Larval feeding in aquaculture species – Types of live feed and its related problems

Aquaculture is the animal production system with the higher growing rate in the recent decades, however, due to the lack of knowledge about larval development and larval requirements, this production has been limited to a few species worldwide.

Larval development is the most critical phase within aquaculture production and when the highest mortality rates can reach up to 70%. The poor development of the digestive system of the larvae and the small size of the mouth limits the use of inert feed during these initial stages, making live feed necessary.

The use of live feed for larval feeding implies higher productive costs, so different alternatives to replace live feed are developed

Larval development

Larval development

Intensive production in aquaculture requires the ability to replicate, in a controlled environment, the reproduction and larval development of the species. Fish larvae are very delicate organisms, whose survival depends on several factors. Feeding is the most important one.

Newly hatched larvae have nutritional reserves stored in the yolk, containing glycogen, free amino acids and one or more fat drops containing triglycerides (Figure 1.). These components are progressively reabsorbed and constitute endogenous feeding. The duration of this nutritional reserve varies between species.

Once the yolk reserves have been reabsorbed, the larvae depend on the capture and digestion of the feed present in the medium. The transition from endogenous to exogenous feeding is a key moment, on which the survival of the larvae will depend.

Fish larvae can be divided into three groups depending on the development of their digestive tract. First, there are species who have a functional stomach from the beginning of exogenous feeding, as in the case of salmonids and different species of catfish. These species have direct ontogeny, this means that they already have all the characteristics of a juvenile at hatching. In these species, it is possible to use inert feed from the initial stages of cultivation.

Secondly, there are species where the development of the stomach takes place during larval development once exogenous feeding has begun. This group includes most of the cultivated marine species.

Third, there are species that do not develop a stomach throughout their life. During the larval development of these species, it is just observed the elongation of the intestine. This group includes most herbivorous species, such as carps and cyprinids.

Figure 2. Hatched larvae with artemia in their digestive tract.
In those species where the development of the digestive tract is not complete once exogenous feeding has started, it is necessary to use live feed during the first cultivation stages (Figure 2).

Live feed

In the natural environment, fish larvae have at their disposal a wide range of preys for feeding, including different microalgae, copepods, ciliates, bivalve mollusk eggs, polychaeta, etc. These preys fit the small mouth and cover all their nutritional needs. Most fish larvae are visual planktonic hunters, regardless of their adult feeding strategy.

The size of the mouth, between 100 – 400 microns depending on the species, is the main limiting factor for the use of inert feed in the earliest stages of fish-farming, together with the lack of knowledge about the nutritional requirements for each species in the initial stages.

In addition, the incomplete development of both the digestive system and sensory organs, that allow the detection and capture of feed, makes it difficult to use inert feed in these early stages.

Therefore, in intensive aquaculture farming, different organisms that resemble the natural environment feed are used. The most common ones are microalgae, rotifers, artemia and copepods.

• Microalgae

Microalgae are single-cell phototrophic organisms. They are generally used for feeding zooplankton, which, in turn, will be used as feed for the larvae.

The most common species in aquaculture are: Nannochloropsis, Tetraselmis, Isochrysis, Skeletonema, Thalassiosira, Chaetoceros, Monochrysis and Haematococcus.

Microalgae have a good nutritional composition, with high levels of essential fatty acids, vitamins and essential amino acids that improve the nutritional quality of zooplankton. In addition, they provide pigments that improve the appearance of meat and skin and promote the development of the reproductive and immune systems.

An application to highlight of microalgae in aquaculture, is the cultivation in green water. This type of culture refers to the addition of microalgae to tanks where fish larvae are grown. The presence of microalgae has the following benefits: it allows to maintain the composition of the live feed, improves the quality of the water, serves as direct feed for the larvae, presents a probiotic effect and causes a shadow or contrast effect that facilitates the capture of zooplankton by the larvae.

• Rotifers

Brachionus genus rotifers are used as the first feed in a great number of aquaculture species. They are ideal planktonic organisms for larval feeding: they have a low swimming speed, a small size (90-350 microns), they are easily cultivated organisms capable of reaching high densities and resistant to different growing conditions.

The life cycle of rotifers presents a type of cyclic reproduction with a parthenogenetic phase, where only amictic females are present. These females can reproduce without being fertilized by males and with extraordinary fertility. The recreation of this cycle under controlled conditions allows to obtain crops of rotifers whose density doubles every 24 hours.

The nutritional composition of rotifers is not the most suitable for larval feeding. However, they are organisms that feed by filtration, which allows the enrichment of these organisms with different components, improving their nutritional quality. This technique is known as in vivo encapsulation of essential nutrients.

The most frequently used rotifers in aquaculture are B. plicatilis and B. rotundiformis.

• Saline artemia

Saline artemia a micro crustacean anostrocean brachiopod that inhabits hypersaline salt flats and coastal lakes. It is the next stage in the trophic chain of marine fish larvae, the successive stage to the rotifer.

Artemia has a life cycle with four phases:

• Cyst: it constitutes a resistance phase with a size between 200-300 microns. It contains a very resistant chorion that protects it from environmental factors, allowing its long-term storage.
• Nauplius: with a size between 400-450 microns, mobile, orange and with hardly any escape response, it is the ideal prey for larval feeding.
• Metanauplius: with a size between 650-700 microns up to 1 mm, allows the feeding of larger larvae.
• Adult: the size of adult individuals varies between 10 and 12 mm (Figure 3).

Currently there are large companies dedicated to the massive production of artemia cysts. Aquaculture industries routinely buy artemia cysts and then they de-capsulate them at their facilities and use them for larval feeding.

• Copepods

They are the main preys of larval feeding in the natural environment. Unlike rotifers and artemia, they have a nutritional composition that adjusts perfectly to the nutritional requirements of fish larvae. At the same time, they have a size range (<45 – 600 microns) that fits the different sizes of the fish’s mouth, ensuring their ingestion.

The main species of copepods with interest in aquaculture are: calanoids, harpacticoids, cyclopoids (Figure 4).

However, despite these characteristics, their use in larval feeding is not widespread. This is because they have longer and more complex life cycles than other living feeds, which complicates their cultivation at an intensive level.

Enrichment of living feed

Fish larvae require a large supply of lipids during their early stages of development. It is especially relevant to ensure the supply of essential fatty acids. They are called essential because larvae are not able to synthesize them and must be provided with feed.

In the case of freshwater fish, the essential fatty acids are omega 3 (ω3) or linolenic acid and omega 6 (ω6) or linoleic acid. Marine aquaculture species can synthesize fatty acids of higher saturation degree from linolenic acid.

In contrast, marine species lack the ability to synthesize fatty acids with a higher degree of saturation. In these species, essential fatty acids are eicosapentanoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (ARA).

Both rotifers and nauplii and metanaupli of artemia are from freshwater habits and lack of high levels of these essential fatty acids. Therefore, it is necessary to enrich these microorganisms prior to their use in larval feeding.

Enrichment is done by exposing rotifers and artemia to emulsions with a large amount of lipids. These organisms filter and accumulate these fatty acids in their body, improving their nutritional composition.

 

Problem associated with the use of live feed

The possibility of intensively produce an aquaculture species, as in other animal productions, depends on the possibility of obtaining larvae in enough quantity and quality to sustain productivity.

The development of larval feeding through rotifers and artemia has enabled intensive cultivation of different species such as seabass, seabream or turbot. However, this type of diet has certain disadvantages.

First, the cost of maintaining these auxiliary crops along with fish farming is extremely high, because it is one of the main production costs. For example, for the cultivation of 300 m² of greater amberjack (Seriola dumerili), 1100 m² of auxiliary facilities are necessary. In addition to a large space to place these facilities, it is necessary to hire specialized workers in the cultivation and maintenance of this type of organisms.

On the other hand, since they are living organisms, especially in the case of rotifers, it is difficult to maintain the same nutritional quality, since it depends on different factors, such as the quality and concentration of the microalgae to feed them, the water quality, etc. In addition, in the case of the artemia, there is a direct dependence on large companies engaged in the production of cysts, which can lead to supply problems.

Therefore, the current trend focuses on the development of micro-feeds or encapsulated feeds capable of replacing live food. However, these initial feeds are not yet fully developed, and further research of specific nutritional requirements of  the initial phases is required during for the different species cultivated.

Intestinal conditioner pronutrients

During the transition from live to inert feed, the digestive tract is not yet fully developed. At this delicate moment it is advisable to add molecules capable of improving the development of the intestinal mucosa and ensuring a correct growth of the larvae to the new feed.

There are products based on intestinal conditioner pronutrients on the market that improve intestinal integrity when added to the starter feeds.

These active molecules of botanical origin increase the expression of genes related to the renewal of enterocytes, improve their integrity, increase the mobility of intestinal microvilli, as well as the synthesis of immune components.

This way, pronutrients improve the absorption of nutrients at the intestinal level (vitamins, amino acids, minerals, etc.) and promote intestinal integrity by preventing the entry of pathogens.

The addition of these active molecules prevents alterations of the intestinal development caused by the diet changes, allow a correct absorption of nutrients and, therefore, an adequate growth.

Conclusion

Nowadays, the larval feeding of most cultivated aquaculture species is based on the use of live food (rotifers and artemia). However, the high cost of maintaining these auxiliary crops makes it necessary to develop inert feeds capable of replacing them.

To produce feeds that can be used at the beginning of exogenous feeding it is necessary to know the nutritional requirements of each species, as well as to develop specific feeds that adapt to the ethological behavior of the larvae.

Due to the fragility of the larvae and the lack of development of the digestive system, it is advisable to add intestinal conditioner pronutrients to the initial feed to ensure proper intestinal development and growth of larvae.