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 (Forsska˚l, 1755) in Kosrae, Federated States of Micronesia, have prompted con- cern regarding long-term persistence of this important cultural and economic resource. 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 in population structure. Many population characteristics, including estimated life span, 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 driven by variable harvest pressure that reflects distribution of the human population and location of emerging commercial harvest operations. Effective management of mangrove crabs is therefore likely to benefit from application of size-based or sex-based restrictions on harvest and might usefully incorporate spatially explicit strategies, 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 genus 1993, Naylor and Drew 1998, Marichamy Scylla are valued as a source of food and and Rajapackiam 2001). They occur from In- income throughout much of the tropical dia and Southeast Asia to parts of Japan and Indo-Pacific and as a consequence have China and throughout the Pacific islands been depleted, in both abundance and size, (Macnae 1968), as an introduced species in throughout much of their range (Brown Hawai‘i, and at higher latitudes in Australia and on southern Africa’s eastern coast (Hill

1 Funding was provided by the MacArthur Founda- tion initiative on Population, Consumption, and Envi- 3 Corresponding author (phone: 707-822-5505; fax: ronment (grant no. 97-49885A-WER to B. Singer, 707-822-5535; e-mail: [email protected]. K.C.E., R. Naylor, and S. Abraham) and the USDA For- 4 National Marine Fisheries Service, Southwest Fish- est Service. Manuscript accepted 14 April 2007. eries Science Center, Fisheries Ecology Division, 110 2 Institute for International Studies, Center for Envi- Shaffer Road, Santa Cruz, California 95060. ronmental Science and Policy, Encina E411, Stanford 5 Department of Fisheries Biology, Humboldt State University, Stanford, California 94305-6055. Current University, 1 Harpst Avenue, Arcata, California 95521. address: Conservation Strategy Fund, 1160 G Street, 6 USDA Forest Service, Pacific Southwest Research Arcata, California 95521. Station, 60 Nowelo Street, Hilo, Hawai‘i 96720. Current address: School of Forest Resources and Conservation, University of Florida, Gainesville, Florida 32611. Pacific Science (2008), vol. 62, no. 1:1–19 7 Division of Forestry and Wildlife, Kosrae Island : 2008 by University of Hawai‘i Press Resource Management Authority, P.O. Box DRC, All rights reserved Kosrae State, Federated States of Micronesia 96944.

1 2 PACIFIC SCIENCE . January 2008

1974, Heasman et al. 1985). Scylla species are cessfully for mates until achieving fully adult harvested for subsistence throughout their morphology (e.g., large claws) upon reaching range, are the focus of small- to moderate- carapace widths of 140–160 mm (Perrine scale commercial fisheries in some areas, and 1978, Heasman et al. 1985, Knuckley 1999). are farmed commercially in Southeast Asia Scylla serrata exhibits sexual dimorphism, with (e.g., Trin˜ o et al. 1999). Despite their impor- male crabs tending to be heavier than females tance, little has been done to examine the of similar carapace width (Chakrabarti 1981). consequences of harvest for wild populations, Reproduction occurs year-round in the especially in developing countries. tropics, with seasonal maxima that appear On the island of Kosrae, in the Federated to coincide with seasonally high rainfall States of Micronesia (FSM), Scylla serrata (Le Vay 2001). As in other Scylla species, cop- (Forsska˚l, 1755) (the sole species of man- ulation takes place directly after the female grove crab present) is one of the most valu- molts and involves an extended period of able resources extracted from the mangrove mate guarding by the male. Ovigerous forest (Naylor and Drew 1998). These crabs females swim substantial distances offshore— have cultural value as a central part of family from a few to many tens of kilometers— feasts and as gifts to expatriates, and eco- where the eggs hatch and larvae enter the nomic value derived from export, domestic plankton (Perrine 1978, Hyland et al. 1984, exchange, and commercial sale to local hotels. Hill 1994). Repeated spawning by females ap- Recent socioeconomic studies show that de- pears to be possible but has not been demon- spite apparent declines in catch-per-unit- strated conclusively (Ong 1966, Perrine 1978, effort and harvest effort (households engaged LeVay 2001). in harvest and number of trips per month) Juvenile S. serrata are most common in during the 1990s, export of crabs for sale and intertidal habitats, whether on mudflats or in as gifts actually increased substantially, by mangroves (Hill 1975, 1978, Hill et al. 1982, factors of 8 and 2.5, respectively, from 1997 Chandrasekaran and Natarajan 1994), mov- to 2000 (Naylor and Drew 1998, Naylor et al. ing into deeper habitats as they grow (Hill 2002). Anticipated economic and human pop- et al. 1982). Larger male S. serrata are com- ulation trends are likely to increase demand monly found in mangrove channels or in as- for this valuable resource, as well as place sociation with burrows located in mangrove greater pressures on the mangrove forests forests, on mudflats, or in the banks of chan- that provide key habitat for the species nels, as well as on neighboring reef flats (Naylor et al. 2002, Hauff et al. 2006). Such (Perrine 1978). These burrows can serve as trends, and a general sense among Kosraeans refugia during low tide and for mating. Fe- that mangrove crabs are becoming increas- male S. serrata, on the other hand, are far ingly scarce, have raised concerns regarding more commonly found on subtidal reef flats the sustainability of current harvest levels than in the mangrove forest proper (Hill and persistence of mangrove crabs as a viable 1975, Perrine 1978, Nandi and Dev Roy and valuable resource. 1990), migrating to and from the mangroves Scylla serrata is a large portunid crab that to mate, commonly around the time of the can reach a maximum carapace width (CW ) new moon (Perrine 1978). Whereas adult well in excess of 200 mm over the course of crabs may move considerable distances out a 3- to 4-yr life span (Perrine 1978, Heasman to sea, previous studies have not found evi- 1980, Robertson 1996). Female S. serrata dence that crabs commonly undertake sub- mature at between 80 and 120 mm CW stantial movements along the coast (Hill (Hill 1975, Heasman et al. 1985, Prasad et al. 1975, Perrine 1978, Hyland et al. 1984). 1990, Robertson and Kruger 1994; see also Most movement on a daily basis occurs at citations in Perrine 1978). Male S. serrata night as crabs move from burrows to channel mature physiologically at 90–110 mm CW and reef habitats to forage. but may not be large enough to compete suc- Mangrove crabs have historically been the Scylla serrata in Kosrae, FSM . Bonine et al. 3 focus of an artisanal fishery on Kosrae, with 1,562 ha and line two-thirds of the coastline almost all crabs being captured by removal (Whitesell et al. 1986, Ewel et al. 2003). The from burrows by hand or with simple tools, island is divided into four municipalities (Ta- by ‘‘torch fishing,’’ which entails walking the funsak, Lelu, Malem, and Utwe), each with a reef flats at night during low tides seeking variety of villages and settlements. At the crabs and other target species, with baited time of this study, the population of Kosrae lines and more recently with baited traps was ca. 7,700 (FSM Census 2002), most of (Perrine 1978, Smith 1992). Most households whom lived on the narrow coastal plain along on Kosrae harvest crabs from nearby man- the northern, eastern, and southern sides of grove forests, but larger-scale family-based the island. The largest population concentra- commercial fisheries emerged in the 1990s tions are in Lelu Island and the village of in Tafunsak and Utwe, where they are said Tafunsak. Smaller settlements lie along the to have substantially increased harvest rates road connecting the two, as well as farther (Naylor et al. 2002). Before this study, there along the road in Malem and Utwe. A village was no restriction on mangrove crab harvest, on the western part of the island (Walung, in but a minimum size limit for retention of 6 the municipality of Tafunsak) was, at the time inches (152 mm) was enacted in 2000. In con- of this study, reachable only by boat. trast to many fisheries on relatively long- As part of a broader study of resource use lived, large crustaceans in North America, in the mangrove forests of Kosrae, the is- Australia, and elsewhere, in which harvest is land’s coast was divided into nine sectors restricted to males, female crabs—including based on the position of rivers and population those bearing eggs—are commonly retained centers (Hauff et al. 2006). We focused much on Kosrae (Naylor et al. 2002). of our sampling effort in four of these sec- In this paper we report on a study con- tors: Lelu Harbor and Pukusrik in the munic- ducted between October 1998 and May 2001 ipality of Lelu, Okat in the municipality of on the island of Kosrae, Federated States of Tafunsak, and a long stretch of coastline ex- Micronesia, to provide information on popu- tending from the village of Utwe to the set- lation characteristics of S. serrata from which tlement of Isra, both in the municipality of inferences regarding population dynamics and Utwe (Figure 1). Additional collections were the effects of harvest can be drawn. The goal made near Walung and the settlement of of this effort is to inform development of Yemulil in the municipality of Utwe, farther management strategies that will ensure con- WNW along the coast from Isra; the data tinued availability of mangrove crabs, yet are from those sites are used in a more limited simple for local resource managers to imple- manner due to differences in the habitat types ment and evaluate. from which the crabs were collected. Although the overall species composition of the mangrove forests shows little system- materials and methods atic spatial variation around Kosrae (Ewel et al. 2003), those forests close to population Study Area, Arrangement of Sampling centers or easily accessible by roads experi- Locations, and Description of Habitats Sampled ence higher rates of removal of mangrove Kosrae (5 19 0 N, 163 00 0 E) (Figure 1) is a wood. Extensive logging often selectively re- high volcanic Pacific island 113 km2 in area, moves valued tree species and generates large much of which is steep forested mountains gaps that may not be recolonized by man- that are surrounded by a narrow coastal plain grove hardwoods (Hauff et al. 2006). Our and a fringing coral reef. The climate is moist Lelu Harbor and Okat sites were similar in and hot, with little interannual variation in that the mangroves in both areas formed a rainfall (5,000–7,000 mm per year) or tem- more or less continuous strip between up- perature (annual average ca. 27C). Diverse, lands and reef flats, penetrated occasionally intact mangrove forests cover approximately by relatively narrow river channels (Figure 4 PACIFIC SCIENCE . January 2008

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 in text.

1). In contrast, the mangrove forests at the might be separated from the open ocean by Pukusrik and Utwe-Isra sites were both reef flat or beach strand; (2) ‘‘riverine,’’ which bounded on the seaward side by berms of lay within 150 m of a riverbank; and (3) ‘‘in- coral sand, containing broad tidal channels terior,’’ which included the remainder of the connected to a larger bay through relatively mangrove forest not part of the previous two narrow gaps (Figure 1). The channel at zones (Hauff et al. 2006). Because the relative Utwe-Isra included substantial reef habitats. extent of each of these zones varied from The mangrove forest in Puksurik rested on sector to sector and either the fringe or riv- sandier soil and contained springs in several erine zone was entirely lacking in some areas places. This mangrove forest was also shorter, (Hauff et al. 2006), we focused most of our probably reflecting a long history of small- sampling in interior mangrove forests. Con- scale harvesting. straints arising from accessibility and our pri- We stratified the mangrove forests where mary method of sampling (traps) prevented we sampled mangrove crabs into three zones: us from sampling areas of the mangrove for- (1) ‘‘fringe,’’ which lay within 100 m of the est close to the landward margin or in slightly seaward margin of the mangrove forest and elevated areas between channels. Scylla serrata in Kosrae, FSM . Bonine et al. 5

Crab Sampling and Estimation of Catch-per- cases where a crab was returned missing a sin- Unit-Effort gle bead of the six, identification was accom- plished using the remaining beads and the We used baited traps as our primary method individual’s carapace width. of collecting S. serrata but also collected crabs opportunistically by hand during surveys of the mangrove forest. All crabs, regardless of Statistical Analysis and Demographic Modeling method of capture, were measured (carapace To estimate catch-per-unit-effort, we defined width in millimeters), weighed (to the nearest effort as a single trap fished at a particular 0.1 kilograms), and identified by sex. location for one night and calculated catch- Traps had approximate dimensions of 1.2 per-unit-effort as the number of crabs trap1 by 0.6 by 0.3 m, were fitted with 2.5-cm wire night1, modeled as a Poisson process. We as- mesh, and were baited with one or more sumed that traps were fishing independently types of bait (e.g., tuna, shark, reef fish, pig and evaluated this assumption by examining remains, etc.). In a typical set, six to eight mean catch-per-unit-effort as a function of traps were deployed at intervals b20 m the number of traps in a set as well as mean along the length of a channel through the catch-per-unit-effort as a function of trap mangrove forest. Traps were deployed at position within a set. arbitrarily selected locations in waterways We estimated allometric relationships be- accessible to a small motor skiff where crabs tween carapace width (L, in millimeters) and were expected or known to occur. Effort con- weight (W, in kilograms) by fitting models centrated on collecting a representative sam- b of the form W ¼ aL using standard non- ple from the sector but was not directed linear regression techniques. Size data for all according to a formal randomized sampling captured crabs were included in the analysis. design. Traps were checked daily, after hav- Based on preliminary examination of the ing fished a complete flood-ebb tidal cycle, data, separate models were fit for male and and were typically fished for 2–4 consecutive female crabs. Preliminary examination of in- nights at a given location. dividual condition (WL3) suggested that Over the course of implementing our sam- among both male and female crabs a small pling program, it became apparent that tuna subset of individuals existed whose weight was clearly the most effective bait, and we clearly exceeded the range typical of the switched to its use exclusively. To estimate population at a given carapace width. We catch-per-unit-effort, we used only data col- therefore divided the population using an lected from traps that were baited with tuna (arbitrary) threshold condition of 2:8 107 and deployed in interior channels. (kg mm3) and fit separate allometric rela- tionships for each group within each sex. To identify size classes, we binned cara- Tagging Study pace widths for all crabs in our data set at All crabs captured were marked by affixing 4 mm intervals over the range of 80 mm a series of six roughly cylindrical glass beads to 220 mm and applied Hasselblad’s (1966) (flat side down, hole oriented vertically) to steepest-descent algorithm for estimating the center of the carapace with cyanoacrylate the parameters of a mixture of normal distri- glue and released at the point of capture. butions. We used two-sample Kolmogorov- Each crab received a unique combination of Smirnov (K-S) tests to compare distributions colored beads to allow identification upon of carapace width among subsets of the data recapture. Local crab harvesters were encour- based on zone, sector, or sex. In addition, dis- aged through various outreach efforts to tributions of carapace width for subsets of the report the day, location, and bead color se- data were compared visually to the overall quence of any tagged S. serrata captured in distribution of size classes to evaluate repre- the course of normal harvesting. In the few sentation of size classes within subsets of the 6 PACIFIC SCIENCE . January 2008 population. We estimated Hiatt’s (1948) rela- recruitment and average size-specific molting tionship for sequential size classes by fitting rates of mangrove crabs do not vary around the model Liþ1 ¼ c þ dLi to the estimated the island. By attrition, we mean the com- mean carapace widths ðLiÞ for each size class bined effects of natural mortality, mortality using least-squares regression. due to harvest, and any size-specific migra- We developed crude estimates of migra- tion to or from the sampled habitat (e.g., tion distance for tagged crabs based on the emigration by larger crabs from interior location of release and recovery. Because lo- channels of the mangrove forest to more cation data were often imprecise, we were subtidal habitats). To avoid confounding our able to quantify movement at a spatial scale analysis with variation in size distributions on the order of a few kilometers (i.e., to re- and sampling effort related to habitat type, solve movement between sectors but with we used only observations collected with little precision within sectors). traps set in interior channels. Because tags are lost when a crab molts, A detailed description of the model, its the longest period that an individual is known construction, and our analytical approach to retain a tag (i.e., the maximum interval are provided in the Appendix and are sum- between the release and recovery of a tagged marized here for convenience. The model individual) provides a lower bound on the describes the development of size-class fre- intermolt interval. The longest of these in- quency distributions according to the rates tervals therefore provides a measure of the of transitions among size classes in a stage- intermolt interval. To estimate minimum in- structured population and, based on these dy- termolt intervals (I in days) as a function of namics, estimates attrition rates necessary to carapace width (L in millimeters), we fit yield predicted size-class frequency distribu- a model of the form I ¼ expfa þ b Lg sub- tions that match our observations. We de- ject to the following constraints: (1) all fined the daily probability of an individual’s observed intervals between release and recap- making the transition from one stage to the ture of a tagged crab had to fall below model next as the inverse of the intermolt interval predictions, and (2) the smallest difference predicted for the mean carapace width of between predicted and observed release- each size class. We lacked the full com- recapture intervals was to be minimized. plement of information necessary to con- This model has a structure similar to con- struct a credible density-dependent model. stant-ratio patterns in intermolt intervals re- We therefore assumed that any density- ported for other crustacean species (Caddy dependent mechanisms affect the population 2003) but instead relates intermolt interval dynamics in such a way that a stable steady to individual size. Because estimation of ex- state exists and that the stable size-class distri- trema is sensitive to sampling effort, and the bution achieved at this steady state is identical smallest crabs are apparently less susceptible to that reached under density-independent to capture, we corroborated this analysis with dynamics with R ¼ 1 (i.e., a constant popula- data reported by Arriola (1940) for intermolt tion size) (Caswell 2001). Because crabs in periods observed for two mangrove crabs smaller size classes (less than ca. 130 mm reared in captivity and a single field measure- carapace width) were either less available or ment of an intermolt interval reported by less susceptible to capture in our traps, we Perrine (1978); however, these data were not included size class–specific catchability in included with those used to fit the model. the process of fitting model-predicted stable We used information obtained on inter- size-class distributions to the data. Prelimi- molt intervals and size-class distributions to nary inspection of the size-class distributions construct a simple model for estimating rates also indicated that attrition rates affecting of attrition affecting the population of man- smaller size classes might differ from those grove crabs susceptible to capture in our sam- affecting larger size classes, so we fit models pling program, under the assumptions that that estimated attrition rates separately for Figure 2. Allometric relationships between carapace width (mm) and weight (kg) for (a) male and (b) female mangrove crabs (Scylla serrata) on Kosrae, FSM. Open circles are for ‘‘light’’ crabs and filled circles are for ‘‘heavy’’ crabs, defined according to a condition threshold; see text for details. Lines indicate allometric relationships between weight (W ) and carapace width (L) of the form W ¼ aL b; parameter values in Table 1; we plot the relation for ‘‘heavy’’ males in (b) for comparison to ‘‘heavy’’ females. Symbols indicate sector: Lelu (a), Pukusrik (j), Okat (s), Walung/Yemulil (v), Utwe-Isra (y). 8 PACIFIC SCIENCE . January 2008

TABLE 1 Allometric Relationships between Carapace Width (L, in mm) and Weight (W, in kg) Estimated by Fitting Models of the 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) 1:23 108 (4:14 1010,2:42 108) ‘‘Heavy’’ males 37 3.16 (2.78, 3.54) 1:73 107 (1:51 107,4:96 107) Normal females 72 2.69 (2.36, 3.02) 6:81 107 (4:85 107,1:85 106) ‘‘Heavy’’ females 6 1.94 (2.23, 6.11) 7:38 105 (1:59 103,1:72 103)

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

sets of contiguous size classes designated on allow a robust allometric relationship to be the basis of apparent breaks in the size-class developed and were not significantly differen- distribution. tiable from any other group, but qualitative examination suggested that they were in the same range as heavy males and exhibited a results similar allometric relationship (Figure 2). We detected no significant differences among Life History and Population Characteristics sectors in the allometric relationships be- sex ratio. Overall, male S. serrata out- tween carapace width and weight for either numbered females (3.3 :1, n ¼ 357, total) re- male or female crabs, but this analysis had gardless of method of capture (trap: 3.2:1, limited power due to small sample sizes. n ¼ 339; opportunistic capture by hand: 5:1, size structure and susceptibility n ¼ 18). The sex ratio varied significantly by to capture. Mangrove crabs exhibited dis- sector; bias toward males in our samples was tinct size classes with little overlap (Figure 3). substantially lower (1.81 :1, n ¼ 90) in Utwe- The progression of mean carapace widths Isra than elsewhere (Lelu Harbor: 6.00 :1, was described by a linear relationship of n ¼ 21; Pukusrik: 4.92 :1, n ¼ 83; Okat: the form Liþ1 ¼ c þ dLi (Hiatt 1948) with 5.87:1, n ¼ 103). None of the captured fe- c ¼ 29:721 (95% CI: 16.565, 42.878) and males was ovigerous. d ¼ 0:912 (95% CI: 0.825, 0.998). Male and allometric relationships. Allometric female crabs exhibited similar size classes, relationships for male crabs differed signifi- and the overall size distribution of crabs did cantly from those for female crabs, with male not differ significantly between sexes (K-S crabs tending to be heavier than females for a test, P ¼ 0:31). The size-frequency distribu- given carapace width (Figure 2, Table 1). For tion of crabs in our samples suggested that both male and female crabs, we observed a the smallest crabs susceptible to capture in distinct subset of individuals that were ap- our traps were ca. 100 mm in carapace width proximately twice as heavy as most other and that susceptibility to capture increased crabs for a given carapace width, a trait more with size. commonly observed among male crabs (37 of stage duration and estimated life 255 were ‘‘heavy’’ [14.5%]) than females (6 of span. Eighty S. serrata were recaptured dur- 78 [7.7%]) (Figure 2, Table 1). Sample size ing the course of our study, 12 of which were limited our ability to estimate the exponential at large for at least 1 month after being term of the allometric relationship for all but tagged and released. The maximum observed the lighter subset of male crabs, but visual in- interval between release and recapture (I spection strongly suggests that a power-law in days) increased with carapace width (L adequately describes the data (Figure 2, in millimeters) according to I ¼ expf0:009 þ Table 1). Heavy females were too rare to 0:0314 Lg (Figure 4), which is consistent Scylla serrata in Kosrae, FSM . Bonine et al. 9

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 proportional representation of each size class in the sample. Thick line shows the composite distribution for the population, formed as the sum of the scaled size class–specific normal distributions.

with a pattern of increasingly long intermolt 1940, Ong 1966), after which our data sug- intervals over the course of a typical crusta- gest a minimum period of 979 days (ca. 2.7 cean life cycle. Information on intermolt yr) to reach the largest observed size class. intervals for two crabs reared in captivity dispersal patterns. Almost all (79 of and two field observations suggests that our 80) recaptures occurred in the same sector model also holds for the smaller size classes where the crab was initially captured or last in our data set (Figure 4) (Arriola 1940, observed, including four of five crabs at large Perrine 1978). for more than 100 days. Crabs that remained To estimate the life span of S. serrata on within a sector traveled no more than ca. 2 to Kosrae, we assumed that (1) the largest crabs 3 km, and it is likely that most distances were observed in our study are representative of substantially smaller (i.e., <1 km, based on the maximum size attained by S. serrata on descriptions of release and recapture loca- Kosrae, and (2) on average, an individual tions). One male crab (166 mm CW ) moved crab requires 6 months to grow to the small- ca. 8.5 km from Walung to Okat during 174 est size class observed in this study (Arriola days at large. 10 PACIFIC SCIENCE . January 2008

Figure 4. Release-recapture intervals for mangrove crabs as a function of carapace width. Closed circles indicate intervals for mangrove crabs tagged and released as part of this study. Open circles connected by solid lines indicate sequential 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 line indicates ‘‘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 to corroborate the model at smaller size classes.

Spatial Variation in Population Characteristics interior channel habitats differed signifi- cantly among sectors (K-S test: Pukusrik ver- Size structure of mangrove crabs varied across sus Utwe-Isra, P ¼ 0:0124; Pukusrik versus the landward-seaward axis of the mangrove Okat, P ¼ 0:0007; Utwe-Isra versus Okat, forest, as well as among sectors around the P < 0:00001), and these patterns reflect spa- island (Figure 5). Crabs captured in traps de- tial variation in proportional representation ployed in fringe channels (near the edge of of size classes (Figure 6). The dominant size the mangrove forest) were significantly larger class observed in Utwe-Isra (178 cm) was al- than crabs trapped in other zones of the for- most absent in Pukusrik and Okat, where the est (Figure 5): Utwe-Isra: K-S test, P < 0:001 dominant size classes were centered at 162 cm (males), K-S test, P ¼ 0:0058 (females); all and 144 cm, respectively (Figure 6). These sectors: K-S test, P < 0:0001 (males), K-S patterns were reflected in separate compari- test, P < 0:0001 (females). sons for male and female crabs (results not Size distributions for crabs captured in shown), although the significance of the dif- Scylla serrata in Kosrae, FSM . Bonine et al. 11

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. ferences tended to decline with sample size, 7). We did not detect significant spatial varia- particularly for females. Although the small tion in attrition from either smaller or larger number of crabs captured in Lelu Harbor size classes. For Pukusrik and Okat, however, limited evaluation of size-class structure and estimated attrition affecting the largest size comparisons with other sectors, size-class dis- classes was significantly higher than attrition tribution there appears to be consistent with affecting the smaller size classes (Table 2, the islandwide pattern and is perhaps most Figure 7). A similar increase in attrition on similar to that observed in nearby Pukusrik the two largest size classes was apparent in (Figure 6). Lelu Harbor and Utwe-Isra, but the presence of only two size classes constrained the preci- sion of our estimate (Table 2, Figure 7). Spatial Variation in Attrition Rates Attrition rates differed more sharply across Catch-per-Unit-Effort transitions between smaller and larger size classes than among sectors, but the size class We estimated Poisson capture rates of 0.592 after which attrition rates increased varied crab trap1 night1 (95% CI: 0.511, 0.674) substantially among sectors (Table 2, Figure for male crabs, 0.161 (0.121, 0.210) for female Figure 6. Variation in representation of size classes of mangrove crabs by sector. Bars (left y-axis) indicate actual counts 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 in Interior Channel Habitats on Kosrae, FSM

Parameter Lelu Pukusrik Okat Utwe-Isra

Attrition (% days1) 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 (% days1) 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 of Kosrae, FSM. Model predictions are based on a stage-structured demographic model for S. serrata in interior channel habitats. Vertical gray lines indicate the transition between lower attrition rates on smaller size classes and higher attrition rates on larger size classes. Estimated attrition rates are provided in Table 2. 14 PACIFIC SCIENCE . January 2008 crabs, and 0.754 (0.661, 0.846) for all crabs. where differences among the four newly We did not detect significant spatial variation divided species may not yet have been estab- in catch-per-unit-effort (Table 2), nor did we lished. detect a significant variation in catch-per- We observed one previously unreported unit-effort over the course of the lunar cycle pattern in the Kosrae population: the pres- or the spring-neap tidal cycle (results not ence of a distinct segment of the population shown). Analysis of mean catch-per-unit- that was substantially heavier than most other effort as a function of the number of traps in crabs for a given carapace width. Predictions a set and as a function of trap position within of carapace width as a function of weight for a set validated our assumption that traps did these crabs suggest that these crabs were ap- not fish competitively when deployed along proaching maximum weight for their size interior channels (results not shown). and possibly nearing a molt (Figure 2). It remains unclear why an apparently bimodal discussion and conclusions distribution of weight in a size class rather than a more continuous distribution is ob- Mangrove crabs (Scylla serrata) on Kosrae, served. It is possible that our sampling was FSM, exhibit biological and ecological char- somehow biased toward crabs of low con- acteristics similar to those reported for the dition and high condition, and crabs of mod- species elsewhere in its range: male crabs are erate condition were simply less available to about twice as heavy as females at a given car- our gear, say, by spending more time in apace width (Figure 2) (Chakrabarti 1981); deeper habitats because they can sustain the males outnumber females in areas that are energetic costs of more extensive seaward more isolated from the open sea (Hill 1975, movements for feeding while also avoiding Perrine 1978, Nandi and Dev Roy 1990); in- predation, yet still have sufficient scope for dividuals tend to move to seaward habitats as growth (increase in mass) to warrant such they grow (Figure 5) (Hill et al. 1982); adult effort. S. serrata exhibit limited movement along the Our study provides compelling evidence coastline (results from the tagging study) for the effects of harvest on the population (Hill 1975, Perrine 1978, Pillans et al. 2005); of S. serrata on Kosrae by detecting spatial and individual crabs can achieve a maximum variation in attrition rates and catch-per- size that exceeds 200 mm CW over a life unit-effort that can be related to the distribu- span on the order of 3 to 4 yr (Figure 4) (Le tion of the human population and the pres- Vay 2001). Although we did not explicitly ence of commercial fishing operations. That evaluate maturity of captured crabs, we ob- harvest is the source of this spatial variation is served no cases (e.g., large, immature crabs) indicated by two patterns: (1) the increase in that contradicted size-maturity relations re- attrition rates between smaller and larger size ported elsewhere (Hill 1975, Perrine 1978, classes, and (2) spatial variation in the size Heasman et al. 1985, Quinn and Kojis 1987, class that marked the onset of the higher Prasad and Neelakantan 1989, Robertson attrition rates. Integrated over the life cycle, and Kruger 1994, Robertson 1996). By dem- estimated attrition rates are highest in Lelu, onstrating that S. serrata on Kosrae share followed in declining order by Okat, Pukus- traits with their conspecifics elsewhere our rik, and Utwe-Isra, which mirrors patterns study implies that insights gained from re- of human accessibility of the mangrove forest search conducted in other parts of the spe- (Table 2, Figure 7). Similarly, the general cies’ range are relevant for developing trend of increasing catch-per-unit-effort from effective management strategies for S. serrata Lelu Harbor to Utwe-Isra and Okat to Pu- on Kosrae. Perhaps more important, infor- kusrik is opposite the trend in estimated attri- mation about S. serrata on Kosrae, where it tion rates (Table 2) and is consistent with the is the only mangrove crab species present, effect of historically high harvest rates (Lelu) can be used unambiguously throughout the and the location of commercial fisheries species’ range, particularly in the tropics, (Okat and Utwe-Isra). We know of no eco- Scylla serrata in Kosrae, FSM . Bonine et al. 15 logical mechanism that could cause molting natural mortality, mortality due to fishing, rates to vary sufficiently among sectors to and net migration to the sampled habitat, we yield the observed differences in size-class can expect our estimates of attrition to exceed distributions if attrition rates were constant natural mortality simply by definition. The across sectors. question therefore becomes not only whether The apparently contrary estimates of attri- total rates of attrition are estimated correctly tion rates and catch-per-unit-effort for Utwe- but also whether harvest, emigration, or other Isra appear to rebut our argument that varia- factors suffice to account for the disparity in tion in harvest underlies spatial variation in rates. size structure of S. serrata on Kosrae. We We are confident that our estimates of at- expect that demographics provides a partial trition are sound and base this conclusion on explanation for this pattern, in that the popu- the validity of the analyses upon which these lation in Utwe-Isra was dominated by larger estimates depend, namely our analysis of size- crabs, which will normally be less abundant class distributions (Figures 3 and 6) and esti- simply due to cumulative natural mortality mation of intermolt intervals (Figure 4). First, even under unfished conditions. We believe, previous studies report size distributions that however, that in Utwe-Isra, attrition rates are are qualitatively consistent with our observa- in fact higher than our estimates suggest— tions and, in some cases, appear to indicate specifically, that a broader range of size distributions in which size classes have been classes is exposed to attrition rates that in- partly obscured by binning the data at a clude substantial harvest mortality—and that coarser resolution than used in our study the apparent discrepancy simply reflects an (Hill 1975, Perrine 1978, Williams and Hill unintended artifact of our sampling program. 1982, Robertson and Kruger 1994, Robertson The reduced degree of male bias in the sex 1996). Second, the observation of a linear re- ratio of crabs and the greater abundance of lationship between the carapace widths at se- larger crabs in general suggest that our habi- quential size classes is consistent with patterns tat designation rules were confounded by the reported for numerous other crustacean spe- extensive presence of readily accessible, pro- cies (Hiatt 1948, Caddy 2003), which lends tected reef flat habitat surrounded by man- further support to the conclusion that the groves in Utwe-Isra and led to the inclusion multimodal size-class distribution reported of collections from ‘‘fringe-like’’ habitats in here is real. Finally, our estimates of inter- our ‘‘interior channel’’ observations. If this is molt intervals are corroborated by indepen- the case, immigration of crabs of moderate dent observations of S. serrata from tropical size from more interior portions of the man- regions (Arriola 1940, Perrine 1978) and lead grove forest to the areas from which our sam- to an estimated life span comparable with that ples were taken is likely to mask or offset the reported for conspecifics elsewhere (Perrine effects of harvest on population size structure 1978, Heasman 1980, Robertson 1996, Le in the areas sampled and thus lead to a rela- Vay 2001). Indeed, we believe our analysis tively low estimate of attrition even though provides a reasonable prediction of average abundance (catch-per-unit-effort) has been intermolt interval in a given size class, al- reduced by harvest. though individuals may exhibit longer inter- Our estimates of attrition rates were sur- molt intervals (Ong 1966, Perrine 1978). In prisingly high and substantially exceed the any case, we note that any error in our esti- only estimate of natural mortality in S. serrata mate of intermolt intervals, say due to un- that we found reported in the literature (41– detected tag losses, will affect our estimate of 60% yr1 for populations in seasonal estuaries absolute attrition rate but will not affect [Hill 1975] versus 82–96% yr1 smaller size spatial variation in relative attrition rates ob- classes and >99% yr1 on larger size classes served between sectors. on Kosrae [this study]), yet we do not believe Although our analysis of spatial variation our estimates to be far off the mark. Indeed, in attrition rates indicates that harvest is a because our definition of attrition subsumes major source of mortality of mangrove crabs 16 PACIFIC SCIENCE . January 2008 on Kosrae, exactly how much harvest contrib- ceans (Pillans et al. 2005; K.C.E., unpubl. utes to attrition is difficult to resolve with the data) suggests that a more sophisticated sys- available data. Measures of attrition rates and tem of harvest ‘‘rotation’’ might allow ade- catch-per-unit-effort among the four sites quate access to larger, more valuable crabs, mutually suggest that harvest effects are not but such a system would be more difficult negligible and are likely to be substantial (see to design, communicate, and enforce. Truly also Naylor et al. 2002). Moreover, the simi- effective management of mangrove crabs, larity between our estimates of catch-per- like any similar species that has a poten- unit-effort and those reported for populations tially dispersive larval stage, will require con- of S. serrata in South African estuaries con- certed attention to sustainable management sidered to be too small to support a commer- at regional scales spanning populations that cial fishery (0.50–0.95 crabs trap1 night1 exchange recruits through larval dispersal [this study]; 0–1.67 crabs trap1 night1 (Mora and Sale 2002). [Robertson 1996]) suggests that the crab The status and dynamics of S. serrata de- population on Kosrae might not be able to scribed in this study provide a suitable basis sustain increased, or perhaps even current, for monitoring to evaluate the effectiveness levels of harvest. More work is needed to ob- of future management strategies on Kosrae. tain more exact estimates of abundance, to Using catch-per-unit-effort as a general pop- quantify the effects of harvest at local scales ulation monitoring tool should be augmented as well as the scale of the entire population by collection and analysis of size-class fre- more precisely, and to understand how re- quency data, which provide a responsive and cruitment contributes to population persis- meaningful measure of changes in population tence and sustainable rates of harvest. In any status (K.C.E., unpubl. data), and which could case, the concerns expressed by Kosraeans re- be readily implemented by local managers. garding the future status and sustainability of The major challenge to effective long-term this important cultural and economic re- management and conservation of mangrove source appear to be well justified. crabs will instead be in development of an ap- Management of the fishery on mangrove propriate management approach that incor- crabs on Kosrae should focus on maintaining porates new biological information to adjust population productivity by reducing mortality harvest regulations appropriately while still rates of immature crabs and increasing sur- accommodating the unique cultural aspects vival of females so that they can reproduce at of Kosrae. natural rates. In other crab populations (e.g., the Dungeness crab, Cancer magister) size- acknowledgments and sex-specific harvest regulations protect female crabs and immature male crabs while We thank Rosamond Naylor, Burton Singer, allowing open harvest on mature males yet Thomas J. Smith III, Thomas G. Cole, James do not appear to compromise the productiv- Baldwin, Stacy Rowe, and Steve Ralston for ity of a stock substantially (Hankin et al. useful discussions and advice during various 1997). Similar regulations have had a positive phases of this work; two anonymous re- effect in Australian mud crab populations viewers for their comments on the manu- (Pillans et al. 2005) and should be considered script; and three anonymous reviewers for for mangrove crabs on Kosrae. Evidence that their comments on an early version of a harvest has localized effects and that adult manuscript based on this work. Thomas G. crabs limit their alongshore movement sug- Cole provided Figure 1. We thank Simpson gests that an adaptive, comanagement strat- Abraham, Robert Hauff, Erick Waguk, and egy that incorporates reserves for mangrove Madison Nena for organizational and logisti- crabs might be an effective tool for managing cal support, and Christo Artusio, Maxwell this species (e.g., Castilla and Defeo 2004, Salik, Jason Jack, Rikitari Lowary, and Hol- Pillans et al. 2005). Evidence for rapid local den Nena for assistance in the field. We are replenishment of larger size classes in crusta- grateful to the many Kosraeans who reported Scylla serrata in Kosrae, FSM . Bonine et al. 17 tagged crabs or otherwise expressed their in- Hauff, R. D., K. C. Ewel, and J. Jack. 2006. terest in the project. Tracking human disturbance in man- groves: Estimating harvest rates on a Mi- Literature Cited cronesian island. Wetlands Ecol. Manage. 14:95–105. Arriola, F. J. 1940. A preliminary study of the Heasman, M. P. 1980. Aspects of the general life history of Scylla serrata (Forska˚l). 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fauna and flora of mangrove swamps and Robertson, W. D. 1996. Abundance, popula- forests in the Indo-West Pacific region. tion structure and size at maturity of Scylla Adv. Mar. Biol. 6:73–270. serrata (Forska˚l) (Decapoda: Portunidae) Marichamy, R., and S. Rajapackiam. 2001. in Eastern Cape estuaries, South Africa. S. The aquaculture of Scylla species in India. Afr. J. Zool. 31:177–185. Asian Fish. Sci. 14:231–238. Robertson, W. D., and A. Kruger. 1994. Size Mora, C., and P. F. Sale. 2002. Are popula- at maturity, mating and spawning in the tions of coral reef fish open or closed? portunid crab Scylla serrata (Forska˚l) in Trends Ecol. Evol. 17:422–428. Natal, South Africa. Estuarine Coastal Nandi, N. C., and M. K. Dev Roy. 1990. Shelf Sci. 39:185–200. Burrowing activity and distribution of Smith, A. J. 1992. Federated States of Micro- Scylla serrata (Forska˚l) from Hooghly and nesia marine resource profiles. FFA Re- Matla estuaries, Sundarban, West Bengal. port No. 92/17. J. 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Perrine, D. 1978. The mangrove crab (Scylla Mar. Biol. (Berl.) 71:187–192. serrata) on Ponape. Marine Resources Di- vision, Ponape, East Caroline Islands, Appendix Trust Territory of the Pacific Islands. Here we construct a simple stage-structured matrix Pillans, S., R. D. Pillans, R. W. Johnstone, P. model for the population of mangrove crabs susceptible G. Kraft, M. D. E. Haywood, and H. P. to capture by traps, which we then use to develop sec- Possingham. 2005. Effects of marine re- tor-specific estimates of total mortality rates that might underlie spatial variation in size structure of mangrove serve protection on the mud crab Scylla crabs inhabiting interior mangrove channels around Kos- serrata in a sex-biased fishery in subtropical rae. The model describes the dynamics for the seven size Australia. Mar. Ecol. Prog. Ser. 295:201– classes we observed in our collections, and has the form: 213. 2 3 2 3 s1ð1 T12Þ 0 00 6 7 Prasad, P. N., and B. Neelakantan. 1989. 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Bonine et al. 19 class–specific probability of survival, and J describes the course of fitting the model to the data by specifying and (constant) rate of recruitment by juvenile crabs to the applying an arbitrary capture ogive of the form [0.2 0.6 smallest observable size class. All rates and probabilities 0.8 1.0 1.0 1.0 1.0] 0 to the predicted stable size-class dis- are specified at a daily time scale. We used numerical tribution. A simple sensitivity analysis demonstrates that simulation to calculate the stable size-class distribution reasonable variation in the capture ogive does not sub- (Caswell 2001) and assume that any density-dependent stantially affect variation among sector-specific estimates mechanisms affect the population dynamics in such a of attrition rates (results not shown). way that a stable steady state exists and that the stable We estimate total mortality rates ðsi ¼ 1 miÞ by fit- size-class distribution achieved at this steady state is iden- ting predictions for catchability-corrected stable size- tical to that reached under density-independent dynamics class distributions to observed size-class distributions, in with R ¼ 1 (i.e., a constant population size). By fitting which each crab is assigned to a size class based on its car- frequency distributions, rather than abundances, we apace width and the size-class distribution identified as effectively factor out the effects of recruitment and can described in the text of the paper. Both observed and focus solely on attrition rates. predicted stable size-class distributions were expressed as Our data suggest that crabs’ availability or susceptibil- frequency distributions to avoid the need to estimate a re- ity to capture in traps increases with size. We therefore cruitment term. Based on patterns apparent from visual assume that crabs smaller than 80 mm carapace width inspection of the size-class distributions, we arbitrarily (CW ) are either not available in the sampled habitats or broke each size-class distribution into a group of small are not susceptible to capture and that availability or sus- size classes and a group of large size classes and fit a sep- ceptibility increases with size to the point that crabs in arate attrition rate to each group. Parameter estimates the 143 mm CW size class are fully available and suscep- were obtained using the ‘‘nlinfit’’ function in Matlab tible to capture. We accommodated this pattern in the (R2006b 7.3 http://www.mathworks.com).