The blue swimming crab (Portunus segnis), locally known as alimasag, masag, or lambay, is one of the Philippines’ most economically important marine species. Recognized by its distinct blue claws and greenish-blue carapace, the species is valued both for its ecological role and its high market demand. Adult crabs can reach up to 23 centimeters in carapace width and are known for their swimming ability, aided by paddle-like hind legs.

Blue swimming crabs (BSCs) inhabit shallow coastal waters, estuaries, and sandy or muddy seabeds up to 50 meters deep. In the Philippines, they are widely distributed across bays and nearshore habitats. Males are easily identified by their blue coloration and triangular abdomens, while females are typically brown to green with rounded abdomens. Juveniles inhabit seagrass beds, while adults occupy deeper sandy and rocky substrates.

The BSC is a vital component of both local diets and export markets. Its meat, known for its sweet and delicate flavor, is sold locally at around Php 500 per kilogram and exported at USD 6-10 per kilogram. However, the species has been experiencing significant decline in recent years.

Status of blue swimming crab fishery

Smallscale fisherfolk, who supply the majority of BSC landings, have reported consistent decreases in catch volume and crab size. Key contributing factors include overfishing, habitat degradation due to coastal development, and limited enforcement of regulations prohibiting the capture of undersized and berried (egg-bearing) females.

The decline of BSC populations has ecological implications as well. As benthic omnivores, these crabs play an essential role in maintaining sediment balance and food web stability in coastal ecosystems.

Science-based stock enhancement

To address declining stocks, Philippine scientists have implemented aquaculture-based stock enhancement strategies. This approach involves breeding and rearing BSC juveniles in controlled hatchery environments and releasing them into natural habitats to strengthen wild populations.

The National Fisheries Research and Development Institute (NFRDI), through its Marine Fisheries Research and Development Center (MFRDC) in Guiuan, Eastern Samar, has developed hatchery protocols specific to the species’ complex life cycle.

In 2023, NFRDI published Blue Swimming Crab Hatchery Production Training Module No. 4, a technical guide designed to train hatchery operators and extension workers in proper production and management techniques.

Inside the hatchery

The hatchery process begins with the selection of gravid (egg-bearing) female crabs, which can produce up to two million eggs per spawning. Selected females undergo disinfection to prevent disease transmission and are then transferred to hatching tanks with controlled water quality parameters—temperature (27–30°C), salinity (28–35 ppt), dissolved oxygen (>4.0 ppm), and pH (7.5–8.5).

Upon hatching, larval crabs (zoeae) are stocked at densities of 80–100 per liter. They are fed rotifers (Brachionus sp.) and brine shrimp (Artemia sp.), supplemented by minced fish and shell meat as they progress through developmental stages. Once the larvae reach the instar stage, they are transferred to nursery tanks with shelters that reduce cannibalism.

Prior to release, crablets are acclimated to natural salinity and temperature conditions. This careful transition helps ensure higher survival rates once released into the wild.

Implementation in Eastern Samar

Eastern Samar, one of the country’s leading BSC-producing provinces, serves as a pilot site for stock enhancement initiatives. Between July 2020 and December 2021, NFRDI-MFRDC and the Bureau of Fisheries and Aquatic Resources Regional Field Office VIII – Guiuan Marine Fisheries Development Center (BFAR-GMFDC) implemented a collaborative stock enhancement project funded by the Department of Agriculture–Bureau of Agricultural Research (DA-BAR).

During this period, 96,234 juvenile crabs were released into Bucaboc and Dinahic Reefs. Since 2009, BFAR-GMFDC has released more than 265,000 crablets in various sites across the region.

Complementary conservation measures, such as Crab Lying-in Cages, allow gravid females to spawn in controlled enclosures before being harvested, ensuring that eggs are released into the wild.

Toward sustainable fisheries

The blue swimming crab stock enhancement program exemplifies a science-based and community-oriented approach to fisheries management. By equipping fisherfolk with hatchery operation skills and conservation tools, the initiative promotes shared responsibility in sustaining BSC populations. However, long-term success depends on continued monitoring, policy support, and active community participation. Adaptive management, informed by scientific data, will be essential in mitigating the combined impacts of overfishing, habitat loss, and climate change.

Efforts to restore the BSC population demonstrate how research, policy, and community action can converge to promote sustainable fisheries. The NFRDI and its partner institutions continue to refine technologies and capacity-building initiatives to ensure the resilience of the BSC fishery. Sustaining the alimasag is not only a matter of species conservation, it represents a broader commitment to food security, ecosystem stability, and the welfare of coastal communities. ### (Norvida Cruz-Gatdula)