River crab pollution-free breeding key points, difficulties and examples

Author: Xu Zaiquan, He Qunyi / Country: Mainland China
Publisher:
Publish Date: 2005-05-01
Features:
4. Temperature-Controlled Development Technology for Fertilized Eggs of Brooding Crabs
To meet the different demands of early spring crab seed and early summer crab seed in crab farming production, the hatching of crab larvae needs to be advanced or delayed to control the hatching time of brooding crabs, enabling planned batch hatching and cultivation. Currently, temperature regulation is used to achieve this purpose. Through meticulous management of continuous aeration, intensive feeding, and frequent water exchange in brooding crab ponds, the water temperature is gradually raised to 15–20°C. The embryonic development of fertilized eggs can be completed in about 20 days, and the larvae will hatch from the membrane. The temperature-raising cultivation is conducted in indoor cement ponds, with efforts to mimic the natural habitat of river crabs. Measures such as shading and placing hiding spots can be adopted to create a comfortable living environment for the crabs. Fresh feed such as sandworms and clam meat should be fed in moderation based on the feeding conditions of the brooding crabs. Meanwhile, water quality should be kept fresh, and the pond bottom should be clean. The speed of embryonic development in river crab fertilized eggs is mainly affected by dissolved oxygen and temperature, so intermittent aeration is required. Temperature control is used to regulate the rhythm of embryonic development.
For hatching before mid-February, the temperature-raising period for brooding crabs should not be less than 40 days. Otherwise, if the temperature is raised too quickly, it may cause asynchronous embryonic development, leading to dead eggs, egg detachment, and poor quality I-stage zoea larvae. If hatching occurs between March and mid-April, since brooding crabs accumulate heat during winter, the temperature-raising speed can be faster, with hatching completed within 20–25 days. After mid-April, since natural water temperatures gradually rise, the P60 embryos can naturally develop in winter ponds. At this stage, simply moving them to indoor ponds with slight temperature adjustment is sufficient for hatching without artificial temperature control.
In the early stages of temperature-raising for early hatching, a faster rate is generally adopted in the beginning and a slower rate later. Before 12°C, the temperature is raised by 1°C every 1–2 days; between 12–16°C, the temperature is raised by 1°C every 3–4 days; and at 16°C, the temperature is adjusted flexibly based on the development of the embryos and the hatching schedule. If the embryonic development is stable, the temperature can continue to be raised at a rate of 1°C every 3–4 days. If development is slower, the temperature can be kept stable at 16°C for a period. Sometimes, to delay the embryonic development of fertilized eggs, brooding crabs can be kept in low temperatures (below 5°C) for an extended period, allowing hatching to last for several months. This makes batch hatching and batch breeding possible.
Currently, there are two common methods for cooling and preserving brooding crabs in river crab hatching production. One is to use underground cement pools in cold storage for indoor wintering, and the other is to place ice blocks in greenhouse earthen ponds for outdoor wintering with temperature control. Since the first batch of hatched larvae has high yield and quality, to ensure that all brooding crabs used in hatching production are first-time brooders, selected parent crabs are separated by sex and temporarily raised in freshwater ponds. Through low-temperature control, the artificial mating and induction of production can also be delayed until mid-to-late April, achieving the purpose of delaying hatching. This allows the use of second-time brooding parent crabs in the second batch of hatching production. P61