Richard Mayrhofer

Improved Sustainable Aquaculture Systems for Small-Scale Farmers in Northern Vietnam

Aquaculture is an important part of the farming system for ethnic Black Thai farmers in the uplands of Son La province, providing cash income and protein rich food for home consumption. The current aquaculture system, with grass carp as its main fish species, is a feed-based system, with leaf material from banana, maize and cassava as well as weeds and by-products from other farming activities, used as key inputs. As with all other feed-based aquaculture systems, this system depends on a constant flow of water in order to supply oxygen; however, the system is limited by feed and water availability and is threatened by a disease that affects only grass carp. This chapter provides a detailed description of the use of aquaculture within the Black Thai’s farming system, an analysis of the feed resources currently used therein, as well as of those resources that may potentially be used. It also provides an analysis of grass carp diseases as well as a description of innovatory aquaculture practices geared towards replacing the disease susceptible grass carp with common carp, as the main species. These suggested modifications are based on the enhanced production of natural food in ponds and the application of supplemental feeds partly based upon on-farm resources. Finally, the potential for earthworms to be used as a high quality feed ingredient, one which can be produced on-farm from currently not used or underutilized resources, is studied in terms of the production of supplemental feed for the common carp.

Pesticide-contaminated feeds in integrated grass carp aquaculture: toxicology and bioaccumulation.

Effects of dissolved pesticides on fish are widely described, but little is known about effects of pesticide-contaminated feeds taken up orally by fish. In integrated farms, pesticides used on crops may affect grass carp that feed on plants from these fields. In northern Vietnam, grass carp suffer seasonal mass mortalities which may be caused by pesticide-contaminated plants. To test effects of pesticide-contaminated feeds on health and bioaccumulation in grass carp, a net-cage trial was conducted with 5 differently contaminated grasses. Grass was spiked with 2 levels of trichlorfon/fenitrothion and fenobucarb. Unspiked grass was used as a control. Fish were fed at a daily rate of 20% of body mass for 10 d. The concentrations of fenitrothion and fenobucarb in pond water increased over time. Effects on fish mortality were not found. Fenobucarb in feed showed the strongest effects on fish by lowering feed uptake, deforming the liver, increasing blood glucose and reducing cholinesterase activity in blood serum, depending on feed uptake. Fenobucarb showed increased levels in flesh in all treatments, suggesting bio-concentration. Trichlorfon and fenitrothion did not significantly affect feed uptake but showed concentration-dependent reduction of cholinesterase activity and liver changes. Fenitrothion showed bioaccumulation in flesh which was dependant on feed uptake, whereas trichlorfon was only detected in very low concentrations in all treatments. Pesticide levels were all detected below the maximum residue levels in food. The pesticide-contaminated feeds tested did not cause mortality in grass carp but were associated with negative physiological responses and may increase susceptibility to diseases.

15N tracer application to evaluate nitrogen dynamics of food webs in two subtropical small-scale aquaculture ponds under different managements

Small, semi-intensively managed aquaculture ponds contribute significantly to the food security of small-scale farmers around the world. However, little is known about nutrient flows within natural food webs in such ponds in which fish production depends on the productivity of natural food resources. 15N was applied as ammonium at 1.1 and 0.4 % of total nitrogen in a traditionally managed flow-through pond and a semi-intensively managed stagnant pond belonging to small-scale farmers in Northern Vietnam and traced through the natural food resources over 7 days. Small-sized plankton (1–60 μ m) was the dominant pelagic biomass in both ponds with higher biomass in the stagnant pond. This plankton assimilated major portions of the applied tracer and showed a high sedimentation and turnover rate. High re-activation of settled nutrients into the pelagic food web was observed. The tracer was removed more quickly from the flow-through pond than from the stagnant pond. A steady nutrient supply could increase fish production.

Leaves from banana ( Musa nana ) and maize ( Zea mays ) have no phyto-prophylactic effects on the susceptibility of grass carp ( Ctenopharyngodon idella ) to Aeromonas hydrophila infection

BackgroundThe ubiquitous and opportunistic bacterial pathogen Aeromonas hydrophila has been associated with ulcerative dermatitis in fish, especially under stressful conditions. It can cause severe losses in fresh water aquaculture and is particularly prevalent in tropical and subtropical regions. Fresh leaves from maize and bananas have been used as feed supplement by fish farmers in Vietnam and it has been reported that they may have phyto-prophylactic benefits. In the present study, a feeding trial was conducted to investigate the benefits of providing maize and banana leaves as feed supplement: to determine if they were taken up and digested by grass carp (Ctenopharyngodon idella), if this uptake resulted in improved growth performance, and if leaf supplementation protected fish when challenged with A. hydrophila by intramuscular injection.ResultsAll fish were fed an identical ratio of commercial pelleted feed relative to biomass. However, in 12/18 tanks, this diet was supplemented with either fresh banana leaves or fresh maize leaves; offered ad libitum. Addition of leaves increased the overall feed conversion ratio (FCR) significantly. However, if only the pellet were taken into account, then no difference was found between treatments. Changes to the isotopic composition of the fish showed leaf nutrient uptake occurred. No prophylactic effects of feeding banana or maize leaves were detected against infection with A. hydrophila, and the diet did not induce changes in the fish haematocrit. However, addition of the maize leaves was associated with significantly reduced severity of the skin lesions, which could improve the market value of the fish.ConclusionsAddition of the leaf supplement did not result in significantly improved growth performance. Similarly, the effect of the supplement on the fish survival to infection was not significant.