Chinese river crab
by Rebecca Sherratt, Features Editor, International Aquafeed
The Chinese river crab (Eriocheir sinensis), also referred to as the Chinese mitten crab, is named so due to its furry claws, which are said resemble mittens. The species is considered relatively new to the aquaculture industry, having only being produced for aquaculture purposes since 1989. As well as China, this crustacean can also be found in temperate waters along Korea, Japan, Taiwan and Eastern Russia.
Much commercial production of Chinese river crabs is carried out in China. This crustacean is typically marketed live, although in recent years processed products have entered the market. Exports are primarily focused within Japan, Hong Kong and Korea. Market rates suggest Chinese river crab is gaining more international attention, as export numbers increased by 70 percent in 2004, compared to the previous year. These rates have continued to increase year-on-year as the Western market establishes itself.
Chinese river crabs typically reside in inland water bodies connected to estuaries. Upon reaching sexual maturity, they relocate to downstream estuaries to reproduce. The species is omnivorous, feeding on small fish, mussels, worms and aquatic plants.
One of the issues most prevalent in Chinese river crab production is the myriad of diseases they can become subject to. Particular diseases to be wary of include Shiver disease, Shell ulcer disease and Sacculina disease, all of which can be caused by a variety of factors such as parasites, viruses and bacteria. Treating these diseases can be very complicated, whilst some diseases (such as Shiver disease) currently have no known cure, so ensuring farm hygiene conditions are optimised is crucial.
In 1995, 41,516 tonnes (t) of Chinese river crabs were farmed for aquaculture purposes. In 2005 this number increased to 378,376t and again to 812,183t in 2016.
Introduction to aquaculture
The Jiangsu province of China first saw an increase in Chinese river crabs following the release of crab seed into lakes in the 1970s. The stocking of seed also began in small lakes along the Yangtze River. The farming of Chinese river crabs was a relatively humble business until it expanded in the 1990s. In this period, crab seed was introduced to various shallow lakes, paddy fields and earthen ponds across China. Nowadays, the Chinese river crab is a key species in Eastern freshwater farming and have also been reportedly farmed in Germany and the US.
This crustacean has been seen as a key contributor to the income of rural farmers in China. Farming of Chinese river crab is still often carried out on a small scale by independent farmers but, in recent years, advances have been made in producing large-scale production facilities, special feed additives and artificial propagation solutions. The aim of these emerging technologies is to improve survival rates, animal welfare and production efficiencies.
Another unique farming method that has been established in China is integrated rice-aquaculture farming (RAF). Praised for its sustainable approach, rice-aquaculture farming encourages rice farmers to produce both rice and crustaceans in the same plot of land. Naturally available food in rice fields sustains crustaceans whilst also serving as a space-efficient and responsible form of farming.
Healthy, adult Chinese river crabs from either open-water or pens will be selected as broodstock in autumn. Males and females will be stocked in different ponds, before being transferred to mating tanks at a male-female ratio of 3:1. After approximately two weeks, male crabs are removed, and females are reared for one month before eggs are hatched.
Two days before the eggs hatch, female Chinese river crabs are relocated to larval rearing tanks, usually at three crabs-per-m3. Operators will recognise when eggs are due to hatch, as they will turn transparent. Larval rearing takes approximately three weeks, wherein the crab larvae will moult five times to reach the 'megalopa' stage. Megalopa will be kept at 22-25°C and fed zooplankton, Artemia nauplii and egg yolk until they are five days old, at which point they will be transferred to seed crab rearing ponds. Megalopa remain in nurseries until they reach between 5-20 grams (g). During this stage, they will grow into juvenile bottom-feeders who feast on aquatic weeds and various grains, as well as molluscs and small fish.
Ongrowing techniques vary depending on the systems used. Chinese river crabs are commonly farmed in ponds, paddy fields, net-pens as well as lakes and reservoirs:
- Semi-intensive pond culture: In ponds, crabs are stocked at 22,500-37,500-per-m2. Chinese river crabs are often also stocked with bighead carp (Hypophthalmichthys nobilis), crucian carp (Carassius carassius) and silver carp (Hypophthalmichthys molitrix) to ensure food availability and optimise water quality.
- Extensive paddy field culture: Paddy field culture is considered the most environmentally friendly farming approach, as it integrates aquaculture with agriculture production. Crabs will rely on the natural food present in rice fields, and often grow to larger sizes as they are stocked in smaller numbers. It is crucial to minimise the use of pesticides on crops, to maintain peak crab health.
- Semi-intensive net-pen culture: This method typically takes place in reservoirs and shallow lakes. Stocking density varies depending upon the size of crustaceans. The bottom of pens will be buried into the soil to prevent crabs escaping, whilst the tops of the nets will be approximately 0.8 metres above the water's surface. Chinese river crabs produced via net-pen culture are often larger and, therefore, go for higher market prices.
- Extensive lake and reservoir culture: Upon establishing a suitable body of water, crabs are stocked at a range of 200-600-per-hectare (ha). Supplementary feed may be used, but this is only recommended when natural food is not available in abundance. Inlets and outlets must be blocked with fencing to prevent crab escape. Fishing must also be monitored, as this can cause damage to the crabs. Whilst this method can result in great economic gains, management of the body of water can prove challenging.