Crab
   
Zooplankton Dynamics and Appropriate Management Approach for Blue-swimming Crab in Kung
Krabaen Bay, Chanthaburi Province, Thailand

W. Tantichaiwanit1, N. Gajaseni1, A. Piumsomboom2 and C. Kunsook
Department of Biology, Faculty of Science, Chulalongkorn University, Phyathai Road Bangkok 10330
Department of Marine Science, Faculty of Science, Chulalongkorn University, Phyathai Road Bangkok 10330


The study aims at assessing zooplankton dynamics; searching for the relationships of crab larvae, other zooplankton and ecological factors; and exploring the appropriate blue-swimming crab management in Kung Krabaen Bay, Chanthaburi Province. 16 stations were selected for bimonthly sample collection during 2004-2005. As the results, it revealed 40 groups of zooplankton from 15 phyla. Among these, four economic important groups were recorded. These groups comprise shrimp larvae, fish larvae, bivalve larvae, and brachyuran larvae (crab larvae). The distribution and density of four groups are different in relation to the influence of monsoon season and specific habitat in the bay. Particularly, brachyuran larvae density was highest (1.4x105-3.8x103 ind/100m-3) during the southwest monsoon (May-October) which had a negative correlation with fish larvae and a positive correlation with Acetes spp.. The results indicated that the highest distribution and density of brachyuran larvae found at seagrass habitat and the bay month. Brachyuran larvae also corresponded to the study of gonad somatic index which found two peaks of the spawning of berried female blue-swimming crab in September and January. Apparently, the spawning of female crab had relationships with the peak density of brachyuran larvae in November. Both of crab larvae and young blue-swimming crab use the seagrass bed as refugium and nursery habitat. With regards to the specific characteristics of this bay, the blue-swimming crab is a dominant economic species which has supported the livelihood of local fisherman communities. At the present situation, the blue-swimming crab production has declined from 120 tonne in 2002 to 80 tonne in 2004 and 62 tonne in 2005 which clearly related to the over-crabbing in this bay by using small mesh size (3.50 cm) of collapsible crab trap. In the fact that the average catching size is also reduced from 11.22 cm in 1999 to 7.31±0.42 cm in 2005 which 70% of caught crab is a pre-reproductive female. To propose an appropriate management approach for blue-swimming crab fishery in relation to enhance the possible survival of crab, it recommends: 1) to conserve seagrass bed by prohibiting a crabbing during the northeast monsoon where the berried female blue-swimming crab and brachyuran larvae have high density; 2) to enlarge mesh size of collapsible crab trap not less than 6.50 cm for selective crabbing particularly the young crab; 3) to ban berried female crabbing; 4) to promote crab restocking and crab culture; and  5) to educate and publicize sustainable crabbing to fisherman communities as well as other stakeholders.

Introduction

Mangrove forest is one of the most important tropical coastal ecosystems. It provides natural protection for coastal erosion and the habitat of the hatchery and nursery of aquatic organisms especially commercial organisms, e.g. fish, crab, mollusk and shrimp, which cannot complete their early stage of life cycles in the open sea (Sikhantakasamit, 2002). These key functions should be maintained by the conservation of mangrove habitat. The mangrove forest has been threatened dramatically on most shorelines by land reclamation for urbanization, tin mining, aquaculture purposes and especially conversion into shrimp farms (Aksornkoae, 1999). Moreover, there are many other factors in relation to a coastal productivity, the ecosystem functions naturally involve not only the nutrient cycling process but also the energy transfer through the food chains and food webs. Therefore, phytoplankton and Zooplankton community are important to support fishery productivity in coastal ecosystem which particularly zooplankton is a linkage between producers and consumer through the higher trophic level. Understanding of community structure, the distribution and production of zooplankton could help to identify areas of potential production of fishery resources (Phapavasit et al., 2003).
Kung Krabaen Bay is the one of coastal ecosystems surrounding of reforested mangrove and aquaculture. It is abundant of marine organisms due to high diversity of habitat type such as rock beach, sand beach, mud beach, mangrove forest and especially seagrass bed (KKBRDSC, 2003). Seagrass bed in Kung Krabaen Bay consists of two species: 1) Enhaulus acoroides is found in the northern Bay near reforested mangrove and 2) Halodule pinnifolia is found in the southern Bay. Seagrass bed is a important nursery and refugium habitat for marine life such as shrimp, mollusk, fish, crab, etc. (Kunsook, 2006). Local fisherman who lives in Kung Krabaen Bay make a living depending on aquaculture (shrimp farming and oyster farming); green mussel and crab harvesting; especially blue swimming crab (Portunus pelagicus). It is an important economic marine organism for the local fisherman because the local fisherman has caught blue swimming crab approximately 80 tonne in year 2004 (valued 4,800,000 bath/annual) by using collapsible crab trap.  To compare the blue swimming crab production in 2002 (120 tonne) and 2004 (80 tonne), it is clear that the production is decreasing 33.4% from year 2002. If local fisherman is continuously crabbing as usual without any implementations of appropriate management its population will possibly decline and extinct in the near future. Therefore, this study aims at assessing zooplankton dynamics; searching for the relationships of crab larvae, other zooplankton and ecological factors in order to conserve blue swimming crab population; and exploring the appropriate management for blue-swimming crab in Kung Krabaen Bay, Chanthaburi Province.

Material and methodsStudy area

Kung Krabaen Bay, located between latitude 12° 34’-12’N and longitude 101°53’-101°55’E, it is a small semi-enclosed estuarine system on the west coast of Chanthaburi Province, the east coast of Thailand (Figure 1). The bay morphological profile is the size of 2.5x4.0 km with an average depth of 2.5 m. The bay is an approximate area of 10 km2 and the total volume of 2.5x107 m3.  Its connection to the Gulf of Thailand is via a channel of 700 m width on the southeastern corner of the bay. The fringing of reforested mangrove forest is approximately 2.58 km2 covers along the northeastern, parts of the northwestern and the southwestern shore where is an appropriate feeding and nursing ground (Chatananthawej, 2002).

Methods

To study on zooplankton dynamics in Kung Krabaen Bay, zooplankton diversity, density, distribution and relationships between zooplankton density especially branchyuran larvae, other zooplankton and ecological factors are focused. Sampling stations were designed and composed of 16 stations in the bay, which each set of 4 stations covered in 4 different habitat types including reforested mangrove near the mouth of discharge canal, seagrass bed, open water in the middle of bay and the bay mouth (Figure 1). Zooplankton were sampled bimonthly in May, July, September  represented the southwest monsoon season, November, January and March represented the northeast monsoon season. The collection of zooplankton used net mesh size 103 µm, were employed for two minute horizontal hauling by low speed boat. Samples were fixed in 7% neutralized formalin. Zooplankton were counted and identified to higher taxonomic groups and density was calculated as the number of individuals per 100 cubic meters (ind/100m-3). Ecological factors were also measured such as temperature, pH, Dissolved Oxygen (DO), salinity, transparency depth, depth and chlorophyll a based on standard methods (Parson et al., 1985). Then, the relationships between zooplankton density and ecological factors were analyzed statistically.


To study population dynamics of blue swimming crab in Kung Krabaen Bay were sampled monthly from January to December 2005 and also considered two seasons as the southwest (May-October) and the northeast (November-April) monsoon season. 25 sampling stations were designed and covered 4 different habitat types corresponding to zooplankton sampling station (Figure 1). The blue swimming crabs were trapped by collapsible crab trap with mesh size 3.50 cm which was set up 3 traps per sampling station. The collected data included sex, carapace width, carapace length, wet weight, stomach content (Williams, 1981) and gonad somatic index of female (Zar, 1984). For data analysis, it was focusing on A) blue swimming crab harvesting rate; B) density and distribution pattern; C) gonad somatic index; and D) stomach content of young and adult blue swimming crab.


 
Figure 1. Study area at Kung Krabaen Bay, Chanthaburi Province,
Showing the zooplankton and blue swimming crab
sampling area in 4 habitat types: I) reforested mangrove
,II) seagrass bed, III) mid bay, and IV) bay mouth.

Results and Discussion Distribution and density of zooplankton

This study found zooplankton 40 groups from 15 phyla, divided into 2 groups: 1) Holozooplankton found 22 groups from 8 phyla, and 2) Merozooplankton found 18 groups from 8 phyla (Table 1). Copepods population (Calanoid copepod, Cyclopoid copepod and Harpacticoid copepod) always dominated at all stations and all year round. It comprised 44.32% - 65.37% of total zooplankton density which corresponded to the report of Marumo et al. (1985). Satapoomin (1999), Aiemsomboon (2000) and Sikhantakasamit (2002) were reported that Copepod is always dominant approximately 71.63%, 88.40% and 40.00% of total zooplankton density respectively. The co-dominant zooplankton group was crustacean nauplii, mollusk larvae, polychaete larvae, larvacean and tintinnid, comprising 7.19%-23.69%, 3.61%-21.64%, 0.80%-11.80%, 1.69%-5.22% and 0.02-4.36% respectively. This group was ranking based on density that indicated occasional (O) and rare (R) situation (Table 1). Opportunistic zooplankton groups could find in the southwest monsoon (rainy season), while Cladocera and Rotifer group almost found in the reforested mangrove area due to its sensitivity to salinity. It also corresponded to the studies of Piumsomboom et al. (1999) that found Cladocera and Rotifer in October 1997 during the southwest monsoon and the salinity is a limiting factor of this groups. Moreover, this study found 4 economic important groups, the groups comprised shrimp larvae, fish larvae, bivalve larvae and brachyuran larvae (crab larvae).

Table 1. Zooplankton diversity, density and distribution in Kung Krabaen Bay, Chanthaburi Province. (F = frequency, 40-59 %; O = occasional, 20-39 %; R = rare, 1-19 %; and NF = not found)

 
Note: * Merozooplankton

In terms of zooplankton distribution, it found that bivalve larvae, shrimp larvae, fish larvae and brachyuran larvae density and distribution were high density in the mid bay zone, in the seagrass area and the bay mouth area during the southwest monsoon season (range of 6.72x105-8.04x105; 237-2,84l; 294-353 and 408-489 ind/100m-3 respectively) (Figure 2, 2A, 2B and 2C). In the northeast monsoon season, only shrimp larvae found high density near the bay mouth and the seagrass ranging of 705-1,411 ind/100m-3. Nateekarnjanalarp (1990) studied fish at Samui Island, Thailand and found some fish living in seagrass bed and some species used seagrass bed for mating and nursing in breeding season such as Lutjanidae. Ziaman (1982) reported that Argopecten irradians larvae (bivalve larvae) lived and depended on seagrass leaf. Furthermore, Williams (1981), Chande & Mgaya (2004), Patel et al. (1971) and Cannicci et al. (1996) studied stomach content of blue swimming crab which found crustacean, mollusk and fish as the major food source of young and adult blue swimming crab.

 
     
(A)   (B)
     
 
     
(C)   (D)
     
Figure 2.

Density and distribution pattern of (A) Bivalve larvae; (B) Shrimp larvae;
(C) Fish larvae (D) Brachyuran larvae in Kung Krabaen Bay.


Relationships between brachyuran larvae, ecological and biological factors

We analyzed the relationships between brachyuran larvae vs. ecological and biological factors. When the study compared the relationships of brachyuran larvae and ecological factors such as depth, transparency depth, salinity, temperature, DO, pH and chlorophyll a, it had no significant difference. On the other hand, brachyuran larvae had negative correlation with fish larvae (p<0.05) but had positive correlation with Acetes spp. (p<0.05). Other zooplankton (Hydromedusae, Chaetognatha, Lucifer sp. and shrimp larvae) indicated their role as a predator which was not correlated with brachyuran larvae (Table 2). Lestang et al.(2003) and Queiroga et al. (2006) reported the brachyuran larvae, first crab stage and young crab migrated from open sea to estuarine near coastal zone. In case of Kung Krabaen Bay, the water current and tidal cycle have influence on zooplankton distribution because of the topographic characteristics with shallow water in the bay. Particularly, the brachyuran larvae densely distributed in the seagrass bed toward the northwest of bay where would be the end of water current to flow in the bay. The water volume exchange rate in Kung Krabaen Bay depends on sea water from open sea flow through the bay mouth and the volume of inflow from open sea is more than 40 times of brackish discharge water inflow from shrimp farms (Seangrungrueang et al., 1999).


Table 2. Correlation coefficient between brachyuran larvae density, ecological and biological factors.

Factors Southwest monsoon Northeast monsoon
Depth -0.068 -0.093
T-depth -0.043 -0.129
Salinity 0.451 0.271
Temperature -0.310 0.205
Dissolve oxygen 0.032 0.224
pH 0.019 0.132
Chlorophyll a -0.200 -0.026
Hydromedusae 0.054 0.237
Chaetognathas 0.107 -0.168
Lucifer  sp. 0.375 -0.162
Acetes  spp. 0.603* -
Shrimp  larvae 0.287 -0.338
Fish  larvae -0.583* 0.376

Note: * significant (p<0.05)

Blue swimming crab situation

The blue swimming crab Portunus pelagicus situation in 2005 indicated that the crab production was 62 tonne/year and lower than 2002 (120 tonne/year).  Furthermore, the average catching size is also reduced from 11.22 cm in 1999 to 7.31±0.42 cm in 2005. As the results, the ratio of male and female crab was 1:1.23 from total of 4,046 crabs (Table 3). Tantikul (1984) studied the sex ratio of blue swimming crab in Chumporn Province and Songkhla Province in the Gulf of Thailand has 1:1.4. The size distribution of blue swimming crab, we found that 70% of caught crab is a pre-reproductive female (≤7.31 cm). The indication of crabbing is likely over harvesting by using small mesh size collapsible traps (3.50 cm), which mainly catch the more female than male. Particularly, the fishermen set up the number of traps in double during spawning season because the ovigerous female has more valuable due to the market demand. Furthermore, the study found 2,816 young blue swimming crabs and 1,230 adult blue swimming crabs (Table 3), which corresponded to the report from Jindalikit et al. (2002). It also indicated that a coastal zone is suitable for young blue swimming crab but adult prefers to live in the open sea. Therefore, it urgently needs a serious concern to change the existing crabbing practices in this bay otherwise the blue swimming crab population is in the risk of extinction. To be clear that the blue swimming crab sustainable management is needed


to implement not only for conserving the crab population but also for sustainably supporting the local economy.

Density and distribution of brachyuran larvae and blue swimming crab.

Brachyuran larvae (megalopa stage) were highly abundant in the northheast monsoon season in the bay mouth and seagrass bed (Figure 3E). Especially in November, it found approximately 1.42x105±3820 ind/100m-3.  Across from the southwest monsoon season, it found low density of brachyuran larvae throughout the bay. Salinity is a major factor for brachyuran larvae development to the first crab stage and the optimal salinity is the range of 30-33 ppt (Tantikul, 1984). The similar report of Pillary and Nair (1972) found high density of brachyuran larvae during February-March (during northeast monsoon season) in the coastal zone of India Ocean. Kuptawatin (2000) reported that seagrass bed area is a good shelter and nursing ground for brachyuran larvae. Moreover, the study of the gonad index of blue swimming crab in Kung Krabaen Bay indicated 2 spawning period; January–March and August–September, corresponded with the high density of brachyuran larvae during November 2004–March 2005 (Figure 4). Blue swimming crab population density in the southwest monsoon season is higher than northeast monsoon season (Figure 3F).

Table 3. The ratio between young and adult blue swimming crab from crabbing.

  Male Female Total
Young crab 1,251 1,565 2,816
Adult crab 564 666 1,230
Total 1,815 2,231 4,046


 
     
(E)   (F)
     
Figure 3.

Density and distribution pattern of (E) brachyuran larvae and (F) blue swimming brab in Kung Krabaen Bay.


 
Figure 4.

Gonad somatic index of female crab blue swimming crab (P.pelagicus).


Stomach content of young blue swimming crab

Stomach content analysis of blue swimming crab from 140 samples by frequency of occurrence method found 8 groups such as crustacean, fish, mollusk, squid, algae, seagrass, organic matter and sand comprise in stomach (83.57, 69.29, 58.57, 27.14, 23.57, 22.14, 18.57 and 10.00 respectively) (Figure 5). This study also corresponded to the reports from Williams (1981), Chande and Mgaya (2004) and Patel et al. (1971). A part of this study indicated the stomach content of young and adult blue swimming crab found the main food composition of crustacean of 45%-75%, which is a major calcium source for the development of crab carapace. Analysis of the stomach content of young blue swimming crab (<7.0  cm) found mollusk, fish, crustacean and sand in stomach, while fish, crustacean, mollusk and organic matter found in adult blue swimming crab carapace (>7.0 cm). Comparison of food quantity between male and female crab found that female crab eat more food than male crab because female crab needs more energy for the process of ovogenesis (Cannicci et al., 1996).



 
Figure 5.

Frequency of stomach content in male and female blue swimming crab by frequency of occurrence method


Appropriate management approach for Blue swimming crab
Therefore, it is necessary to propose an appropriate management approach for blue-swimming crab fishery in relation to enhance the possible survival of crab larvae, the recommendations are:

  1. to conserve seagrass bed by prohibiting a crabbing during the northeast monsoon where the berried female blue-swimming crab and brachyuran larvae have high density;
  2. to enlarge mesh size of collapsible crab trap not less than 6.50 cm for selective crabbing particularly increasing the survival rate of young crab;
  3. to prohibit a berried female crabbing;
  4. to promote a crab restocking and a crab culture;
to educate and to publicize sustainable crabbing practices to local fisherman communities as well as other stakeholders.

Conclusion

Zooplankton dynamics in Kung Krabaen Bay is revealed 40 groups from 15 phyla. Among these, four economic important groups were recorded. These groups comprise shrimp larvae, fish larvae, bivalve larvae, and Brachyuran larvae (crab larvae). Copepod is dominant group could found all year round.
Zooplankton density in the northeast monsoon season is higher than in the southwest monsoon season. Distribution pattern of brachyuran larvae found high density in the seagrass bed and the bay mouth more than the reforested mangrove.
Blue swimming crab population in Kung Krabaen Bay is clearly decline from 120 tonne in 2002 to 62 tonne in this study in 2005. We also found the proportion of the population of young to adult blue swimming crab was 2.3 times. The young crab mostly lives in the seagrass bed, while the adult crab lives in open sea.
The spawning period of blue swimming crab is identified by the gonad somatic index that shows 2 periods; January–March and August–September. It is likely indicated the relationships of spawning blue swimming crab and brachyuran larvae that brachyuran larvae density is high in the northeast monsoon (November-March) about overlap to both spawning periods.
In case of the stomach content of blue swimming crab revealed both of young and adult blue swimming crabs forage mainly on crustacean and followed by mollusk, squid, algae and organic matter.
Regarding the blue swimming crab population and production in Kung Krabaen Bay, it urgently needs to propose an appropriate management approach for young blue-swimming crabbing in relation to enhance the possible survival of crab larvae by actively conserving seagrass bed area; implementing the use of larger mesh size of collapsible crab trap; agreeing on banning berried female crabbing; promoting a crab restocking and crab culture; and educating and publicizing sustainable crabbing to local fisherman and other stakeholders. However, we also recommend to do further study in blue swimming crab migration by tagging technique as well as in social participatory approach for adaptive management in the future.



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