Post‑cultivation management of river crabs

2026-06-05

Every year, August and September represent the critical period when Chinese mitten crabs accumulate nutrients, build up their roe, and increase in weight. During this time, the crabs begin their final molt one after another; the molting‑to‑weight‑gain ratio of this last molt directly determines the marketable size of the crabs and, consequently, the farm’s ultimate profitability. Next, let’s explore how to manage ponds in the late stage of culture to enhance both the molting rate and the weight‑gain ratio.

I. Water Level

Crab molting requires a certain accumulated thermal unit; excessively high water levels can hinder the process. During the later stages of culture, water levels should be appropriately lowered: when air temperatures exceed 35°C, maintain the pond water level at 60–70 cm; once temperatures drop below 32°C, reduce the water level to 40–50 cm. Ensure adequate pond illumination to promote gonadal maturation and facilitate earlier market release.

II. Aquatic Plants

In the later stages of culture, it is inadvisable to maintain an excessive amount of aquatic vegetation in the pond. Overabundant plants can lead to low dissolved oxygen levels at night, which in turn impairs feed intake by the river crabs and results in delayed molting and slow shell hardening. It is recommended to retain only about one-third of the pond’s total aquatic plant cover. Additionally, during periods of high temperature, aquatic plants are prone to die and decompose; dead vegetation should be removed promptly, and a potent bottom‑conditioning agent (6 mu per package) should be applied to improve the pond bottom. For ponds with no natural vegetation, consider introducing some floating species—such as water hyacinth, foxtail algae, or duckweed—that are resistant to decay, providing shelter for the crabs and minimizing adverse effects on their molting process.

III. Water Quality

In the later stages of culture, most ponds accumulate substantial amounts of decaying aquatic vegetation, leftover feed, and feces at the bottom, leading to severe oxygen depletion. Since Chinese mitten crabs consume even more oxygen during molting, apply a powerful bottom‑decomposition agent (one package per 6 mu) every 5–7 days to break down these oxygen‑depleting materials and reduce oxygen demand. At the same time, extend the operating hours of the aerators—ideally turning them on at 10 p.m. and leaving them running until 7 a.m. the next morning—to boost overall dissolved oxygen levels and facilitate smooth molting. Moreover, molting is an osmotic swelling process; in the late culture phase, ponds often harbor significant accumulations of toxins. During molting, these toxins can be absorbed into the crab’s body, resulting in low survival rates, incomplete molts, or even post‑molting mortality. To mitigate this, apply Bishui Jiedu’an (one bottle per 2 mu) every two weeks to detoxify the water, while simultaneously using EM Jinlu (one bottle per 2 mu) to decompose harmful substances such as ammonia and nitrite, thereby enhancing water fertility.

IV. Nutrition of River Crabs

Fattening river crabs primarily involves increasing the weight of the liver and gonads, as well as the thickness of the carapace, thereby boosting the crabs’ body weight. It is recommended to apply calcium‑magnesium rapid‑supplement (one packet per 8–10 mu) combined with Life‑Enhancing Agent (one packet per 2 mu) via water spraying to accelerate gonad development and promote molting and shell hardening.