Van Someren,Vernon D.

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A NOTE ON THE FOOD OF YOU~G BLUE GILL SUNFISH, LEPOMIS
MACROCHIRUS, IN KENYA COLONY.
by VernonD. vanSomeren,M.B.E., Ph.D.
INTRODUCTION.
The AmericanBlue Gill Sunfish,Lepomis macrochirus(Centrarchidre),was
firstintroducedintoKenyaColonyin 1940by theGameDepartment.Theoriginal
stockwasbroughtupfromSouthAfrica,to whichcountrythey had beenimported
previouslyfromAmerica,andtheintroductionwasundertakenin orderto provide
a sportingandediblefishsuitablefor cultivationin damsin Kenya. TheseBlue
Gillshavesincebredwell,andarebeingdistributedto variouspartof theColony.
Like theirnearrelativesamongthe Percidre,BlueGillsin theirnativecountry
of North Americaareknownto be mainlycarnivorous,but their foodin Kenya
Colonyhashithertonotbeendetermined.Examinationsof thetypesof foodeaten
by any introducedspeciesof fisharehowever,valuablein so far as theyprovide
an indicationof wheresuchnon-indigenousfishwill taketheirplacein theecology
of theotherindigenousanimalspresentin thewaters.
In severalinstances,theseBlueGillshavebeenplacedin damsalreadystocked
with theindigenousAthi River" Carp,"TilaPia nigra, whichin thepasthasbeen
the speciesmostusedfor stockingdamsin Kenya. The questionof a possible
foodcompetitionthusarises. Stomachexaminationsof T. nigra froma fewwaters
haveshownthatthisspecies,likemanyof theothercichlidfishwidelydistributed
in East and CentralAfrica, is almostexclusivelyvegetarian;their staplefoods
arealgal,thoughat certainseasonstheymay be caughtreadilyon rod and line
baitedwith a worm,or on troutflies.
In December,1944,Mr. H. Copleyof the Kenya GameDepartment,kindly
gaveme12fingerlingBlueGills (3-4inchesin length)with whichto stocka small
damin thegroundsof theVeterinaryLaboratoryat Kabete. Thesewereliberated
in this damon 12/12/44.
Duringdryseasonsthisdamisnotmorethanhalf-an-acrein extent,thedeepest
portionbeingabout3f feet. The waterhas becomeprogressivelymoreturbid
duringthelastfewyears,becausethedamis usedforwateringstock. At thetime
of writing(22/1/46)it is impossibleto seethroughthewaterfor morethananinch
or so. The bottomis composedof a thickchocolate-colouredmudwhichis very
fluid in places. Aquaticvegetationis very scanty; therearetwo smallbedsof
floatingNymPhaea,andthe westernedgeis fringedwitha smallbedof emergent
TyPha and Cyperaceaesp. reeds.
The aquaticfauna is likewisescanty. There is a sparseplanktonfauna
of CladocerandCopepoda,withoccasionalHydracarina. Amongaquaticinsects,
onlyDytisC1$Sandchironomidlarvreareat all abundant,thoughaquaticHemiptera
such as notonectidsare also present,togetherwith a few surface-dwelling
Hydrometridre.In shelteredspotsamongthereeds,wherethereis no disturbance
by cattle,thereis a fairlyrichmicrofloraandmicrofaunaof protozoa,rotifersand
algre. TheClawedFrogXenopusis seasonallyabundant.
In January, 1938,the damwasstockedwith a few TilaPia nigra whichhave
sincemultipliedgreatly. The averagesizeof theseis about2-3ozs.only,though
the biggestreachi-lb. A shorttimeprior to introducingthe Blue Gills, about
200 TilaPia weretrappedout to destockthe damandimprovethe growthrate.
The stomachsof someof theseTilaPia wereexaminedandall containedvegetable
matteronly,mainlyeuglenids.
64
June 1946 VANSOMEREN,A Noteon theFood of Young Blue Gill Sunfish 65
In January, 1946,it becameapparent that the dam might dry up completely
beforethe long rains set in. Orthodox fishing for Blue Gills had proved fruitless,
and it was decidedto net the pond and removewhateverheadof fish might still be
presentto damselsewhere,to preventlosingthe wholelot during the droughtperiod.
Accordingly, on 13/1/46,the pond was netted by four hauls with a 100yard,
1 inch meshseine. This resultedin the capture of about 80 lbs. of TilaPia of all
sizes,and 54 young Blue Gills about 3-4 inches long.
Of theseBlue Gills, 24 died within twenty-four hours of capture before they
could be transferredelsewhere. After slitting the abdominalwall, thesedead fish
werepreservedin 5% formalin, and at a later datethey weredissected,the stomachs
beingcut out for examinationof the food contents.
The stomach contents were washed on to a slide, sorted under a low-power
binocular microscopeand assessedaccordingto Frost's (1943)modification of the
methodusedby Swynnertonand Worthington (1940). In this method, points are
allocatedaccordingto the frequencyand bulk of the various food organismspresent
in the stomach,having regardto the fullnessof the gut. It is in effectan assessment
of bulk, andhasbeenfoundin nearly300stomachexaminationsfrom Kenya Rainbow
Trout to give a very satisfactory estimate on a percentagebasis (van Someren,
unpublishedresults).
In thiitstudy no attempt has beenmadeto give other than a very generalised
classificationof the food present. Such a classificationis sufficiently informative
for all practicalpurposesand the food material,which hasbeenpreserved,will allow
of a more detailedstudy of specieslater when more is known about the freshwater
invertebratesof East Africa.
RESULTS.
34.0
25.3
24.7
9.6
4.0
1.6
0.8..
43
32
31
12
5
2
1
Cladocera
Copepoda
HemiPtera(sub-aquatic)
Ostracoda
Chironomid larvre
Hydracarina
Nematoda(free-living)
Table I summarisesthe resultsobtained,and it will be seenthat Blue Gills of
this age (about 6-9 months) are exclusivelycarnivorous; no vegetablematter was
found in any of the stomachs.
TABLE I. Food of 24young Blue Gill Sunfishat Kabete, 13/1/46.
Food animal. No. of points. % proportions. % ofstomachsin
whichoccurring.
87.5
75.0
54.0
46.0
8.3
8.3
4.2
Total 126 100.0
Average size of fish 8.1 cms.
Range of size of fish 6.1-9.6 cms.
The main food of this age-classin the Kabete dam at this time of the year is
cladoceranandcopepodanimals(waterfleas),thesetwo typesbeingfoundin greatest
bulk in the greatestpercentageof the stomachsexamined. Cladoceraare more
commonlyeatenthan copepods,perhapsbecausethey may be morecommonin the
water; theseCladoceraareall of theonespecies,characterisedby a slightlysculptured
carapacewith a very long posterior spine. The Copepodaareof at least two free-
swimmingspecies.
66 VAN SOMEREN, A NoteontheFoodof YoungBlueGillSunfish VOL. XIX
Aquatic Hemipteraof varioustypes (corixid,notonectid,and micronectid
'.'waterboatmen") area closesecondto copepodsas an articleof diet,but are
grazeduponby fewerfish. Blue Gills feedinguponthesehemipteransarethose
mainlyofthelongerlength~groups;thismightbeexpectedbecausesuchhemipterans
arelargeranimalsthanwaterfleas.
Small ostracods(anothertypeof waterflea),comefourthonthelist, but are
muchmorescantyin the stomachsthan the first threetypesof food animals.
Chironomid(midge)larvae,andhydracarinewatermitesarepresentin twostMllachs
only in smallnumbers,and onestomachcontainsa small free-livingnematode
worm..
The interestingfeatureof thes~resultsis thatmostof thefoodtakenconsists
ofmidwaterandplanktonicanimals. Nostomachscontainedanyaerialorterrestrial
foodanimals,andbottom-dwellerssuchasmidgelarvae,nematodesandto a large
extenttheostracods,aretakenin verysmallquantitiesonly. Thewatermitesmay
beplanktonic,butarescantyin thepond. Midgelarvaearehoweververynumerous
in thebottommudandit wouldappearthatthefeedinghabitsof BlueGillsof this
ageareselective;thefishchoosemainlythemidwaterfoodavailable,andarenot
II bottom-grubbers"to anyextentasaretheirrelativesamongthe Cyprinidreand
Cichlidre.
It is evidentalsothatsuchBlueGillsarenot in anywaydirectlycompetitive
for foodwiththeTilaPia in sucha dam,andthereis no doubtthatthetwospecies
shouldthrivetogetherin thesamewater,aslongasphysicalovercrowdingdoesnot
occur.
Moreinformationis, however,still requiredon thefoodof adultBlue Gills in
thisColonybeforeit canbeshownthattheyarenot,forexample,directlypredatory
uponTilaPia fry. That theyarecarnivorousfish,shouldalsobea warningthat if
BlueGillsareaccidentallyor deliberatelyintroducedintoanyof theriversstocked
withtroutin thisColony,theymaycauseanupsetin thepresentfoodsupplyin these
riversif they areableto establishthemselves.Every precautionshouldbetaken
to ensurethatthisdoesnothappenexceptasa controllablexperiment.
REFERENCES.
Frost,W. E., 1943.TheNaturalHistoryof theMinnow,Phoxinusphoxinus.J. Anim.Eeol.,
12(2),139-162.
SWynnerton,G. H. andWorthington,E. B.; 1940. NoteontheFoodof Fish in Haweswater
(Westmorland).J. Anim.Eeol.,9 (2),183-187.
CORRIGENDUM.
Vol. 18,Nos. 1 and2, p. 84,Line 17: "Two mole-rats,Otomys... "should
read" Two Groove-toothedRats, Otomys.... " Line 19: "A Groove-toothed
Rat, Tachyoryctesrex" shouldread" A mole-rat,Tachyoryctesrex."


A Note on the food of young bluegill Sunfish, Lepomis marochirus, in Kenya Colony

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A Note on the food of young bluegill Sunfish, Lepomis marochirus, in Kenya Colony

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