Gonad maturity and gonadal somatic index of blue swimming crab Portunus pelagicus harvested from Spermonde Archipelago, South Sulawesi, Indonesia

DOI: 10.13170/ajas.5.2.19187 Reproductive biology is one of the biological aspects that needed to formulate responsible management of blue swimming crab (BSC). The crab is one of the commercial fisheries commodities in South Sulawesi, Indonesia. Presently no information on the reproductive biology of this crab from Spermonde Archipelago, Sout Sulawesi. Therefore, the study aimed to analyze and compare gonad maturity stage (GMS) and gonadal somatic index (GSI) of the BSC caught on three ecosystems namely coral reef, seagrass, and mangrove in Salemo Island, Spermonde Archipelago. GMS and GSI were analyzed descriptively for five months from March to July 2015. Results indicate there was a difference in GMS of the BSC caught in mangroves, seagrass, and coral reef. Generally, mangrove was dominated by immature BSC with GMS I and GMS II, while the BSC caught in the seagrass and coral reef BSC were dominantly mature and spawn GMS III, IV, and V. GSI BSC caught in mangrove ecosystem are smaller than GSI BSC caught in seagrass and coral reef ecosystems. Seagrass and coral reef ecosystems suitable for development no-take zone of the BSC.


Introduction
Spermonde archipelago is located in the center of the Coral Triangle of which marine biota is highly diverse (Máñez et al., 2012;Sawall et al., 2013). These islands sit on the Makassar strait (Timm et al., 2017). The highly valued commodity from this region is blue swimming crabs (Portunus pelagicus), which is mainly captured surrounding Salemo island, is an island in the archipelago (Nurdin et al., 2016;Nurdin et al., 2020).
Blue swimming crab (BSC) is an important commodity due to its high value in the global market fishery (Sahoo et al., 2011;Mehanna et al., 2013;Sabrah et al., 2020). Increasing demand for BSC is mainly in the form of frozen and canned meats (Lai et al., 2010), and freshly captured (Yusfiandayani and Sobari, 2011). The market demand triggers continuous exploitation of BSC in Salemo Island (Nurdin et al., 2019a).
Presently, the BSC population in Salemo Island is facing multiple stressors. Over-exploitation has caused changes in reproductive biology patterns indicated by the shorter duration of spawning peak season (Ali et al., 2004;Redjeki et al., 2020). Apart from being heavily harvested, habitat degradation due to destructive fishing also contributes to factors that affect the BSC population (Kurnia et al., 2014;Astuti et al., 2020;Johnston and Yeoh, 2020).
Continuous and intensive exploitation without management intervention on BSC fishery in Salemo Island can lead to local extinction (Adam et al., 2006). To retain sustainable production of BSC to contribute to the local economy, conservation efforts must be imposed. Conservation and management efforts must be supported by information from science to make the implementation more effective. Therefore, the objective of the present study was to assess the reproductive biology of blue swimming crab especially on the aspects of gonad maturity stage (GMS) and gonadal somatic index (GSI) of BSCs in three ecosystems, namely seagrass meadows, coral reefs, and mangrove nearby the Salemo Island. This information is crucial to plan better conservation and management strategies of the BSC.

Materials and Methods Site and time
The study area covers seagrass meadows, coral reefs, and mangroves in Salemo Island coastal waters, Pangkep Regency ( Figure 1). Sampling was conducted from May to July 2015. Samples were collected using traps and gillnets. Samples obtained were put in the coolbox filled with ice and transported were transported to Aquatic Productivity Laboratory, Hasanuddin University for further analysis.

Data collection
A total of 457 samples of blue swimming crab were obtained during the study. Of these, 92 BSC was captured in seagrasses, 250 samples were obtained from mangroves, and 115 samples from coral reef ecosystems. Parameters observed include body weight, gonadal weight, and GMS. The body weight was measured using an analytical balance with 0.001 g accuracy. The width of the carapace was measured using a digital caliper with 0.1 mm accuracy. GMS was classified descriptively based on Kumar et al. (2000), and Castiglioni and Fransozo (2006) which divides the maturity stage into (i) GMS I is indicated by the non-developed ovary and the color is pale yellow; (ii) GMS II is when the size of the ovary is extended with yellow golden color, eggs structure are yet been seen and the ovary is located above hepatopancreas; (iii) GMS III the size of ovary become larger and eggs granule becomes visible even though they are still covered by an oil membrane; (iv) GMS IV, eggs become larger and become visible with orange color, they can be easily segregated due to the decreasing the oil membrane that covers them, and; (v) GMS V ovary is contracted with some unleashed eggs during spawning remain on the abdomen. In this research, crabs with GMS I and II were classified as immature, crabs with GSI III and IV were classified as mature, and those with GSI V was classified as spawned.

Data analysis
Gonadal somatic index was computed based on the ratio between gonad weight and body weight with the formula following (Johnson, 1971;Muchlisin et al., 2010): GSI = Gonadal somatic index; GW = Gonad weight (g); BW = Body weight (g).

Gonad maturity stage
The number of samples captured in five months interval is described in Figure 2. From the above figure, it can be seen that samples captured in mangroves were dominated by immature BSC with gonad maturity is in GMS I and GMS II. In the meantime, BSC captured in seagrass and coral reefs were dominated by mature and spawn (GMS III, IV, and V).

Gonadal somatic index
The gonadal somatic index in seagrass and coral reefs develops relatively faster during observations within this study. The GSI of females captured in the mangrove ecosystem is smaller compared to that of seagrasses and coral reefs. In general, the GSI in mangroves, seagrasses, and coral reefs ranged from 3.78-5.73%, 5.52-8.83%, 2.69-12.87% (Figure 3).

Discussion
The GMS of female BSC captured in mangrove ecosystem (represent estuaries) with those captured in seagrasses and coral reefs which represent marine ecosystem are different. The GMS of BSC captured in estuaries is smaller compared to the other two ecotypes. Female BSC found in the estuary are those in the younger life stages (GMS I and II) and those which are found in marine habitats (seagrass and coral reefs) are those which are in the adult phase (mature and spawn).
The distinction of GSIs representing estuary and marine habitats support studies reported by (Potter and De Lestang, 2000;Safaie et al., 2013) who stated that BSC are migrated to the sea to spawn from estuaries. This behavior is explained by the catadromous characteristic of BSCs (Sahoo et al., 2011) who spend their juvenile and young adult phase in the estuary and moved to the sea for spawning (Kangas 2000;Adam et al., 2006).
Based on a previous study by (Nurdin et al., 2019b), from March to July there are always BSCs at the GMS V phase which are expected to release their eggs at any time. Recruitments of BSCs in Salemo Island occurs continuously in a year with recruitment percentages range between 1.9 -22.11%). This phenomenon can also be used as an indicator that BSC spawn throughout the year (Dineshbabu et al., 2008;Kamrani et al., 2010;Dash et al., 2013;Kunsook et al., 2014). According to Bagenal (1978), aquatic animals with GSI below 20% can spawns more than one time in a year due they can have eggs more than once in a season depending upon their body size (Kumar et al., 2003;Johnson et al., 2010).
Seagrass and coral reef ecosystems are considered as a spawning ground for the BSC due to the high number of matured BSC captures within the two ecosystems (Nurdin and Haser, 2018). Therefore, the two ecosystems are expected to be preserved to become the "no-take zone". The larger the conservation area, the more BSCs will be protected. Apart from biological and scientific aspects, the social aspect of conservation preparation such as consulting fishery stakeholders including fishermen is crucial to avoid conflict (Nurdin et al., 2019a).

Conclusions
There are some distinctions found in GMSs of BSC captured within the three ecotypes. Those which captured in estuaries comprise of immature that are still in GMS I and GMS II phases, whilst those who were captured in seagrasses and coral reefs are in the GMS phases III, IV, and V. The GSIs of BSC captured in mangrove are smaller than those captured in seagrasses and coral reefs. The latter two ecosystems are considered as the spawning ground of the BSC because of the larger number of mature females caught in the two marine ecosystems. Therefore, they can be proposed as the "no-take zone" area to protect the BSCs population.