Population dynamics of the mud crab Scylla serrata - a mark and recapture study on Mafia Island, Tanzania.

Transcription

1 Population dynamics of the mud crab Scylla serrata - a mark and recapture study on Mafia Island, Tanzania. Emma Björkvik Degree project for Bachelor of Science in Biology 30 hec Department of Marine Ecology University of Gothenburg Supervisor: Per-Olav Moksnes Local supervisor: Humphrey Matala Mahudi Mafia Island Marine Park, Tanzania Examiner: Kristina Sundbäck

2 ABSTRACT A mark and recapture study was accomplished on Mafia Island, Tanzania during an 8 weeks period in February and March The aim of the study was to assess population variables such as habitat distribution, dispersal, growth, population size and mortality in the mud crab Scylla serrata. The aim was also that the results would contribute in the development of a grow-out aquaculture of mud crabs on Mafia Island. Sampling was performed in two study areas (MIMP and Changarama) with a hierarchical design to assess dispersal distances. The habitat distribution was significant size-specific. Juveniles smaller than 40 mm in carapace width had the highest abundance on the mangrove back-flat. The dispersal of crabs was limited (on average 282 m during the 2 month study period), and the exchange between the two areas was small (5%), which means that the two areas could be managed as two local populations. The growth rate was decreasing with size and the time from settlement to maturity was estimated to 8 months. The population sizes were relatively small where the highest abundance of crabs was in Changarama (11923 number of mud crabs/425 mud crabs per ha. mangrove habitat). This means that together with the long generation time the populations could be overexploit with the wrong type of management. Because of a low recapture rates mortality was not possible to estimate. These results are important when assessing the effects of local fishing for adults and collection of juvenile seed-crabs for aquaculture on the natural populations.

3 1. BACKGROUND The mud crab Scylla serrata is a large portunid crab which inhabits mangrove habitats in the Indo-Pacific region. It lives both free-ranging and in burrows, subtidal, intertidal and inside the mangrove forests (Hill et al. 1982, Hyland et al. 1984, Mahika et al. 2005) and can reach a size well over 200 mm in carapace width (CW; Robertson 1996). The mud crab is an economically and commercial important species because of its tasteful and nutritiously meet (Keenan 2003) which generate high prices on an international market (Cowan 1984). Still, little is known about the mud crab s basic ecology and biology (Keenan 2003), especially in East Africa and regarding the ecology of juveniles In Southeast Asia, aquacultures of mud crabs have successfully developed and mud crabs survive and grow well in ponds or pens (Rodriguez et al. 2007). The market of mud crabs has increased rapidly in recent years and the aquaculture turns over a multibillion dollars industry (FAO 2007). The unmet demand has lead to an overexploitation of natural populations in many countries (Rodriguez et al. 2007). The fishery for mud crabs in East Africa is still low compared to Southeast Asia but the tourism is growing and consequently the demand for mud crabs (ASDI-VOCA 2005). So far in Tanzania, only small scale pilot farms have been established by various NGOs. In those farms the technique crab fattening has been used. In contrast to Southeast Asia were small juveniles are used as seed crabs (Rodriguez et al. 2007), subadults and small adults are used as seed-crabs in crab fattening farms ( g, equivalent to mm CW). The crabs are feed and kept in small individual cages for 1-4 months until they reach market size (>700 g, equivalent to >150 mm CW; Keenan 2003, Shipton and Hecht 2007, Mahika et al. 2005). The mortality of a mud crab in a crab-fattening farm (10% per month (ACDI-VOCA 2005) is higher than the estimates of natural mortality of mud crabs (2% per month; Mahika et al. 2005) which means that it is more sustainable to fish for adults compared to keep them in farms. Poorly developed crab-fattening farms together with an increasing demand for mud crabs could possible lead to overfishing of seed-crabs and a depletion of natural populations (Mahika et al. 2005). If a more sustainable way for aquaculture of mud crabs is developed in East Africa, the mud crab as a resource may have the potential to develop into an important alternative livelihood for resource poor coastal communities (Barnes et al. 2002). A more sustainable and eco-friendly way could be aquacultures based on small juveniles as seed-crabs, similar to the small-scale, grow-out mud crab farms recently developed in the Philippines (Trino and Rodriguez 2002). The small farms can be incorporated in the mangrove habitat which minimize environmentally damages (Trino and Rodriguez 2002) and enables a sustainable management of the farm on a local level. Recent studies on Mafia Island shows that small juvenile mud crabs could easily be collected on the mangrove back-flat (Karlsson 2009) and that small juvenile mud crabs have a higher predation mortality compared to sub-adults and adults (Palmqvist 2009). This indicates that the use of small juveniles as seed-crabs for farming would have a lower impact on local populations than collection of large mud crabs for crab-fattening farms. Though, to understand how a local population would respond to a local fishery of small seed crabs one must know more about the juvenile s population ecology. How big is a population in total numbers and what is the geographical distribution of a local juvenile population? How far can a juvenile and adult mud crab disperse over a day and a month? How fast can a juvenile mud crab reach sexual maturity? Does juvenile and adults use the same part of the mangrove habitat? Does population estimates of natural mortality support the experimental estimates of

4 size-dependent mortality? These are very important questions which need an answer to assess the potentially negative impact a seed-crab fishery could have on local populations. Mark and recapture techniques have successfully been used to asses migration and distribution of juvenile Scylla serrata in Australia (Hill et al. 1982, Hyland et al. 1984) and to estimate migration, growth, mortality and population size of juvenile Scylla paramamosain in Vietnam (Le Vay et al. 2007). Mark-recapture methods have also been used to assess growth and population densities of adult mud crabs in South Africa (Hill 1975, Roberson & Piper 1991) and in Tanzania (Barnes et al. 2002), but there is a lack of studies and understanding regarding migration, growth and mortality rates in juvenile mud crabs in East Africa. The aim of the study was to use mark and recapture techniques to estimate population variables such as population size, dispersal, growth, habitat distribution and mortality in the juvenile and adult mud crab, Scylla serrata. The aim was also therefore that the study should contribute to the development of a sustainable, grow-out aquaculture of the mud crab, on Mafia Island, Tanzania. These results are important for future resource management in the evaluation of environmental impacts and in the designing of appropriate management and conservation protocol. 2. MATERIALS AND METHODS 2.1 Study area The study area was located inside the marine park of Mafia Island (MIMP), Tanzania (Fig 1). To assess population dynamics of the mud crab, Scylla serrata, mark and recapture techniques, similar to those described by Le Vay et al (2007), were used in two adjacent areas inside the park during 8 weeks in February and March Average salinity in the waters outside of Mafia is 34.4 ppt and average yearly water temperature is 27 C (Newell 1959). Semi-diurnal tide affects Mafia, with average amplitude of 3.3 m (McClanahan 2000).

5 Fig. 1. Map of Mafia Island and the centres of the two study areas MIMP (A) and Changarama (B). 2.2 Sample sites Fourteen different sites for sampling in the two study areas, MIMP and Changarama, were used. The two areas were separated by 3.16 km (Fig. 2.). A hierarchical spatial sampling design was used to assess dispersal of juvenile crabs. There were 7 different sites of sampling within each study area, where a core-site (where marked crabs were released) constituted the centre of each area (MIMP and Changa; Fig. 2.). Around each core site, 6 sites were chosen at a distance of approximately 500, 1000 and m in each direction. The exact location of the sites was determined by the presence of juvenile habitats such as mangrove back-flats and tidal creeks (Fig. 2, Table 1). The core-sites were chosen because of high abundance of juveniles crabs (Karlsson 2009). The design allowed an assessment of dispersal distance up to 11 km (the distance between the two outermost sites, N4 and W4; Fig. 2), which tested if the two areas were one or two separate local populations. In contrast to mangrove back flats and fringes, where juvenile mud crabs are almost never seen during the day (Karlsson 2009), a few juvenile mud crabs were observed in small tidal creeks during the day in pilot studies. To assess if tidal creeks constituted a more permanent juvenile habitat for mud crabs, all crabs collected in tidals creeks (e.g. site N4, E3 and W4) were release in the same creek after tagging. Thus, a total of 5 releasing sites were used. Three different mangrove habitats were distinguished within the sites: fringes, back-flats (openings behind or between mangrove trees) and tidal creeks (Table 1.). The dominating mangrove species along the fringe and on the back-flat were Sonneratia alba. In the tidal creek Sonneratia alba dominated the fringe and Rhizophora mucronata vegetated further in the forest. The tidal creeks varied in length and width. The longest creek was at the site N4 with a length of over 1500 m in contrast to the other creeks' length, which varied between m. Changarama core-site was the widest, shallowest creek, and the creek with the largest volume. The average width was around 20 m and the depth at low tide varied between

6 20 cm to 40 cm. The other three creeks had a similar width and depth, where the width varied from 1 to 3 m and the depth on falling tide varied between 60 cm to 100 cm. Fig. 2. The study area on Mafia Island together with the sites. Table 1. The sampling sites with location from core-site, release site and habitat classification. Distance from Release site Habitat Sites core site (m) N m N N4 Tidal creek N3 930 m N MIMP Back-flat and fringe N 560 m N MIMP Back-flat and fringe MIMP 0 m MIMP Back-flat and fringe S2 430 m SW MIMP Back-flat and fringe S3 810 m SW MIMP Back-flat and fringe S m SW MIMP Fringe E m E Changa Fringe E3 910 m E E3 Tidal creek E2 570 m E Changa Fringe Changa 0 m Tidal creek W2 650 m W W2 Tidal creek W3 900 m W Changa Fringe W m W Changa Fringa 2.3 Sampling To assess population dynamics in different life stages, the data of collected mud crabs were separated into four size classes, <40 mm, mm, mm and mm in carapace width (CW). Mud crabs with a CW below 100 mm are recognized as juveniles (Hill et al. 1982). The sampling was carried out on foot during dark hours and on a falling low tide. Torches and handhold nets were used for catching crabs. All encountered mud crabs

7 (including adults) were caught and brought back to the lab for tagging which was located at Mafia Island Marine Park headquarters, close to the MIMP site. Depending on the targeted crab size, the sampling methods were different. When searching for the smallest juveniles on the mangrove fringes, back-flat and along the fringes of the tidal creeks, small aquaria nets were used, and the shallow water (< 10 cm depth) along shore and in puddles were carefully searched for small crabs. Larger crabs were sampled using larger hand-nets on a stick in up to knee-deep water while walking slowly in a falling low tide along the mangrove fringes and in the tidal-creeks. To estimate dispersal rate and distances from the two core-sites and assess geographic size of the population, all the sites of each area were sampled during three different 8 days periods, each period separated by two to three weeks. During each 8 days period, the sites at MIMP were sampled during 4 days and the sites at Changarama were sampled during 4 days this to assess short time dispersal. Because there was no possibility to sample all 7 sites at each area during one night, the core-site plus the 3-sites 500, 100 and m away from the core-site in one direction were sampled each night, changing direction each consecutive night. Thus, during each 8 days period, all sites were sampled two times. With this design, the core-sites were sampled 12 times and all the other sites 6 times each during the 2-month study. Because of dispersal which could occur in either direction from the coresite, the 4 different dispersal distances assessed (0-200 m, 500 m, 1000 m and m) were each sampled 12 times in each area. Because of no captured mud crabs at site N3, the site was not included in the results. The smallest mud crabs (<40mm) were found in shallow pools at the back flat and between mangrove roots. Bigger mud crabs were most successfully caught when the tide was on its way out, in water depth cm on the tidal flats along the mangrove fringe or in the tidal creeks. 2.4 Tagging and recapture After collection, the mud crabs were transported in buckets back to the lab. Over the night the caught mud crabs were kept in net-covered buckets filled with branches and sand. The next morning every individual was tagged with sequentially coded micro-wire tags (CWT-tags). The CWT-tags does not affect the crabs growth or survival (Van Montfrans et al. 1986, Ut 2002). The mud crabs were released the following night. The CWT-tags were injected into the base of the third walking leg on the right side of the crab using a handheld single shot injector (Northwest Marine Technology inc.). After the implantation, the mud crabs were checked with the hand held magnetic detector for CWT-tags (Northwest Marine Technology inc.) for successful tagging. After tagging, the mud crabs were put in buckets filled with water to help sealing the injection wound, the water was pored out after one hour to avoid an anoxic environment. Earlier studies have shown that micro-wire tags can be used on crabs as small as 15 mm in carapace width (Le Vay et al. 2007).Mud crabs smaller then 15 mm were instead marked with nail polish on the carapace. Notes were taken on carapace width (CW), weight, sex and collection site on all tagged crabs. Mud crabs newly moulted with soft shell were not tagged. All mud crabs collected after the first release of tagged crabs were checked for tags with the tag-detector. After recapture of a tagged mud crab, the tag was excised using dissection and read at the lab. Because of every sequentially tag unique number, it was possible to identify the recaptured mud crabs to individuals and estimate dispersal and growth since release. The dissection meant a great negative impact of the mud crab s future living so therefore recaptured crabs were killed using freezing prior to the dissection.

8 2.5 Population estimates Schnabel s census-method was used to estimate population size with mark and recapture data (Schnabel 1938): N = Ʃ(C*M)/ Ʃm N is the population estimation, C is the number of individuals caught in sample t, M is the number of individuals in the population marked before sample t and m is the number of recaptures in sample t. The method assumes that the population is closed (the population size (N) is constants during the whole study period), that all individuals in the population have the same chance of being caught, the probability of being caught is not affected by the marking, there is no loss of tags between two samples and all recaptured individuals are reported (Seber 1973). A 95% confidence interval was recorded using the ratio of N and Ʃ(C*M) in Chapman's poission table (Seber 1973). To estimate abundance of mud crabs per ha, the areas of each study area were estimated by multiplying the mangrove width and the length of the mangrove fringe at each study area. The sites areas were added and divided by the estimated number of individuals. The sites search-length and mangrove width was roughly estimated by data from a GPS. 2.6 Dispersal distance The distances between every site were registered by a GPS. Therefore, after recapture and reading the tag the recapture site was compared to the collection site. 2.7 Growth The size (mm CW) of all collected crabs was measured by using a ruler. To estimate the growth rate in recaptured crabs, the growth increment between release and recapture was divided by the number of days since the crabs was released (size at recapture - size at tagging / number of days = mm day -1 ). Using these growth rates, the average growth rate per size class was estimated. To obtain a crude estimate of the time from settlement (4 mm CW) to maturity (100 mm CW), the average growth rate for all size classes <100 mm CW was used Statistics To assess if the distribution of mud crabs was habitat or size-specific, the mean number of mud crabs caught per 100 m transect were tested in a two factors model of analysis of variances (ANOVA) as the dependent factor where habitat and crab-size were used as independent variables. Cochran s c-test was used to test the homogeneity of variances. Student-Newman-Keuls method was used in a multiple comparison post-hoc test.

9 3. RESULTS 3.1 Summation of capturing During the 8-week study-period a total of 556 mud crabs were caught in the two study areas. The catches were dominating by mud crabs in the size-classes mm CW (37%) and mm CW (33%). The abundance of smaller juveniles (e.g. <40 mm CW) was relatively low (a total number of 71 mud crabs caught), which indicates a period of low recruitment (Fig.2). MIMP was the area where smaller juveniles (<76 mm) were most common (62%) in contrast to Changarama where bigger juveniles and adults (>75 mm) were most common (60%). MIMP core-site and the tidal creek at MIMP N4 distinguished from the other sites with a high density of mud crabs smaller than 40 mm (Fig. 3). Total no. of caught Scylla serrata < Crab size (mm CW) Fig. 2. Pooled number of mud crabs found at all sites over a 8-week sampling period in 4 predefined size-classes (CW in mm). 40 Total no. of caught Scylla serrata m N 560 m N MIMP MIMP 430 m SW 910 m E 1470 m E 1810 m SW 810 m SW Distance from core-site 650 m W Changa 570 m E Changarama 900 m W 1400 m W <40 mm mm mm mm

10 Fig. 3. All sites with distance to core-site and total catch from the whole study period. 3.2 Habitat distribution Analyses of the three habitat types (mangrove back-flat, creek and fringe) showed a size-specific distribution of crabs resulting in a significant interaction effect between crab size and habitat (F=3.65, df=6,48 p=0.0045). The small juvenile mud crabs (<40 mm CW) were mainly found high in the intertidal on the mangrove back-flat where their abundance was significantly higher than on the other habitats, and significantly higher compared to two largest size-classes (>75 mm in CW) in the same habitat. In contrast, the larger mud crabs (>75 mm CW) dominated on the mangrove fringe and in the tidal creeks where their density were significantly higher then on mangrove back-flats. Their abundance on the fringes was significantly higher than the small juvenile mud crabs caught in the same habitat, though in the tidal creeks no significant difference between the crab-sizes was registered. The larger juveniles (41-75 mm CW) were most abundant in the tidal creeks, but their density did not differ significantly between habitats or compared to the other size classes (Fig. 4). Mean no. Scylla serrata per 100 m transect a a a a a a b b b b Back-flat Tidal creek Fringe < Crab size (mm CW) Fig. 4. Mean number mud crabs (+SE) per 100 m transect in three different habitats in four predefined sizeclasses (CW in mm). Different letters above indicate significantly different means between habitats within each size-class at p< Mark and recapture Of the total 556 caught mud crabs, 525 were tagged with CWT-tags and released. A total number of 21 tagged mud crabs were recaptured, representing 4% of all releases (Appendix A). Of these, 19 tags could be read (two were lost during dissection). Five of the identified mud crabs had moved from the releasing site.

11 The proportion recaptures within the released sites varied between 5.5% at the MIMPsite to 0-2.5% at the 4 tidal creek sites (Table 2). Recapture rate varied between 3.3% and 5.3% in the four size-classes (Table 3). Table 2. Total number released mud crabs at the 4 different releasing sites and the number of crabs recaptured at the site of release. Sites Habitat No. released No. recaptures at release site Prop. recaptures at release site N4 Tidal creek % MIMP Back-flat and fringe % E3 Tidal creek Changa Tidal creek % W2 Tidal creek Table 3. Summation of released and recaptured mud crabs in 4 size-classes. < Total no. released Total no. recaptures Proportion recapture/capture 5.0% 5.3% 3.3% 3.8% No. migrated crabs Proportion migrated crabs 0% 40% 14% 28% Dispersal distance Most of the recaptured mud crabs (74%) were caught at their release-sites and had therefore moved less then 100 m (Fig. 5). Only 5 mud crabs were recaptured outside the release areas, which had dispersed between 430 m and 5530 m (Table 4), giving an average distance moved from the releasing site of 282 m for all recaptured mud crabs. Mean dispersal distance within the group of mud crabs that had moved outside of the releasing site was 1878 m. All mud crabs recaptured outside the release areas had been released at the MIMP core-site and they had all moved in a southwest direction. Only one mud crab had moved across the two study areas, from the MIMP core-site to the site E3 of Changarama area. This mud crab had moved over 5.5 km in 5 days. All mud crabs that were caught outside of the releasing area were bigger then 61 mm, but there was no clear indication that the dispersal increased with size (Fig. 6, Table 3). The low number of recaptures prevented a test of size-dependent dispersal.

12 16 No. of recaptures Distance from release site (m) Fig. 5. Total number of recaptured mud crabs at different dispersal distances (m) from the release site in the two study areas. Dispersal distance implicated the distance a mud crab have moved in either directions outside the release site. Mud crabs that moved between m mean that they have been recaptured in the release site. Table 4. Summation of the mud crabs that have moved outside the release site. Site Site Time Sex Tagged Total Movment released recaptured free (days) size (mm) movement (m) (m/day) MIMP S4 40 M MIMP S2 5 M MIMP E3 5 F MIMP S3 15 F MIMP S3 15 M Distance from release site (m) Crab size (mm CW) Fig 6. Dispersal distance outside release areas (m) plotted against crab size (CW in mm) for the 19 recaptured mud crabs.

13 3.3.2 Growth A total of 6 mud crabs had increased in size between marking and recapture. The proportional growth increment in carapace width varied between 15% and 30%, which indicates that each mud crab had only been moulting ones. These moults had occurred over a period between 13 and 40 days. The two smallest mud crabs (36 and 44 mm in CW) have increased body size the most (Table 5). Mean growth per day varied between 10.2 mm and 0 mm and maximum period of time without moulting varied between 5 to 27 days in the 4 sizeclasses (Table 6; for more details see Appendix A). The average growth rate per size class decreased with size from 0.62 mm day -1 for crabs <40 mm CW to no detectable growth for crabs >100 mm CW. The average growth rate for all juvenile size classes was mm day - 1. Using this growth rate, the time between settlement and maturity was roughly estimated to 243 days or 8 months. Table 5. Summation of growth. Site Site Time Sex Tagged Recaptured Increased released recaptured free (days) size (mm) size (mm) size (%) MIMP MIMP % MIMP MIMP 14 M % MIMP S4 40 M % Changa Changa 13 M % Changa Changa 18 F % Changa Changa 33 M % Table 6. Mean growth and the maximum time without moult in size-classes. Size-class Mean growth/day (mm/day) Max no. days without moult <40mm mm mm mm Population estimates Schnabel s model was used to estimate population size (N) in size-classes, and at the two study areas. The population size at MIMP was estimated to 3826 individuals, with a 95% confidence interval of individuals representing 117 mud crabs per ha. At Changarama, N was estimated to with a 95% confidence interval of representing 425 mud crabs per ha. The population size within each size-class varied at MIMP between 53 to 796 individuals and at Changarama between individuals (Table 7). Because of lacking recaptures in size-classes <40 mm and mm at Changarama there was no estimation done in those two classes. The large confidence intervals in the population estimations of separate size classes were due to the low number of recaptures and indicate that these estimates are less dependable.

14 Table 7. Estimated population size in size-classes at each location with 95% confidence interval. MIMP Changa N - + N - + <40 mm mm mm mm Pilot study - juvenile mud-crabs, <15 mm Twenty-three juveniles were too small to tag with CWTs and were instead marked with nail polish. Three of the caught mud crabs were caught during the day and two of them were recaptured representing 8% of all mud crabs released with nail polish. The rest were caught during night. The recaptures were recaptured in the same area (20 m transect) as they were released, after one day. 4. DISCUSSION 4.1 Summary The habitat distribution was size-specific and small juvenile mud crabs (<40 mm) aggregate mainly on the back-flat during the night. The dispersal of mud crabs was limited (on average 282 m) and there was a low exchange between the study areas. The low exchange suggested that the two study areas, MIMP and Changarama can be viewed as two local populations for each cohort of settled crabs. The time from settlement to maturity was roughly estimated to 8 months. The relatively small population sizes within the two study areas (3826 and number of mud crabs), makes them sensitive to overexploitation. Because of a low number of recaptures, the natural mortality could not be estimated. These population variables will be important knowledge necessary in the assessing of what impact fishing for adults and juvenile seed-crab fishery would have on the local populations of mud crabs on Mafia Island.

15 4.2 Habitat distribution Mangrove back-flat seems to be an important habitat for small juvenile mud crabs (<40 mm CW) during the night. This is consistent with an earlier study on Mafia Island, where small juveniles also had a higher abundance on the back-flat compared with the fringe (Karlsson 2009). In the nail-polish study, a few small juveniles were recaptured during the day, which were in moulting or did moult before re-release. It may be possible that if a juvenile mud crab is in the process of moulting it will stay in the intertidal also during the day. After recapture they were re-released but were not caught the following night. This indicates that the small juveniles are not resident on the mangrove back-flat and seems to live subtidally during the day and migrate to the intertidal with the tides during dark hours. The results are consistent to an earlier study on Mafia Island where no juvenile crabs were found during the day, only at night (Karlsson 2009). Maybe this is because of their inability to dig burrows (Walton et al. 2006) or to avoid predation and dehydration (Karlsson 2009). The diurnal movement of young juvenile mud crabs on Mafia Island is not consistent with studies of mud crab from other areas. Juvenile Scylla serrata within a large mangrove creek in Southern Kenya showed similar densities at day and night along the mangrove fringe (D. Mirera unpubl. data). A study of Hill et al. (1982) in Australia showed that juvenile Scylla serrata smaller then 100 mm were found to be intertidal resident in the mangroves and on the tidal flats. In Vietnam, newly settled juvenile (0-9 mm) of Scylla paramamosain were found during day among the pneumatophores and in puddles in low tide (Walton et al. 2006). The juvenile behaviour seems to differ with different mangrove areas, maybe because of different conditions concerned mangrove species, sediment types and ecosystem dynamics. In the present study, juveniles were also found in high densities in tidal creeks, which haven t been shown earlier on Mafia Island. Mud crabs bigger then 40 mm CW seemed to migrate from the intertidal out to the subtidal with the falling low tide during dark hours; this pattern hasn t been studied before in East Africa. Though, this is consistent with an earlier study in Australia were mud crabs bigger then 100 mm migrated to the intertidal to feed and with the falling low tide migrate back to the subtidal (Hill et al. 1982). On the back-flat, the optimal time for capture was during the night on a falling tide when the water was just on its way to uncover the flats and puddles were formed. In the puddles juveniles smaller then 40 mm CW were most common. Along the fringes and in the tidal creeks the optimal tide for capture was with a water depth of cm in a falling tide. Along the fringe mud crabs bigger then 75 mm were mostly caught, and in the tidal creek juveniles smaller then 40 mm could be found on the fringe of the creek. In the tidal creek mud crabs bigger then 40 mm were caught in the middle of the creek and on the mud-flats when the tidal water was cm. 4.3 Dispersal The average dispersal distance of all recaptured crabs was less then 300 m. Only 5 of the 19 recaptured crabs were caught outside the release area during the 2 month study, and the average dispersal distance within the group of mud crabs that had moved outside of the releasing area was only 1.8 km. Although the low recapture rates make estimates of average dispersal distances less dependable, the data clearly suggest that dispersal among the studied mud crabs was limited. Thus, even though juvenile and adult mud crabs tend to migrate between the subtidal and intertidal (Hill et al.1982, Karlsson 2009), most seem to stay within a limited area along the coast (<300 m). This result indicates a relatively limited dispersal

16 area, which are consistent with earlier studies. A study in Australia with Scylla serrata have shown that the mean dispersal distance was less then 5 km with an exception for females which move longer distances probably to extrude eggs offshore (Hyland et al. 1984). Because of a low recapture rate it was not possible to compare dispersal distance between males and females and none of the caught females were carrying eggs. In a study of Scylla paramamosain in Vietnam, little migratory movement was found. The maximum recorded distances were 5.8 km and 12.3 km (Le Vay et al. 2007). In this study, no movement was recorded with mud crabs smaller then 40 mm. The recapture rate in the smallest size class (<40 mm) was the highest which indicates that the smallest juveniles are relatively stationary. This corresponds to earlier studies in Australia where it is suggested that juveniles smaller then 100 mm lives intertidal during the whole day (Hill 1982). However the results is not consistent compared to a study on the juvenile blue crab Callinectes sapidus in North Carolina, USA were post-settlement juveniles disperse by swimming in the plankton, and can move 10's of kilometers using storm driven transport (Etherington and Eggleston 2000). Only one mud crab had moved across the two study areas. This crab had moved 5.5 km in 5 days, indicating an exchange of crabs between the two study areas of only 5%. MIMP and Changarama were not separated by an actual barrier except for a 1 km opening in the mangrove. Because of the low exchange, the population estimates are presented as two separate populations. It is important to point out that the two areas are not separated in an ecological sense because of a perceived high level of larval exchange between the two populations of mud crabs. Females extrude their eggs offshore and the larvae are transported by oceanic currents and may disperse over long distances before settlement (Hill 1994). There is no pattern of genetically separation of Scylla serrata throughout the Indo West Pacific ocean (Gopurenko et al. 1999). 4.4 Growth The estimates of growth per size class decreased with size from 0.62 mm day -1 for crabs < 40 mm CW to 0.21 mm day -1 for crabs mm CW, which is consistent with earlier studies (Hill 1975). A rough estimation of the time from settlement (4 mm in CW) to maturity (100 mm in CW) was estimated to be 8 months. No growth rates of wild juvenile Scylla serrata has been published, but the estimated growth rate is comparable to the result from a similar study by Le Vay et al (2007) of Scylla paramamosain which also estimated a time from settlement to maturity of 8 months. The recapture study will continue for a total of 6 months and hopefully there will be more recaptures which will be helpful in obtaining a more accurate estimate of a growth rate in Scylla serrata. 4.5 Population size The populations were relatively small. The abundance of mud crabs in all size-classes at MIMP was estimated to 3826 crabs representing 117 crabs per ha of mangrove. At Changarama the population size was estimated to representing 426 crabs per ha of

17 mangrove. Barnes et al. (2002) estimated population size of Scylla serrata by using burrow occupancy on Chole Island (adjacent small island to the present study area) were the abundance in the inner mangrove was 328 crab per ha and the abundance on the mangrove fringe was 1228 crabs per ha, which indicates a lower abundance mud crabs in the present study. In a study in Vietnam, densities of juvenile Scylla paramamosain were estimated to 1102 crabs per ha of mangrove (Le Vay et al. 2007), which indicates a higher abundance compared to the present study. The total population size in number of individuals is probably very local and adjusted after the condition of the environment and therefore comparables between other studies is not so relevant. The mangroves in the present study were relatively narrow ( m wide in most parts), dominated by Sonnertia alba also in the back-flats and with a sand sediment in most areas, which may not be the optimal habitat for mud crabs. Schnabel census method estimates relies on the assumption that the population is closed and constant during the study period. Schnabel census method did not therefore allow recruitment, mortality and migration, which clearly was not fulfilled in a natural population. However, only one recaptured crab had moved from MIMP to Changarama, which suggests that there is little movement between the two study areas which supports the assumption of two closed populations. The low capture rates of mud crabs smaller then 40 mm indicates a period of low recruitment, and because of the short study period the new recruitments was likely not a problem in the estimations. Though, some mortality surely occurred, particularly for the smaller juveniles (Palmqvist 2009), so this asssumption was likely not fully met. This violation would lead to overestimations of the total number of mud crabs, so the real populations are likely smaller. The low number of recaptures gave large confidence intervals and made the estimations less dependable. The population estimations were conducted over a limited time period. A long term mark and recapture study should be conducted to provide a more robust estimation of crab abundance (Robertson and Piper 1991). 4.6 Mortality The low number of recaptures prevented an estimation of natural mortality. The recapture rates varied within the size-classes and the recapture rate was highest in the two smallest size-class (<75 mm) indicating a higher mortality for the larger crabs. This is not supported by earlier studies. On Mafia Island a tethering experiment showed that predation mortality decrease with size in mud crabs (Palmqvist 2009). Mortality is high in many benthic juvenile organisms, and is creating a bottleneck in survival (Gaines and Roughgarden 1987; Smith and Herrnkind 1992; Eggleston and Armstrong 1995; Gosselin and Qian 1997; Moksnes et al.1998; Moksnes 2002). However, the lower recapture rate in larger mud crabs in this study may also indicate a higher dispersal rate out of the core areas in the larger size-classes, which is supported by the migration data of mud crabs bigger then 40 mm. The low numbers of recaptures make it difficult to come to a reasonable conclusion. 4.7 Implications for aquaculture and resource management The results shows that habitat distribution is size-specific, which is important knowledge in the collection of small juvenile seed-crabs (<40 mm in CW) for aquaculture and also in the

18 fishing of adult crabs. The mangrove back-flat is recommended fishing grounds for juvenile seed-crab fishing. Tidal creeks seem to be an important habitat for crabs in all sizes, not previously sampled on Mafia, and may also work as fishing ground for aquaculture. On Mafia Island, the two areas MIMP and Changarama can be treated as two local populations for each recruitment period of mud crabs because of the low exchange of benthic crabs between the areas. The limited movement enables detection of effects of fishing on the local populations A growth rate from settlement to maturity is very important both in an ecological and economic perspective in the development of an aquaculture. The growth rates found in the present study suggest that it would take 328 days for a juvenile with 20 mm in CW to reach market size (150 mm CW), which is important knowledge for estimations of needed feed for aquaculture and how to adjust the farming period to the market and tourist seasons. The growth rate is also important for assessing the impact of fishing for adults and for collection of juvenile seed-crabs for aquaculture. Because of the relatively low total number of individuals in the two local populations studied in the present study, a relatively low fishing pressure of mud-crabs may possible lead to overexploitation of local populations seasonally. For example, if fishermen collect 20 mud crabs every night for 8 months, they will collect over 4800 crabs, which constitutes more than the whole estimated population at MIMP. Because of the estimated 8 months period from settlement to maturity, the population would be depleted before it had a chance to reproduce. This would create an unsustainable fishing and lead to an overexploitation of the local and seasonal mud crab population. However, a limited fishing for small seed-crabs may still be sustainable during peak recruitment season in June-September, when up to 0.5 small juvenile mud crabs (<30 mm CW) per m 2 could be found at the MIMP core site (H. Mahudi, unpubl. data). Since these small mud crabs suffer high mortality from predation in nature (Palmqvist 2009), a large proportion may be collected with limited effects on local populations. Further studies are required during the peak recruitment season to assess this possibility. Either way, strictly controlled use (both in terms of fishing of mud crabs and collection of juvenile seed-crabs for aquaculture) is necessary to avoid overexploitation. ACKNOWLEDGMENTS I would like to thank everyone who made the completion of this thesis possible. Especially thanks to my supervisor, Per-Olav Moksnes for giving me this wonderful opportunity and for all the support during the fieldwork and in the writing process. Thanks to Humphrey Matala Mahudi for a warm welcome to Mafia Island and for the invaluable help during the field work. And finally, thanks to my sampling team on Mafia Island, Hannes, Hashim, Nasoro, Abdallaha (including Humphrey) who made all the late nights of sampling out in the mangroves to a pleasurable and unforgettable time.

19 APPENDIX A. All recaptured crabs Site Site Time Tagged Recaptured Increased Total Movment released recaptured free (days) size (mm) size (mm) size (%) movement (m) (m/day) MIMP MIMP % MIMP MIMP MIMP % - - MIMP S % ,25 MIMP MIMP Changa Changa % - - MIMP MIMP MIMP S MIMP MIMP N4 N MIMP MIMP Changa Changa

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