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With economical development and life style changes, billions of people want to diversify their diets by consuming less plant starch and more animal protein. Compared to other animals, fish are highly efficient at converting feed into protein. Health concerns have also prompted some people to shift consumption from beef and pork to poultry and fish. These resulted in the quick development of fish culture. According to FAO statistics, aquaculture output expanded from 13 million tons in 1990 to 48 million tons in 2005, growing by about 9 percent a year. China is the leading producer, contributing 83.6% of the increment. In 2005, China's aquaculture production is made up 70.5% of the world total. China has a long history in aquaculture back to 2000 years ago. Since 1970s, under the reform policies and driven by the economic benefits, the rapid development of China's aquaculture both in fresh waters and sea waters has been the focus of the world attention. China's freshwater fisheries output has increased rapidly by more than ten times in the last 50 years, reaching more than 19 million tons (mt) in 2007, which is equivalent to about 14% of the total world catch. The most commonly farmed species are grass carp (3.55 mt), silver carp (3.07 mt), bighead carp (2.22 mt), common carp (2.13 mt), crucian carp (1.94 mt), which accounts for 65.5% of the total freshwater aquaculture. Why these traditional species are farmed so abundantly? We should have an insight into the characterization of Chinese freshwater fish culture. Freshwater fish culture in China mainly refers to pond fish culture, which accounts for about 70% of the total freshwater aquaculture production. Technological system for pond fish culture is basically the ancestors' traditional Chinese fish farming technology, which is refined and improved through knowledge and experience from research and development efforts of last five decades. One of the outstanding features in pond system is mixed and polyculture with high density: Several species are cultured in the same pond. This experience was explored and accumulated by the Chinese fish farmers in a long period of practical production activities. Different species of fishes are cultured in the same pond according to their biological characteristics, food and feeding habits, and the water column inhabited by different species, so as to fully utilize the water space and the available natural feed to maximize production per unit area. Over time, China has evolved a remarkably efficient fish polyculture using four types of carp that feed at different levels of the food chain, in effect emulating natural aquatic ecosystems. Silver carp and bighead carp are filter feeders, eating phytoplankton and zooplankton respectively. The grass carp, as its name implies, feeds largely on vegetation, while the common carp is a bottom feeder, living on detritus on the bottom. China's aquaculture is often integrated with agriculture, enabling farmers to use agricultural wastes, such as pig or duck manure, to fertilize ponds, thus stimulating the growth of plankton. Now, Fish polyculture which typically boosts pond productivity over that of monocultures by at least half also dominates fish farming in India. The above is the reason why cyprinid fish holds an overwhelming proportion in freshwater fish culture. The silver carp (Hypophthalmichthys molitrix) is a species of freshwater cyprinid fish, which is a native species of China and has been raised in ponds since historic times. Silver carp, like all Hypophthalmichthys species, have no stomachs; they are thought to feed more or less constantly. Silver carp are thought to feed largely on phytoplankton; they also consume zooplankton and detritus. As silver carp is a plankton feeder fish, it occupies the upper region (surface waters) of culture ponds. As plankton feeders throughout life, silver carp is indispensable for polyculture, which is widely practiced in China. Silver carp is not very competitive with other adult fish, is less susceptible to disease, and grow rapidly. As it is a fast-grower fish able to feed on the lowest level of the food chain (phytoplankton). Silver carp is also used as a means of biological control of algal populations due to its high filtering rates. And it is usually raised in polyculture with other carp species. The overall aim of this practice is to make the best use of the production in the water body and to ensure equilibrium between the different species in their different ecological niches. Silver carp is chosen the principal species if ponds have been heavily fertilized. The above several desirable characteristics that are sought for by aquaculturists, also are the reason why Silver carp is imported to so many countries. Silver carp are usually farmed in polyculture with other Asian major carps, or sometimes Indian major carps or other species. Nowadays, this species is farmed in almost every major continent. In fact, silver carp is the most widely fish species cultured in the world. In China, the production cycle of silver carp lasts for two years: in the first year silver carp are grown to 50 g, and reach 500-1000 g after the second year. At this point, fish are sold. Despite the fact that freshwater fish resources in China have expanded rapidly, their distribution sphere and storage period are very limited since most freshwater fish are transported as live fish without treatment, such as freezing and processing. Compared with marine resources, freshwater fisheries resources lack the diversity of consumption because of the backward transportation and processing technologies. A big gap existed between China and some developed countries in the world in the fish processing industry, particularly in fishery products finely processed. Only 30% of landed fish are processed compared to 70% in developed countries such as Japan. Freshwater fish are almost always sold alive. Although the sector of freshwater fish processing has gained a great development, the volume of the processed aquatic product from freshwater fish only occupies 10% of the total according to the statistics in 2004, even less with fine processing. Due to backward post-harvesting technologies compared with fast increment of the production of freshwater fish, overproduction is the problem in harvesting season in some fish farming regions. The market value of freshwater fish drops because of the excess of supply over demand, which definitely would hinder a further development of fish farming. To develop freshwater fisheries environmental friendly and sustainably, the development of processing industry is essential to reduce losses through poor handling of fish, to utilize over catch fish, and to raise the value of fish products. In addition, improving fish quality, flavor, healthy and safe fish products are another goals for Chinese government. In some means, processing becomes the neck of the development of freshwater fisheries. To address the post-harvest problems and fully utilization of the abundant animal protein resource, Japan and China initiated a collaborative research project titled "Development of freshwater fish surimi". Because surimi is generally manufactured by applying a simple technology of washing minced meat by water, dehydration, and mixing with cryoprotectants and has a long frozen shelf-life, it is an intermediate foodstuff with high potential for free productivity of texturized products, such as imitation crab. Because of the low cost for aquaculture and low market value, silver carp maybe a good materiel for making surimi. Our research group has investigated on the utilization and processing of freshwater fisheries resources efficiently including silver carp since 1996. We proposed that silver carp has good gel forming ability, which is comparable to that of surimi made from Pollack, which is a typical commercial marine fish surimi that accounts for more than 60% of world surimi production. Silver carp showed a setting effect (suwari), texture-enhancing treatment by pre-incubating salted surimi at temperatures lower than 40 ℃ before cooking at high temperature (85 ℃). As silver carp is a temperate fish, large temperature changes occur in a year, we found the thermal stability of myosin, which is a main component in fish myofibrils, differed seasonally. This also resulted in the different gel forming ability of silver carp surimi which was prepared in different seasons. Now the industrial use of silver carp surimi is conducted. It is promising that many kinds of seafood products, ready-to-fry fish portions, surimi-based products, and fish sausages and cakes maybe processed from freshwater fish species. It is believed that the improved processing technologies of freshwater fish should ensure the sustainable development of freshwater fisheries.
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