Detailed Explanation of Key Points and Methods for Artificial Breeding Technology of Catfish

I. Materials and Methods

1. Experimental Site and Facilities

The experiment used the original spawning ponds and hatching ponds for artificial breeding of carp, crucian carp, and the four major Chinese carps. For the spawning nests, palm bark was steamed and disinfected, then strung together with bamboo poles or ropes and suspended in the middle and upper layers of the spawning pond, with the pond bottom covered with palm bark.

2. Source and Cultivation of Broodstock

Broodstock were mainly collected from wild fish in Biliuhe Reservoir in Dalian City and subjected to intensive pond cultivation. Broodstock were fed freshwater trash fish and freshwater mussel meat. The average body weight of males was 500 g per fish, and that of females was 750 g per fish. Feeding was done twice daily, with light used to attract feeding at night. The daily feeding rate was 2%-5%.

3. Sex Identification and Pairing Ratio of Broodstock

Male and female broodstock were distinguished by the tail fin fork: the tail fork of females was shallow and inconspicuous, while that of males was deeper and obvious. The pairing ratio of females to males was (2.2-2.1):1.

4. Spawning-Induction Drugs and Their Dosages

The spawning-inducing agents used were a combination of SGnRH-A (salmon gonadotropin-releasing hormone analog) and DOM (domperidone maleate), at a dose of 8 micrograms + 8 milligrams per kilogram of body weight, administered as a single injection.

II. Experimental Results

1. Injection and Latency Period

Injection was performed at the base behind the dorsal fin, with the needle pointing toward the head at a 45-degree angle to the body, at a depth of about 1.5 cm, and the drug was slowly pushed in. To facilitate injection, broodstock were placed in transparent plastic bags. The latency period was closely related to water temperature, broodstock maturity, water flow, type of spawning-inducing drug, and injection dose. The suitable water temperature for estrus and spawning was 18-30 degrees C, with 24-25 degrees C being optimal. The time from the onset of estrus to spawning was relatively long, about 2-5 hours, but shortened as water temperature increased. Flushing water had a certain stimulating effect on broodstock estrus, especially for those with poorer maturity, and appropriately increasing the flushing volume could produce obvious results. Usually, a state of slight water flow was maintained from the time broodstock entered the spawning-induction pond, and the water flow was increased 1-2 hours before estrus. During estrus, the male wrapped its body around the female's abdomen, pressing against each other to release eggs and sperm.

2. Hatching

After spawning was completely finished, the spawning nests and eggs were thoroughly disinfected and then moved to a hatching circular channel for incubation, with slight water flow maintained in the channel and palm bark sheets laid on the bottom so that detached eggs would not become densely packed and suffocate to death. The time for fry to hatch and break the membrane was related to water temperature: at 17-23 degrees C, the membrane-breaking time was 72-80 hours; at 24-26 degrees C, the membrane-breaking time was 44-50 hours; at 27-30 degrees C, the membrane-breaking time was 24-40 hours. Newly hatched fry were milky white, with a total length of 0.5-0.6 cm and a large yolk sac on the abdomen, and the fry generally gathered around the spawning nests. Afterwards, body surface pigments gradually deposited, and after 3-4 hours they appeared grayish black, the yolk sac then basically disappeared, and they could swim horizontally by wagging their tails; at this point, they should be transferred to ponds in a timely manner according to specific conditions.

3. Results

In this artificial breeding session, a total of 409 groups were induced to spawn, with a spawning-induction rate of 69.4%, an average egg output of 11,200 eggs per group, a total egg yield of 3.18 million eggs, a fertilization rate of 61.3%, a hatching rate of 43.1%, and 840,000 fry hatched.

III. Problems and Discussion

1. Nutritional Problems During Broodstock Cultivation

During broodstock cultivation, whether the nutritional content of the feed can satisfy gonadal development is very important. This experiment adopted the method of feeding dead fish, which has many disadvantages. First, some feed could not be utilized and, after rotting, polluted the water quality, seriously affecting the normal life of the broodstock in the pond; second, regular feeding of a single bait lacked necessary nutritional elements and readily led to diseases such as skin rot and fatty liver.

2. Problems of Separate Cultivation of Male and Female Broodstock and Pairing

Male and female fish should be cultured in separate ponds, and separate pond culture became even more necessary after the water temperature rose to 15 degrees C. During the breeding season, pairing should strictly take broodstock maturity into account. This experiment adopted a group breeding method, and some broodstock that were not yet fully mature were also induced to spawn, thereby affecting the total egg output.

3. Problems of Broodstock Egg Output

In the experiment, a total of 3.18 million eggs were obtained, with an average of 11,200 eggs per group. Although this was relatively high overall, it was found during spawning induction that few large female fish were completely spent, indicating that large individuals had poorer maturity than some smaller individuals; second, the size difference between male and female individuals was great, the males were uneven in size, and there were many small individuals, whose chasing intensity and ability to wrap and press the female abdomen were weaker, affecting female estrus; furthermore, the injection dose of the drug should be accurately controlled according to the body weight of the broodstock. If the injection dose was low, it would cause asynchronous egg release and sperm release in the broodstock, resulting in a low fertilization rate.

4. Problems of Broodstock Selection and Retention

Most of the broodstock used in the experiment were 2-3 winters old (female body weight 0.75-1.00 kg per fish, male 0.30-0.75 kg per fish). The previous practice of the Biliuhe Reservoir Fish Seed Farm in selecting and retaining broodstock was to return most of them to the original pond for further cultivation after spawning induction. Although this could avoid the problem that first-time spawning females generally had low egg output because the broodstock age was too young, long-term cultivation caused diseases such as skin disease and liver disease.

5. Problems of Fertilization Rate and Hatching Rate

In this experiment, due to improper pairing ratio of females to males (more females and fewer males), and because most eggs with poor egg quality detached and sank to the bottom, the fertilization rate of the eggs was affected. At the same time, during the hatching process, excessive density and too little water flow caused most of the fish eggs in the bottom layer to die, resulting in water quality pollution and the death of some hatched fry.

6. Several Measures to Improve Breeding Technology

(1) Strengthen broodstock cultivation. It is best to use special pelleted compound feed for catfish broodstock, which has the advantages of comprehensive nutrition and convenient storage and transportation. During cultivation, attention should be paid to checking broodstock maturity so as to adjust cultivation methods in a timely manner. In the reserve of broodstock in batches, fish should be raised separately according to different ages, different individual sizes, newly selected and retained or originally retained in ponds, first-time pre-spawning or previously spawned, so as to implement different cultivation measures. In early spring or about one month before spawning, male and female fish should be strictly cultured in separate ponds to avoid sporadic natural spawning caused by stimulation such as rain during the breeding season.

(2) Accurately identify maturity and pair reasonably. If natural breeding is adopted after spawning induction, maturity should be accurately identified and the proportion of males increased. The female-to-male ratio can be increased to 2:3 or 3:5, especially when there is a size difference between male and female fish. The minimum size of male fish participating in breeding should be 0.5 kg per fish. It is recommended to use male broodstock over 3 years old and cultivate them with nutritionally comprehensive feed. This can not only ensure a relatively high egg-bearing capacity but also improve the spawning-induction rate and egg output.

(3) Accurately determine spawning-inducing drugs and injection doses. The spawning-inducing drugs were injected as a mixture of SGnRH-A and DOM. If the catfish maturity is good, the spawning-induction rate can reach more than 85%. This method has obvious advantages compared with the previous use of LRH-A plus DOM or LRH-A plus pituitary or HCG (human chorionic gonadotropin). When determining the injection amount of spawning-inducing drugs, each fish should be weighed as much as possible so as to accurately calculate the drug dose.

(4) De-adhesion hatching. Catfish eggs are adhesive eggs and can be hatched in flowing water after de-adhesion by drugs. This hatching effect requires less time than hatching together with spawning nests, has a higher hatching rate, and produces robust fry, but the choice of de-adhesion method still needs further experimentation.