Nutrition of Aquatic Animal Larvae (2)

Relationship between larval development stage, food digestive tract residence time and enzyme activity level

Formulated feeds provide essential nutrients for the growth and survival of fish and shrimp larvae. The speed at which food passes through the intestines of fish and shrimp larvae and the digestibility of nutrients significantly affect the efficiency of formula feeds. When European lobster larvae were fed with biological baits, it was found that their feeding rate decreased with growth and development, while the average retention time of food in the digestive tract increased. When the feeding habits of crustacean larvae with different feeding habits or different developmental stages of the same species change from herbivorous to carnivorous, their trypsin activity shows a downward trend. Since the feed consumed by decapod larvae (herbivorous) contains lower levels of energy, although they show higher enzyme activity and rapid cell turnover rate, the absorption efficiency is still relatively low. Among them, the lowest level of enzyme activity corresponds to the longest residence time of feed in the intestine. Therefore, as the amount of food consumed per unit time decreases, the retention time of food in the intestine is prolonged, and the enzyme activity decreases accordingly. Carnivorous larvae consume less feed per unit time, so the intestinal retention time is relatively long. This type of food is only digested by the larvae at a relatively low level of enzyme activity and is generally high in energy and easily digestible. Therefore, compound feeds for carnivorous larvae must have high digestibility. Some early herbivorous larvae have a high feeding rate and a short intestinal retention time, but relatively high enzyme activity. Therefore, the development of efficient compound feeds for different stages of larval development must take into account the digestive physiology (enzyme activity and feeding rate) of the corresponding developmental stage. For example, a compound feed for early herbivorous larvae may not require very high digestibility. Such a compound feed can contain a slightly lower protein content, but the premise is that the protein can be well utilized by the larvae and is of good quality (amino acid composition).

During larval development, the digestive capacity of the digestive tract is also an important factor. It is not easy to produce a compound feed that is easily digestible for carnivorous fish and shrimp larvae because the larvae initially have sufficient enzyme activity in the intestine, but the quality of the enzymes is limited. The raw materials and ingredients that provide nutrients in the compound feed should be adapted to the digestive capacity of the animal, which is particularly important in the early stages of larval intestinal development. Larval feeds should never be prepared simply by crushing and grinding adult commercial feeds.

Nutritional fortification of biological feed

Much of the information on the nutritional requirements of fish and shrimp larvae has been obtained by fortifying biological diets, such as rotifers and Artemia nauplii. The fatty acid and vitamin content of rotifers can be altered by emulsification in marine oils for short periods of time (<8 h). This method does not control the nutritional composition of the biological diet and often results in excessive fat levels. Long-term fortification techniques provide multiple nutrients based on a carrier with a stable nutritional composition. A variety of commercial products are used to fortify rotifers with vitamins, such as fat-soluble vitamins or water-soluble vitamin derivatives (such as vitamin C palmitate) combined with emulsifiers, spray-dried single-cell algae powders, etc. For example, concentrated Chlorella spp. is commonly used for vitamin supplementation. Culture Selco (CS) and AlgaMac are commercial fortification products that fortify rotifers and Artemia nauplii to specific levels of DHA and EPA. For rotifers, the concentrations accumulated by fortification are much higher than those achieved by feeding mixed single-cell algae and/or brewer’s yeast.

Compound feed

The high cost and lack of stable nutritional quality of biological feeds make it obvious that compound feeds can completely replace biological feeds. However, the development of compound feeds that are widely used and reliable like biological feeds has not yet been developed, which is also a bottleneck restricting the development of marine fish farming worldwide.

Research on the development and evaluation of complete compound feeds for larvae has been carried out for more than 30 years. Among them, most compound feeds can only serve as a supplement to biological feeds at best, but cannot completely replace biological feeds. For some aquaculture species, the research and development of compound feeds in terms of nutritional and physical properties has achieved certain success, which can greatly reduce the use of biological feeds before the transition period. Part of the reason for the lack of complete success is the lack of confidence that micro-pellet feeds can completely and ideally replace biological feeds. To achieve the successful development of micro-pellet feeds for larvae, it is necessary to recognize that the production of practical feeds is not complicated, and the improvement of the production process requires a trade-off between various feed characteristics. Micro-pellet feeds should have inherent flexibility and can be widely used in a variety of aquaculture species. Moreover, in terms of the nutritional composition of micro-pellet feeds for larvae, simply imitating compound feeds for juveniles may not be appropriate because there are significant differences in physiological development between the two. In addition, the metabolic rates of fish and crustaceans at the juvenile stage are generally considered to be higher than those of corresponding juvenile fish and shrimp. Therefore, higher levels of nutrients and energy should be provided in the juvenile feed.

In the past 10 years, aquaculture seed production has made great progress through the development of micro-particle feeds such as micro-adhesive feeds, micro-encapsulated feeds or micro-coated feeds. Micro-encapsulated feeds are encapsulated by a membrane or wall material composed of a cross-linked protein or lipid. They can be produced by spraying micro-droplets on the surface of the feed ingredients. Another method of production is to spray micro-particles of feed ingredients (including binders) into a liquid solution to activate the binder. In some cases, the cost of preparation and the lack of satisfactory results are due to the poor digestibility of micro-encapsulated feeds by animals and the relatively high level of non-nutritional substances in the wall.

Micro-adhesive feeds are the simplest and most commonly used form of micro-particle feed. The feed ingredients are thoroughly mixed with the binder, and the combination of the binder and the feed raw materials provides the micro-particles with the desired physical stability. Such binders are generally agar, calcium alginate, carrageenan, casein, gelatin, zein or carboxymethyl cellulose. The binding is usually achieved by appropriate temperature or chemical reaction. Finished feeds are usually dried, crushed and sieved to the appropriate particle size. The use of binders requires special attention, because although the use of binders can achieve better physical stability, it may also reduce the availability of nutrients. Microencapsulated feeds are microencapsulated feeds with a lipid or lipoprotein membrane to reduce the loss of water-soluble components. Currently, a lot of research has been done on lipid wall encapsulation of water-soluble nutrients. Under the wall material, the level of water-soluble nutrient loss is significantly reduced. The outer membrane is then embedded in the microparticles. At present, the application of this process to effectively provide water-soluble nutrients to larvae is still in the development stage, but it has great potential in determining nutritional needs and the way nutrients are provided. It is also very attractive for the supply of substances such as hormones or antibiotics.

The preparation of microgranules can already be achieved using equipment from the pharmaceutical industry. Two pelleting technologies, using a combination of a pelletizer and a radial discharge extruder (micro-expanded spherical pelletization, MEM) or without an extruder (particle-assisted rotational polymerization, PARA), have been able to produce water-stable and palatable pellets (about 500 μm in size) that have been fed to walleye perch juveniles with some success. Micro-pellets produced by the PARA process are smaller in size and lower in density than pellets produced by the MEM process. These micro-pellets are expected to find applications in the study of qualitative and quantitative nutritional needs of juveniles.

Currently, some micro-pellets have been successfully used in the commercial nursery industry, but they can only partially replace biological feeds. Liquid feeds used in commercial nurseries are essentially micro-pellets suspended in a liquid medium.