By Dr. Rafael D. Guerrero III
The blue swimming crab (BSC) or “alimasag” (Portunus pelagicus) is a widely-distributed crustacean in the coastal waters of our country and other countries in the Indo-Pacific Region.
As a seafood, it is highly delectable for its “sweet, delicate flavor and tender meat.” The Philippines was the fourth largest producer of the BSC in 2020 next to Indonesia, China and Thailand. We were the third largest exporter of BSC frozen meat and fat to the United States with a value of US$45 million.
Small-scale or municipal fisherfolk comprise the largest number of fishers or crabbers of the BSC in nearshore waters using crab pots or traps, gillnets and lift nets. The Visayan Sea produces about 40% of the country’s BSC harvest (13,000 metric tons) and has 50% of the picking stations (processors) that provide employment to about 10,000 workers in Region 6.
With overfishing, there has been a decline in the catch of BSC in the country. Prior to 2014, there were no regulations for the catching of BSC juveniles and berried (egg-bearing) females which have led to the drop in production.
In 2014, a Joint Administrative Order of the Bureau of Fisheries and Aquatic Resources of the Department of Agriculture (DA) and the Department of Interior and Local Government was enacted for the conservation of blue swimming crabs. The regulations limited the catching of BSC with a minimum size of 10.2 centimeters (cm) for the carapace (upper shell of the crab) width and the holding of berried females in cages to allow them to spawn. Another conservation strategy initiated by the National Fisheries Research and Development Institute (NFRDI) of the DA was the artificial spawning and rearing of BSC larvae in hatcheries and the release of crablets into the sea for stock enhancement.
The first observations on the reproduction and larval development of the BSC were reported in 1989 by Jose Ingles and Erich Bkaum of the University of the Philippines Visayas. BSC females from the Ragay Gulf in Bicol were found to be mature with carapace width of 10.5 cm and weight of 72 grams. Berried females were found throughout the year with two peak spawning periods in February to April and July to October. In tanks, mature females can spawn as many as 13 times.
The BSC males have blue spots on their carapace (upper shell) while females have brown spots. Mature males have a slender and triangle-shaped lower shell on their abdomens while that in females are broad and oval-shaped. The BSC can live up to three years. Nonita Cabacaba and Jimmy Salamida reported on the spawning and production of crablets of the BSC at the NFRDI’s Guiuan Mariculture Research and Development Center (GMRDC) in Eastern Samar in 2015. Wild-caught berried females are placed in circular tanks with seawater (salinity 32-34 ppt) for ovoposition (egg-laying). The eggs hatch after 5-6 days into zoea, the first larval stage, which are fed with rotifers (zooplankton) for the first four days and then with cultured brine shrimp (Artemia) and Chlorella for 10-12 days. After four zoea stages, the larvae become megalopae (second larval stage) in 3-5 days. The megalopae become crablets after 15-17 days.
The crablets are then cultured in outdoor nursery ponds with seawater and fed minced fresh fish and mussel meat for 21 days before they are released into fish sanctuaries and marine protected areas. BSC juveniles inhabit shallow water areas with seagrass and seaweeds at the bottom while adults dwell in deeper waters with sandy bottoms. They grow to maturity in about 10 months.
After stocking 96,234 crablets into the sea in October 2020, the GMFRDC researchers found a slight increase in the CPUE (catch per unit effort) of municipal fishers from 1.49 kg/day in October 2020 to 1.63 kg/day in November 2021.
Photo courtesy of National Fisheries Research and Development Institute