June 12, 2017 – Six (6) months or close to twenty four (24) weeks is what it takes for a blue swimming crab “alimasag” larvae to reach its marketable size, based in an experimental study conducted in Rapid Hatchery, Brgy. Old Poblacion, Escalante City from October 6, 2016 to March 15, 2017.
Philippines is the second largest exporter of pasteurised crab meat in Southeast Asia, next to Indonesia. The continuous collection of this commodity since early 90s has put pressure on the wild stocks. A decline in catch size and volume has been seen over the years. Statistics has already shown that this species is heavily overfished (Ingles, 1999; Ingles, 2004). In a study of Ingles and Flores (2000) in Negros Occidental waters, stocks have exhibited both growth and recruitment overfishing.
Initiatives that safeguard existing stocks are currently in place and conducted through campaign awareness programs, data collection, policy development, and establishment of hatcheries to produce crab juveniles for stock enhancement and aquaculture.
Stock enhancement is seen as a possible approach to restore crab stocks. This intervention technique is widely used to replenish vulnerable or exploited stocks caused by overfishing and to mitigate the detrimental impacts that could possibly result from excessive environmental disturbance. However, success of rebuilding stocks often depends on addressing the factors responsible for the decline (Hillborn, 2004), which may include understanding life cycle, physiological behavior, migration, genetics, and the habitats and its carrying capacities (Seitz et al., 2008). Stock enhancement must be carefully integrated with sound fishery management to conserve existing and future stocks (Bell et al., 2006).
Larval culture of Portunus pelagicus started in 1970s (Moto et al., 1978) and refinements of the technology continue up to the present times (Ikhwanuddin et al., 2012; Cabacaba and Salamida, 2015). While seed production is relatively successful, farming in grow-out systems remains a bottleneck. Attempts have been made but due to high cost of production and bottom low survival, blue crab culture is not economically feasible.
An experimental study on growth and survival of Philippine blue swimming crab, Portunus pelagicus were conducted under highly controlled conditions from October 2016 to March 2017. Broodstock or gravid crabs, containing orange to brown egg mass were obtained from a nearby crab landing area. Larval rearing was conducted, modified from the protocol described by Fujaya et. al. (2016) in South Sulwesi, Indonesia until crabs reach the 4th crab instar. 1,164 crab instar were taken for initial length and body weight prior to stocking to 10-tonner concrete tanks, earthened and provided with artificial shelter. Sampling was done every 2 weeks and optimal water parameters were maintained. A combination of trash fish and artificial feeds were given twice daily in the morning and afternoon.
At 21 weeks from the 4th crab instar, crabs reached the marketable size with an average carapace width (CW) of 9.73 cm and average body weight (ABW) of 62.1 g. At the end of the rearing period, survival of 4.03% was obtained among the test animals.
In a recent study of P. pelagicus in Indonesia, first sexual maturity of male and female crabs was observed as early as 7.6 cm and 7.2 cm CW, respectively (Sara et al., 2016). In the Philippines, average sexual maturity for females is at 10.6 cm CW (Sukumaran & Neelakantan 1997). Allowable catch size for BSC is often based from size at maturity.
Age to maturity of P. pelagicus is reported to be at 10 to 12 months (Smith, 1982; Sumpton et al., 1994) contradictory to the results of the present study. Factors affecting these inconsistencies such as genetics, food and habitat can be further investigated.
This study was conducted to aid in the propagation of blue swimming crab stocks and inform possible research areas to enable grow-out culture. As one of the most exploited fishery commodities, alternatives to restoring the BSC population are being explored.
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