A guide to mastering large-water-area river crab farming from five aspects

River crab, also known as hairy crab or crab, with the scientific name Chinese mitten crab (Eriocheir sinensis), commonly inhabits waters with fresh water quality and abundant aquatic plants, and has the characteristic of being active at night and resting by day, generally coming out at night to move about and feed. Each year from June to September is the peak period for river crab activity and feeding. Farming river crabs in large water areas has the resource advantages of flowing live water, a large wind-exposed surface, sufficient dissolved oxygen and relatively abundant natural bait, so using large water areas to farm river crabs can significantly reduce production costs and improve farming efficiency. Now, combining many years of farming experience in our town, the technical key points are summarized as follows.

           1 Preparatory work before stocking           1.1 Selection of water area           According to the biological characteristics of river crab, waters with fresh water quality, abundant sunlight and rich aquatic plants (preferably submerged plants such as tape grass and Hydrilla verticillata, and emergent plants such as reeds and cattails), and with relatively many snails and clams (if snails and clams are insufficient, they can be stocked artificially), should be selected. In this way, river crabs can obtain sufficient animal and plant bait, which helps to satisfy and promote the growth of river crabs. Where aquatic plants are scarce, partial artificial planting of aquatic plants can be adopted to make up for it.           1.2 Establishment of escape-prevention facilities           Use netting and tree stakes (bamboo stakes) to enclose the farming water area. For the blocking net, polyethylene netting is suitable, and the mesh size should be such that crabs cannot escape; double-layer enclosure netting is best. The height of the blocking net should be more than 1 m above the year-round average water level. The bottom of the net should use stone cages as sinkers to sink into the bottom mud, and barbed stakes should be used to insert the netting into the mud; on the inner layer of netting, plastic film or calcium-plastic board should be used to make an inward-facing brim, to prevent river crabs from climbing over the net and escaping. At certain intervals, insert tree (bamboo) stakes into the mud 0.5-1.0 m to reinforce the blocking net and prevent impact from wind and waves.           1.3 Clearing wild species           Before stocking juvenile crabs, various methods should be used in advance to remove wild and miscellaneous fish from the water area, reduce natural enemies of river crabs, and provide a suitable farming environment for river crabs.           2 Crab seed stocking           2.1 Strictly control the quality of crab seeds           For farming efficiency, seedlings and seeds are crucial; when purchasing seedlings and seeds, the following points should be noted.           2.1.1 The purchased juvenile crabs (crab seeds) should be uniform in size, robust in constitution, and free from disease and injury; this is the primary and most basic condition.           2.1.2 In the Jianghuai region and the Yangtze River basin, Yangtze River crab is the preferred crab seed for purchase; its growth rate and recapture rate are better than those of other crab seeds. Under the same conditions, Liao River crab matures sexually earlier than Yangtze River crab, and the average individual size is naturally much smaller.           2.1.3 Crab seeds cultivated in inland pure freshwater are better than crab seeds cultivated in coastal areas. Generally speaking, the water used for cultivation in coastal areas has a certain salinity, and the crab seedlings cultivated are not adapted to inland waters, tend to mature early, and their growth rate is not very ideal, so if conditions permit, it is best to cultivate juvenile crabs yourself.           2.1.4 From many years of farming results, ecological earthen-pond seedling rearing is better than industrialized cement-pond seedling rearing; the seedlings cultivated by the former are more adapted to natural stocking, especially large-water-area stocking.           2.2 Establish the stocking time in good time           Large-water-area stocking should mainly be winter stocking and spring stocking; each year December and the following year March to April are the best stocking seasons. In cold winter and when frozen, stocking is generally not suitable, so as to avoid freezing injury to crab seeds and affecting the survival rate.           2.3 Determine a reasonable stocking quantity                    The determination of stocking quantity should be based on the natural bait in the water area and one's own economic strength, acting according to ability. Stock crab seeds of 200-300 individuals/kg at a density of 400-600 individuals/667 sq m. Where the area is large, bait is sufficient, water quality is good, and funds are guaranteed, the stocking quantity may also be appropriately increased, but it must not exceed the natural bait supply capacity of the water area.           2.4 Adopt scientific stocking methods           Before stocking, place the crab seeds on a boat and release them as evenly as possible over a larger range. If the crab seeds were purchased from elsewhere, because they have undergone long-distance transport and a relatively long period of dehydration, they should not be directly released; instead, the juvenile crabs should first undergo acclimation treatment, that is, together with the net bag, soak them in water for 1-2 minutes, take them out and leave them for 3-5 minutes, and repeat this 2-3 times; after the juvenile crabs have fully absorbed water and adapted to the water environment, stock them according to the above method.          3 Feeding management           3.1 Bait feeding           In large water areas, aquatic plants are generally relatively abundant, and in the early stage no feeding is generally needed. July to September is the period of vigorous growth for river crabs. In order to effectively protect aquatic plants from large-scale damage, part of the artificial bait should be fed in good time at this time, such as snails, river clams, fish and shrimp meat, etc., and plant-based bait such as grains, beans, shredded pumpkin, shredded sweet potato, etc., to better promote rapid growth of river crabs. Generally feed 2 times per day, once each at 8-9 in the morning and 17-18 in the afternoon, and the daily feeding amount should be 5%-7% of the river crab body weight. In the morning, feed in the deep-water area of the aquatic plants; in the afternoon, feed in the shallow-water area of the aquatic plants.           3.2 Water quality management           Large water areas generally have smooth water flow, and water quality is not easily deteriorated, but when aquatic plants are bitten off by river crabs and float on the water surface, they should be removed in good time, so as to avoid rotting in the high-temperature season and spoiling the water quality. At the same time, pollution of water quality by organophosphorus and pyrethroid drugs is prohibited. In addition, every 15 days, quicklime should be used at 25-30 kg/667 sq m, dissolved in water and splashed over the whole pond, to improve water quality.           3.3 Daily management           First, strengthen blocking net management, and check in good time whether the enclosure netting has holes; if holes are found, repair them in good time, especially in the flood season, when river crabs can very easily escape with the flow of water.           In addition, during the growth period of river crabs, it is prohibited to gather aquatic plants, prohibited to release ducks, and prohibited to use fishing gear for underwater operations, because these can very easily cause injury or death to river crabs, especially at the peak of river crab molting, when the harm is even greater.           4 Disease prevention and control           4.1 Pond clearing and disinfection of the water area should be carried out as far as possible.           4.2 During operations, movements should be gentle and quick, and try not to injure the crab body.           4.3 Adopt a combination of regular splashing of quicklime and mixing internal medicine into bait for feeding, to reduce disease occurrence as much as possible. Commonly used internal medicines include norfloxacin, Yigan Su, FRC, etc.           5 Adult crab harvesting      Farming river crabs in large water areas is different from pond crab farming, and harvesting should be carried out in good time. Starting in mid-September, a considerable number of river crabs attempt to climb and escape outside the net enclosure, so determining the harvesting time in good time is very important for increasing the recapture rate and reducing river crab escape. Therefore, for large-water-area net-enclosure crab farming, harvesting must be started in advance, generally choosing early September, concentrating efforts on organized harvesting before the natural reproductive migration of adult crabs, usually using ground cages and crab traps (maze traps) for fixed capture, which gives relatively good harvesting results.