Mud crab pen culture: replacement of fish feed requirement and impacts on mangrove community structure

Brackishwater pond culture has been a major factor in mangrove loss in Southeast Asia, hence, the need to develop environment‐friendly technologies such as mud crab Scylla (Portunidae) culture in mangrove pens exists. This study evaluated the effects of mud crab netpen systems in central Philippines on mangrove macroflora, and the replacement of dietary fish with low‐cost pellets. Wild or hatchery‐sourced Scylla olivacea and Scylla serrata were stocked at 0.5–0.8 m⁻² in 167–200 m² nylon netpens (2.3 cm stretched mesh) in Avicennia‐dominated mangrove habitats. The feeding treatments were: (A) Zarraga: (1) no feeding (natural productivity), (2) no feeding for 1 month+supplementary feeding, (3) fish biomass and (4) low‐cost pellets, and (B) Batan: (1) fish biomass and (2) pellets+fish biomass. Feeds were given ad libitum twice daily. Growth and survival rates of S. olivacea in Zarraga pens were not significantly different among treatments, although crabs fed fish biomass had the highest survival, body weight and production. Similarly, growth and survival of S. serrata were not significantly different between the Batan treatments. Economic analysis of the latter gave a 38.5% return on investment (ROI) and 2.6 years payback period (PP) for pellets+fish biomass treatment compared with 27.5% ROI and 3.6 years PP for fish alone. Sensitivity analysis showed an improved economic performance of the pellets+fish biomass treatment by increasing the survival rate. Evaluation of mangrove community structure showed that crab culture reduced species diversity, numbers and biomass of seedlings and saplings, but not of mangrove trees. Therefore, mud crab pen culture is recommended for mangrove sites with mature trees, but not seedlings and saplings, and low‐cost pellets can reduce dependence on fish biomass.     

Global voices of science: Mangroves, fishponds, and the quest for sustainability

Aquaculture, the farming of shrimp and other useful aquatic and marine plants and animals in artificially confined and tended ponds, pens, and cages, ranks as a phenomenal success story in global food production. In 1975, aquaculture contributed 8% to the overall yield of the world's fish harvest; now it provides more than one-third of the yield. Total aquaculture production in 2003 was 54.8 million metric tons valued at 67.3 billion in U.S. dollars. More than 90% of this output comes from Asia, where aquaculture has its origins and where this month's essay author has lived and worked all of her life. In her essay, Jurgenne H. Primavera, senior scientist of the Aquaculture Department of the Southeast Asian Fisheries Development Center based in Iloilo, Central Philippines, traces the recent history of aquaculture and the socioeconomic and environmental challenges that its rapid growth has wrought, especially for the mangrove ecosystems in which much of brackishwater pond aquaculture occurs. With an eye on all stakeholders, Primavera lays out how aquaculture is now falling short of the goal of sustainability and what steps might be taken to move the industry in that direction.     

Coastal ecosystem-based management with nonlinear ecological functions and values

A common assumption is that ecosystem services respond linearly to changes in habitat size. This assumption leads frequently to an "all or none" choice of either preserving coastal habitats or converting them to human use. However, our survey of wave attenuation data from field studies of mangroves, salt marshes, seagrass beds, nearshore coral reefs, and sand dunes reveals that these relationships are rarely linear. By incorporating nonlinear wave attenuation in estimating coastal protection values of mangroves in Thailand, we show that the optimal land use option may instead be the integration of development and conservation consistent with ecosystem-based management goals. This result suggests that reconciling competing demands on coastal habitats should not always result in stark preservation-versus-conversion choices.     

Observations on artificial fertilization of eggs and the embryonic and larval development of milkfish,Chanos chanos (Forskal)

Hydrated eggs obtained from a female milkfish, Chanos chanos, were artificially fertilized with the milt collected from a male injected with acetone-dried pituitaries of salmon. The fertilized eggs (1.1–1.25 mm in diameter) developed normally in seawater in basins and petri dishes at a salinity of 30–34‰, and successfully hatched in 25–28.5 hours at a temperature of 26.4–29.9°C. The yolk was completely absorbed in about 2.5 days and during this period many postlarvae died. A few larvae were reared up to 5 days but all died on the 6th day. Attempts were made to feed the postlarvae with freshly hatched trochophore larvae of oysters obtained from eggs artificially fertilized in the laboratory.     

Identifying mangrove areas for fisheries enhancement; population assessment in a patchy habitat

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A COMPARISON OF TWO PRAWN (Penaeus monodon) BROOD STOCK SYSTEMS—LAND-BASED TANKS AND MARINE PENS1

The paper compares two prawn (Penaeus monodon) brood stock systems—land-based tanks and marine offshore pens. Construction costs and annual maintenance and operating costs total US$1,430 and $2,550 for the tank and $1,500 and $3,680, respectively, for the pen. The 12 m³ ferrocement tank requires power for a 24-hour seawater flowthrough and can stock 50 animals/run for a projected yearly production of 8×10⁶ nauplii for 1.2 × 10⁶ juveniles (0.1 to 0.5 g body weight). The 16×16×4 m pen made of bamboo and nylon net materials can accommodate 300 brood stock/run with a projected annual production of 30 × 10⁶ nauplii or 4.5 × 10⁶ juveniles. Discussion includes general requirements, mortality and maturation rates, and criteria for choosing a brood stock system.     

Socio-economic impacts of shrimp culture

Farmed shrimp contributed 27% of total world shrimp production in 1995 with a volume of 712 000 tonnes. Undoubtedly, the shrimp culture industry earns valuable foreign exchange for developing countries and generates jobs across the industry from fry gatherers to growers and processors. However, grave socio-economic consequences – including conversion, expropriation and privatization of mangroves and other lands; salinization of water and soil; decline in food security; marginalization of coastal communities, unemployment and urban migration; and social conflicts – have followed in the wake of, shrimp farm development in the Philippines and other tropical countries. The paper focuses on mangrove ecosystems: the valuation and cost-benefit analysis of their goods and services, and the mangrove-offshore fisheries connection. Research gaps in these areas and the need to internalize the ecological and socio-economic costs (‘externalities’) of shrimp farming are highlighted. Other recommendations include mangrove conservation and rehabilitation, enforcement of existing legislation, and introduction of environment-friendly aquaculture within the broader framework of community-based, integrated coastal area management, e.g. the traditional, extensive polyculture ponds in Indonesia.