Environmental Requirements and Management Techniques for Hatching in Rice Field Eel Aquaculture

The hatching rate of fertilized rice field eel eggs is affected by various environmental factors, among which the main ones are water temperature, dissolved oxygen and harmful organisms.

(1) Water temperature The embryonic development of rice field eel is extremely closely related to water temperature. This is mainly shown in three aspects: first, embryonic development must be carried out at a suitable water temperature; too high or too low will cause a decline in hatching rate or embryonic deformity. Studies show that the suitable water temperature for embryonic development of rice field eel is 21-28°C, and the optimal water temperature is 24-26°C; second, within the suitable water temperature range, too large or too rapid a water temperature difference can also cause embryonic deformity or death, and it is usually required that the temperature change within a short time should not exceed 3°C, and preferably not exceed 1°C; third, the length of embryonic development time of rice field eel is directly affected by water temperature. At a water temperature of about 30°C (28-36°C), embryonic development takes 5-7 days, and at a water temperature of about 25°C, embryonic development takes 9-11 days.

(2) Dissolved oxygen The embryos of rice field eel cannot use oxygen from the air and can only absorb dissolved oxygen in water through blood vessels and other tissues. Too low dissolved oxygen in water will cause delayed or arrested development, and even suffocation and death. Experiments show that at a water temperature of 24°C, the oxygen consumption per hour of 100 rice field eel eggs is: 0.29 mg at the cell division stage, 0.46 mg at the blastula stage, and 0.53 mg at the gastrula stage, far higher than the oxygen consumption of embryonic development in the “four major Chinese carps”. Therefore, during the hatching process, the oxygen content of the hatching water should approach or reach the saturation level of dissolved oxygen in the water body.

(3) Water quality Fresh water quality plays a great role in improving the hatching rate. Water polluted by pesticides or industry must never be used as hatching water. It is best to build a reservoir or arrange a dedicated pond to provide hatching water, and before use it should be filtered to prevent harmful organisms and pollutants from entering and affecting the improvement of the hatching rate. The pH of the water should preferably be neutral.

(4) Harmful organisms Under artificial breeding conditions, larger harmful organisms are easy to remove (such as tadpoles, small fish, small shrimp, etc.), but smaller organisms such as cyclops are easily overlooked. In fact, cyclops pose a relatively large threat to eel eggs and fry. They can use their appendages to pierce the egg membrane or bite the eel fry, and then suck the eel eggs and eel fry as nutrition. The damaged eel eggs or eel fry soon die. The best way to prevent cyclops is to filter the hatching water, with the filter net installed at the water inlet.