Essential tips in rearing rainbow trout larvae with emphasis on nutrition

Raising fish larvae
The transformation of a baby into a baby fish is considered the biggest change in the life of a fish. When the baby’s swim bag fills with air – which enables the baby fish to swim like larger fish – and external feeding begins, the baby stage ends and the baby fish stage begins. There is a major difference between juvenile and adult fish. At this stage, the baby fish has a large head and a narrow body, and the gills and digestive system are not yet well developed.
At this stage, the caudal fin of the fish is complete, and apart from the reproductive organs, which remain undeveloped, the appearance of the fish resembles an adult fish, and the skeletal structure of the body gradually changes from cartilaginous to bony. Therefore, it is possible to observe losses due to the vulnerability of larvae and juvenile fish. In addition to all the baby’s essential needs, such as sufficient amount of oxygen, suitable temperature, removal of waste materials and control of enemies, the baby fish also needs external food, which must be suitable in terms of quantity and quality.
At the beginning, the baby guava has a yolk sac, which is enough to meet food needs for one to four days, depending on the type of fish. The yolk sac gives the baby time to learn how to find natural food. Once the entire food bag has been absorbed, the juvenile fish must be able to find suitable food, otherwise it will weaken and die. Lack of proper food is the most important reason for the death of the young Norse fish.

Feeding management

Start active feeding and detect when to start feeding fish
Fish can be divided into two categories according to the time of the start of active feeding:
A) Fishes that have the ability to take and digest food after hatching, their digestive system is active in this state and the stomach is ready to absorb food, for example, among marine fishes, we can mention oilfish and haddock species. In these fish, the time from hatching to the start of the first feeding is shorter than the time from the start of active feeding to the complete absorption of the yolk sac.
b) Larvae of many species such as salmon and carp need a longer time to use foreign food. The egg size of the mentioned species is larger than the previous category and the food reserve of the yolk sac is proportional to the length of the feeding period.
Basically, the most important factor in the initiation of active feeding of the larvae is the water temperature and the type of fish species. It is estimated that the maximum delay time until the start of active feeding is about 25% of the hatching time until the absorption of the yolk sac. The time of transition from internal feeding to external feeding is called the “critical stage of larval development”. In aquaculture, the detection of the start of active feeding is very important in the survival and growth of larvae. It is very important that high-quality food is provided to the larvae at the time of external feeding and food search.
Lack of feeding in fish larva breeding can have very bad and irreparable consequences. This situation is referred to as an irreversible point in aquaculture and larval rearing. In this situation, the larvae suffer from extreme hunger, lose their ability to feed, their activity decreases, and they lose their ability to swim and cannot maintain their buoyancy.
Larvae that emerge from larger eggs have more tolerance when there is a delay in receiving the first external food (hence, proper feeding of broodstock in order to produce thicker eggs is one of the important management strategies in order to reduce fish larval losses. is fostered).
In salmon farming, timely detection of the start of active feeding can greatly help the economy of production and increase the survival of larvae. At this stage, if the feeding is delayed and the larvae remain hungry, they will maintain their survival by consuming the remnants of the yolk sac. If hunger continues or the food ration is inappropriate, they become thinner. These larvae can be recognized by their black appearance, big head and thin and elongated body, and they are known as “needle larvae”, and when viewed from above, the head appears larger than the body. Therefore, to prevent the occurrence of larval losses, the stage of active feeding must be recognized correctly.

The importance of choosing the right food

The mouth of baby Norse fish is very small and it cannot use large-sized food. On the other hand, at this stage, the sensory organs are not fully developed, and as a result, the larva finds its food mostly with the help of the eyes, so the rearing environment must be bright. In the natural environment, juvenile Norse fish use a range of zooplankton as food. These creatures are the size of a fish’s mouth, and because they float and move very slowly, they remain in the field of view of the fish and are easily hunted.
In addition to the optimal quality, the food for the juvenile Norse should be available in sufficient quantity to provide optimal growth conditions. At the time of choosing the formulated feed, attention should be paid to the morphological and physiological characteristics of the larvae, especially their underdeveloped digestive system. In the larval stage, the short length of the digestive tube and its lack of development increases the rate of emptying of food from the intestine. In addition, the amount of production of digestive enzymes in the larval stage is low, and this situation reduces the digestibility of food – especially food of poor quality. Digestion of formulated foods by free-living fish larvae is better and more suitable than other omnivorous or herbivorous fish.
Basically, the food digestibility of fish larvae is divided into three categories:
1) Larvae of free fish and other fish that have a clear and active stomach are able to receive formulated food from the beginning of active feeding and digest and absorb the food well.
2) Larvae of fishes whose stomachs develop during growth stages and are not developed at the beginning of active feeding, such as Corgonus fish, the larvae of these fishes must first feed on live foods.
3) Fish without a specific stomach, such as carp, whose digestive system has difficulty in digesting formulated foods, so it is very important to choose high-quality feed.
Nowadays, due to the extensive problems and limitations related to the production of live food and on the other hand, the development of dense and ultra-dense systems, synthetic starter feeds have replaced live foods to a significant extent, and aquaculture researchers are trying to find the profiles in the structure Simulate live food and increase the willingness to accept artificial food (Figure 2). Among the points that are prioritized in feed simulation are the following.

Quantity and quality of food consumed

The quantity and quality of food plays a big role in the growth and development of juvenile fishes. In the past, fresh food (slaughterhouse waste) was used to provide vitamins and unsaturated fatty acids needed by fry. But currently, due to the many problems that wet food causes in the breeding environment of young fish, the use of this type of food is obsolete in almost all reproduction centers, these problems are:
1) Severe water pollution: due to the loss of fresh food when it comes into contact with water (about 30% of food is thrown away at this stage). The presence of such a volume of food in addition to the physical adverse changes in the water environment (water color change) causes an increase in oxygen consumption.
2) Physical irritation of the gills and occurrence of diseases: Uneaten food after being suspended in the water environment causes damage to the immature gills of fish. Bronchial stimulation also affects the secretion of mucus and provides the basis for the occurrence of salivary diseases.
At present, the managers of fish breeding and production centers also prefer to use dry and concentrated foods instead of fresh foods, which have unpleasant consequences. Formulated and dry foods are used based on the tables proposed by the feed producing companies. Starter feed is usually used to feed salmon fry. This feed is prepared in the form of granules or puree, and its size varies according to the size of the fish. (Refer to the Betta Trout Starter Feed Page)
Almost all salmon breeding centers use dry pellet foods. The feeding of larvae and fry can be continued 24 hours a day, provided that sufficient light is provided. Automatic feeders can be used to ensure optimal feeding of fish fry. At this stage, fry eat up to 10% of their body weight per day, and the amount of food consumed during the week is twice their total weight. In optimal conditions, the weight of baby fish in the best farms and at a temperature of 10 degrees and during a period of 120 to 130 days after fertilization reaches 4-5 grams (long finger stage).
Formulated foods for larval stages should contain sufficient amounts of unsaturated fatty acids, amino acids, vitamins and minerals. Decreasing these nutrients in the larva’s diet reduces growth and increases the percentage of deaths and physical abnormalities of larvae.
In addition to being essential for growth, vitamin C in feed can also be effective in reducing the stress caused by manipulation of larvae. Basically, vitamin C is responsible for the maintenance and support of connective tissue such as collagen. Therefore, its deficiency can cause typical scoliosis, lordosis and spinal curvature. Fish with vitamin C deficiency may develop anemia