Modification of the biological intercept model to account for ontogenetic effects in laboratory-reared delta smelt (Hypomesus transpacificus)*

We investigated age, growth, and ontogenetic effects on the proportionality of otolith size to fish size in laboratory-reared delta smelt (Hypomesus transpacificus) from the San Francisco Bay estuary. Delta smelt larvae were reared from hatching in laboratory mesocosms for 100 days. Otolith increments from known-age fish were enumerated to validate that growth increments were deposited daily and to validate the age of fish at first ring formation. Delta smelt were found to lay down daily ring increments; however, the first increment did not form until six days after hatching. The relationship between otolith size and fish size was not biased by age or growth-rate effects but did exhibit an interruption in linear growth owing to an ontogenetic shift at the postflexon stage. To back-calculate the size-at-age of individual fish, we modified the biological intercept (BI) model to account for ontogenetic changes in the otolith-size−fish-size relationship and compared the results to the time-varying growth model, as well as the modified Fry model. We found the modified BI model estimated more accurately the size-at-age from hatching to 100 days after hatching. Before back-calculating size-at-age with existing models, we recommend a critical evaluation of the effects that age, growth, and ontogeny can have on the otolith-size−fish-size relations

Production of freshwater prawns in the Mekong Delta

Currently there are 2 sources of freshwater prawn Macrobrachium rosenbergii production in Vietnam - natural production from the rivers and canals and aquaculture production from ponds and ricefields. An account is given of the production of this prawn in the Mekong Delta. The prawns are harvested primarily by a common small scale method using shelter traps. The total annual freshwater prawn production in Vietnam during 1985-90 was reported to vary from 5000 to 8000 tons, most of it from natural fisheries; although there are no official statistics on the production of farmed prawns, the amount is believed to be very small. The grow-out system includes intensive monospecies pond culture, and semi-intensive or extensive integrated rice-prawn or vegetable-prawn culture.

Rice-freshwater prawn (Macrobrachium rosenbergii) farms in the Mekong Delta, Vietnam

Aquaculture in riceland has been practiced in Mekong Delta, Vietnam for a long time and integrated rice-freshwater prawn (Macrobrachium rosenbergii) farming has become more and more popular. The integrated farming systems developed and practiced by farmers in the area to produce more food and more cash crops are presented and discussed.

Integrated rice-prawn farming in the Mekong Delta, Vietnam: a route towards sustainable and profitable farming systems?

Economic analysis and performance of the integrated rice-prawn farming systems in the Mekong Delta (Vietnam) are reviewed, including the problems and constraints of this integrated system technology.

Clam farming in the Mekong Delta, Vietnam

The Mekong Delta region in southern Vietnam has high potential for coastal aquaculture, including mollusc culture. Many mollusc species are cultured for domestic and export markets including white clam (Meretrix lyrata Showerby) and blood cockle (Arca granosa). Techniques for clam farming include the nursery and grow-out phases. At present, there are approximately 600 coastal families engaged in clam farming over a total area of 1,870 ha, of which 82.63% is used for the grow-out phased and 17.7% for the nursery phase. Nursery areas are near the coast and receive less than 5 hours of sunlight per day. The average area for a nursery is 3-4 ha and it is fenced with a net or bamboo stakes to prevent clams from escaping and to prevent water currents from carrying them away. Grow-out farm areas are further from the coast and are exposed to sunlight for only 2-3 hours/day. Average farm area for grow-out is 5-6 ha, and may or may not be fenced. Average operating cost is US$1100 per ha for nursery and US$757 per ha for grow-out (the cost of capital assets are not included) with loans being the main source of financial. Problems for clam farmers in the area include natural phenomena, inadequate culture techniques, lack of financing or credit systems, and marketing. Environment-related problems that cause clam mortality include flooding, and freshwater effluent and siltation or sedimentation from Mekong River. Other problems that constrain the development of clam culture in the area are: marketing problems such as lack of buyers and price fluctuations; exploitation of the natural clam populations.

Multidisciplinary research and management: fisheries in the Central Delta of the Niger River (Mali)

This contribution gives an overview of the fisheries in the Central Delta of the Niger River (Mali) and highlights the results of a multidisciplinary research and management program conducted in the area to assess the status of the fisheries and improve their management. The importance of multidisciplinary approaches is emphasized to fully elaborate the problems impacting fisheries and the measures for their resolution. Directions for increased decentralization, participation of fishers and ecosystem preservation for improved management of the Central Delta fisheries are briefly discussed.

Potentials for different models for freshwater aquaculture development in the Red River Delta (Vietnam) using GIS analysis

This study was an attempt to apply land-based GIS analysis for freshwater aquaculture planning in the Red River Delta of Vietnam. It was based on diverse data sources in order to help decision makers at the site and also to contribute to the modelling of selection processes for aquaculture development planning in the region.

Improvement of traditional shrimp culture in the Mekong Delta

Improvements to traditional brackishwater shrimp culture in the Mekong Delta, Vietnam are discussed. A technical support program has been implemented based on a so-called improved extensive shrimp culture method, as previously developed and tested by the Artermia and Shrimp Research and Development Center (ASRDC). The program focuses on: 1) the use of hatchery-produced postlarvae (of Penaeus monodon and P. merguinensis) nursed for three to four weeks, and 2) the application of low-cost pond management practices including predator control, supplementary feeding and frequent water renewal. A credit program, managed as a revolving fund was made available. A dialogue among participating farmers was encouraged through the organization of group meetings before and after each production cycle.

Expansion of improved-extensive shrimp culture in the Mekong Delta

This paper describes the expansion of improved-extensive shrimp (Penaeus monodon) farming in the Mekong Delta, southern Vietnam, emphasizing management decisions, technological constraints and economics.

Autonomous adaptation to climate change by shrimp and catfish farmers in Vietnam’s Mekong River delta

The Mekong River delta of Vietnam supports a thriving aquaculture industry but is exposed to the impacts of climate change. In particular, sea level rise and attendant increased flooding (both coastal and riverine) and coastal salinity intrusion threaten the long-term viability of this important industry. This working paper summarizes an analysis of the economics of aquaculture adaptation in the delta, focusing on the grow-out of two exported aquaculture species—the freshwater striped catfish and the brackish-water tiger shrimp. The analysis was conducted for four pond-based production systems: catfish in the inland and coastal provinces and improved extensive and semi-intensive/intensive shrimp culture.