Artificial propagation techniques for large icefish: Practical experience from Hongshan Reservoir

Hongshan Reservoir is located in the lower reaches of the Laoha River in the northeast of Chifeng City. It is a large plain-type reservoir integrating flood control, irrigation and aquaculture, with a catchment area of 24,500 sq km and a total storage capacity of 2.56 billion cubic meters. The reservoir@#@233@#@area ranges from 80,000 mu to 130,000 mu between the dry season and the wet season. The reservoir introduced fertilized eggs of large icefish in 1995, and the catch reached 2.5 kg/mu in 1998. In the following 2 years the yield declined somewhat, but in recent years, thanks to relatively well-developed propagation management measures, the resource has gradually recovered.

I. Strictly enforce the closed fishing system

Based on the growth and reproduction patterns of large icefish, a closed fishing system was formulated, and the reservoir management bureau supervises the aquaculture company in its implementation. The period from June to September, when large icefish grow and gain weight, and the period from December to February of the following year, when large icefish reproduce and hatch, are designated as closed fishing periods; the area within 3 km of the main dam in the reservoir area, where the substrate is gravel and the natural spawning and hatching rate of large icefish is relatively high, is set as a permanent closed fishing area; nets with a mesh smaller than 3 cm and shrimp traps are designated as closed-fishing gear.

II. Carefully carry out reproductive monitoring of large icefish

The peak spawning period of large icefish is closely related to the abundance or scarcity of food each year, climatic conditions and water quality. In early December each year, gill nets are used every day to catch parent fish in different areas to check the maturity rate. When the maturity rate is about 25%, large-scale fishing begins for artificial fertilization. The peak spawning period of large icefish in Hongshan Reservoir generally falls within the 10 to 15 days between December 5 and January 10 of the following year. On December 13, 2004, data such as the maturity status of parent large icefish in Hongshan Reservoir were tested; details are shown in the table below. As can be seen from the table, the parent large icefish in Hongshan Reservoir are relatively large in individual size, have a relatively high egg-carrying capacity and moderate egg diameter, and mid-December is exactly the peak spawning period of large icefish.

III. Capture and preservation of parent fish

Mature parent fish are captured by ice operation using single-layer bottom gill nets. During operation, the following must be observed:

1. Set nets within the designated area and control the number of nets set, thus both protecting the natural spawning area and controlling the number of parent fish caught.

2. The mesh should be small rather than large, and the time should be short rather than long. To ensure the parent fish are caught by the head rather than the chest or abdomen and remain alive, the mesh must not exceed 1.5 cm and the net-setting time must not exceed 1.5 hours.

3. When hauling the net, quickly place the net together with the fish into an insulated box and transport it by motorcycle to the egg-collection room. When removing fish, take one fish at a time as each is removed, and keep the rest in the insulated box with added ice at low temperature.

IV. Artificial insemination

When sufficient parent fish are available, in order to improve the hatching rate, the selection of parent fish basically follows these rules: those with food in the intestine are not used, those whose bodies are whitish and hard are not used, and those where the female-to-male ratio is less than 1:2 are not used. Our specific practice over the years has been: wipe the selected parent fish dry with a dry towel or toilet paper, take a female fish with the head up and tail down and squeeze the abdomen from front to back to squeeze the mature eggs into a plastic bag, aim the anal region of the male fish at the eggs in the bag and gently squeeze the testes of 2 to 3 male fish with both hands so that the semen hangs at the anal fin and urogenital pore, stir the sperm and eggs with the anal region of the male fish and place the male fish in the bag, add 20 mL to 30 mL of filtered reservoir water and gently shake, let stand for 2 minutes and then add about 250 mL of filtered reservoir water, tie the bag mouth and move it into the temporary incubation room for rinsing and temporary incubation.

V. Temporary incubation of fertilized eggs

For many years, the temporary incubation tool for fertilized eggs has been a hard plastic bathtub that is relatively small in volume, smooth on the inner wall, easy to move and convenient to operate. Fertilized eggs that have been cleaned, floated and freed of debris are placed into the bathtub in order according to fertilization time for incubation, with a temporary incubation density of 2 million eggs per sq m. The indoor temperature is controlled at 3degC plus or minus 0.25degC, the water is changed once a day and the water in the tub is stirred 2 times a day, and debris, whitened eggs and moldy eggs are removed in time. Note that the fertilization rate should be tested after each daily water change. Stirring the water in the tub every day during temporary incubation helps eggs whose development has stopped float to the surface for timely removal; otherwise it affects the development of the fertilized eggs. Testing of the fertilization rate showed that the fertilization rate of eggs stirred on schedule was far higher than that of eggs not stirred. A comparative experiment was conducted in December 2004. Under the same conditions, based on development to the late gastrula stage, the fertilization rate of eggs stirred every day was 68.4%, while the fertilization rate of eggs not stirred was only 19.2%.

VI. Release of fertilized eggs

According to the reservoir water temperature, on the day before releasing the fertilized eggs, adjust the water temperature of the temporary incubation room to be equivalent to that of the reservoir water. After testing the fertilization rate, the counted fertilized eggs are put into plastic bags and transported with an insulated box with added ice. The release site is selected at the reservoir bay at the south end of the dam, where the substrate is relatively hard, the water depth is about 3 m, motor vehicles do not pass, it faces the sun and is sheltered from the wind, and plankton is relatively abundant. The ice holes for releasing eggs are 50 cm square, and the spacing between ice holes is not less than 50 m. After temperature adjustment, the eggs are released at a density of 1 million eggs per ice hole, ensuring that 200 million effective eggs are returned each year. During release, the eggs settle relatively slowly, so it is necessary to prevent the eggs from sticking to the ice walls; plastic film can be used to protect the ice walls, and the stocking effect in this way is very ideal.