Detailed Explanation of Key Technical Points for Broodstock Cultivation of Little Yellow Croaker
The little yellow croaker (Pseudosciaena polyactis Bleeker) belongs to the order Perciformes, suborder Percoidei, family Sciaenidae, genus Larimichthys (yellow croakers). It is distributed in the Bohai Sea, the Yellow Sea and the northern East China Sea. It is a warm-temperate demersal fish and was formerly one of the four major marine economic fish species in China. Since the 1950s, the little yellow croaker resource has declined severely. In this article, the author introduces broodstock cultivation techniques for the little yellow croaker. Through observation of the growth process of the little yellow croaker, growth parameters were recorded, laying the preliminary groundwork for artificial breeding and artificial farming of the little yellow croaker.
I. Capture of little yellow croaker broodstock
The little yellow croaker has 2 geographical populations in the Yellow Sea and Bohai Sea region: the northern population and the Lusi population. The northern population overwinters in the central and southern Yellow Sea and feeds and reproduces along the northern and Bohai Sea coasts; the Lusi population is larger, with a shorter seasonal migration distance, mainly moving east-west between deep and shallow waters in the southern Yellow Sea. In March, the little yellow croaker begins its reproductive migration from the overwintering grounds, and spawns in coastal shallow waters from April to May.
The little yellow croaker broodstock captured by the author were located in the southern Yellow Sea and northern East China Sea, belonging to the Lusi population. In May, small offshore fishing boats from Dongji, Putuo, using the "small trawl" encircling operation method in the Zhongjieshan fishing ground, caused relatively light abrasions to the little yellow croaker; although the quantity was small, surviving little yellow croakers could be captured; by July, a total of nearly 150 person-times were organized to go to sea with boats on 24 capture trips, and more than 400 live little yellow croakers were captured. After intermediate temporary holding and live-transport maintenance, only 16 survived.
II. Cultivation of little yellow croaker broodstock
After the little yellow croaker broodstock were transported to the Aoshan Experimental Base of the Zhoushan Fisheries Research Institute, they were bathed in a certain concentration of antibacterial drugs. After stabilizing for a period in the nursery pond, they were moved into soil-pond cages for cultivation. When the seawater temperature in the sea area dropped to 10°C, the broodstock were moved indoors to overwinter.
1. Soil-pond cage cultivation. During the soil-pond cage cultivation process, various physical and chemical factors of the seawater in the soil pond were measured regularly, and the water exchange ratio was determined according to water quality conditions so as to keep the water fresh. The feed organisms were mainly fresh and live small trash fish, fed twice a day, with a daily feeding amount of 5%-10% of the total body weight of the broodstock, and a small amount of medicated feed was fed regularly to avoid the occurrence of fish diseases. The cages were replaced regularly, and a short-term medicated bath treatment was carried out during the process of turning the cages. The growth of the little yellow croaker was measured monthly, and body weight, total length and body length were recorded. On July 19, the average body weight of the little yellow croaker was 27.4 g, the average total length was 15.2 cm, and the average body length was 12.6 cm; by December 19, the average body weight of the little yellow croaker was 53.6 g, the average total length was 18.4 cm, and the average body length was 15.3 cm.
2. Indoor overwintering cultivation of broodstock. When the water temperature of the soil pond dropped to about 10°C, the little yellow croaker broodstock were moved indoors for cultivation. The indoor cement pond was 4.0 m x 8.0 m x 1.2 m, with slight aeration; the water temperature was controlled at 12°C-15°C, and the salinity was controlled at 22‰-30‰. Feed organisms were fed twice a day and the tank was siphoned. During the overwintering process, broodstock gonad cultivation was carried out at the same time, using the method of adding an appropriate amount of vitamin E to the feed organisms. Because the captured little yellow croakers were mostly one-year-old fish and their gonads could not reach maturity requirements, they were placed in shallow-sea cages on April 5, 2004 for continued cultivation. During overwintering, it was found that the fish grew relatively quickly, which may be closely related to factors such as feed organisms and the environment. On January 20, the average body weight of the little yellow croaker was 69.4 g, the average total length was 18.6 cm, and the average body length was 15.5 cm; by the 5th of the next month, the average body weight of the little yellow croaker was 89.6 g, the average total length was 21.8 cm, and the average body length was 18.6 cm.
3. Shallow-sea cage cultivation of two-year-old broodstock. Because two-year-old broodstock have relatively strong swimming ability, and in order to better simulate the actual farming environment, the author's research group decided to conduct a shallow-sea farming experiment and moved the little yellow croakers into shallow-sea cages in April 2004. The cultivation techniques for shallow-sea cages were basically the same as those for soil-pond cages, but it was found that the growth rate of the fish was not as fast as in soil-pond cages. On May 5, the average body weight of the little yellow croaker was 8 g.2 g, the average total length was 22.1 cm, and the average body length was 18.6 cm; by December 6, the average body weight of the little yellow croaker was 109.3 g, the average total length was 23.0 cm, and the average body length was 20.5 cm.
4. Indoor overwintering of two-year-old broodstock. On December 6, 2004, the little yellow croakers were again moved indoors for cultivation, and gonad maturation promotion was carried out. On January 6, the average body weight of the little yellow croaker was 117.4 g, the average total length was 23.6 cm, and the average body length was 21.2 cm; by March 25, the average body weight of the little yellow croaker was 135.6 g, the average total length was 25.0 cm, and the average body length was 22.6 cm.
III. Summary
1. Broodstock live-keeping. Because the little yellow croaker is easily injured during capture and scales fall off rather seriously, fish body disinfection and medicated baths must be carried out during the temporary holding process to increase the survival rate of the little yellow croaker. However, due to limitations of objective conditions such as high temperature, the survival rate of broodstock in this study was not high, but a great deal of practical operating experience was still summarized, laying a foundation for future work.
2. Control of physical and chemical factors. The little yellow croaker can grow normally within a water temperature range of 6°C-32°C. According to observations of the growth of the little yellow croaker over 2 years, it was found that the optimum growth water temperature for the little yellow croaker is 15°C-25°C; at the same time, the little yellow croaker is a euryhaline fish, and the optimum growth salinity is 20‰-35‰.
3. Growth environment. The author compared the growth of the little yellow croaker under the above 3 different farming methods and found that the little yellow croaker has relatively stable requirements for its living environment, and the water flow cannot be too rapid. Through comparative cultivation experiments between shallow-sea cages and soil-pond cages, it can be preliminarily concluded that soil-pond cages are more suitable for the cultivation of the little yellow croaker.
4. Overwintering management. During overwintering, the water temperature is best controlled above 12°C, so that it neither affects the growth of the little yellow croaker nor wastes too much energy. During the overwintering process, special attention should be paid to disease prevention work for the little yellow croaker; feeding should be appropriate, and excrement and residual feed at the bottom of the pond should be cleaned up in a timely manner.
5. Farming prospects. From the experimental process, the little yellow croaker has relatively strong disease resistance and is suitable for artificial farming; from the experimental results, the growth rate of the little yellow croaker also meets farming requirements. In addition, the little yellow croaker has relatively high economic value, so the prospects for artificial farming are broad.