Several Key Control Points for Large-Specification Hairy Crab Culture in the Yellow River Estuary
In order to build up the "Yellow River Estuary" aquatic product brand and enhance the market competitiveness of "Yellow River Estuary" hairy crab, in 2005 Kenli County, Shandong Province, carried out a large-specification "Yellow River Estuary" hairy crab culture trial in a ten-thousand-mu river crab demonstration park, achieving good economic and social benefits. We now discuss several key technical issues in the culture process with fellow practitioners.
1. Pond ecology
Influenced by traditional concepts, many units @#@235@#@ follow @#@233@#@ methods when building ponds and rearing, with poor results: the survival rate of "Yellow River Estuary" hairy crab is low, the market size is small, and economic benefits are poor. This is because: (1) the effective bottom area is small and water body utilization is low; (2) water quality is difficult to manage, and the weight gain rate of "Yellow River Estuary" hairy crab during growth and molting is low; (3) there are no attachments or shelter in the pond, so "Yellow River Estuary" hairy crab burrows at a high rate, easily escapes, has many chances for cannibalism, and is vulnerable to predators; (4) under low dissolved oxygen conditions, organic matter at the pond bottom decomposes and produces toxic gases that attack the crab body and affect the taste of the flesh.
The living habits of "Yellow River Estuary" hairy crab differ from those of fish, and the ecological conditions it requires are also different. It requires waters with a relatively high coverage of aquatic plants, a shallow but not polluted mud bottom, pH 7.5-8.5, as well as clean water quality and suitable water depth. Soil and water sources are generally fixed, and suitable water depth and pH are also easy to control, but aquatic plants, as an important ecological factor, have different ecological effects depending on their species and cultivation form. For example: Hydrilla verticillata, Vallisneria natans, and Ceratophyllum demersum are favored as food by "Yellow River Estuary" hairy crab, yet cannot form stable communities and are difficult to establish effective ecological functions; Alternanthera philoxeroides can form stable communities, but like Typha it pollutes water quality rather heavily; Zizania caduciflora has well-developed root fibers, and its young shoots and tender stems are favored as food by "Yellow River Estuary" hairy crab, with relatively strong regeneration and water-purifying capacity, except that ponds stocked with crab seed earlier need careful cultivation every year; Phragmites australis, although not favored as food by "Yellow River Estuary" hairy crab, has strong water-purifying capacity and does not pollute water quality after withering. The planting form for both Zizania caduciflora and Phragmites australis is strip-block shaped with shallow trenches, which facilitates feeding and recapture, while also providing a certain living space and allowing ventilation and increased light. Generally, an aquatic plant coverage of 70%-80% is appropriate. From an ecological perspective, if the pond area is too small, it is difficult to create a good ecological environment; if the pond area is too large, it brings difficulties to management and recapture. Therefore, a pond area of 1 mu to 15 mu is appropriate.
2. Crab seed stocking
1. Stocking time. Crab seed is stocked as early as October on the solar calendar and as late as June on the solar calendar. In the former case, crab seed is mostly polycultured in adult crab ponds, and when adult crabs are harvested it causes limb loss, death, and mixing of sizes, affecting overwintering and spring management; in the latter case, it coincides with the first molting period of "Yellow River Estuary" hairy crab, which not only causes large transport losses but also delays the crab body's adaptation to the new environment, resulting in a low growth starting point. Therefore, seeding time should begin after the pond is cleared following the sale of adult crabs early the previous year, and end as late as mid-April. The smaller the size of the seed stocked, the more appropriately earlier the stocking time should be.
2. Crab seed quality requirements. The size should be uniform and reach a certain weight, so that the market size of 100 g/crab to 150 g/crab can be reached in the same year of culture. According to the author's measurements, the weight gain rate per molt in adult crab culture is 47.3%, with the best reaching 61.3%; from April to September there are at least 2 molts and at most 4 molts. Considering weight gain accumulated from non-molting growth, assuming a weight gain rate of 80% for 4 molts, and requiring a market size of 100 g/crab, the minimum size of crab seed is calculated to be no less than 52 g/crab; at general culture levels, where the weight gain rate per molt during growth is between 50% and 60%, a crab seed size of 40 crabs/kg to 80 crabs/kg is appropriate. Second, artificially reared juvenile crabs are smaller in size than natural crab seed, and a considerable proportion reach sexual maturity in the same year, with biological individual weight clearly declining. On the one hand, this is because artificial seedling technology and the developmental requirements of river crab are still not fully coordinated; on the other hand, it is due to severe degradation of river crab germplasm, and the mortality rate of sexually mature individuals at the first molt in spring is above 60%. Therefore, when selecting and purchasing crab seed, one should not only pay attention to size but also select improved varieties, while also checking sexual maturity and the proportion of such individuals.
3. Stocking density. When crab seed is stocked before mid-April, with aquatic plant coverage in the pond at 80%, its effective area can reach 100%-150%, and the mud is shallow but not polluted. For crab seed size of 40 crabs/kg, stocking density is 1.5 crabs/sq m to 2.0 crabs/sq m, and culture survival rate is 85%-90%; for crab seed size of 60 crabs/kg, stocking density is 2.0 crabs/sq m to 3.0 crabs/sq m, and culture survival rate is 80%-90%; for crab seed size of 80 crabs/kg, stocking density is 3.0 crabs/sq m to 4.0 crabs/sq m, and culture survival rate is 75%-800; for crab seed size of 100 crabs/kg, stocking density is 4.0 crabs/sq m, and culture survival rate is 70%-80%. The stocking density may also be increased or decreased as appropriate depending on the quality of conditions.
3. Feed
The "Yellow River Estuary" hairy crab has a broad diet and strong selectivity in feeding, so continuous feeding of a single feed variety has many drawbacks: (1) single nutrition and slow crab body growth; (2) low feed utilization rate and high feed conversion ratio; (3) serious pollution that ruins water quality. Through trial comparisons, with feed mixed and prepared from two types of raw materials, high-grade and low-grade, the selectivity of "Yellow River Estuary" hairy crab for it is stronger than for feed of a single ingredient, the feed conversion ratio is clearly reduced, and the crab body grows faster. At the same time, even when commercial feed is available, "Yellow River Estuary" hairy crab still needs to consume a certain amount of aquatic grass, so no matter how sufficient the concentrated feed is, some aquatic grass should still be appropriately fed.
4. Management
1. Management during the molting period. In units that culture crabs relatively well, the molting peak lasts 10 days, while in some it takes 20 days to 30 days. Obviously, an excessively long population molting period is not conducive to weight gain and grow-out. As far as protecting soft-shell crabs is concerned, the measures can only be reflected in management during the molting process. To make "Yellow River Estuary" hairy crab molt in a concentrated manner, the key points include: (1) accurately grasp the time of each molt; (2) before and after each molt, the animal protein and other nutritional components in the feed should reach 50%-80%; (3) once molting crabs are found, splash quicklime at 15 kg/mu to 20 kg/mu, while adding and changing a large amount of water, and make the new water level 10 cm higher than the original water level to increase the area of attachments; (4) after molting enters the peak period, only add water without changing it; (5) during the peak period, increase the number of feeding trays to ensure that soft-shell crabs feed sufficiently. After molting, soft-shell crabs harden in about 2 days, but indoor rearing requires 10 days. Practice has shown that light intensity has a relatively large effect on soft-shell hardening, so pond embankments should not be planted with shade-forming trees.
2. Spring management. In early spring, the temperature difference between day and night is large. Because "Yellow River Estuary" hairy crab is relatively sensitive to temperature, when the afternoon water temperature reaches above 7°C it emerges from the water and becomes active; when the nighttime temperature gradually drops to 2°C, its walking legs become stiff, and just as in late autumn and early winter, the phenomenon of "lingering on the embankment" appears. Therefore, pond patrols must be maintained morning and evening; frozen-stiff "Yellow River Estuary" hairy crab placed in warm water at about 20°C can recover to normal. Timely feed a small amount of high-quality feed. Adult crabs kept in the pond should be sold promptly, otherwise they are prone to mating and dying when the water temperature reaches above 13°C. Individuals that are too small for market and whose gonads have matured should be cultured separately by sex until the end of the first molt, at which point the gonads have degenerated to stage III-stage I. Ponds should be cleared in a timely manner, silt removed, weeds removed, and calcium fertilizer applied.
3. Summer management. Control the feeding amount; feed must not be excessive. Pay attention to changes in water quality and prevent the production of toxic gases at the pond bottom. Regularly use microbial preparations or substrate improvers to improve water quality; quicklime may also be applied regularly, and water changed regularly. During the flood season, prevent crabs from escaping.