by Zhang jin, Dong Qiufen and Zhang Song, Guangzhou Nutriera Group Co., Ltd; Ye Xing, Pearl River Fisheries Institute, Chinese Academy of Fishery Sciences.

 

Mandarin fish, Siniperca chuatsi (Basilewsky) (Order Perciformes, Family Percichthyidae), distributes in the major rivers and subsidiary lakes in east China. It is a commercial fish native to China and has been hailed as one of 'four Chinese famous freshwater fishes.' The flesh of the mandarin fish is delicate, sweet-tasting, and has extremely high medicinal value.

The Compendium of Materia Medica describes, 'The flesh of mandarin fish is mildly sweet, non-toxic and is effective in the treatment of consumptive diseases by strengthening the spleen and stomach and through the kidney-tonifying and body strength.'

It is also the main ingredient in the signature Anhui cuisine dish, stinky mandarin fish. After the successful attempt of artificially breeding of mandarin fish in 1984, the mandarin fish industry started to grow, and the yield of mandarin fish increased yearly.

In 1993, the yield was only 9,000 tonnes, however, it reached 150,000 tonnes in 2003 and doubled to more than 300,000 tonnes in 2018, at a value of 21 billion CNY. As such, mandarin fish has become one of the most popular species cultured by prosperity-seeking Chinese farmers.

At present, farmed mandarin fish in China are mainly fed with dace and other wild fish. Producing one kilogram mandarin fish requires at least 4.5kg ofother forage fish, which results in a huge waste in aquaculture area.

Currently, a stocking density around 4.5 pieces of fish/m2 has been adopted in intensive mandarin fish culture, before, the indiscriminate increase in intensive stocking density has led to a rising trend in outbreaks of aquaculture diseases in mandarin fish farming.

In 2018, survival rate of mandarin fish fry was merely 30 percent, therefore, changes of farming methods are inevitable. Commercial feed has advantages of high efficiency, environmental friendliness and suitability of industrialised production. Thus, it has been regarded as the best alternative to live food for mandarin fish farming.

After nearly two decades of research and development, Chinese researchers have achieved breakthroughs on various aspects, such as the feeding physiology of mandarin fish, mechanisms of habituation of ingestion behavior and nutrition requirements.

Mandarin fish feed has evolved from frozen fresh fish to moist feed, which contain fish paste during the transitory phase, and eventually to extruded feed with major breakthroughs, which have been achieved in feed manufacturing techniques.

Therefore, the undertaking of studies on mandarin fish feed is necessary for the sustainable development of the Chinese mandarin fish industry. This article provides a review of the research progress on the feed nutrition of mandarin fish in China, which may serve as reference for mandarin fish farmers.

 

Physiological characteristics of feeding in mandarin fish

In contrast to other carnivorous fish species such as the largemouth bass and northern snakehead, which are primarily fed on plankton during the fry stage, mandarin fish only switch to a diet consisting of other fish or shrimp after reaching a certain stage.

Mandarin fish are carnivores when born, but intake other newborn fry and reject other food such as plankton or formulated feed, therefore, Mandarin fish are known as 'underwater tigers.'

Under natural conditions, mandarin fish lay eggs at nearly the same time as the other species of China's 'four famous domestic fishes', however, the incubation period of mandarin fish eggs is shorter than that of the fish eggs of the other species.

Newly hatched mandarin fish fry have strong swimming abilities that enable them to pursue fry of other fish species, and their teeth appear three days after hatching, which enable the direct biting of food.

Mandarin fish usually dwell in the clumps of aquatic vegetation with slight water movements, and detect moving prey using vision or the lateral line system. Fish and shrimp are captured through ambush predation and are directly swallowed. Group predation is usually practiced by mandarin fish, and predatory activity is most intense in the morning and evening.

Only moving prey are pursued by mandarin fish while static food is ignored, therefore, formulated feed that is directly dispensed remains largely uneaten. During the predatory process, mandarin fish may occasionally swallow other food by accident, however, their highly developed taste buds facilitate the detection and expulsion of food swallowed unintentionally.

Therefore, mandarin fish feeding is a process that involves the collaboration of multiple sense organs, i.e., vision, the lateral line, and taste.

Mandarin fish have a highly developed digestive system with a large number of pyloric ceca that are distributed at the bottom of the stomach, which increase the surface area for digestion and absorption of food and ensure that food is digested in a timely fashion. Therefore, the understanding of the physiology and feeding characteristics of these 'natural-born carnivores' forms the basis for the habituation of mandarin fish to formulated feed.

 

Habituation of mandarin fish and selective breeding of easily habituated breeds

The studies on the feeding physiology of mandarin fish have indicated that the species identifies the shape and size of food through vision, determines the distance and position of food through the lateral line, and detects the taste and hardness/softness of food through taste buds in the oral cavity to select palatable food for ingestion.

These three aspects are indispensable in the feeding process of mandarin fish. Mandarin fish exhibit an optomotor response, that is their eyes show strong tracking responses to moving bait fish of certain shapes, and bait fish are further detected and located through the lateral line before an attack is launched.

Therefore, to habituate mandarin fish to formulated feed, besides addressing the issues of the nutrient requirements of mandarin fish and palatability, a habituation process to 'deceive' the vision and lateral line must be formulated.

The successful habituation of mandarin fish to formulated feed can be achieved by harnessing the conditioned reflex of feeding in fish. In a previous study researchers established, in mandarin fish, a conditional reflex of food snatching at the water surface through the dispensing of live bait.

Subsequently, the dispensed feed was replaced with dead fish, fish chunks, and eventually soft pellet feed. After 15 days of habituation, mandarin fish fry weighing 5-245g is habituated to soft pellet feed, and a habituation rate of up to 70 percent was achieved.

In contrast, for mandarin fish that did not undergo the conditional reflex establishment process with live bait, it will reject any fish chunks or pellet feed. By establishing a conditioned reflex of feeding and following habituation procedures, the habituation of mandarin fish to formulated feed can be successfully achieved in small water bodies such as net pens or concrete ponds.

However, the application of this strategy in fish pond culture has resulted in low habituation success rates due to the complexities in fry habituation procedures and difficulties in providing support for soft pellet feed preparation, and this limits the development of formulated feed for mandarin fish.

During the process of habituating mandarin fish to formulated feed, it was found that a small minority of mandarin fish can be easily habituated, and habituation can be achieved in a relatively short period of time, while a certain proportion of fish still reject formulated feed despite attempts at habituation.

Additionally, it was found that habituation can be achieved effortlessly in another mandarin fish species native to Yalu River in China, the leopard mandarin fish (Siniperca scherzeri).

Therefore, the existence of 'easily habituated' mandarin fish genes in nature makes the selective breeding of easily habituated mandarin fish breeds possible. The current strategies for the habituation of mandarin fish to formulated feed are primarily based on the selective breeding of new and easily habituated breeds and consists of two main strategies:

1) The cross-breeding of mandarin fish and leopard mandarin fish, followed by the selection of easily habituated offspring

2) The directed screening of easily habituated mandarin fish. Both strategies require molecular biology methods to assist the directed screening of mandarin fish with 'easily habituated' genes for selective breeding.

With the advancements in modern molecular biology techniques, rapid development with the use of molecular markers to aid in mandarin fish breeding has been achieved. At present, Chinese researchers have already established a genetic diversity database of the mandarin fish breeds worldwide, elucidated the physiological mechanisms of mandarin fish with formulated feed, and successfully constructed a high-density genetic linkage map of mandarin fish in which the loci of feeding and growth-related genes have been identified.

This work has laid a solid foundation for the research on directed selective breeding of easily habituated mandarin fish with the use of molecular markers. In 2017, a research team at Huazhong Agricultural University, specialising in research on mandarin fish, successfully bred the first new breed of mandarin fish, called 'Huakang No. 1', after eight years of selective breeding efforts.

The breed has gained market popularity due to its exceptional 'easily habituated' characteristics. In a previous study on easily habituated mandarin fish crossbreeds, a Chinese fishery successfully bred a new crossbreed, the golden mandarin fish, with a habituation success rate to extruded feed that exceeded 70 percent.

The use of molecular markers to aid breeding has greatly shortened the breeding period of mandarin fish and made directed selective breeding possible, and it is estimated that new breeds of easily habituated mandarin fish will be selectively bred within the next decade.

 

Nutrition requirements of mandarin fish and feed preparation technologies

The earliest studies on the protein requirements of mandarin fish, which can be traced back to 1989, involved the use of internationally accepted force-feeding methods for the preliminary investigation of protein requirements in mandarin fish.

Specifically, casein was used as the protein source, and a series of protein concentrations were established by a dilution with dextrin. Seven types of formulated feed, with crude protein content ranging from 2.56 percent-to-62.13 percent, were used to force-feed juvenile mandarin fish, and it was concluded that a higher crude protein content leads to a higher relative rate of weight gain.

With improved understanding of feeding habits in mandarin fish, methods of habituating mandarin fish to formulated feed have been developed on the basis of conditioned reflexes. In previous studies, researchers investigated the protein, fat, and energy requirements of mandarin fish through the habituation of mandarin fish to soft moist pellets.

Early studies indicated that with white fish meal, as a good source of protein, and chicken intestine and cod liver oil, as sources of fat, the appropriate protein and fat content of feed for mandarin fish fingerlings (54.91g) were 53 percent and six percent, respectively, at a feeding rate of five percent.

The appropriate protein and fat content of feed for adult mandarin fish (378.08g) at a feeding rate of three percent were 47 percent and 12 percent, respectively. Subsequently, a mandarin fish habituation experiment to formulated feed (average initial weight: 65g) was conducted in the USA by using white fish meal as the source of protein, at six different protein levels.

By analysing indicators, such as the specific growth rate (SGR) and protein efficiency ratio (PER), the optimum protein content of formulated feed for mandarin fish was determined to be between 44.27 percent and 48.41 percent .

Additionally, some studies have shown that mandarin fish have a higher tolerance towards fat in feed, while carbohydrates are almost entirely unutilised. The fat content of feed has a significant influence on growth and protein efficiency in mandarin fish, where a feed fat content of 9-12 percent results in relatively higher SGR and PER.

Continuous innovations have also been made for research on mandarin fish feed preparation. To satisfy the shape, movement, and palatability requirements of mandarin fish towards food, soft moist pellet feed was initially invented.

Such feed can only be prepared just before feeding by adding pre-formulated meal into freshly made fish paste and shaping the feed mixture into strips. During feed dispensing, movement of the feed occurs through the propulsion of water flow, which promotes ingestion by mandarin fish.

However, the use of soft moist pellet feed was quickly aborted due to its complex preparation process and low habituation success rate. In recent years, the advancements in feed preparation technologies have propelled mandarin fish feed into the era of extruded feed.

The pre-mixed mandarin fish feed, developed by Guangzhou Nutriera Biotechnology Co, Ltd., has effectively replaced fish paste by meeting the flavor and texture requirements of the mandarin fish, and it has high feed attractiveness.

Through the selective breeding of easily habituated mandarin fish breeds, by the research team at Huazhong Agricultural University, the role of the lateral line in mandarin fish feeding has been weakened, thus making direct habituation to extruded feed possible.

Therefore, extruded feed, specially formulated for the easily habituated fish, may be revolutionary for the entire mandarin fish industry.

 

Conclusions

Mandarin fish have been a celebrated Chinese fish since ancient times as well as been mentioned in a verse of a famous Tang poem: 'Egrets hovering over Xisai Mountain, mandarin fish thriving in peach blossom-covered waters.'

At present, the annual yield of mandarin fish has exceeded 300,000 tonnes, with an annual value of more than 21 billion CNY. Based on the FCR of 1.5, the annual consumption of mandarin fish feed exceeded 450,000 tonnes, therefore, breakthroughs in formulated feed will undoubtedly create a huge revolution in the entire mandarin fish industry.

With different unit prices US $2,200-2,600 per metric tonnes and $1,100-1,500 per metric tonnes of commercial feed and trash fish, feeding with commercial feed can bring 40 percent profits more than feeding with trash fish.

With the rapid advancements of modern molecular breeding techniques, the transition and upgrade of China's feed industry and feed preparation technologies, as well as the increase in the consumption of Chinese aquaculture products, will promote and accelerate the development of mandarin fish feed. It will be exciting to see who takes the lead in this imminent revolution of the mandarin fish industry.

 

Acknowledgement

The research project on nutrition requirement and feed technology of mandarin was supported by the China-ASEAN Fisheries Resources Conservation and Exploitation Fund.

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