Historical and present status of the pearl oyster, Pinctada margaritifera, at Pearl and Hermes Atoll, Northwestern Hawaiian Islands

2006·5.5 MB·English

by  Elizabeth E Keenan

HISTORICALAND PRESENT STATUS OFTHE PEARLOYSI EK, PI\CT\DA
MARGARITIFERA,AT PEARLAND HERMESATOLL, NORIHW ESlERN
HAWAIIAN ISLANDS
BY
ELIZABETH E. KEENAN', RUSSELL E. BRA1NARD^ and LARRY V. BASCH'
ABSTRACT
Populations ofthe black-lipped pearl oyster, Pinctada margaritifera, at Pearl
and HennesAtoll in the Northwestern Hawaiian Islands were first reported in 1928
and heavily harvested overthe next 2 years.Approximately 150,000 pearl oysters were
eitherexported orkilled during the exploitation. An expedition in 1930 to assess post-
hai"vestpopulation status found 480 P. margaritifera and determined the population to
be severely depleted. Limited surveys in 1994 and 2000 found only a few pearl oysters
and led to the conclusion thatthe population was still depleted. In 2003, the National
Oceanic andAtmospheric Administration (NOAA)-led multi-agency marine debris
removal team spent several months conducting surveys at Pearl and HermesAtoll that
includedquantitative observations ofPinctadamargaritifera. Data were collected on
location, size, depth, habitat, and orientation ofindividual pearl oysters on the reef
Analyses ofthe 1930 and 2003 data sets revealed similar size-frequency distributions of
theP. margaritifera population. The population has a spatial distribution within theAtoll
similarto the 1930 post-harvest distribution, and some sustained level ofreproduction.
Density and depth distribution comparisons from the two survey periods suggest that
pearl oysters are significantly more abundant in the shallow waters where they were
harvested during the fisherybut at a similardensity overall as they were during the 1930
survey. Although no estimates ofabsolute population size are available for any time
period, the large numberofoysters harvested priorto the 1930 survey, togetherwith
estimates ofoysterdensity in 1930 and 2003. suggest that the population may neverhave
recovered to its pre-exploitation level.
INTRODUCTION
Thepearl oyster, like other shellfish and many othermarine animals (e.g.,
abalone; Tegneret al., 1996), has a long history ofexploitation throughout the world.
'JointInstituteforMarineandAtmospheric Research,UniversityofHawaiiandNOAAPacific Islands
FisheriesScienceCenter, 1125-BAlaMoanaBlvd.Honolulu.HI 96814USA. E-mail: ekeenan(amail.nmfs.
hawaii.edu
^NOAAPacific IslandsFisheries ScienceCenter,2570DoleStreet, Honolulu. HI 96822 USA
'U.S. National ParkService, PacificIslandsCoralReefProgramandUniversityofHawaiiManoa,Hawai'i-
PacificIslandsCooperativeEcosystem StudiesUnit.Honolulu, HI USA
334
Records from the pearl industries in India and Venezuela document the discovery,
hai'vest, and eventual over-exploitation ofthese populations (Arunachlam, 1952; Romero
et al., 1999). Pearl oysters have been prone to exploitation due to the considerable value
ofthe pearls and the nacre, or"motherofpearl", ofthe shell, andbecause ofthe animal's
sessile nature and tendency to occur in sufficient densities at shallow depths forrelatively
easy collection.
The first documented discovery ofPinctada margaritifera atPearl and Hermes
Atoll in the Northwestern Hawaiian Islands (NWHl) (Fig. 1) was in May 1928 by
Captain William B. Anderson ofthe Lanikai Fishing Company(Amerson et al., 1974).
Forthe next 2 years, the pearl oysters were heavily harvested fortheirnacre. This shiny
portion ofthe shell w/as exported to the U.S. mainland where it was used primarily to
make buttons. Although documents concerning the harvest are wanting, conservative
estimates are that the shells ofapproximately 100,000 oysters were exported (Galtsoff,
1933). It is estimated that about 50,000 more oysters were killed and discarded, some due
to theirpoor shell quality and others in the search forpearls (Galtsoff, 1933).Afterthe
extent ofthe hai'vest was realizedby the Hawaii Temtorial government, an expedition
was undertaken to assess the population and atemporaryban onharvesting wasput in
place. This six-weekexpedition, ledby P. Galtsoffinthe summerof1930, utilized several
Filipino divers and produced a lengthy report including data on pearl oyster size, weight,
location (Fig. 2, modified from Galtsoff(1933)) and survey effort. Galtsoff(1933) found
480 P. margaritifera andpronounced the population too depleted to sustain further
harvesting. At this time theTerritory ofHawaii made the taking ofpearl oysters illegal
without pemiission, and a resurvey was suggested in five years to assess the recovery of
the population. Subsequently the industry collapsed, coinciding with replacement ofpearl
shell withplastic forbutton making and the advent ofcommercial pearl oysterfarms. Due
to the lack ofinterest in furtherfishing of/! margaritifera in Hawaii, the suggested 5-
yearresurvey at Pearl and Hermes was not conducted; however, the species has remained
under state protection since that time.
175°W 170°W 165"W 160°W 155°W
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^-^' Haw
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125 250 500 NauticalMiles
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175°W IZO-W
Figure 1. MapoftheHawaiianArchipelago.
335
Figure2. (FromGaltsoff, 1933). Pearl oystersur\'eysitesat PearlandHermesAtoll fromthe 1930sur\'ey.
Thesingleblackcirclerepresentsthehighestrelativeabundancefound(33 oysters/diverhour)andis
representedas 100percent. Othercircles indicatetheproportionalrelativeabundancefoundatothersitesin
the 1930survey.Whitecirclesaresitesatwhichnooysterswerefound.
There have been three recent surveys at Pearl and Hermes which included
documentation ofpearl oysterpresence: (1) by the U.S. National Marine Fisheries
Service (NMFS) in 1993 (Moffitt, 1994); (2) the 2000 NWHI ReefAssessment and
Monitoring Program (NOWRAMP) expedition (Maragos and Gulko, 2002); and (3) the
NOWRAMP
2002 expedition (Basch, unpublished data). Each ofthese surveys reported
onlyafewpearl oyster sightings, suggesting that the population did not rebound from the
harvesting event andremained severely depleted. However, researchers had insufficient
datato determine an accurate status ofthe pearl oysterpopulation. The 1993 NMFS effort
was a two-day survey ofthe general areas which had the highest pearl oysterdensities
in Galtsoff's (1933) survey. The densities at the three sites assessed in 1993 were found
to be lowerthan in 1930 (Moffitt, 1994). One problem with the 1993 survey is that the
methods usedto detemiine the locations ofsurvey sites in 1930 were not sufficiently
accurate. Theywere comprised ofcalculations using triangulation ofdistant markers
336
and dead reckoning; moreover, these methods were made less accurate bythe scientists'
inability to navigate straight lines through the shallow, reticulated reef. Consequently,
it is notpossible to locate the 1930 sites with enough accuracy to make site-by-site
comparisons overtime, particularly considering the patchy distribution characteristic of
pearl oysters.
NOWRAMP
The 2000 and 2002 ciiiises were not specifically focused on
surveying forpearl oysters. The relatively small areas sui-veyed were selected to record
detailed infonnation on the fish, algae, corals, and otherinvertebrate species present.
Pearl oysters were also recorded on some ofthese transects. Areport documentingthe
results ofthe 2000 cruise states that only a few oysters were found, and they were smaller
than those taken in 1930 (Maragos and Gulko, 2002). The transects were purposely
located on vaiyinghabitattypes and many were not inpreferredpearl oysterhabitat. The
few observations made on the status ofthe pearl oysterand the constraints ofthe surveys
limitthe usefulness ofthese surveys fordetennining the status ofthe population.
The purposes ofthis study were to: (1) accurately documentthe recent status of
the pearl oysterpopulation at Pearl and Hermes by means ofa systematic, quantitative,
and broad-scale survey oftheP. margaritiferapopulation at theAtoll, and (2) make initial
comparisons betweenhistorical andrecent surveyresuhs.
MATERIALSAND METHODS
Data Collection
As part ofan ongoingNOAA-ledmulti-agency effortto remove derelictfishing
gear and othermarine debris from the coral reefecosystems ofthe NWHI, divers from
NOAAs Coral ReefEcosystem Division (CRED) have methodically and systematically
surveyed large areas ofthe shallow waterreefhabitats at Pearl and HermesAtoll
(Donohue et al., 2001). Since 2003, the survey protocols have included extensive pearl
oysterobservations. Divers surveyed reefs using snorkel gearwhile swimming orbeing
towed along patch orreticulated reefs.Areas surveyed were recorded using a Garmin
Geographic Positioning System (GPS) 12 (NAD84) in a small boat closely following
divertracks. For each pearl oysterobserved, latitude, longitude, size, and depth were
recorded. Maximum shell length and width were measured. Lengthwas measured
as the maximum dorsal ventral measurement (DVM), andwidth was recorded as the
measurement ofthe shell perpendicularto the length. Fora small number (approximately
10 percent) ofthe oysters no measurements, depths, or locations were recorded. Since
identification ofjuvenile recmits to species requires more time and greatertaxonomic
skills than were available, and usually requires obsei-vation inthe laboratory,juvenile
oysters (<1.5 cm)were not included inthe data analysis.
For a subset ofobservations, additional datawere collected, when time allowed,
on habitat (substratum, dominantbiotic covercategory), and orientation ofindividual
oysters on the reef. For40% percentofobservations substrate was documented, and for
59% percent orientation was recorded. Habitatwas characterizedbypercent coverofthe
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substratum types in the 1 m- area centered on an oyster. The substrate categories were
recorded as algae, sand, coral, and coral cement. The algae category consisted ofmacro-
algae only, which were not identified to species. The coral cement category encompassed
coral rubble and dead coral either exposed or with associated turfor coralline algae.
Orientation, the angle between the plane ofthe oyster's shell and the substrate it was
attached to, was classified as horizontal, vertical ordiagonal.
DataAnalysis
Field data were transcribed daily to an Excel worksheet containing all parameters
foreach oyster. The GPS tracklines and waypoints were imported into ESRl Arcview
3.2 Geographic hiformation System (GIS) software, where they were used to map both
the reefs surveyed and the point location ofoysters on those reefs (Fig. 3). The total area
ofthe reefsurveyed during 2003 was detenninedby creating polygons inArcView which
delineated the reefcontours ofareas where divers swam. These polygons were created
using an Ikonis satellite image ofthe atoll (Fig. 4). The areas ofall polygons were added
to obtain total reefarea suneyed.
Several manipulations ofGaltsoff's (1933) observations were performed to
enable comparison ofhis and oursurveyresults. Galtsoff(1933) reported survey effort
in diverminutes. He reported thatthe divers covered a reefat the speed of42.7 ft/min
(0.01 km/min), and that in orderto coverthe entire breadth ofthe reefthe divers swam a
zigzagpattern with a width of60 to 100 ft. Galtsoffreported survey effort only for areas
where oysters were found. In orderto compensate forthe rest ofthe survey area effort,
we assumed that the average effort fora site with no oysters reported was approximately
the same as forthe sites where oysters were observed. There were 32 siteswith oysters
and 32 without, so the total minutes were doubled for a best approximation ofsuney
effort. To estimate the distance sui"veyed from effort we multiplied the survey rate (0.01
km/min) by total minutes (4,562 min) fora result of58.9 km. We multipliedthis distance
bythe associated width (60 to 100 ft. or0.01 to 0.03 km) to estimate the survey area
covered, with a result of 1.1 km- to 1.8 km-.
RESULTS
Atotal of 1,057 pearl oysters were found at Pearl and HermesAtoll during the
2003 summer survey. The pearl oysters were distributed primarily throughout the inner
lagoonarea (Fig. 2) with the exception often observations where individuals were found
on the sandflats orouter fringing reefs. The lagoon habitat was surveyed by swimming
only; we did not factor in the towed-diver survey areas in ourdensity estimates as they
were largely performed over sand and on habitat unsuitable forpearl oysters. This
facilitated comparisons with Gahsoff's (1933) results as observers in the 1930 surv'ey
intentionally avoidedthe sandflats. Area computations using GIS resulted in a total
lagoon survey area of5.9 km-. With an observed total of1,047 pearl oysters in this area,
we calculated an average density of 177 pearl oysters/km- in the lagoon area surveyed.
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Figure3. Distribution mapof2003 pearl oystersightings. PearlandHennesAtoll.Areassurveyedby
swimmingaredisplayedasblack lines,andoystersarerepresentedaswhitesquares. Onlythesurveyareas
andoysters inthe innerlagoonwereusedinthedensitycalculations.Theblackboxrepresentsthearea
portrayedinFigure4.
This density estimate assumes that all oysters present were observed bythe divers, when
in reality some oysters were missed. Therefore, this is not an estimate ofabsolute oyster
density, but a density estimate that can be compared to the 1930 survey, assumingthat in
each study there was the same probability that an oysterpresent in the surveyed areawas
m m
observedby the divers. Pearl oysters were found at depths ranging from 0.31 to 6.1
with amean of1.36 m and standarddeviation (sd) of0.87 m (Fig. 5).
The average shell length ofpearl oysters measured was 20.2 cm (sd=4.76 cm,
n = 963). Shell length ranged from 1.5-33.0 cm. Pearl oysters smallerthan 1.5 cm were
excluded from analysis since oysters ofthat size could notbe accurately identified
339
Miles
Figure4.Anenlargementofasectionofthe lagoonatPearland HenriesAtoll (Areaportrayed isoutlined
bytheblackboxinFigure3).Theblackareasarethepolygonscreated inArcview3.2todelineatethe
surveyedreefareaatPearlandHermesduring2003. Pearloystersarerepresentedbywhitesquares.
to Species in the field. TheP. margaritifera size frequency distribution (Fig. 6) has a
single mode. However, immature oyster recruits, or spat (shell length <5 cm), were
excluded from the size-frequency data set. The mean shell length of20.2 cm found in
2003 is remarkably similarto the mean shell length of20.23 cm forthe 164 adult pearl
oysters measuredby Galtsoff(1933), although the distributions have different shapes
(Fig. 6). For419 (40% oftotal surveyed) pearl oysters observed, data were recorded on
substratum type. Within the lagoon, the typical substratum composition consisted of: sand
11% (sd=24.8), coral 13% (sd= 17.3), algae 28% (sd= 32.1), and coral cement 48% (sd
= 34.6). The oysters were found in various orientations. In the subset ofoysters
forwhich orientation datawere collected (n = 624), mostwere horizontal (53%)). Ofthe
remaining oysters, 32%) were vertical, and 15% were diagonal.
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Percent of Population (%)
2 4 6 8 10 12 14 16 18
0.305
1.22
2.135
3.05
-^4.575
Q T
5.49
7.32
8.845
10.37
12.2
14.03
Figure5. Depthrange frequencydistribution forpearl oystersurveys in 1930and2003. Percentvaluesfor
the 1930datasetareestimatesbasedonthegiven meandepth rangeandminimum andmaximumdepths
fromGaltsoff(1933).
25
2- 20
c
o
iS 15
3
Q.
O
Q. 10
0O)
<u
Q.
9 11 13 15 17 19 21 23 25 27 29 31 33
Shell Length (cm)
Figure6. Size-frequencydistribution ofthepearl oysterpopulationatPearl andHennesAtoll.
NorthwesternHawaiianIslandsin 1930(N=197)and2003 (N=963).
341
DISCUSSION
In distribution maps (Figs. 3 and 4) \\c indicate that pearl oyslers are wides|-)icad
throughout theAtoll lagoon. When comparing our results with GaltsolTs 1933) post-
(
harvest data, we considered whetherchanges in the population occurred since that
time. By visually comparing the maps from the 1930 and 2003 surveys (Figs. 2 and
3), a general idea ofthe difference in spatial distribution can be obtained. Although the
locations from the 1930 map (Galtsoff, 1933) are only roughly estimated, no oysters were
recorded in 1930 forthe reefs in the southeast and south ends ofthe islands. When those
areas were surveyed in 2003, relatively high levels ofpearl oysters were observed on
almost all reefs. It is likely there has been new recruitment and population expansion into
this region since the 1930 survey. Moreover, reefs in the south central and north central
lagoon, where some oysters were seen in 1930, were not surveyed in 2003; some oysters
maybe present in these areas.
Determining whetherPiiictada margaiitifera populations have recovered to
pre-harvest levels is complicated by the factthatthere are no estimates ofpre-harvest
population density, and no estimates ofabsolute oyster abundance at Pearl and Hermes
forany time period. Comparisons between early and recent post-harvest data sets are
facilitatedbythe fact that Galtsoff(1933) did report numbers ofoysters found and the
survey effort, which can be used to estimate oysterdensity in the 1930 survey. After
converting Galtsoff's (1933) reported effort into survey area, we determined an average
densityof209 to 349 pearl oysters/km- during his surveys. In our2003 survey, we
estimated an average density within the lagoon areas of 77 pearl oysters/km-, lower but
1
ofthe same order ofmagnitude as the density found in 1930, and presumably lower than
the densityjustpriorto exploitation. Given the lack ofdatabetween the two surveys,
changes in pearl oysterabundance during the intervening 73 years cannot be detennined.
However, ifabundance has not reached pre-e.xploitation levels, it is useful to ask why.
One explanation forthis wouldbe that adultpearl oysterdensities were reduced by
exploitation below a threshold whereAUee effects (or inverse density dependence) came
into play (Levitan, 1995). Pinctadamargaiitifera is abroadcast spawnerwith planktonic
larvae (Pouvreauet al. 2000); consequently, reduced adult densities could have imposed
a directbottleneck on fertilization success, and subsequent embryonic, larval, and
recruitment success (Pouvreau et al., 2000). With a lowered adult density there would be
less likelihood that female gametes wouldbecome fertilized in the watercolumn, as has
been shown foroctocorals, sea urchins, abalone, and other sessile or sedentaiy benthic
marine invertebrates (Levitan, 1995; Tegneret al., 1996; Coma and Lasker, 1997).
Subsequently, ifa larva was produced and dispersed proximate to a suitable settlement
site, the likelihoodthat it would encountera settlement cue associated with an adult shell
alsowould be more remote. In otherwords,Allee effects would be furtherenforced given
that pearl oyster larvae tend to settle gregariously on the shells ofadult oysters (Pascal
and Zampatti, 1995; Zhao et al.. 2002).
Comparison ofthe pearl oysterpopulation depth distribution between the 1930
and2003 surveys shows some intriguing differences (Fig. 5). In 1930, oysters were
reported as ranging from 2.5 to 15.0 m, and were most abundant from 4.4 to 8.3 m.
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Galtsoff(1933) also reports that, according to CaptainAnderson, when the oysters were
first discovered they were very abundant in waterdepths of 1 to 3 m. In our 2003 survey,
we found oysters from 0.3 to 6.0 m depth, but animals were most abundant in the 0.5 to
2.2 m range (detennined using the mean of 1.36 m + 0.87 m sd). In Figure 5, we illustrate
the difference in the depth ranges between the two studies. The absence ofany oysters
in waters shallowerthan 2.5 m in 1930 is evidence ofthe heavy harvesting effort at
these shallow depths in the immediately preceding years. Seventy-three years later, we
found oysters to be very abundant at these shallow depths, suggesting that the remaining
population contributedto areseeding ofshallow areas ofthe reef What remains elusive
at this time is an explanation forthe apparent scarcity ofoysters at deeperdepths in our
survey. The most likely explanation is that oysters still occur in higherabundance at these
greaterdepths but that our sampling did not detect them. Since the lagoon surveys were
perfoiTnedby snorkeling, the divers spent most oftheirtime at ornearthe surface while
surveying. Oysters which may have been on the deeperreefslopes mayhave been missed
because oftheirsmaller size in relationto othersearch images (since the primarymission
was to locate generally largermarine debris) and the greaterdistance with depth fromthe
divers. We have no reason to believe that the lagoonal reefs have changed in a way that
would impose biological limits to the depth range ofthe oysters.
Alternatively, though less likely, pearl oysters may recruit preferentially to
shallower depths and may be less abundant in deeperareas due to this preference in
combination with (1) reduced adult densities at depth sustained overthe post-harvest
period and (2)Allee effects. Adirected survey forpearl oysters at Pearl and Hermes
conducted along multiple-depth contours would help determine the distribution and other
population parameters ofoysters at deeperdepths.
The present study indicates that thePinctada margaritiferapopulation at Pearl
and HenriesAtoll is reproducing at some level, as indicated by individuals ofabroad
range ofsize classes, including recniits. The mean shell length ofthe 963 pearl oysters
measured in the 2003 survey was 20.2 cm. The oysters were foundpredominantly
on coral cement and macro-algae dominated habitat. This observation contrasts with
Galtsoff's (1933) report that most oysters were found "confined exclusivelyto those
sections where the bottom is covered with corals." Initially, we thoughtthatthe difference
between surveys in composition ofoyster-occupied habitat might be attributed to
differences in the depth range, but examination ofthe data showed similar coral percent
coveratall depths.
The shell orientation ofthe oysters was measured in our surveybecause ourinitial
observations oforientation were inconsistent with a comment in Galtsoff's (1933) report.
Galtsoff(1933) noted that oysters were found in a vertical orslightly inclinedposition,
while we commonly observed oysters in a horizontal position. Our results indicate that
only about 1/3 ofoysters were oriented vertically, and> 1/2 were horizontal. These
differences in orientation maybe aresidual artifactofharvesting, ormayreflect depth-
related differences in the nature ofnear-boundary layerwatermovements which the
animals mayrespondto by orienting themselves, eitherto minimize drag dueto sheer
forces, orto optimize filter-feeding efficiency in differentflowregimes.
We report the first systematic, quantitative survey forpearl oysters throughout