Food of Cobia, Rachycentron canadum, from the Northcentral Gulf of Mexico

7 Pages·1996·4.8 MB·English

by  Gabriele H Meyer

GulfRescaichReports,Vol.9.No.3,161-167,1996 ManuscriptreceivedAugust23.1995;acceptedSeptember18,1S>95
FOOD OF COBIA,RACHYCENTRONCANADUM,
FROM THE NORTHCENTRAL GULF OF MEXICO
GabrieleH.Meyer*andJamesS.Franks^
^UniversityofSouthernMississippi.DepartmentofBiologicalSciences,
Box5018,Hattiesburg,MS39406-5018,USA
^GuifCoa^ResearchLaboratory,P.O.Box7000,Oceanbrings,Mississippi39566-7000,USA
AB^RACT Thestomachcontentsof403cobLa,Rachycentroncanadum,caughtinthenorthcentralGulfof
Mexicorecreational fishery from April throughOctoberof1987-1990wereexamined. Cobiaranged
from 373-1,530mminforklength. Ofthe403stomachs,287(71.2%)containedatleastoneidentifiableprey
taxon. Crustaceans,consisting{uimarilyofpoTtunidcrabs,werethepredominantfood. Crustaceansoccurredin
79.1%ofthestomachsandcomprised77.6%ofthetotalnumberofidentifiableprey. Thesecondmostimportant
preycategorywasfishwhichwasdominatedbyhardheadcatfish,Ariasfelis,andeels. Fishoccurredin58.5%
ofthestomachsbutonlyaccountedfor20.3%ofthetotalnumberofprey.Theimportanceoffishaspreyincreased
withincreasingsize(length)ofcobia,withthelargestsizeclassofcobia(1,150-1,530mmFL)showingthehighest
percentfrequencyoccurrenceoffishprey(84.4%).Therewerenosignificantdifferencesbetweenthedietsofmale
andfemalecobia. Speciescompositionofthedietindicatedthatcobiaexaminedinthisstudyweregeneralist
carnivoresintheirfeedinghabitsandfedprimarilyonbenthic/epibenthiccrustaceansandfishes. However,the
occurrenceofpelagicpreyprovidedevidenceofdiversityintheforagingbehaviorofcobia. Feedingincobia
indicatedtheirdependenceuponpreyavailabilityratherthanuponafewspecificfoodorganisms.
Introduction Theseresearchersfoundthatcrustaceansandfishmadeupthe
dietofR.canadum,althoughtheirconclusionsvariedonthe
Rachycentron canadum,commonlyknownascobia relativeimportanceofeachpreytype.
orling,isawidelydistributed,pelagicfishwhichoccurs Knowledgeofthefoodhabitsofcobiaisnecessaryfor
worldwideintropical,subtropical,andwarm temperate understandingtheroleofdietintheirgrowthandsurvival
seas, except in the central and eastern Pacific Ocean andforcomprehendingthedynamicsofthefishery. The
(ShafferandNakamura1989),InthewesternAtlantic,the purposeofthisstudywastodescribethedietofcobiafrom
cobia occurs from Massachusetts to Argentina (Briggs thenorthcentralGulfofMexico.
1958),butismostcommonin!heGulfofMexico(Migdalski
and Fichter 1983), where it supports an important MaterialsandMethods
recreationalfishery. IntheGulfofMexico(Gulf)cobia
rangefromKeyWest,FloridaalongthecoasttoCampeche,
Mexico(Dawson 1971). Cobiatypicallymigrateduring Cobiaexaminedinthisstudywerecaughtbyhook-and-line
spring and summer from their wintering grounds off inthenorthcentralGulfrecreationalfisheryfromAprilihrou^
southern Florida to spawning/feeding grounds in the Octoberof1987-1990.CobiaweretakenoffsoulheastLouisiana,
northernGulfandreturntotheirwinteringgroundsinlate Mississippi,Alabama,andnorthwestFloridabetween
fallandearlywinter(BiesiotetaL1994,Franksetal,1991). lat. 30^.0'-29’^.(yN and long. 86^-89^0.0^, The
The diet ofR. canadum from the GulfofMexico, majorityofspecimensweretakenoffcoastalMississii^i.Some
particularlythenorthernGulf,ispoorlyknown. Mostof fishwereprovidedbystateandfederalfisheriesagencies.
thepreviousresearchonthefeedinghabitsofcobiawas Fish were well-iced from the time of capture until
limitedtosimpledescriptionsofpreyitemsfoundinafew Stomachswereremovedatfishingdocksorcoastalfishing
mm
stomachs. Miles(1949)reportedthestomachcontentsof tournaments. Forklength(FL)wasmeasuredin and
11 cobia from Aransas Bay, Texas, and Knapp (1949, the sex was recorded. Most stomachs were placed in
1951)notedthepreyfoundin24cobiatakenfromthesame sealableplasticbagsandstoredinaniceslurryforshort-
area. Reid(1954),Boschung(1957),andChristmasetal, term storage, usually 4-6h. Stomachs were then either
(1974)commentedonfeedinginasmallnumberofcobia frozen or placed in 10% buffered formalin for later
fromCedarKey,Florida(onefish),coastalAlabama(four examination.Occasioiudly,whentimepermitted,stomachs
fish)andoffshoreMississippi(elevenfish),respectively. wereremovedfromfish,opened,andprocessedinthefield.
161
MeyerANDFranks
Stomachs were thawed or removed &om fornialin, compared. Acontingencytableanalysisandpost-hoctest
opened, and scored as either containing foodorempty. (Freeman-Tukeytransformation)forproportionaldatawere
Stomachcontentsweregentlyrinsedwithfreshwaterinto usedtodeterminesignificantdifferences(«=fi.05)between
a0.5mmmeshsieve.I^yitemswereseparated,identified classesforeachmajorpreycategory(Zar 1984).
to the lowest possible taxon, and counted. Accurate Major prey of male and female cobia were also
identificationandcountscouldbemadeinmostcasessince compared. Sincemalestendedtobesmallerthanfemales,
foodsweregenerallyswallowedwhole. Somepreyitems onlycobiawithinthesizerange590-1,045mmFLwere
were in advanced stages ofdigestion and could not be selected. TTiisrangecontainedmostofthemalessampled
identified to species; however, those prey were often and reduced the confounding effect of size. Tests for
identifiabletothefamilyororderlevel. significantdifferences(«=0.05)weremadeusingaFisher
exacttestcorrectedforcontinuity.
Analyses
All analyses were based on stomachs containing at Results
least one identifiable taxon. Prey loo far digested for
identification were not used in any computations. The stomach contents of403R. canadum, ranging
Additionally,someitemsfoundinstomachswereexcluded from373-1,530mmFL,wereexamined.Ofthesestomachs,
becausetheywereprobaWyingestedincidentaUy.Examples 287(71.2%)containedatleastoneidentifiablepreytaxon.
oftheseweretubesofChaetopteruswonns,fragmentsof Prey consisted of crustaceans, fishes, and cephalopods
bivalveandgastropodshells,Sarga&sumweed,andpieces (Table 1). Another 35 (8.7%) stomachs containedonly
of coral, wood, and leather. Parasitic nematodes and badlydecomposed,unidentifiableremains.Theremaining
acanthocephalonswhichoccurredinsomeofthestomachs 81 stomachs(20.1%)wereempty.
werealsonotconsideredinthedietanalyses.
Numeric abundance, frequency of occurrence and Invertebrates
percentfrequencyofoccurrence(%F)weretabulatedforall
identifiable prey. In addition, major prey categories Crustaceans were the iMiraary food of cobia and,
(crustaceans, fish, and cephalopods) were analyzed for essentially,dominatedthediet. Crustaceansoccurredin
percentnumericabundance(%N)andpercentfrequencyof 79.1%ofthestomachsandrankedfirst(77.6%)innumeric
occurrence. importance among prey (Figure 1). Crustaceans were
Threedifferentforklengthsizeclassesofcobia,small representedbyeightfamiliesofdecapodsandtwofamilies
(373-945 mm), medium (950-1,145 mm), and large ofstomatopods(Table1).
(1,150-1,530mm), wereselectedbased onnaturalbreaks Portunid crabs werenotonly the predominant taxa
within the size frequency distribution, and the percent among invertebrates consumed (Table 1) but also
firequency ofoccurrence ofmajorprey within each was represented 60.7%N oftotal food items in the diet and
occurredin72.8%ofthestomachs. Thelesserbluecrab,
Callinectessimilii, was themostabundantprey species
foundin the diet, comprising 36.5%N andoccurring in
48.8% ofthe stomachs. The iridescentswimmingcrab,
Portunusgibbesih (12.5%N,26.5%F) and the ladycrab,
Ovalipesfloridanus,(9.0%N,23.3%F)werethenextmost
importantfoodsinthediet.
Following the portunids in importance were the
sicyoniids and penaeids (combmed=9.6%N). Other
decqxxls, ).e.,calllanassids. calappids,majids,pagurids
andxanthids,occunedinfiequeittly(Table1).Stomatopods,
predominantlySquUlidae,comprised6.9%Nofthediet.
Cephalopodscomprisedtheotherprimaryinvertebrate
CfU3taco«na Fish Cephatopods prey group and were represented by two families,
Figure 1, Percentnumericabundance(%N)andpercent LoUginidae, the predominant group, and Octopodidae.
frequencyofoccurrence(%F)ofmajorpreycategoriesof Cephalopodswerefoundin13.2%ofthestomachsbutonly
RachycenirancanadumtromthenorthcentralGulfofMexico. madeup2.2%Nofpreyconsumed(Figure 1).
162
FoodofCobiafromNorthcentralGulfofMexico
TABLE
1
Preyitemsoccurringinstomachsofcobia,Rachycentroncanadum^ffomthenorthcentralGulfofMexico,1987-90.
PercentfrequencyofoccurrencebasedonN=2S7,
Totalnumb^
Frequencyof Percentfrequency
Prey ofindividual
occurrence ofoccurrence
preyitems
INVERTEBRATES
Crustaceans
Decapoda
Pcnaeidae
Penaeusaztecus 3 1 0.3
Penaeussetiferus 1 1 0.3
Penaeussp- 34 8 2.8
Trachypenaeussp. 37 9 3.1
Sicyoniidae
Sicyoniabrevirostris 62 15 5.2
Sicyoniasp. 102 18 6.3
Callianassidae
Callichirusislagrande 1 1 0.3
Paguridaesp. 2 2 0.7
Calappidae
Calappaflammea 2 1 0.3
Hepatusepheliticus 2 2 0.7
Majidae
Libiniaemarginata 1 1 0.3
Portunidac
Arenaeuscribrarius 16 8 2.8
Callinectessapidus 5 5 1.7
Callinectessimilis 909 140 48.8
Ovalipesfloridanus 224 67 23.3
Portunusgibbesii 312 76 26.5
Portunussayi 1 1 0.3
Portunusspinicarpus 16 3 1.0
Portunusspinimanus 30 17 5.9
Xanthidae
Menippeadina 1 1 0.3
Stomatopoda
Lysiosquillidae
Lysiosquillascabricauda 2 2 0.7
Squillidae
Squillachydaea 2 2 0.7
Squillaempusa 78 21 7.3
Squitlaneglecta 1 1 0.3
Squillasp. 88 40 13.9
Cephalopods
Loliginidae
Lotigopealei 1 1 0.3
Unid.loUginids 47 33 11.5
Octopodidae
Octopussp. 6 4 1.4
FISH
Squatinidae
Squatinadumeril 1 1 0.3
Dasyaddae
Dasyatissp. 7 7 2.4
Torpedinidae
t^arcinebrasiliensis 4 3 1.0
Anguillifonnes 133 52 18.1
163
MeyerANDFranks
Totalnumber
Fi^equencyof Percentfrequency
Prey ofmdividual
occurrence ofoccurrence
preyitems
Clupeidae
Brevoortiapatronus 19 3 1.0
Brevoortiasp. 2 2 0.7
Unid.clupeids 4 4 1.4
EngrauUdae
Anchoasp. 2 1 0.3
Unid.engraulid 1 1 0.3
Axiidae
Ariasfelis 138 70 24.4
Ophidiidae 5 4 1.4
Ogcocephalidae
Halieuiichthysaculeatus 1 1 0.3
Syngnatliidae 2 2 0.7
Triglidae
Prionotussp. 48 7 2.4
Serranidae
Diplectrumbivitatum 33 2 0.7
Unid.serranids 2 1 0.3
Carangidae
Decapteruspunctatus 26 18 6.3
Serioladumerili 1 1 0.3
Unid.carangid 1 1 0.3
Lutjanidae
Latjanuscampechanus 3 3 1.0
Sparidae
Lagodonrhomboides 10 10 3.5
Unid.sparid 1 1 0.3
Sciaenidae
Menticirrhussp. 3 3 1.0
Micropogoniasundulatus 9 3 1.0
Cynoscionsp. 1 1 0.3
Lehstomusxamhurus 1 1 0.3
Mugilidae
Mugilsp. 5 3 1.0
Uranoscopidae
Astroscopusy-graecum 5 5 1.7
Trichiuridae
Trichiuruslepturus 3 1 0.3
Stromateidae
Peprilusburti 1 1 0.3
Peprilussp. 3 1 0.3
Bothidae
Citharichthyssp. 12 3 1.0
Etropuscrossotus 1 1 0.3
Etropussp. 2 1 0.3
Soleidae
Symphurusplagiusa 1 1 0.3
Symphurussp. 1 1 0.3
Balistidae
Batistescapriscus 1 1 0.3
Unid.balistids 4 3 1.0
Tetiaodontidae
Chilomycterusschoepfi 2 2 0.7
Unid.tctraodontids 6 3 1.0
Total 2,491
Numberofstomachsexamined 403
Number(and%)ofstomachscontainingidentifiableprey 287 (71.2)
Number(and%)ofstomachscontainingonlydecomposed,unidentifiableremains 35 (8.7)
Number(and%)ofemptystomachs 81 (20.1)
164
FoodofCobiafromNorthcentralGulfofMexico
Fish Incontrast,theimportanceoffishaspreyincreased
withincreasingsizeofcobia.thelargestsizeclassshowing
AlthoughcontributingsubstantiaUytothediversityof
thehighestpercentfrequencyofoccurrence (84.4%)
thediet,fishwerenotasimportantascrustaceans. Fish
occurred in 58.5% of the stomachs and accounted for (Figure2). Theincreaseinfishoccuirencewasattributable
20.3%Nofallpreyconsumed(Figure1). Awidevariety tothehard-headcatfish,Ariusfelis,whichincreasedfrom
offisheswasconsumed,includingtwentyfamiliesofbony 7.0%Finthesmallsizeclassto43.8%Finthelargecobia
fishesandthreefamiliesofcartilaginousfishes(Table1). (Table2). Again,con^gency table analysis(x^27.77,
The hardhead catfish. Ariusfelis, and eels (Order df=2,/kO.001)andpost-hoctests indicated that all size
Anguilliformes)werebyfarthepredominantfishesinthe classesweresignificantlydifferentfromeachother.
diet.Ariusfetis^foundin24.4%ofstomachs,exhibitedthe Thepercentageofcephalopods(predominantlysquid)
highest numeric percentage (273%) among fish and remained consistently low across the three size classes
contributed5.5%NtothetotaldietEelsoccurredin18,1% (Figure2,Table2). Nosignificantdifferenceswerefound.
ofstomachs, comprised 26.3%N offish inthe diet,and
accountedfor5.3%Noftotalitemsinthediet. Comparisonofthedietsofmaleandfemalecobia
Fishlessfrequentlyencounteredinthedietincluded
roundscad,£>ecupre/‘U5puncraruj(Carangidae)andpmfish, Ihedietofmaleandfemalecobiawithinthesizerangeof
Lagodon rhomboides (Sparidae). Other identified fish 59()-l,045inmFLappearedtobesimilar(Table3).Crustaceans
occurredonlyrarely(Table 1). were the dominantprey in both sexes. Althoughfemales
showedahigherpercentfrequencyofoccurrence(86.8%)of
Comparisonofdietamongsizeclassesofcobia crustaceansihandidmales(79.2%),thesedifferenceswerenot
Crustaceansdominatedthedietofthesmall(77.2%F) significant, Portunid crabs were themajor componentof
andmedium(84.8%F)sizeclassesofcobia,andmadeup crustaceansingestedbybothsexes.
aprimaryportion(65.6%F)ofthelargesizeclass(Figure2). Fish occurred with greater frequency in the dietof
Despitethesehighfrequencies,contingencytableanalysis males (60.4%F) than in the diet of females (46.2%F),
(x^10.25,df=2,p<0.05)andthecorrespondingpost-hoc partiallyduetoagreateroccurrenceofeelsinthemalediet
tests indicated all three size classes were significantly (Table3). Males,however,fedlessfrequentlyoncatfish.
different from each other. Portunid crabs, particularly Aswiththecrustaceanprey,nosignificantdifferenceswere
CalUnectessiwi/is,werethemostimportantpreyconsumed found betweenthediets ofmale and female cobia with
inallsizeclassesofcobia(Table2). respecttofishorcephalopodprey.
TABLE! TABLES
Percentfrequencyofoccurrenceofmajortaxainthestomachs IN^rcentft^uencyofoccurrenceofm^jortaxafromthestomachs
of three size classes ofRachycentron canadum from the ofmaleandfrmaleRorhycenl^oncanadumfromthenorthcentral
northcentralGulfofMexico. GulfofMexico. Sizerangefrom590-1045mmFL.
Male Female
Forklength(mm) 373-945 950-1145 1150-1530
iV=48 N=106
N==57 N=164 N=64 Crustaceans
Crustaceans (AllCrustaceans) (79.2) (86.8)
(Portunidcrabs) (63.2) (80.5) (64.1) (Portunidcrabs) (70.8) (80.2)
CalUnectessimilis 35.1 53.0 51.6 CalUnectessimilis 33.3 52.8
Portunusgibbesii 17.5 31.1 23.4 Portunusgibbesii 20.8 32.1
Ovalipesfloridanus 16.7 30.2
Ovalipesfloridanus 19,3 28.0 15.6
Stomatopods 14.6 19.8
Stomatopods 24.6 19.5 25.0
Fish
Fish (AllFish) (60.4) (46.2)
Anguilliformes 14.0 19.5 18.8 Anguilliformes 27.1 17.9
Ariusfelis 7.0 22.0 43.8 Ariusfelis 6.3 17.0
Cephalopods Cephalopods
Loliginidae 17.5 9.1 14.1 Loliginidae 10.4 14.2
165
,
MeyerANDFranks
Discussion
Wefoundcrustaceans,primarilyportunidcrabs,tobe
the dominant foods ofcobiaboth in terms ofnumeric
abundanceandpercentfrequencyofoccurrence. Fishes
weresecondinorderofimportance. Theseresultsvary
somewhatfromthefindingsofotherresearchers. Miles
(1949)reportedcrabs,shrimps,andfishesinnearequal
numbersinthestomachsofcobiatakenfromAransasBay
Texas,and,similarly,diristmasetal,(1974)foundthe
numbersoffishesandcrustaceanstobeapproximatelythe
same in their samples from northern Gulf waters off
Mississippi. Insharpcontrast,Knapp(1951)observeda Forkler\gth(mm)
predominanceoffishes(83.3%F),followedbystomatopods
(58%F),penaeidshrimps(46%F)andcrabs(42%F)inthe Figure 2. Percent frequency ofoccurrence ofmajor prey
categories for three size classes ofRachycentron canadum
diet of cobia caught near Aransas Bay. Texas. The fromthenorthcentralGulfofMexico.
conclusions reached in previous studies werebasedon
examinationsofalimitednumber(24orless)ofstomachs.
Althoughcobiaexaminedinourstudywerecollectedby epibenthic crustaceans and fishes, although some
hook-and-lineand,therefore,didnotrepresentarandom feedingdidoccurinthewatercolumnandnearsurface.
sample,webelieveourfindingsrepresentamorcdefinitive Additionally,ourresultsindicate thatthecobiaisan
descriptionofthedietofcobiainthenorthernGulfof opportunistic carnivore and that feeding appears to
Mexico,due,inpart,toourhighsamplenumber(7V=287) depend more on prey availability rather than upon a
andextensivegeographicalrange. few specific food organisms.
Althoughcrustaceans were thedominant food, our
resultsalsoindicatedthatlargercobia,malesandfemales
Acknowledgments
alike,consumedfishwithsignificantlygreaterfrequency
thandidsmallercobia.Thismayreflectanontogenetic
shifttoward fish as prey in largercobia. Our results, WethankTomMcllwainforhisadvocacyofthiswork.
however,showednosignificantdifferencesinthedietof We would also like to thank coastal sportfishermen
maleandfemalecobiawithinarangeofcomparablesizes and the directors of coastal fishing tournaments in
whichmaybeattributabletotherelativelylowsamplesize MississippiandFloridafortheirspiritofcooperation
of males. Although not statistically different, we did in helping us obtain samples. We are grateful to
encounterfishmorefrequentlyinthestomachsofmales RichardHeardandBruceComynsfortheirhelpinidentifying
than females which also may be an indication of an the invertebrates and the fishes, respectively. The field
ontogeneticshifttowardfishpreysincemostofthelarge assistance providedby Carol Cleveland, Mike Buchanan
males, and not the large females, were includedin the andT.J.Beckerwasgreatlyappreciated.Wegiveourthanks
male>femalecomparativeanalysis. toTerryMcBeeforhishelpinthefieldandforhisvaluable
The species composition of the diet revealed that assistancewithdataanalysis. Ourgratitudeisextendedto
cobiafedprimarilyonorneartheseafloor.Theportunids, BarbaraVLskupandDavidAbomforreviewinganearlier
sicyoniids,penaeids,andstomatopods,thoughcapableof versionofthemanuscript.JefleryLotzalsoprovidedhelpful
swimming,areprimarilybenthicorcpibenthicinhabitants. suggestions which improved the manuscript. The
Octopi,as wellas many ofthe fishprey (e.g., bothids, constructivecommentsoftwoanonymousreviewerswere
uranoscopids,airids,triglids,dasyatids,eels),alsoreside alsoappreciated. Wethankmarineenforcementpersonnel
on or near the bottom. However, other prey such as oftheMississippiDept, ofWihllife,FisheriesandParksfor
carangids,clupeids,andsquidarepelagicorganisms,and providinguswithsomespecimensofcobia.Fundingforthis
their presence in the diet indicated flexibility in the studywasprovidedbytheU.S,Dept,oftheInterior,U.S.
foragingbehaviorofcobia. Fish and Wildlife Service, Federal Aid in Sportfish
Insummary,wefoundthattheprimaryfoodsofcobia RestorationProjectF-91, throughtheMississippi Dept,of
from the northcentral Gulfof Mexico were benthic or MarineResources.
166
FoodofCobiafromNorthcentralGulfofMexico
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