten. The fact that these epipelaglc fishes seem tofeed rarely on vertically-migrating Eupiiausia although these animals areabundantly caught by the net during the night at the same level where thesefishes live, indicates that they are principally day-feeders. They abundantly feedon non-migrating Stuloctieiron which stay at the same level as these animalsduring the day.

NI G HT

/-, /-, r>.I P I I EZ I I S I<:> <:> <:>

lX1Xl~-~---i:1

300·

Fig. 1. Diagrammatic representation of the links among the pelagtc communitiesof the tropical Pacific. Solid lines represent feeding activity. Dotted lines - non­feeding activity; circles - epipelagrc species; triangles - vertically-migratingspecies; squares - deep sea species. Arrows indicate main paths of energytransfers through predator-prey relationship. Pe-phvtoplankton : EZ=epipelagiczooplankton : S=stylocheiron .euphausnds: EF=epipelagic fish (e.g. Bramidaeand Gempylidae). T=tuna; MZ=vertlcally-migrating zooplankton: E = Euphausia.euphausiids: MF=vertically migrating fish (e.g. Myctophidae): DF=deep sea fish

Thus, the feeding rhythm of the predator (here the epipelagic fishes) hasbeen determined as a result of the knowledge of vertical distribution and dallyvertical migration of some of its prey.

The diagram of the community relationship is thus complete. However,two gaps remain in this relationship: 1. the role of cephalopods which are animportant food source for tuna in the food chain: 2. the feeding rhythms of theother plankton organisms such as copepods and arnphlpods.

Let us assume that in these groups also there exists, like in euphausiids,

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Claude Roger

migra ting night-feeding species (" MZ" in F'ig. 1) and non-migrating day-feeders.species (HEZ" in Fig. 1).

It is by far th e si mplest way to illus trate ' the poin t whi ch )n reality maybefar more complicated. Nevertheless, summing up all the features noted above,t he general t rend of t h e community st r ucture can be sh own diagramma tically(Fig. 1).

During the day :- At depths lower than 300-400 m, .on ly the representativesof the "ep tpelagic system" (circles in Fig. 1) are en countered. These ,are : phyto­plankton (HP"), epipelagic zooplankton (" EZ"), Stuloctieiroii euphausiids ("S") ,eplpelaglc micronektonic fish ("EF") : and ' tuna ("T"). ""ll these are dependenton one another for their trophic needs, and all of them are, day-feeders,. . . . :

At depths greater than 300-400 in, the vertically-migrating forms (seet r iangles in Fig. 1), the deep sea fauna (squares in Fig. 1) ; vertically-migratingzooplankton ("MZ"), Euptuiusia euphausilds (HE"), verttcally-migrattng micro­11ektonic , fish ("MF") and deep sea fish ("DF") are found.

At night: - At depths shallower than 300-400 m. epipelagic fauna is stillthere but it is not feeding ,(dot ted lines in Fig. 1).. It is preyed upon by theve r ti ca lly- m igr ati n g fauna which is feeding very actively.

Deep sea fishes still remain at depths gr eater than 300-400 m.

CONCLUSIONS

, The conclusions drawn from the pelagic communit y are as follows : Threesystems exist which are linked in an interesting way forming a. chain. Along eachlink ot the chain, energy transfer occurs and the pathway s for energy are deter­mined by the vertical distribution of animals , their daily vertical migration andf eeding rhythm. '

The fi rst system in th e food web is eprpelagrc in which all the links a red ay-feeders and they are "sleeping" at night i.e., they do not feed (d ot t ed linesin Fig. 1) . This system is self sufficient, that is it do es not need any en er gy inputfrom the outside except perhaps solar en er gy and nutrient enrichment.

The second is the vertically-migrating system (t r ian gles in Fig. 1). Theanimals in this system are mainly night-feeders ; their main source of food is theepipelagic system.

The third sys te m is formed of de ep fauna, which receive en er gy by preyin gupon the mi grating system during the day time.

The important point to be considered in this organization is that theepipelagic system is not fe eding during t h e night wh en the vertically-migratings ystem is f eeding on it. Hence . an ener gy transf er occurs only in on e direction,fro m the epipelagic system towards the vertically-migrating system. The epi­pelagic syst em is n ot a ble to make us e of the migrating forms as food because itis not feeding at the time when they both inhabit the sa me layers . Thus, thetuna can not m a ke us e of th e t remen dous biomass represented by the vertically­mi grating species as food .

Thus, there exists a mechanism wh ich could be called an "energy safetyvalve " which allows energy to be transmitted from subsurf ace layers t o deep erlayers but it preven ts the backward transfer of ener gy because the epipel.agicsyste m is not feeding a t n ight.

It seems desirable to poin t ou t that this ty pe of investiga ti on is simple

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to undertake as it does not need any sophisticated equipment or analytical tech­niques. It needs only a few things such as facilities to work at sea and a teamof specialists to cover the important taxonomic groups. Such investigation canimprove ' our understanding of the pelaglc communities of the other oceanswhich appear to be simpler than those of coastal waters and estuaries.

REFERENCES

GRANDPERRIN, R. AND MICHEL, A. 1970. Emploi du chalut pelagique Isaacs-Kidd10 pieds dans les eaux equatoriales du Pacifique. Mar. Biol.,7 . 273-284.

LEGAND, M., BOURRET, P., FouRMANOIR, P. GRANDPERRIN, R., GUEREDRAT, J. A., MICHEL,A., RANCUREL, P., REPELIN, R. AND ROGER, C. 1972. Relations trophiqueset distributions verticales en milieu pelagique dans l'ocean Pacifiqueintertropical. can. ORSTOM ser. oceanoar., 10, 303-393.

MICHEL, A. AND GRANDPERRIN, R. 1971. Traitement des recoltes micronectoniques.Mar. tnoi., S, 238-242.

OMORI, M. 1965. A 160 cm opening-closing plankton net. I. Description of thegear. J. Oceanoqr, Soc. Japan, 21. 212-218.

ROGER, C. 1973a. Recherches sur la situation trophique d 'un groupe d 'organtsmespelagiques (Euphausiacea). I. Niveaux trophiques des especes, Mar.moi.. 18,312-316.

ROGER, C. 1973b. Recherches sur la situation trophique d'un groups d'organismespelagiques (Euphausiacea). Il. Comportements nutritionnels. Mar. Biol.,18, 317-320.

ROGER, C. 1973c. Recherches sur la situation trophiqued'un groupe d 'organismespelagiques (Euphauslacea), Ill. Potentiel alimentaire du groupe. Mar.Biol:, 18, 321-326.

ROGEII, C. 1973d. Recherches sur la situation trophique d'un groupe d 'organismespelagtques (Eu ph a usia cea). IV. Relations avec les autres elements dumicronecton. Mar. su»; 19, 54-60.

ROGER, C. 1973e. Recherches sur la situation trophique d 'un groupe d 'organismespelagtques (Euph a usiacea). V. Relations avec les thons. Mar. B iol., 19,61-65.

ROGER, C. 1973/. Recherches sur la situation trophique d'un groupe d'organismespelagiques (Euph a usia cea) . VI. Conclusions sur le role des euphausiacesdans les circuits trophiques de - l'ocean Pacifique intertropical. Mar.moi., 19, 66-68.

ROGER, C. 1974a. Les euphausiaces du Pacifique equatorial et sud tropical : zoo­geographte, ecologie, biologie et situation trophique. Memoires ORSTOM.,71, 1-265 (ISBN. 2. 7099.0335.0. ).

ROGER, C. 1974b. Repartttions bathyrnetriques et migrations verticales des euphau­slaces -(cr ust aces) dans les zones de peche au thon du Pacifique sudtropical. Cah. ORSTOM ser . oceanour., 12, 221-239.

ROGER, C. 1975. Rythmes nutritionnels et organisation trophique d 'une populationde crustaces pelaglques (Euphauslacea). Mar. B iol., 32. 365-378.

ROGER, C. AND GRANDPERRIN, R. 1976. Pelagic food webs in the tropical Pacific.Limnol. Oceanoqr., 21, 731-735.

ROGER, C. AND WAUTHY, B. 1968. Sur une technique de determination de groupesde tailles applicable a I'etude de certains organismes planctonlques.J. cons., 2. 216-225.

DISCUSSION

OMORI

316

You have shown a very clear diagram in trophic relationshipsamong different trophic levels. Howev er, I feel it is rather

;~ ;

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ROGER

RAYMONT

ROGER

SILAS

ROGER

SILAS

ROGER

SILAS

ROGER

Claude Roger

difficult to treat your data for quantitative consideration.Your data showed that the contribution of euph a usiids as anindirect energy source to tuna was only less than 10%. Wasthere any other organism: which contributed indirectly greaterthan euphausnds? Ii'there is any, we must put that speciesinto consideration. Otherwise; the analyses may bring mis­leading results. May I ask your opinion about it?

I don 't think that any other group contributes significantlymore than euphausl1ds to the diet of macronektonic fisheseaten by longline tunas. I believe that copepods and amphl­pods, for example, are of simllar importance. But I fully agreethat quantitative assumptions are hardly possible from ourresults - they would require informations on other groupsthan euphausiids. Our objective was to bring into evidencemechanisms of trophic relationships as exemplified by thoseregarding euphausiids, not to quantify energy transfers.

Do species of Euphausia feed mainly on phytoplanktonwhereas Siulocheiron feed carnivorously?

'Yes. Stuloctieirori species are typically carnivorous. Euptuiusiaspecies are either omnivorous or phytophagous depending onspecies; slight variations occur also with regions, an omni­vorous species feeding ' more on zooplankton when phyto­plankton is scarce.

Did you find any relationship between the occurrence ofAlepisaurus spp. and of tuna in a given area due to competi­tion for food?

The number of AZepisaurus and tunas caught is not sufficientto carry out a comparative study of the geographic distribu­tion of the species. But Alepisaurus concerns tunas in twoopposite ways: 1. It is undoubtedly a competitor of tunas, asstomach contents of the two fish are very similar (In ciden ­tally, stomach contents of AZepisaurus are always in a per­fect condition. I don 't know why, whether di gestion ispostponed, or alternatively occurs in another part of thedigestive tract). 2. Juvenile AZepisaurus are a common foodfor tunas.

What is the ' depth of the Deep Scattering Layer in the areaof investigation?

The DSL is somewhat diffuse during daytime; when tracesare pr esent, I should say it lies around 400 m depth. By night,the DSL is usually conspicuous between 50 and 150 ID depth.

Was any shift in the food chain noticed between the in shoreareas and the op en sea?

All our investigations have been done in hi gh sea. Coastalregions are much more difficult to st udy because a very grea tnumber of larval forms from cor al reef organisms interactwith pelagic animals which feed on them. Thus, I cannot drawany comparison between food chains occurring in high seasand coastal waters.

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Have you' taken any: tuna below the 350-450 isobath area andif so, what do they feed on?

We have caught few albacore ( T . alalunga) and yellowfin.( T. atoacaresi below that depth, therefore data on feeding ofthese deep water individuals are not reliable. A third species.

. of tuna namely bigeye tuna (T. obesus) lives deeper than theother two species and does feed on deep and vertically mi­grating animals. Thus, it does not ' fit very well with thesuggested diagram but it is too scarce in the investigated areato give a reliable description of its position in the food web.

Roger Claude. (1977).

The use of a group of macroplanktonic

organisms (Euphausiid crustaceans) in the

study of warm water pelagic food webs.

In : Proceedings of the symposium on warm

water zooplankton. Goa : National Institute of

Oceanography, 309-318.

(Special Publication of NIO). Symposium on

Warm Water Zooplankton, Goa (IND),

1976/10/14-19.