Cetacean Species Richness and Relative Abundance Around the Bar Reef Marine Sanctuary, Sri Lanka

2008·2.7 MB·

by  A D Ilangakoon

Journal ofthe Bombay Natural History Society, 105 (3), Sep-Dec 2008 274-278
CETACEAN SPECIES RICHNESS AND RELATIVEABUNDANCE AROUND
THE BAR REEF MARINE SANCTUARY, SRI LANKA
A.D. Ilangakoon1
'MemberCetaceanSpecialistGroupofIUCNSpeciesSurvivalCommission,215GrandburgPlace,Maharagama,Sri Lanka.
Email: [email protected]
TheBarReefMarineSanctuary,offnorth-westernSriLanka,anditsadjacentwatersweresuspectedtobeanimportant
Cetacean habitat, but a dedicated survey had never been undertaken. Therefore, a one-year survey was carried out
fromApril 2004toMay 2005 tofill the vacuum ofknowledgeonCetaceanspeciesofthearea, andtogatherbaseline
data for management and conservation. Surveys were carried out twice every month. Thirty three sightings ofeight
cetacean species were documented. The species recordedwere Balaenoptera acutorostrata, Balaenoptera musculus,
Physeter macrocephalus Kogia sima, Peponocephala electra, Stenellci longirostris, Tursiops truncatus and Sousa
,
chinensis.ThefirstconfirmedsightingofS. chinensisinSri Lanka’swaterswasrecordedwhileK.simaandP.electra
were sighted off the west coast for the first time. The northern and central parts of the Sanctuary can be termed as
’cetaceanhotspots’duetohighspeciesrichnessandyear-roundabundance. Baselinedatafromthissurveycanbeused
immediatelyformanagementpurpose,thoughfurtherresearchisrecommended.S.chinensisneedsspecialconsideration
as the newly discovered population is heavily dependent on the PuttalamLagoon which is underintense human use,
making these dolphins vulnerable to a multiplicity ofanthropogenic threats.
Key words: Cetacean, species richness, relative abundance, Sousa chinensis, Kogia sima Peponocephala electra
, ,
Sri Lanka
INTRODUCTION The present survey was undertaken to fill the
information lacunaoncetaceanfaunainandaroundBRMS.
The Bar Reef Marine Sanctuary (BRMS) is one of The survey was designed with the primary objective of
thefewmarineprotectedareasinthewatersoffthenorthern gatheringdataonthespeciesrichnessandrelativeabundance
Indian Ocean island of Sri Lanka. This Sanctuary of cetaceans occurring in the area. This was deemed
is 306.7 sq. km in area and is located between 8° 16' 00"- necessary inordertomakeinformedmanagementdecisions
8° 32' 00" N and 79° 44' 00"-79° 46’ 70" Eoffthe Kalpitiya based on scientific data, which would lead to a long-term
Peninsula on the north-western coast of Sri Lanka. It was conservationandrational managementofimportantmarine
demarcated under the countries Fauna and Flora Protection fauna.
Ordinancein 1992,andisdivided intoabufferzoneandcore
METHODOLOGY
area, within which varying degrees of human activity take
place (Rajasuriyaetal. 1995).
TheBRMS anditssurroundingwatersweresuspected Vessel-basedcetaceansightingsurveyswereconducted
to be important for cetaceans on the basis of opportunistic twice a month from April 2004 to March 2005. One survey
observationsandsporadicsightingsinthearea(Leatherwood wasdonewithintheSanctuary,usinga25-hpoutboardengine-
et al. 1984; Rajasuriya el al. 1995; Dayaratne et al. 1997; poweredfibreglassvessel andoneoffshore surveywasdone
Ilangakoon 2002). Though some information was available seaward of the Sanctuary boundary using a larger 3.5 ton
for the area, a dedicated cetacean survey had never been motorised fishing boat. A pre-planned saw-tooth patterned
carried out in this marine sanctuary or its immediate transect line was covered each month in order to maximise
surroundings. Therefore, the waters within the BRMS, the the coverage areain the available time andresources.
Puttalam Lagoon immediately adjacent to it, and the deeper Offshore surveys to look for cetaceans beyond the
waters immediately seaward ofit were selected asthe study seaward boundary of BRMS were conducted during all
area for the present survey. The water depth within the months except in June 2004 when the survey had to be
PuttalamLagoonvariesfrom 1 to5m,whilethewaterswithin abandoned half way through due to adverse weather
the BRMS straddle the 20 m contour towards its western conditions.The Sanctuary survey was notconducted during
boundary. Beyond the Sanctuary’s western boundary, the May, June and July as the weather was very rough. In all,
depth increases rapidly, with the 20 m and 100 m contours 21 days were spent at sea actively searching for cetaceans
located in close proximity toeach other. during the one-yearsurvey period.
CETACEAN SPECIES RICHNESSAND RELATIVE ABUNDANCEAROUND BAR REEF MARINE SANCTUARY
35.0% The data on cetaceans encountered wasrecordedon a
30.0% standard data sheet with particular attention to position at
25.0% sighting,speciesidentification,groupsize,presenceofcalves
3OS) 20.0% and general behaviour. Other parameters such as
a
S 15.0% environmental conditions and associated fauna were also
a>
a. 10.0% recorded. Positions were recorded using a hand-held
5.0% GARMINe-trexglobalpositioningsystem;photographswere
00% taken once the cetaceans were approached.
.
^
AT #vp \«Y A')5’ A* RESULTS
^ M2> / * /j* y / y
r 0oT The survey yielded a total of 33 sightings of eight
Species cetaceanspecies.Theeightspeciesofcetaceansincludedtwo
species from Suborder Mysticeti, Family Balaenopteridae,
Fig. 1: Relative abundanceofCetaceans (N=33 sightings) namelyBlueWhaleBalaenopteramusculusandMinkeWhale
Balaenopteraacutorostrata. The othersix speciesbelonged
There were three observers on board to look for to Suborder Odontoceti and included the Sperm Whale
cetaceans. The observers rotated through three positions - Physeter macrocephalus of Family Physeteridae, Dwarf
port observer, bow observer and starboard observer - at Sperm Whale Kogia sima ofFamily Kogiidae, and Melon-
30-minute intervals. The port and starboard observers headed Whale Peponocephala electra Indo-Pacific
,
surveyed the area from the beam of the vessel to Humpback Dolphin Sousa chinensis Long-snouted Spinner
,
approximately 10 degree from the bow, while the bow Dolphin Stenella longirostris and Common Bottlenose
observer surveyed the area directly ahead ofthe vessel in a Dolphin Tursiops truncatusall ofFamily Delphinidae.
20degreecone,andalsoactedasthedatarecorder.Thesurvey CetaceansweresightedinallmonthsexceptJune,when
effortwasmadewhentheseastatewasbelowBeaufort4and the survey was abandoned midway due to bad weather
visibility was good.When the sea state exceededBeaufort4 (Table 1). Species richness was high in February, April,
orheavyrainmade visibilitypoor,theeffortwas suspended. August,andSeptember.Approximately50%ofthesightings
Cetaceansorcetaceangroupsencounteredwere approached were within the BRMS (Fig. 2). Species richness was high
to a distance where species identification and group size within the Sanctuary, with sixofthe eight species sighted at
estimates werepossible.Apairof7x50binoculars was used leastonce.Mostsightingswereclusteredaroundthewestern,
to verify the distant sightings anddetermine species. seaward boundary of the northern and central parts of the
Table 1: NumberofCetacean sightings by month and species in and aroundthe Bar Reef Marine Sanctuary
Month B. mus B. acu P. mac K. sim P. e/e T. tru S. Ion S. chi Total
April - 1 - - - 2 - 3 6
May - - - 1 - - - - 1
June - - - - - - - - 0
July - - - - - - - 1 1
August 1 1 1 - - - - 1 4
September - - 2 - - - 1 2 5
October - - - - - - 2 - 2
November - - - - - 1 3 - 4
December - 1 - - - - 2 - 3
January - - - - - - 1 - 1
February - - - - 1 - 2 2 5
March - - - - - - - 1 1
Note: Complete species names in column headingsofthe aboveTable
B. mus=Balaenoptera musculus, B. acu=Balaenoptera acutorostrata, P mac= Physetermacrocephalus, K. sim=Kogiasima,
P. e/e=Peponocephala electra, T. tru= Tursiops truncatus, S. Ion=Stenella longirostris, S. chi=Sousachinensis
J. Bombay Nat. Hist. Soc., 105 (3), Sep-Dec 2008 275
CETACEAN SPECIES RICHNESSAND RELATIVE ABUNDANCE AROUND BAR REEF MARINE SANCTUARY
BRMS, with no sightings towards the southern boundary.
The most common cetacean encountered was Long-
snouted Spinner Dolphin Stenella longirostris (33.3%)
(Fig. 1). Italsohad the widest distributionandwas recorded
throughoutthecentralpartoftheBRMSandindeeperwaters
seaward of the Sanctuary boundary. This species was also
sighted in very large schools during February, April,
August, and September, with some groups containing over
1,000animals,includingmanyjuveniles.SightingoftheIndo-
Pacific Humpback Dolphin Sousa chinensis (30.0%) was
restricted to the Puttalam Lagoon and the north-western
extremity of the Sanctuary (Fig. 2). These are the first IntheSanctuary E3SeawardofSanctuary InLagoon
scientifically documented sightings of this species in
Fig. 2: Distributionofcetaceanspeciesin relationtosightingarea
Sri Lankan waters.
Minke Whale Balaenoptera acutorostrata Sperm EasternTropical Pacific, the associationwiththeYellow-fin
,
Whale Physter macrocephalus and Common Bottlenose Tuna in northern Indian Ocean has not been extensively
Dolphin Tursiops truncatus accounted for9.0% each ofthe documented (Ilangakoon 2002). While Ilangakoon (2000)
total recorded sightings. Minke Whale sightings were mentionsthatsomesightingsduringa 1994studyonthewest
clustered in a small area within the north-central part ofthe coastindicatedanassociationbetweenthesetwospecies,the
BRMS (Fig. 2). Common Bottlenose Dolphin were sighted present survey shows avery strong association in which the
fromnorthtosouthalongtheseaward(western)boundaryof occurrence of large schools of this dolphin coincides with
the Sanctuary, and Sperm Whale sightings were limited to the Tuna fishing season during September-October. Tuna
deep offshore areas beyond the seaward boundary of the fishermen in the BRMS study area were aware of this and
Sanctuary (Fig. 2). actually looked for dolphins to find the Tuna. Despite
BlueWhale, Dwarf-SpermWhale, and Melon-headed containing manyjuveniles, these dolphin schools interacted
Whale were recorded only once (3.0%) each. While Dwarf- freely with the tuna fishing boats and our survey vessel,
SpermWhaleandMelon-headedWhaleweresightedindeep bow-riding often and showing no fear.
waters,theBlueWhalesightingwaswithintheBRMS where Although the Indo-Pacific Humpback Dolphin
waterdepth ranges from shallow to moderate (Fig. 2). accounted for 30% sighting during the present survey, the
occurrenceofthisspecieshasneverbeforebeendocumented
DISCUSSION in any part ofSri Lanka’s waters. Although the species was
known to be present around the west coast, based on two
Twenty-seven cetacean species have been recorded museum specimens collected in the 1880s and 1934
fromSriLanka'swatersto-date(Ilangakoon2002).Theeight respectively(Blanford 1891 Deraniyagala 1945;Ilangakoon
;
species recordedin this survey account forabout 30% ofall 2002) and a possible unconfirmed sighting from an aerial
the species. It is noteworthy that a survey earned out overa survey fordugongs in the early 1980s around Dutch Bay in
shortperiodofone year, with 21 fielddays,recordedalmost the Puttalam Lagoon (Leatherwood etal. 1984), there have
30%ofall speciesinthe SriLankanwaterswithin thissmall been no confirmed sightings ofthe Indo-Pacific Humpback
survey areaoffthe north-westcoast. Dolphin tillthe presentsurvey. Itispossible thatthe species
The Long-snouted Spinner Dolphin was the most is common only off the north-west coast of Sri Lanka
common species of cetacean in this area. This agrees with including the shallow Puttalam Lagoon, and was never
the other surveys conducted offSri Lanka’s west, south and properly documented due to the lack ofdedicated previous
eastcoastsinthe 1980sand 1990s(Leatherwoodetal. 1984; cetacean surveysinthe area.Thisissubstantiatedbythefact
Ailing 1986; Leatherwood and Reeves 1989; Ilangakoon et thatthe skullcollectedin 1891 wasfromtheGulfofMannar
al. 2000;Ilangakoon2002).However,theverylargeschools offnorth-westSriLankaandtheunconfirmedsightinginthe
sightedduringFebruary,April,August,andSeptember,often 1980s was also in the Puttalam Lagoon.
in association with the Yellow-fin Tuna Thunnusalbacares, TheMinkeWhaleaccountedfor9%ofthesightingsin
during the present survey have not been reported before. the present survey, but this species has not been commonly
While this dolphin species has been documented in Tuna- sighted during the surveys of other parts of Sri Lanka.
Dolphin associations inotherparts ofthe world, such as the Leatherwood etal. (1984) reported a single sighting offthe
276 J. Bombay Nat. Hist. Soc., 105 (3), Sep-Dec 2008
CETACEAN SPECIES RICHNESS AND RELATIVE ABUNDANCE AROUND BAR REEF MARINE SANCTUARY
east coast, and Ilangakoon (2002) reported one sighting off reportedfromthenorth-eastcoast(Leatherwoodetal. 1984).
the west coast and another off the north-west coast near Duringthepresentsurveyboththesespecieswereencountered
Thalawila,justsouthofthe present study area.Accordingly, in deeper offshore waters beyond the BRMS, and sightings
the present results in conjunction with the previous ofboth species wererecordedoffthe westcoastforthe very
observationby Ilangakoon (2002)indicatethatthe watersin firsttime.
and around the BRMS may be an important habitat for this Toconclude,thesurveysucceededinfillingthelacunae
species in Sri Lanka. on information on cetacean species richness and relative
TheSpermWhaleandtheCommonBottlenoseDolphin abundance in this area and added new knowledge on the
also accounted for9% each ofthe total numberofsightings distributionofthecetaceansinSri Lanka’swatersasawhole
recordedinthepresentstudy. Severalstudiesandsurveysoff by documenting one species never sighted previously and
SriLankahavereportedthattheSpermWhalesarecommon two others that had not been sighted off the west coast of
in deeper waters all around the islands and this has been the island. While the entire study area off north-western
particularly well-documentedofftheeastcoast(Ailingetal. Sri Lanka has a rich cetacean diversity, it is suggested that
1982; Whitehead et al. 1983; Leatherwood et al. 1984; the area from the northern boundary of the BRMS to its
Ilangakoon2002).WhileIlangakoon(2002)reportedprevious central sectionbeconsidered a ‘cetaceanhotspot’ duetothe
sightings offthe north-west coast, more recently the Ocean high species richness and numerous sightings made
AllianceresearchvesselR/VOdysseyreportedlargenumbers throughout the year. This should also be taken into
inthedeeperwatersoftheGulfofMannaroffnorth-western consideration while planning future conservation and
Sri Lanka (OceanAlliance 2003). The Common Bottlenose management strategies for the BRMS, and ideally the core
Dolphinwasnotasnumerousorcommon inthepresentstudy areashouldbeextendedinordertoprovidebetterprotection
area as has been reported forother areas offSri Lanka. The to these cetaceans. The role of the Puttalam Lagoon in
specieshasbeenreportedasbeingcommonly sightedoffthe providing an essential habitat to the newly discovered
east coast (Ailing 1986) and off the south and west coasts population of the Indo-Pacific Humpback Dolphin should
(Ilangakoonetal. 2000; Ilangakoon 2002).The presentdata be investigated. Since this population was only discovered
is not sufficient to offer an explanation for the Common throughthepresentsurvey, moredetailedstudiesareneeded
BottlenoseDolphinbeinglesscommon inthestudyarea,but to ensure its long-term survival through management
sightings along the seaward boundary ofthe BRMS, where measures based on sound scientific data. This is urgent and
there is a change in waterdepth, indicate that this may be a extremely important because the Puttalam Lagoon is under
preferred feeding areaforthe species. intensivehuman use posing amultiplicity ofanthropogenic
The Blue Whale, Dwarf-Sperm Whale and Melon- threatstothese dolphins. Finally, it should be notedthat the
headed Whale were not commonly seen but were recorded data from this survey have only provided a baseline upon
onlyonceeachduringthis survey. However, the BlueWhale which further studies could be undertaken on the cetacean
is common around Sri Lanka, especially offthe east, south fauna of the area. While these baseline data can be used
and west coasts (Ilangakoon 2002). It is of interest to note immediatelyformanagementpurposesmoredetailedstudies
that the Blue Whale sighting during the present survey was are recommended as the area appears to provide a habitat
within the BRMS in moderately shallow waters and not in for globally threatened species, such as Blue Whale and
thedeeperwatersbeyondaswouldbeexpected. However,as Sperm Whale.
ACKNOWLEDGEMENTS
statedbyIlangakoon(2002),thespecieshaspreviouslybeen
sighted in near-shore continental shelfwaters in other areas
offthewestcoastofSriLanka. BoththeDwarf-SpermWhale Financial support for this survey was provided by
and the Melon-headed Whale are not commonly sighted the Asian Development Bank and the Government of the
species anywhere in Sri Lanka’s waters, but have been Netherlands, through the Coastal Resources Management
frequentlyrecordedinthefisheriesbycatcharoundtheisland Project ofthe CoastConservation Department in Sri Lanka.
(Leatherwood and Reeves 1989; Ilangakoon 1997, 2000; I thank Koen Broker and Ramani iayewardene who
Ilangakoon etal. 2000). While there have been no previous were involved with the field work throughout the project,
sightings of either species off the west coast of the island. all others who participated in surveys and the Coastal
Ailing(1986)reportedasingle sightingofthe Dwarf-Sperm Resources Management Project staff for providing
Whale from the east coast. A possible, but unconfirmed, invaluable logistical support and assistance during the
sighting of the Melon-headed Whale was also previously project.
J. Bombay Nat. Hist. Soc., 105 (3), Sep-Dec 2008 277
CETACEAN SPECIES RICHNESS AND RELATIVE ABUNDANCE AROUND BAR REEF MARINE SANCTUARY
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