Detailed Explanation of Key Points in Mullet Fry and Fingerling Production Technology

The fry and fingerling production of mullet and Liza mullet includes two major steps: artificial propagation and fry and fingerling rearing. The former is further divided into broodstock rearing, induced spawning (artificial induction of ovulation), artificial insemination, and artificial incubation; the latter is further divided into fry rearing and fingerling rearing.

Research on artificial propagation of mullet dates back to 1930, when Italy completed the first artificial spawning of mullet. It was not until the past one to two decades that it attracted widespread attention from relevant countries around the world and became an important subject of international biological research. China, the United States, India, the former Soviet Union, Japan and Israel all actively carried out a large amount of experimental research and made great progress. Fry and fingerling production technology has reached a production level, yet many theoretical issues and key technical problems remain to be further studied and explored.

I. Broodstock Rearing

Rearing high-quality broodstock for induced spawning is the primary key to artificial propagation of mullet and Liza mullet, and is the material basis and prerequisite for the entire artificial propagation process. Only on the basis of fully mature broodstock gonads, supplemented by appropriate induced spawning measures, can artificial propagation achieve the desired goal. The role of spawning-inducing agents lies only in stimulating the ovary and other organs to promote a series of responses such as maturation division of the egg, follicle rupture, and ovulation. Whether this chain of reactions can occur depends on whether the ovary can accept such stimulation, that is, it depends on the internal factor of the maturity state of the ovary at the time of induced spawning. Practice has proved that only ovaries at the end of stage IV, after stimulation by a certain water temperature, can enter the sensitive period and accept such external stimulation. If the broodstock has not been well reared and the gonads are not yet mature, or have not entered the sensitive period, even the best induced spawning techniques (using highly efficient, large-dose spawning-inducing agents) will be of no avail, or will be far from ideal. This shows the important role of broodstock rearing in the artificial propagation process.

Rearing broodstock means creating good conditions according to the gonadal development patterns of mullet and Liza mullet to promote the maturation of their gonads.

(I) Broodstock Rearing Ponds

1. Pond conditions There are no special requirements for broodstock rearing ponds, but in order to facilitate the growth, development and feeding management of broodstock, the following should be noted as much as possible when selecting rearing ponds: close to a water source, with convenient drainage and filling of seawater and freshwater, so that water salinity can be adjusted at any time and pond water quality can be improved; close to the induced spawning base, in a relatively quiet environment, convenient for transportation and feeding management. A rectangular fish pond with an area of 2-3 mu is suitable. If the area is too large, management is inconvenient, harvesting is also difficult, and with a large pond and many fish, batch induced spawning causes multiple seine nettings in one pond, which easily injures the broodstock and causes gonadal degeneration. The water depth should be about 1.5 m, the pond bottom flat, the pond slope ratio 1:2.5-3, and the pond bottom should have an appropriate amount (5-10 cm) of fertile mud layer. Such a pond is conducive to the propagation of basic bait organisms and harvesting operations.

2. Cleaning and preparation of rearing ponds Broodstock rearing ponds should be cleaned and prepared once a year to remove harmful animals, bacteria and some aquatic plants, dig out excess sludge, improve water quality, and carry out pond embankment repairs at the same time. Cleaning and preparation are generally carried out after the artificial propagation production is completed. Before stocking broodstock, pond disinfection is carried out with drugs such as derris extract, bleaching powder, tea seed cake, ammonia water, and quicklime (the drug dosage can refer to shrimp pond disinfection). Then seawater is filtered in through 30-40 mesh silk bolting cloth, and base fertilizer (chemical fertilizer or dried manure) is applied to propagate basic bait organisms for the broodstock. Alternatively, the pond water can first be drained dry, wild and miscellaneous fish removed, organic fertilizer applied, and after 3-5 days of sun exposure, water let in to fertilize the pond.

(II) Sources and Stocking of Broodstock

1. Sources of broodstock Currently there are three sources of broodstock used for artificial propagation: first, broodstock caught from the sea in autumn or early spring from natural growth, which after short-term intensive rearing are promoted to mature (such as in Shandong, Liaoning and other places); second, individuals selected from artificial culture in brackish water with salinity above 5%, which after 1 year of intensive rearing in dedicated ponds develop into mature broodstock (such as in Tianjin and Hebei); third, Liza mullet selected in late autumn or early spring from culture in freshwater or brackish water (salinity 0.5-5.0‰), in which before or after overwintering the salinity is gradually transitioned to 13-18‰ to promote their development and maturation (such as in Jiangsu).

2. Broodstock transportation It is best to obtain broodstock locally and rear them locally, avoiding long-distance transportation as much as possible. Transportation means include trucks, live fish boats, etc. Broodstock can be placed in wooden barrels, canvas buckets, plastic fish baskets or oxygenated nylon bags. The fish can also first be placed in perforated nylon bags, one fish per bag, and then the fish and bags placed together in the water of a container to avoid collision. Broodstock transportation is best carried out at a water temperature of about 10°C, with sparse loading and fast transport. During transportation, efforts should be made to keep the water fresh and oxygen sufficient. Where conditions permit, oxygenated nylon bag anesthesia transportation can be used. Commonly used anesthetics include quinaldine, sodium barbital, ether, MS-222, chloral hydrate, tert-butanol, chlorobutanol, metaldehyde, tert-amyl alcohol, etc.

3. Selection and retention of broodstock For Liza mullet retained for use, generally males are over 2 years old with a body weight of 0.5-0.75 kg, and females are over 3 years old with a body weight of over 1-2 kg; after rearing to the following year, they can be used. For mullet, generally females of 4-6 years old with a body weight of 2 kg and males of 4 years old with a body weight of 1.2 kg are used as broodstock. In Taiwan, China, broodstock caught from the sea are about 4 years old, with a body length of 32-50 cm and a body weight of 1.0-2.0 kg. Mullet broodstock reared in freshwater ponds should be over 3 years old.

Generally speaking, it is better to select and retain larger individuals, which not only have higher fecundity but also produce offspring that grow faster. However, attention must be paid to the ratio of females to males. Since it is difficult to distinguish males from females by appearance, when selecting and retaining, an appropriate number of individuals with a body weight of 0.5-1 kg should be selected to prevent one-sided selection of large individuals, which would result in insufficient males at the time of induced spawning. The broodstock selected and retained should be robust in constitution, uninjured and disease-free.