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This study aims to identify suitable density of suspended trays and care regime, which was conducted from February to November 2022 at Tay Van Boi, Lan Ha Bay, Cat Ba, Cat Hai - Hai Phong to improve growth, survival rates and helping to shorten the time for snout otter clam grow-out culture. The experiment was carried out with 3 treatments: Densities of 4, 8 and 12 trays/4 m2 combined with 3 care regimes during grow-out culture. The results showed that densities of 4 and 8 trays/4 m2 were higher growth than the remaining treatment, specifically length was from 32.9 - 36.8 mm and survival rate was over 84%, with a significant difference, compared to a density of 12 trays/4 m2 with 3 different care regimes. The results show that to increase productivity and reduce production costs and care time, a density of 8 trays/4 m2 with 3 care regimes/a culturing period should be chosen. The study has also propounded disease prevention and treatment measures for the Snout otter clam to help control the disease well and proposed solutions on farming techniques that contribute to high and sustainable economic efficiency.

There are questions remaining about many aspects relating to fish food, including, the type of organisms consumed, mechanisms of consumption, and the digestibility and nutritive value of plankton. Common practice has been to divide fish into different types according to foraging in natural habitats, that is, fish are described as feeding on phytoplankton, zooplankton, benthic animals, macrophytes, or detritus. The distinction between the different feeding habits is not always absolute. Although species such as rohu, Labeo rohita, mrigal, Cirrhinus mrigala (Indian major carp), and Nile tilapia, Oreochromis niloticus (African origin) are omnivorous, their adaptability in different climatic conditions varies. A study was carried out to determine the optimal stocking ratio of tilapia and Indian major carp (rohu and mrigal) and with common carp, Cyprinus carpio; silver carp, Hypophthalmichthys molitrix; and big head carp, Aristichthys nobilis in polyculture. Four ratios of Nile tilapia and Indian and Chinese carp (60%, 40%, 20% and 0%) were tested in pig and in cow manure fertilized ponds. Results of this study indicated that the fish ratio and manure type significantly influenced fish survival and biomass over time. The interaction between species ratio, pH, and alkalinity was found to be significant. The stocking density of rohu, mrigal, and tilapia influenced fish survival and biomass production. Ratio 1 with 60% Nile tilapia was the poorest performing treatment in both types of manure. Ratio 3 (20% Nile tilapia, 20% silver carp, 10% big head, 20% rohu and 20% mrigal) was the best performing ratio in pig treatments. Fish production in ratio 2 and ratio 4 with cow manure treatments were not significantly different. In general, fish production in pig manure fertilized ponds was significantly higher than in cow manure treatments. [ABSTRACT FROM PUBLISHER]

Efficient usage of fertilizers in organic or inorganic forms show conflicting results in terms of net fish production under various climatic conditions. Manures applied to polyculture ponds as organic fertilizer, require a process of decomposition before the nutritional contents are released, assimilated, and utilized by plankton. On the other hand, inorganic (chemical) fertilizers are granular and concentrated with primary nutrients. Major nutrients such as nitrogen (N) and phosphorus (P) readily dissolve in water in ionic forms of nitrate, ammonium and orthophosphate. The differences in fertilizer solubility (nutrient release rate) and decomposition processes (organic fertilization) make the water quality and biological response distinctively different in inorganic fertilizer applied ponds. Previous research has indicated that one grass carp, Ctenopharyngodon idella, can support the growth of three silver carp. This means the excreta of herbivorous fish can be utilized to fertilize the water and produce plankton for filter-feeding fish to consume. Due to an inconsistent supply of grass foliage, farmers in North Vietnam found it difficult to include an adequate percentage of grass carp to support polyculture operation without supplementary fertilization to maintain adequate levels of natural foods in the pond. This supplementary fertilization, in particular manure application, often impacted water quality, which negatively affected health of grass carp. In the present study, an attempt was made to evaluate the production of grass carp polyculture when N:P were maintained through supplementary organic fertilization. An assessment of organic and inorganic fertilizer application on fish production was also undertaken and compared with grass carp polyculture performance. Fish biomass change and fertilizer treatments had significant interaction (P = 0.001). The fish production was found to be significantly different for organic fertilizer, grass carp + organic fertilizer and inorganic fertilizer treatments. The net fish production was highest (5.6±0.03 tonnes/ hectare) in grass carp + organic fertilizer treatment followed by organic fertilizer (4.0±0.06 tonnes/hectare). The inorganic fertilizer treatment yielded the least with 3.0±0.05 tonnes/hectare. The control ponds with no fertilizer was applied produced only 0.29±0.01 tonnes/hectare. [ABSTRACT FROM PUBLISHER]