Li De-shang

Closed recirculating system for shrimp-mollusk polyculture

This paper deals with a new system of aquaculture, i.e., a closed recirculating system for shrimp-mollusk polyculture. The culture system consisted of several shrimp ponds, a mollusk water-purifying pond and a reservoir. During the production cycle, water circulated between the shrimp and mollusk ponds, and the reservoir compensated for water loss from seepage and evaporation. Constricted tagelus,Sinonovacula constricta, was selected as the cultured mollusk, and Pacific white shrimp,Litopenaeus vannamei, as the cultured shrimp. The main managing measures during the production cycle were: setting and using the aerators; introducting the probiotic products timely into the shrimp ponds; adopting a “pen-closing” method for controlling shrimp viral epidemics; setting the flow diversion barriers in the mollusk pond to keep the circulating water flowing through the pond along a sine-like curve and serve as substrate for biofilm; no direct feeding was necessary for the cultured mollusk until the co-cultured shrimp was harvested; natural foods in the water from the shrimp ponds was used for their foods. Two sets of the system were used in the experiment in 2002 and satisfactory results were achieved. The average yield of the shrimp was 11 943.5 kg/hm2, and that of the mollusk was 16 965 kg/hm2. After converting the mollusk yield into shrimp yield at their market price ratio, the food coefficient of the entire system averaged at as low as 0.81. The water quality in the ponds was maintained at a desirable level and no viral epidemics were discovered during the production cycle.

Comparative studies on nitrogen budgets of closed shrimp polyculture systems

April to October, 1997 comparative studies on the nitrogen budgets of closed shrimp polyculture systems showed that, in all the studied polyculture systems, nitrogen from feeds and fertilizers were the main input items, which comprised 70.7%–83.9% of the total input nitrogen 3.2%–7.4% of which was provided by nitrogen fixation. It was in monoculture enclosures (Y-4, Y-11 and Y-12) that the percentage reached the maximum value. The output nitrogen in harvested products comprised 10.8%–24.6% of total input nitrogen, and the highest percentage, 24.6%, was found in shrimp-fish-tagelus polyculture systems. In shrimp monoculture and shrimp-fish polyculture systems, they were 19.1% and 21.9% respectively. The nitrogen utilization efficiency was different and varied from 12.2% was found in shrimp-tagelus polyculture systems. The lowest, 12.2%, was found in shrimp monoculture systems. All the nitrogen utilization efficiencies in shrimp-fish systems or shrimp-scallop systems seemed to be higher than that of the monoculture system, but they showed little statistical difference. The main outputs of nitrogen were found in sediment mud, and comprised 48.2%–60.8% of the total, input, the lowest percentage was found in shrimp-fish-tagelus polyculture systems, and the highest percentage in shrimp-scallop systems. During the experiment, nitrogen lost through denitrification and ammonia volatilization comprised 1.9%–6. 2%, averaged 2.8%, of the total input, and the loss through seepage comprised 5.9%–8.9% of the total. The estimated nitrogen attached to the enclosure wall comprised 3.7%–13.3% of the total, and was highest in shrimp monoculture systems. Compared with the classic shrimp farming industry, the closed shrimp polyculture systems may improve the nitrogen utilization efficiency, and hence reduce the environmental impacts on coastal waters. The nitrogen discharging rates for all the studied polyculture systems ranged from 3.0% to 6. 0% of total input nitrogen.

Comparative studies on the feeding capacity of silver carp and bighead carp

Study of the feeding capacity of silver carp and bighead carp by means of experimental ecology showed that the filtering frequency of silver carp was slightly greater than that of bighead carp, but that the latters suction volume was much greater than the formers so the filtering rate (filtering frequency multiplied by the suction volume) of silver carp was smaller than that of bighead carp. The filtering efficiency of silver carp for phytoplankton was greater than that of bighead carp for them. The removal rates of silver carp for phytoplankton were greater than those of bighead carp, but for zooplankton the former were smaller than the latter. For food particles about 70 μm both removal rates were almost equal. The feeding habits of the two species in natural waters is also discussed in the paper.

Carbon cycle in shrimp polyculture mesocosm

The carbon cycle in shrimp polyculture mesocosm ecosystems was studied in the shrimp farm of the Huanghai Fisheries Group Corporation in Shandong Province from May to August, 1997. The results showed that the plankton community respiration rate fluctuated between 0.07 and 2.28 mgC/(L·d), average of 0.82±0.42 mgC/(L·d), which was 49 percent of the rate of phytoplankton gross production; that the average respiration rates (mgC/(L·d)) of micro-, nano- and pico-plankton were 0.07, 0.38 and 0.31, which were 175, 30 and 207 percent of the corresponding sized phytoplankton production rates; that the sediment respiration rate (mgC/(m2·d)) varied from 178.64 to 373.23, average of 262.60±48.68, and increased gradually with the lapse of culture time; and that the organic carbon accumulation and the respiration in the sediment of the shrimp monoculture mesocosm was higher than that in the polyculture mesocosms. The total sediment respiration per 25 m2 mesocosm in the culture period averaged 571.16 gC, which was 10 percent of the total organic carbon input of the mesocosm.

Summary of studies on closed-polyculture of penaeid shrimp with fishes and moluscans

Closed-polyculture of Chinese penaeid shrimp, Taiwan red tilapia, and molluscans(constricted tagelus and bay scallop) was studied from 1995 to 1997 with the enclosure-experiment method, on a shrimp farm in Haiyang County, Shandong Province. Four structure-optimized closed-polyculture systems, i. e., the “shrimp-tagelus” system, the “shrimp-scallop” system, the “shrimp-tilapia” system and the “shrimp-tilapia-tagelus” system, were worked out. All these polyculture systems were superior to shrimp monoculture system in economic and ecological efficiencies. The order of these polyculture systems in efficiencies was “shrimptilapia-tagelus” > “shrimp-tagelus”>“shrimp-tilapia”>“shrimp-scallop”. The “shrimp-tilapia-tagelus” system raised the production by 28% and the utilization efficiency of input nitrogen by 85%. These closed-polyculture systems reduced the nitrogen discharge ratio to 6%–8%, instead of 40%–90% in the usual open culture systems. The ecological features of the systems were also investigated and many meaningful results have been obtained. The mechanism enhancing the efficiencies of these systems, the ways to enhance the efficiencies further and those to reuse the sedimented materials are disscussed.

Preliminary study on limiting nutrients in seawater ponds

A method for monitoring biological production of oxygen was used in the July 10–September 15, 1995 study on the limiting nutrients in eight land-based 5 m×5 m enclosures equipped with water stirrers in a seawater pond in the study region (Yellow Sea). Chicken manure only, chicken manure combined with chemical fertilizers, and chemical fertilizers only, were put into the enclosures, respectively. The results showed that phosphorus was the limiting nutrient; and that the optimal N:P ratio of the fertilizers was 7.25–7.54 (mean of 7.37), which was significantly lower than the ratio measured in the water, as phosphorus deposited is easily absorbed by sediments and is excessively uptaken by phytoplankton. The above optimal N:P ratio is suitable for daily fertilization with combination fertilizer in the seawater ponds.

BACTERIOPLANKTON SECONDARY PRODUCTION ESTIMATES FOR ARTIFICIALLY FERTILIZED SHRIMP POND

Experiments were conducted from June to September, 1995 in a controlled integrated culture pond-enclosure ecosystem. The principal objective of this study was to quantify the rate of heterotrophic bacterioplankton productionin situ in a fertilization pond ecosystem. This paper presents a method by which bacterial production was estimated through incubation in situ and measurement of increased bacterial abundance with time. Bacterial growth rates, production and turnover per day during the periods of culture were estimated. The influence of zooplankton grazing, substrate limiting and water temperature on the bacterial growth rates and production were studied also.