Population Characteristics of the Mangrove Crab Scylla serrata

1

Population Characteristics of the Mangrove Crab Scylla serrata(Decapoda: Portunidae) in Kosrae, Federated States of Micronesia:

Effects of Harvest and Implications for Management1

Kimberly M. Bonine,2,3 Eric P. Bjorkstedt,4,5 Katherine C. Ewel,6 and Moses Palik7

Abstract: Apparent declines in abundance of mangrove crabs Scylla serrata(Forsskal, 1755) in Kosrae, Federated States of Micronesia, have prompted con-cern regarding long-term persistence of this important cultural and economicresource. To support development of effective management strategies, we gath-ered basic biological information about mangrove crabs on this island, where S.serrata is the only mangrove crab species present. In particular, we were inter-ested in understanding movement patterns and evaluating spatial variation inpopulation structure. Many population characteristics, including estimated lifespan, ontogenetic shifts in habitat use, sex-specific allometric relationships,male-biased sex ratios, and evidence for limited (<2 km) alongshore movement,are similar to those reported elsewhere in the range of the species. Therefore,insights from S. serrata populations elsewhere might usefully inform manage-ment of the species on Kosrae. Moreover, information reported in this study,for which there is no ambiguity about species identification, has broader rele-vance. Spatial variation in size structure of the population appears to be drivenby variable harvest pressure that reflects distribution of the human populationand location of emerging commercial harvest operations. Effective managementof mangrove crabs is therefore likely to benefit from application of size-based orsex-based restrictions on harvest and might usefully incorporate spatially explicitstrategies, such as partial or complete reserves. Development and implementa-tion of effective management will necessarily depend on cultural as well as sci-entific information.

Mangrove or mud crabs of the genusScylla are valued as a source of food andincome throughout much of the tropicalIndo-Pacific and as a consequence havebeen depleted, in both abundance and size,throughout much of their range (Brown

1993, Naylor and Drew 1998, Marichamyand Rajapackiam 2001). They occur from In-dia and Southeast Asia to parts of Japan andChina and throughout the Pacific islands(Macnae 1968), as an introduced species inHawai‘i, and at higher latitudes in Australiaand on southern Africa’s eastern coast (Hill

Pacific Science (2008), vol. 62, no. 1:1–19: 2008 by University of Hawai‘i PressAll rights reserved

1 Funding was provided by the MacArthur Founda-tion initiative on Population, Consumption, and Envi-ronment (grant no. 97-49885A-WER to B. Singer,K.C.E., R. Naylor, and S. Abraham) and the USDA For-est Service. Manuscript accepted 14 April 2007.

2 Institute for International Studies, Center for Envi-ronmental Science and Policy, Encina E411, StanfordUniversity, Stanford, California 94305-6055. Currentaddress: Conservation Strategy Fund, 1160 G Street,Arcata, California 95521.

3 Corresponding author (phone: 707-822-5505; fax:707-822-5535; e-mail: [email protected].

4 National Marine Fisheries Service, Southwest Fish-eries Science Center, Fisheries Ecology Division, 110Shaffer Road, Santa Cruz, California 95060.

5 Department of Fisheries Biology, Humboldt StateUniversity, 1 Harpst Avenue, Arcata, California 95521.

6 USDA Forest Service, Pacific Southwest ResearchStation, 60 Nowelo Street, Hilo, Hawai‘i 96720. Currentaddress: School of Forest Resources and Conservation,University of Florida, Gainesville, Florida 32611.

7 Division of Forestry and Wildlife, Kosrae IslandResource Management Authority, P.O. Box DRC,Kosrae State, Federated States of Micronesia 96944.

1974, Heasman et al. 1985). Scylla species areharvested for subsistence throughout theirrange, are the focus of small- to moderate-scale commercial fisheries in some areas, andare farmed commercially in Southeast Asia(e.g., Trino et al. 1999). Despite their impor-tance, little has been done to examine theconsequences of harvest for wild populations,especially in developing countries.

On the island of Kosrae, in the FederatedStates of Micronesia (FSM), Scylla serrata(Forsskal, 1755) (the sole species of man-grove crab present) is one of the most valu-able resources extracted from the mangroveforest (Naylor and Drew 1998). These crabshave cultural value as a central part of familyfeasts and as gifts to expatriates, and eco-nomic value derived from export, domesticexchange, and commercial sale to local hotels.Recent socioeconomic studies show that de-spite apparent declines in catch-per-unit-effort and harvest effort (households engagedin harvest and number of trips per month)during the 1990s, export of crabs for sale andas gifts actually increased substantially, byfactors of 8 and 2.5, respectively, from 1997to 2000 (Naylor and Drew 1998, Naylor et al.2002). Anticipated economic and human pop-ulation trends are likely to increase demandfor this valuable resource, as well as placegreater pressures on the mangrove foreststhat provide key habitat for the species(Naylor et al. 2002, Hauff et al. 2006). Suchtrends, and a general sense among Kosraeansthat mangrove crabs are becoming increas-ingly scarce, have raised concerns regardingthe sustainability of current harvest levelsand persistence of mangrove crabs as a viableand valuable resource.

Scylla serrata is a large portunid crab thatcan reach a maximum carapace width (CW )well in excess of 200 mm over the course ofa 3- to 4-yr life span (Perrine 1978, Heasman1980, Robertson 1996). Female S. serratamature at between 80 and 120 mm CW(Hill 1975, Heasman et al. 1985, Prasad et al.1990, Robertson and Kruger 1994; see alsocitations in Perrine 1978). Male S. serratamature physiologically at 90–110 mm CWbut may not be large enough to compete suc-

cessfully for mates until achieving fully adultmorphology (e.g., large claws) upon reachingcarapace widths of 140–160 mm (Perrine1978, Heasman et al. 1985, Knuckley 1999).Scylla serrata exhibits sexual dimorphism, withmale crabs tending to be heavier than femalesof similar carapace width (Chakrabarti 1981).

Reproduction occurs year-round in thetropics, with seasonal maxima that appearto coincide with seasonally high rainfall(Le Vay 2001). As in other Scylla species, cop-ulation takes place directly after the femalemolts and involves an extended period ofmate guarding by the male. Ovigerousfemales swim substantial distances offshore—from a few to many tens of kilometers—where the eggs hatch and larvae enter theplankton (Perrine 1978, Hyland et al. 1984,Hill 1994). Repeated spawning by females ap-pears to be possible but has not been demon-strated conclusively (Ong 1966, Perrine 1978,LeVay 2001).

Juvenile S. serrata are most common inintertidal habitats, whether on mudflats or inmangroves (Hill 1975, 1978, Hill et al. 1982,Chandrasekaran and Natarajan 1994), mov-ing into deeper habitats as they grow (Hillet al. 1982). Larger male S. serrata are com-monly found in mangrove channels or in as-sociation with burrows located in mangroveforests, on mudflats, or in the banks of chan-nels, as well as on neighboring reef flats(Perrine 1978). These burrows can serve asrefugia during low tide and for mating. Fe-male S. serrata, on the other hand, are farmore commonly found on subtidal reef flatsthan in the mangrove forest proper (Hill1975, Perrine 1978, Nandi and Dev Roy1990), migrating to and from the mangrovesto mate, commonly around the time of thenew moon (Perrine 1978). Whereas adultcrabs may move considerable distances outto sea, previous studies have not found evi-dence that crabs commonly undertake sub-stantial movements along the coast (Hill1975, Perrine 1978, Hyland et al. 1984).Most movement on a daily basis occurs atnight as crabs move from burrows to channeland reef habitats to forage.

Mangrove crabs have historically been the

2 PACIFIC SCIENCE . January 2008

focus of an artisanal fishery on Kosrae, withalmost all crabs being captured by removalfrom burrows by hand or with simple tools,by ‘‘torch fishing,’’ which entails walking thereef flats at night during low tides seekingcrabs and other target species, with baitedlines and more recently with baited traps(Perrine 1978, Smith 1992). Most householdson Kosrae harvest crabs from nearby man-grove forests, but larger-scale family-basedcommercial fisheries emerged in the 1990sin Tafunsak and Utwe, where they are saidto have substantially increased harvest rates(Naylor et al. 2002). Before this study, therewas no restriction on mangrove crab harvest,but a minimum size limit for retention of 6inches (152 mm) was enacted in 2000. In con-trast to many fisheries on relatively long-lived, large crustaceans in North America,Australia, and elsewhere, in which harvest isrestricted to males, female crabs—includingthose bearing eggs—are commonly retainedon Kosrae (Naylor et al. 2002).

In this paper we report on a study con-ducted between October 1998 and May 2001on the island of Kosrae, Federated States ofMicronesia, to provide information on popu-lation characteristics of S. serrata from whichinferences regarding population dynamics andthe effects of harvest can be drawn. The goalof this effort is to inform development ofmanagement strategies that will ensure con-tinued availability of mangrove crabs, yet aresimple for local resource managers to imple-ment and evaluate.

materials and methods

Study Area, Arrangement of SamplingLocations, and Description of Habitats Sampled

Kosrae (5� 19 0 N, 163� 00 0 E) (Figure 1) is ahigh volcanic Pacific island 113 km2 in area,much of which is steep forested mountainsthat are surrounded by a narrow coastal plainand a fringing coral reef. The climate is moistand hot, with little interannual variation inrainfall (5,000–7,000 mm per year) or tem-perature (annual average ca. 27�C). Diverse,intact mangrove forests cover approximately

1,562 ha and line two-thirds of the coastline(Whitesell et al. 1986, Ewel et al. 2003). Theisland is divided into four municipalities (Ta-funsak, Lelu, Malem, and Utwe), each with avariety of villages and settlements. At thetime of this study, the population of Kosraewas ca. 7,700 (FSM Census 2002), most ofwhom lived on the narrow coastal plain alongthe northern, eastern, and southern sides ofthe island. The largest population concentra-tions are in Lelu Island and the village ofTafunsak. Smaller settlements lie along theroad connecting the two, as well as fartheralong the road in Malem and Utwe. A villageon the western part of the island (Walung, inthe municipality of Tafunsak) was, at the timeof this study, reachable only by boat.

As part of a broader study of resource usein the mangrove forests of Kosrae, the is-land’s coast was divided into nine sectorsbased on the position of rivers and populationcenters (Hauff et al. 2006). We focused muchof our sampling effort in four of these sec-tors: Lelu Harbor and Pukusrik in the munic-ipality of Lelu, Okat in the municipality ofTafunsak, and a long stretch of coastline ex-tending from the village of Utwe to the set-tlement of Isra, both in the municipality ofUtwe (Figure 1). Additional collections weremade near Walung and the settlement ofYemulil in the municipality of Utwe, fartherWNW along the coast from Isra; the datafrom those sites are used in a more limitedmanner due to differences in the habitat typesfrom which the crabs were collected.

Although the overall species compositionof the mangrove forests shows little system-atic spatial variation around Kosrae (Ewelet al. 2003), those forests close to populationcenters or easily accessible by roads experi-ence higher rates of removal of mangrovewood. Extensive logging often selectively re-moves valued tree species and generates largegaps that may not be recolonized by man-grove hardwoods (Hauff et al. 2006). OurLelu Harbor and Okat sites were similar inthat the mangroves in both areas formed amore or less continuous strip between up-lands and reef flats, penetrated occasionallyby relatively narrow river channels (Figure

Scylla serrata in Kosrae, FSM . Bonine et al. 3

1). In contrast, the mangrove forests at thePukusrik and Utwe-Isra sites were bothbounded on the seaward side by berms ofcoral sand, containing broad tidal channelsconnected to a larger bay through relativelynarrow gaps (Figure 1). The channel atUtwe-Isra included substantial reef habitats.The mangrove forest in Puksurik rested onsandier soil and contained springs in severalplaces. This mangrove forest was also shorter,probably reflecting a long history of small-scale harvesting.

We stratified the mangrove forests wherewe sampled mangrove crabs into three zones:(1) ‘‘fringe,’’ which lay within 100 m of theseaward margin of the mangrove forest and

might be separated from the open ocean byreef flat or beach strand; (2) ‘‘riverine,’’ whichlay within 150 m of a riverbank; and (3) ‘‘in-terior,’’ which included the remainder of themangrove forest not part of the previous twozones (Hauff et al. 2006). Because the relativeextent of each of these zones varied fromsector to sector and either the fringe or riv-erine zone was entirely lacking in some areas(Hauff et al. 2006), we focused most of oursampling in interior mangrove forests. Con-straints arising from accessibility and our pri-mary method of sampling (traps) preventedus from sampling areas of the mangrove for-est close to the landward margin or in slightlyelevated areas between channels.

Figure 1. Map of Kosrae, Federated States of Micronesia, showing general distribution of mangrove forests (in gray),boundaries of sectors defined in text (all capital letters), and location of villages and other settlements mentioned intext.


Crab Sampling and Estimation of Catch-per-Unit-Effort

We used baited traps as our primary methodof collecting S. serrata but also collected crabsopportunistically by hand during surveys ofthe mangrove forest. All crabs, regardless ofmethod of capture, were measured (carapacewidth in millimeters), weighed (to the nearest0.1 kilograms), and identified by sex.

Traps had approximate dimensions of 1.2by 0.6 by 0.3 m, were fitted with 2.5-cm wiremesh, and were baited with one or moretypes of bait (e.g., tuna, shark, reef fish, pigremains, etc.). In a typical set, six to eighttraps were deployed at intervals b20 malong the length of a channel through themangrove forest. Traps were deployed atarbitrarily selected locations in waterwaysaccessible to a small motor skiff where crabswere expected or known to occur. Effort con-centrated on collecting a representative sam-ple from the sector but was not directedaccording to a formal randomized samplingdesign. Traps were checked daily, after hav-ing fished a complete flood-ebb tidal cycle,and were typically fished for 2–4 consecutivenights at a given location.

Over the course of implementing our sam-pling program, it became apparent that tunawas clearly the most effective bait, and weswitched to its use exclusively. To estimatecatch-per-unit-effort, we used only data col-lected from traps that were baited with tunaand deployed in interior channels.

Tagging Study

All crabs captured were marked by affixinga series of six roughly cylindrical glass beads(flat side down, hole oriented vertically) tothe center of the carapace with cyanoacrylateglue and released at the point of capture.Each crab received a unique combination ofcolored beads to allow identification uponrecapture. Local crab harvesters were encour-aged through various outreach efforts toreport the day, location, and bead color se-quence of any tagged S. serrata captured inthe course of normal harvesting. In the few

cases where a crab was returned missing a sin-gle bead of the six, identification was accom-plished using the remaining beads and theindividual’s carapace width.

Statistical Analysis and Demographic Modeling

To estimate catch-per-unit-effort, we definedeffort as a single trap fished at a particularlocation for one night and calculated catch-per-unit-effort as the number of crabs trap�1

night�1, modeled as a Poisson process. We as-sumed that traps were fishing independentlyand evaluated this assumption by examiningmean catch-per-unit-effort as a function ofthe number of traps in a set as well as meancatch-per-unit-effort as a function of trapposition within a set.

We estimated allometric relationships be-tween carapace width (L, in millimeters) andweight (W, in kilograms) by fitting modelsof the form W ¼ aLb using standard non-linear regression techniques. Size data for allcaptured crabs were included in the analysis.Based on preliminary examination of thedata, separate models were fit for male andfemale crabs. Preliminary examination of in-dividual condition (W L�3) suggested thatamong both male and female crabs a smallsubset of individuals existed whose weightclearly exceeded the range typical of thepopulation at a given carapace width. Wetherefore divided the population using an(arbitrary) threshold condition of 2:8� 10�7

(kg mm�3) and fit separate allometric rela-tionships for each group within each sex.

To identify size classes, we binned cara-pace widths for all crabs in our data set at4 mm intervals over the range of 80 mmto 220 mm and applied Hasselblad’s (1966)steepest-descent algorithm for estimatingthe parameters of a mixture of normal distri-butions. We used two-sample Kolmogorov-Smirnov (K-S) tests to compare distributionsof carapace width among subsets of the databased on zone, sector, or sex. In addition, dis-tributions of carapace width for subsets of thedata were compared visually to the overalldistribution of size classes to evaluate repre-sentation of size classes within subsets of the


population. We estimated Hiatt’s (1948) rela-tionship for sequential size classes by fittingthe model Liþ1 ¼ c þ dLi to the estimatedmean carapace widths ðLiÞ for each size classusing least-squares regression.

We developed crude estimates of migra-tion distance for tagged crabs based on thelocation of release and recovery. Because lo-cation data were often imprecise, we wereable to quantify movement at a spatial scaleon the order of a few kilometers (i.e., to re-solve movement between sectors but withlittle precision within sectors).

Because tags are lost when a crab molts,the longest period that an individual is knownto retain a tag (i.e., the maximum intervalbetween the release and recovery of a taggedindividual) provides a lower bound on theintermolt interval. The longest of these in-tervals therefore provides a measure of theintermolt interval. To estimate minimum in-termolt intervals (I in days) as a function ofcarapace width (L in millimeters), we fita model of the form I ¼ expfaþ b � Lg sub-ject to the following constraints: (1) allobserved intervals between release and recap-ture of a tagged crab had to fall below modelpredictions, and (2) the smallest differencebetween predicted and observed release-recapture intervals was to be minimized.This model has a structure similar to con-stant-ratio patterns in intermolt intervals re-ported for other crustacean species (Caddy2003) but instead relates intermolt intervalto individual size. Because estimation of ex-trema is sensitive to sampling effort, and thesmallest crabs are apparently less susceptibleto capture, we corroborated this analysis withdata reported by Arriola (1940) for intermoltperiods observed for two mangrove crabsreared in captivity and a single field measure-ment of an intermolt interval reported byPerrine (1978); however, these data were notincluded with those used to fit the model.

We used information obtained on inter-molt intervals and size-class distributions toconstruct a simple model for estimating ratesof attrition affecting the population of man-grove crabs susceptible to capture in our sam-pling program, under the assumptions that

recruitment and average size-specific moltingrates of mangrove crabs do not vary aroundthe island. By attrition, we mean the com-bined effects of natural mortality, mortalitydue to harvest, and any size-specific migra-tion to or from the sampled habitat (e.g.,emigration by larger crabs from interiorchannels of the mangrove forest to moresubtidal habitats). To avoid confounding ouranalysis with variation in size distributionsand sampling effort related to habitat type,we used only observations collected withtraps set in interior channels.

A detailed description of the model, itsconstruction, and our analytical approachare provided in the Appendix and are sum-marized here for convenience. The modeldescribes the development of size-class fre-quency distributions according to the ratesof transitions among size classes in a stage-structured population and, based on these dy-namics, estimates attrition rates necessary toyield predicted size-class frequency distribu-tions that match our observations. We de-fined the daily probability of an individual’smaking the transition from one stage to thenext as the inverse of the intermolt intervalpredicted for the mean carapace width ofeach size class. We lacked the full com-plement of information necessary to con-struct a credible density-dependent model.We therefore assumed that any density-dependent mechanisms affect the populationdynamics in such a way that a stable steadystate exists and that the stable size-class distri-bution achieved at this steady state is identicalto that reached under density-independentdynamics with R ¼ 1 (i.e., a constant popula-tion size) (Caswell 2001). Because crabs insmaller size classes (less than ca. 130 mmcarapace width) were either less available orless susceptible to capture in our traps, weincluded size class–specific catchability inthe process of fitting model-predicted stablesize-class distributions to the data. Prelimi-nary inspection of the size-class distributionsalso indicated that attrition rates affectingsmaller size classes might differ from thoseaffecting larger size classes, so we fit modelsthat estimated attrition rates separately for


Figure 2. Allometric relationships between carapace width (mm) and weight (kg) for (a) male and (b) female mangrovecrabs (Scylla serrata) on Kosrae, FSM. Open circles are for ‘‘light’’ crabs and filled circles are for ‘‘heavy’’ crabs, definedaccording to a condition threshold; see text for details. Lines indicate allometric relationships between weight (W ) andcarapace width (L) of the form W ¼ aL b ; parameter values in Table 1; we plot the relation for ‘‘heavy’’ males in (b) forcomparison to ‘‘heavy’’ females. Symbols indicate sector: Lelu (a), Pukusrik (j), Okat (s), Walung/Yemulil (v),Utwe-Isra (y).

sets of contiguous size classes designated onthe basis of apparent breaks in the size-classdistribution.

results

Life History and Population Characteristics

sex ratio. Overall, male S. serrata out-numbered females (3.3 : 1, n ¼ 357, total) re-gardless of method of capture (trap: 3.2 : 1,n ¼ 339; opportunistic capture by hand: 5:1,n ¼ 18). The sex ratio varied significantly bysector; bias toward males in our samples wassubstantially lower (1.81 : 1, n ¼ 90) in Utwe-Isra than elsewhere (Lelu Harbor: 6.00 : 1,n ¼ 21; Pukusrik: 4.92 : 1, n ¼ 83; Okat:5.87 : 1, n ¼ 103). None of the captured fe-males was ovigerous.

allometric relationships. Allometricrelationships for male crabs differed signifi-cantly from those for female crabs, with malecrabs tending to be heavier than females for agiven carapace width (Figure 2, Table 1). Forboth male and female crabs, we observed adistinct subset of individuals that were ap-proximately twice as heavy as most othercrabs for a given carapace width, a trait morecommonly observed among male crabs (37 of255 were ‘‘heavy’’ [14.5%]) than females (6 of78 [7.7%]) (Figure 2, Table 1). Sample sizelimited our ability to estimate the exponentialterm of the allometric relationship for all butthe lighter subset of male crabs, but visual in-spection strongly suggests that a power-lawadequately describes the data (Figure 2,Table 1). Heavy females were too rare to

allow a robust allometric relationship to bedeveloped and were not significantly differen-tiable from any other group, but qualitativeexamination suggested that they were in thesame range as heavy males and exhibited asimilar allometric relationship (Figure 2).We detected no significant differences amongsectors in the allometric relationships be-tween carapace width and weight for eithermale or female crabs, but this analysis hadlimited power due to small sample sizes.

size structure and susceptibility

to capture. Mangrove crabs exhibited dis-tinct size classes with little overlap (Figure 3).The progression of mean carapace widthswas described by a linear relationship ofthe form Liþ1 ¼ c þ dLi (Hiatt 1948) withc ¼ 29:721 (95% CI: 16.565, 42.878) andd ¼ 0:912 (95% CI: 0.825, 0.998). Male andfemale crabs exhibited similar size classes,and the overall size distribution of crabs didnot differ significantly between sexes (K-Stest, P ¼ 0:31). The size-frequency distribu-tion of crabs in our samples suggested thatthe smallest crabs susceptible to capture inour traps were ca. 100 mm in carapace widthand that susceptibility to capture increasedwith size.

stage duration and estimated life

span. Eighty S. serrata were recaptured dur-ing the course of our study, 12 of which wereat large for at least 1 month after beingtagged and released. The maximum observedinterval between release and recapture (Iin days) increased with carapace width (Lin millimeters) according to I ¼ expf0:009þ0:0314 � Lg (Figure 4), which is consistent

TABLE 1

Allometric Relationships between Carapace Width (L, in mm) and Weight (W, in kg) Estimated by Fitting Models ofthe Form W ¼ aL b for Groups of Mangrove Crabs from Kosrae

Group n a (95% CI) b (95% CI)

Normal males 218 3.54 (3.36, 3.73) 1:23� 10�8 (4:14� 10�10, 2:42� 10�8)‘‘Heavy’’ males 37 3.16 (2.78, 3.54) 1:73� 10�7 (�1:51� 10�7, 4:96� 10�7)Normal females 72 2.69 (2.36, 3.02) 6:81� 10�7 (�4:85� 10�7, 1:85� 10�6)‘‘Heavy’’ females 6 1.94 (�2.23, 6.11) 7:38� 10�5 (�1:59� 10�3, 1:72� 10�3)

Note: Point estimates are followed in parentheses by upper and lower bounds on 95% confidence intervals for the estimatedparameters.


with a pattern of increasingly long intermoltintervals over the course of a typical crusta-cean life cycle. Information on intermoltintervals for two crabs reared in captivityand two field observations suggests that ourmodel also holds for the smaller size classesin our data set (Figure 4) (Arriola 1940,Perrine 1978).

To estimate the life span of S. serrata onKosrae, we assumed that (1) the largest crabsobserved in our study are representative ofthe maximum size attained by S. serrata onKosrae, and (2) on average, an individualcrab requires 6 months to grow to the small-est size class observed in this study (Arriola

1940, Ong 1966), after which our data sug-gest a minimum period of 979 days (ca. 2.7yr) to reach the largest observed size class.

dispersal patterns. Almost all (79 of80) recaptures occurred in the same sectorwhere the crab was initially captured or lastobserved, including four of five crabs at largefor more than 100 days. Crabs that remainedwithin a sector traveled no more than ca. 2 to3 km, and it is likely that most distances weresubstantially smaller (i.e., <1 km, based ondescriptions of release and recapture loca-tions). One male crab (166 mm CW ) movedca. 8.5 km from Walung to Okat during 174days at large.

Figure 3. Size classes, based on carapace width, of trap- and hand-caught mangrove crabs (Scylla serrata) on Kosrae,FSM. Bars (scale on left y-axis) indicate observed frequency of crabs in 4 mm size bins. Thin lines (scale on right y-axis) indicate normal distributions that correspond to size classes with mean (SD), in mm, of 104.3 (5.9), 122.6 (6.6),143.5 (6.2), 162.4 (5.4), 177.6 (3.4), 189.0 (3.7), and 203.1 (3.6). Size class distributions are scaled by the proportionalrepresentation of each size class in the sample. Thick line shows the composite distribution for the population, formedas the sum of the scaled size class–specific normal distributions.


Spatial Variation in Population Characteristics

Size structure of mangrove crabs varied acrossthe landward-seaward axis of the mangroveforest, as well as among sectors around theisland (Figure 5). Crabs captured in traps de-ployed in fringe channels (near the edge ofthe mangrove forest) were significantly largerthan crabs trapped in other zones of the for-est (Figure 5): Utwe-Isra: K-S test, P < 0:001(males), K-S test, P ¼ 0:0058 (females); allsectors: K-S test, P < 0:0001 (males), K-Stest, P < 0:0001 (females).

Size distributions for crabs captured in

interior channel habitats differed signifi-cantly among sectors (K-S test: Pukusrik ver-sus Utwe-Isra, P ¼ 0:0124; Pukusrik versusOkat, P ¼ 0:0007; Utwe-Isra versus Okat,P < 0:00001), and these patterns reflect spa-tial variation in proportional representationof size classes (Figure 6). The dominant sizeclass observed in Utwe-Isra (178 cm) was al-most absent in Pukusrik and Okat, where thedominant size classes were centered at 162 cmand 144 cm, respectively (Figure 6). Thesepatterns were reflected in separate compari-sons for male and female crabs (results notshown), although the significance of the dif-

Figure 4. Release-recapture intervals for mangrove crabs as a function of carapace width. Closed circles indicateintervals for mangrove crabs tagged and released as part of this study. Open circles connected by solid lines indicatesequential intermolt intervals observed for a pair of mangrove crabs reared in captivity (Arriola 1940). Triangles indi-cate intervals at large for tagged crabs that molted (s) and did not molt (v) reported by Perrine (1978). Dotted lineindicates ‘‘minimum’’ intermolt interval (see text for description of fitting method) inferred from the tagging experi-ment reported here; data from Arriola (1940) and Perrine (1978) are not used in this analysis and are presented only tocorroborate the model at smaller size classes.


ferences tended to decline with sample size,particularly for females. Although the smallnumber of crabs captured in Lelu Harborlimited evaluation of size-class structure andcomparisons with other sectors, size-class dis-tribution there appears to be consistent withthe islandwide pattern and is perhaps mostsimilar to that observed in nearby Pukusrik(Figure 6).

Spatial Variation in Attrition Rates

Attrition rates differed more sharply acrosstransitions between smaller and larger sizeclasses than among sectors, but the size classafter which attrition rates increased variedsubstantially among sectors (Table 2, Figure

7). We did not detect significant spatial varia-tion in attrition from either smaller or largersize classes. For Pukusrik and Okat, however,estimated attrition affecting the largest sizeclasses was significantly higher than attritionaffecting the smaller size classes (Table 2,Figure 7). A similar increase in attrition onthe two largest size classes was apparent inLelu Harbor and Utwe-Isra, but the presenceof only two size classes constrained the preci-sion of our estimate (Table 2, Figure 7).

Catch-per-Unit-Effort

We estimated Poisson capture rates of 0.592crab trap�1 night�1 (95% CI: 0.511, 0.674)for male crabs, 0.161 (0.121, 0.210) for female

Figure 5. Size distribution of mangrove crabs captured in interior (upper panel ) and fringe (lower panel ) channels.Black bars indicate size distributions observed for Utwe-Isra only; gray bars indicate distributions for all sampled areas,including Utwe-Isra.


Figure 6. Variation in representation of size classes of mangrove crabs by sector. Bars (left y-axis) indicate actualcounts of crabs in 4 mm size intervals for each sector. The thin and thick lines (right y-axis) are, respectively, the esti-mated size class–specific and composite distributions based on the islandwide analysis and illustrated in Figure 2.

TABLE 2

Size Class–Specific Attrition Rates and Estimated Catch-per-Unit-Effort (CPUE) for Mangrove Crabs Captured inInterior Channel Habitats on Kosrae, FSM

Parameter Lelu Pukusrik Okat Utwe-Isra

Attrition (% days�1) 0.89 (0.61, 1.31) 0.50 (0.42, 0.57) 0.51 (0.29, 0.91) 0.48 (0.33, 0.72)Size classes (mm CW ) 104–178 104–162 104–144 104–178

Attrition (% days�1) 5.38 (0, 99.95) 3.27 (2.27, 4.69) 3.08 (2.30, 4.13) 2.04 (0.24, 15.31)Size classes (mm CW ) 189–203 178–203 162–203 189–203

CPUE (n/trap night) 0.50 (0.31, 0.76) 0.95 (0.75, 1.19) 0.85 (0.68, 1.05) 0.63 (0.49, 0.79)

Note: Point estimates are followed in parentheses by upper and lower bounds on 95% confidence intervals for the estimated param-eters. Size classes indicate the mean carapace width (CW ) of size classes for which the preceding attrition rate applies.

Figure 7. Observed (filled bars) and modeled (open bars) size-class distributions for S. serrata in four sectors ofKosrae, FSM. Model predictions are based on a stage-structured demographic model for S. serrata in interior channelhabitats. Vertical gray lines indicate the transition between lower attrition rates on smaller size classes and higherattrition rates on larger size classes. Estimated attrition rates are provided in Table 2.

crabs, and 0.754 (0.661, 0.846) for all crabs.We did not detect significant spatial variationin catch-per-unit-effort (Table 2), nor did wedetect a significant variation in catch-per-unit-effort over the course of the lunar cycleor the spring-neap tidal cycle (results notshown). Analysis of mean catch-per-unit-effort as a function of the number of traps ina set and as a function of trap position withina set validated our assumption that traps didnot fish competitively when deployed alonginterior channels (results not shown).

discussion and conclusions

Mangrove crabs (Scylla serrata) on Kosrae,FSM, exhibit biological and ecological char-acteristics similar to those reported for thespecies elsewhere in its range: male crabs areabout twice as heavy as females at a given car-apace width (Figure 2) (Chakrabarti 1981);males outnumber females in areas that aremore isolated from the open sea (Hill 1975,Perrine 1978, Nandi and Dev Roy 1990); in-dividuals tend to move to seaward habitats asthey grow (Figure 5) (Hill et al. 1982); adultS. serrata exhibit limited movement along thecoastline (results from the tagging study)(Hill 1975, Perrine 1978, Pillans et al. 2005);and individual crabs can achieve a maximumsize that exceeds 200 mm CW over a lifespan on the order of 3 to 4 yr (Figure 4) (LeVay 2001). Although we did not explicitlyevaluate maturity of captured crabs, we ob-served no cases (e.g., large, immature crabs)that contradicted size-maturity relations re-ported elsewhere (Hill 1975, Perrine 1978,Heasman et al. 1985, Quinn and Kojis 1987,Prasad and Neelakantan 1989, Robertsonand Kruger 1994, Robertson 1996). By dem-onstrating that S. serrata on Kosrae sharetraits with their conspecifics elsewhere ourstudy implies that insights gained from re-search conducted in other parts of the spe-cies’ range are relevant for developingeffective management strategies for S. serrataon Kosrae. Perhaps more important, infor-mation about S. serrata on Kosrae, where itis the only mangrove crab species present,can be used unambiguously throughout thespecies’ range, particularly in the tropics,

where differences among the four newlydivided species may not yet have been estab-lished.

We observed one previously unreportedpattern in the Kosrae population: the pres-ence of a distinct segment of the populationthat was substantially heavier than most othercrabs for a given carapace width. Predictionsof carapace width as a function of weight forthese crabs suggest that these crabs were ap-proaching maximum weight for their sizeand possibly nearing a molt (Figure 2). Itremains unclear why an apparently bimodaldistribution of weight in a size class ratherthan a more continuous distribution is ob-served. It is possible that our sampling wassomehow biased toward crabs of low con-dition and high condition, and crabs of mod-erate condition were simply less available toour gear, say, by spending more time indeeper habitats because they can sustain theenergetic costs of more extensive seawardmovements for feeding while also avoidingpredation, yet still have sufficient scope forgrowth (increase in mass) to warrant sucheffort.

Our study provides compelling evidencefor the effects of harvest on the populationof S. serrata on Kosrae by detecting spatialvariation in attrition rates and catch-per-unit-effort that can be related to the distribu-tion of the human population and the pres-ence of commercial fishing operations. Thatharvest is the source of this spatial variation isindicated by two patterns: (1) the increase inattrition rates between smaller and larger sizeclasses, and (2) spatial variation in the sizeclass that marked the onset of the higherattrition rates. Integrated over the life cycle,estimated attrition rates are highest in Lelu,followed in declining order by Okat, Pukus-rik, and Utwe-Isra, which mirrors patternsof human accessibility of the mangrove forest(Table 2, Figure 7). Similarly, the generaltrend of increasing catch-per-unit-effort fromLelu Harbor to Utwe-Isra and Okat to Pu-kusrik is opposite the trend in estimated attri-tion rates (Table 2) and is consistent with theeffect of historically high harvest rates (Lelu)and the location of commercial fisheries(Okat and Utwe-Isra). We know of no eco-


logical mechanism that could cause moltingrates to vary sufficiently among sectors toyield the observed differences in size-classdistributions if attrition rates were constantacross sectors.

The apparently contrary estimates of attri-tion rates and catch-per-unit-effort for Utwe-Isra appear to rebut our argument that varia-tion in harvest underlies spatial variation insize structure of S. serrata on Kosrae. Weexpect that demographics provides a partialexplanation for this pattern, in that the popu-lation in Utwe-Isra was dominated by largercrabs, which will normally be less abundantsimply due to cumulative natural mortalityeven under unfished conditions. We believe,however, that in Utwe-Isra, attrition rates arein fact higher than our estimates suggest—specifically, that a broader range of sizeclasses is exposed to attrition rates that in-clude substantial harvest mortality—and thatthe apparent discrepancy simply reflects anunintended artifact of our sampling program.The reduced degree of male bias in the sexratio of crabs and the greater abundance oflarger crabs in general suggest that our habi-tat designation rules were confounded by theextensive presence of readily accessible, pro-tected reef flat habitat surrounded by man-groves in Utwe-Isra and led to the inclusionof collections from ‘‘fringe-like’’ habitats inour ‘‘interior channel’’ observations. If this isthe case, immigration of crabs of moderatesize from more interior portions of the man-grove forest to the areas from which our sam-ples were taken is likely to mask or offset theeffects of harvest on population size structurein the areas sampled and thus lead to a rela-tively low estimate of attrition even thoughabundance (catch-per-unit-effort) has beenreduced by harvest.

Our estimates of attrition rates were sur-prisingly high and substantially exceed theonly estimate of natural mortality in S. serratathat we found reported in the literature (41–60% yr�1 for populations in seasonal estuaries[Hill 1975] versus 82–96% yr�1 smaller sizeclasses and >99% yr�1 on larger size classeson Kosrae [this study]), yet we do not believeour estimates to be far off the mark. Indeed,because our definition of attrition subsumes

natural mortality, mortality due to fishing,and net migration to the sampled habitat, wecan expect our estimates of attrition to exceednatural mortality simply by definition. Thequestion therefore becomes not only whethertotal rates of attrition are estimated correctlybut also whether harvest, emigration, or otherfactors suffice to account for the disparity inrates.

We are confident that our estimates of at-trition are sound and base this conclusion onthe validity of the analyses upon which theseestimates depend, namely our analysis of size-class distributions (Figures 3 and 6) and esti-mation of intermolt intervals (Figure 4). First,previous studies report size distributions thatare qualitatively consistent with our observa-tions and, in some cases, appear to indicatedistributions in which size classes have beenpartly obscured by binning the data at acoarser resolution than used in our study(Hill 1975, Perrine 1978, Williams and Hill1982, Robertson and Kruger 1994, Robertson1996). Second, the observation of a linear re-lationship between the carapace widths at se-quential size classes is consistent with patternsreported for numerous other crustacean spe-cies (Hiatt 1948, Caddy 2003), which lendsfurther support to the conclusion that themultimodal size-class distribution reportedhere is real. Finally, our estimates of inter-molt intervals are corroborated by indepen-dent observations of S. serrata from tropicalregions (Arriola 1940, Perrine 1978) and leadto an estimated life span comparable with thatreported for conspecifics elsewhere (Perrine1978, Heasman 1980, Robertson 1996, LeVay 2001). Indeed, we believe our analysisprovides a reasonable prediction of averageintermolt interval in a given size class, al-though individuals may exhibit longer inter-molt intervals (Ong 1966, Perrine 1978). Inany case, we note that any error in our esti-mate of intermolt intervals, say due to un-detected tag losses, will affect our estimate ofabsolute attrition rate but will not affectspatial variation in relative attrition rates ob-served between sectors.

Although our analysis of spatial variationin attrition rates indicates that harvest is amajor source of mortality of mangrove crabs


on Kosrae, exactly how much harvest contrib-utes to attrition is difficult to resolve with theavailable data. Measures of attrition rates andcatch-per-unit-effort among the four sitesmutually suggest that harvest effects are notnegligible and are likely to be substantial (seealso Naylor et al. 2002). Moreover, the simi-larity between our estimates of catch-per-unit-effort and those reported for populationsof S. serrata in South African estuaries con-sidered to be too small to support a commer-cial fishery (0.50–0.95 crabs trap�1 night�1

[this study]; 0–1.67 crabs trap�1 night�1

[Robertson 1996]) suggests that the crabpopulation on Kosrae might not be able tosustain increased, or perhaps even current,levels of harvest. More work is needed to ob-tain more exact estimates of abundance, toquantify the effects of harvest at local scalesas well as the scale of the entire populationmore precisely, and to understand how re-cruitment contributes to population persis-tence and sustainable rates of harvest. In anycase, the concerns expressed by Kosraeans re-garding the future status and sustainability ofthis important cultural and economic re-source appear to be well justified.

Management of the fishery on mangrovecrabs on Kosrae should focus on maintainingpopulation productivity by reducing mortalityrates of immature crabs and increasing sur-vival of females so that they can reproduce atnatural rates. In other crab populations (e.g.,the Dungeness crab, Cancer magister) size-and sex-specific harvest regulations protectfemale crabs and immature male crabs whileallowing open harvest on mature males yetdo not appear to compromise the productiv-ity of a stock substantially (Hankin et al.1997). Similar regulations have had a positiveeffect in Australian mud crab populations(Pillans et al. 2005) and should be consideredfor mangrove crabs on Kosrae. Evidence thatharvest has localized effects and that adultcrabs limit their alongshore movement sug-gests that an adaptive, comanagement strat-egy that incorporates reserves for mangrovecrabs might be an effective tool for managingthis species (e.g., Castilla and Defeo 2004,Pillans et al. 2005). Evidence for rapid localreplenishment of larger size classes in crusta-

ceans (Pillans et al. 2005; K.C.E., unpubl.data) suggests that a more sophisticated sys-tem of harvest ‘‘rotation’’ might allow ade-quate access to larger, more valuable crabs,but such a system would be more difficultto design, communicate, and enforce. Trulyeffective management of mangrove crabs,like any similar species that has a poten-tially dispersive larval stage, will require con-certed attention to sustainable managementat regional scales spanning populations thatexchange recruits through larval dispersal(Mora and Sale 2002).

The status and dynamics of S. serrata de-scribed in this study provide a suitable basisfor monitoring to evaluate the effectivenessof future management strategies on Kosrae.Using catch-per-unit-effort as a general pop-ulation monitoring tool should be augmentedby collection and analysis of size-class fre-quency data, which provide a responsive andmeaningful measure of changes in populationstatus (K.C.E., unpubl. data), and which couldbe readily implemented by local managers.The major challenge to effective long-termmanagement and conservation of mangrovecrabs will instead be in development of an ap-propriate management approach that incor-porates new biological information to adjustharvest regulations appropriately while stillaccommodating the unique cultural aspectsof Kosrae.

acknowledgments

We thank Rosamond Naylor, Burton Singer,Thomas J. Smith III, Thomas G. Cole, JamesBaldwin, Stacy Rowe, and Steve Ralston foruseful discussions and advice during variousphases of this work; two anonymous re-viewers for their comments on the manu-script; and three anonymous reviewers fortheir comments on an early version of amanuscript based on this work. Thomas G.Cole provided Figure 1. We thank SimpsonAbraham, Robert Hauff, Erick Waguk, andMadison Nena for organizational and logisti-cal support, and Christo Artusio, MaxwellSalik, Jason Jack, Rikitari Lowary, and Hol-den Nena for assistance in the field. We aregrateful to the many Kosraeans who reported


tagged crabs or otherwise expressed their in-terest in the project.

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Appendix

Here we construct a simple stage-structured matrixmodel for the population of mangrove crabs susceptibleto capture by traps, which we then use to develop sec-tor-specific estimates of total mortality rates that mightunderlie spatial variation in size structure of mangrovecrabs inhabiting interior mangrove channels around Kos-rae. The model describes the dynamics for the seven sizeclasses we observed in our collections, and has the form:

n1

n2

..

.

n7

266664

377775

tþ1

¼

s1ð1� T12Þ 0 � � � 0 0

s1T12 s2ð1� T23Þ 0 � � � 0

0 s2T23. .

. ...

..

. . ..

s6ð1� T67Þ 0

0 � � � 0 s6T67 s7

2666666664

3777777775

�

n1

n2

..

.

n7

266664

377775

t

þ

J

0

..

.

0

266664

377775

where the n terms describe abundance in each of theseven size classes observed in our trap data, the Tij termsdescribe the probability of a crab in size class i moltingand making the transition to size class j, si is the size


class–specific probability of survival, and J describes the(constant) rate of recruitment by juvenile crabs to thesmallest observable size class. All rates and probabilitiesare specified at a daily time scale. We used numericalsimulation to calculate the stable size-class distribution(Caswell 2001) and assume that any density-dependentmechanisms affect the population dynamics in such away that a stable steady state exists and that the stablesize-class distribution achieved at this steady state is iden-tical to that reached under density-independent dynamicswith R ¼ 1 (i.e., a constant population size). By fittingfrequency distributions, rather than abundances, weeffectively factor out the effects of recruitment and canfocus solely on attrition rates.

Our data suggest that crabs’ availability or susceptibil-ity to capture in traps increases with size. We thereforeassume that crabs smaller than 80 mm carapace width(CW ) are either not available in the sampled habitats orare not susceptible to capture and that availability or sus-ceptibility increases with size to the point that crabs inthe 143 mm CW size class are fully available and suscep-tible to capture. We accommodated this pattern in the

course of fitting the model to the data by specifying andapplying an arbitrary capture ogive of the form [0.2 0.60.8 1.0 1.0 1.0 1.0] 0 to the predicted stable size-class dis-tribution. A simple sensitivity analysis demonstrates thatreasonable variation in the capture ogive does not sub-stantially affect variation among sector-specific estimatesof attrition rates (results not shown).

We estimate total mortality rates ðsi ¼ 1�miÞ by fit-ting predictions for catchability-corrected stable size-class distributions to observed size-class distributions, inwhich each crab is assigned to a size class based on its car-apace width and the size-class distribution identified asdescribed in the text of the paper. Both observed andpredicted stable size-class distributions were expressed asfrequency distributions to avoid the need to estimate a re-cruitment term. Based on patterns apparent from visualinspection of the size-class distributions, we arbitrarilybroke each size-class distribution into a group of smallsize classes and a group of large size classes and fit a sep-arate attrition rate to each group. Parameter estimateswere obtained using the ‘‘nlinfit’’ function in Matlab(R2006b 7.3 http://www.mathworks.com).


Documents

Population Characteristics of the Mangrove Crab Scylla serrata