I-INTRODUCTION.

We should consider that, in nature, nutrition of fish evolves as a function of age following a scheme that plays from the fertilized egg to fry, all stages of its evolution and zoological group. So the embryos feed on egg yolk, newborns eat algae, which are stored in the still not functional stomach and digest them in the intestine, and in intermediate ages they introduce foods until  reaching the nutritional scheme of adults.

In nature most current adult marine fish feed on invertebrates, annelids worms (polychaetes) crustaceans (arthropods with chitinous exoskeleton), and molluscs (soft body protected by a calcareous shell); algae and other plant and animal remains. So since its appearance  480 million years ago, the late Cambrian and early Ordovicivo, fish have evolved to form three large groups if we consider nutritional criteria: herbivores, carnivores and omnivores.

In any case the origin of food is mostly of aquatic origin.

Third is important to consider that fish have developed along the digestive tract itself (mouth, esophagus, stomach and intestine) several related glands, such as the liver and pancreas whose secretions affect the digestion, and a sensory system that gives them the sense of taste and smell that also influence their nutritional habits.

Therefore the design of proper nutrition in aquaculture must take into account the natural features: evolution, anatomy and physiology of the digestive tract and related senses.

To end this introduction we must point out three characteristics that the aquaculture industry has introduced in the natural conditions of the aquaculture nutrition:

(1) The policy decision to reduce the use of flour and fat of marine origin, trying to turn carnivore fish to herbivores.

(2) The introduction of land-based foods, specially vegetables, in the nutrition of animals that have never lived on land.

(3) The introduction of concentrated foods at an early age when the digestive tract of fish has not yet matured enough to make the correct digestive functions related to feed.

These changes, together with the need to achieve adequate production parameters, make necessary  the introduction of management plans for food (pelleting, extrusion), of additives

to improve the digestive physiology, related glands and sensory organs, preservative additives and mycotoxin binders.

II. Aqualculture nutrition. Anatomic evolution of the digestive tract and attached glands

The digestive system of fish, while presenting adaptations by nutritional model (herbivore, omnivore, carnivore) is available as in other higher animals in the mouth, pharynx, esophagus, stomach, intestine and attached glands (liver and pancreas).s.

The mouth of the fish may have teeth or not according to the nutritional group (non-existent or small in herbivores and are more developed in carnivores) and may be located in the palate, tongue or pharynx. In all cases the mission is crusher. Salivary glands are not present but mucus producing glands exist to facilitate swallowing of preys.

Chemoreceptors of taste in the mouth (taste buds) are located in the nasal passages and the olfactory nerve terminals that connect to the olfactory lobes of the frontal area of ​​the brain where the sense of smell is located.

The operation of these anatomical structures is effective upon hatching.

The pharynx acts primarily as a filter preventing particles from water passing the gill filaments.

The esophagus connects the pharynx to the stomach, being generally thick walls, allowing it to relax for the passage of prey or food.

The current basic stomach has (more curved in carnivores) sigmoid shape with a blind sac of different size depending on the species, directed caudally. The interior can be divided into three regions (cardia, fundus and pyloric portion). The gastric mucosa is arranged in numerous folds and contains glands secreting hydrochloric acid and pepsin.

During the embryonic period and early weeks of age the stomach is not functional as evolutionarily the stomach of fish was developed as an expansion of the original intestine (uniform tube as it exists in Ciona intestinalis, an ancestor of modern animals).

The intestine begins at the pylorus and ends at the anus. It is shorter in carnivores and longer in herbivores and divided into four sections:

(1) pyloric caeca, (2) cranial intestine, (3)caudal intestinal and (4) rectum.

The pyloric caeca are simple finger-blind sacs or branched formed in the transit between the pylorus of the stomach and caudal intestine, and they can be communicated independently or through a common orifice. Their number is variable, scarce, between 3 and 5 in Sparus genus (porgy and bream), approximately 70 in salmonids and hundreds in tunas.

The mucosa of the blind folds presents numerous glands secreting enzymes (protease, carbohydrase, lipase) and enterocytes with digestion and absorption function. May be absent in species with  lack of stomach.

The same histological structure is present in the cranial intestinal mucosa, although the frequency of the folds decreases progressively in flow direction up to the intestinal valve that separates it from rectum. Rectum mucosa folds are short.

The intestine is functional in embryonic period because its function is to digest and absorb nutrients from the yolk. However posthatching it acquires more and more enzymatic capabilities especially in regard to enzymes capable of digesting carbohydrates.

The liver is located immediately after the heart caudally but separated from it by a septum. Its size and color, dark brown in carnivorous fish and clear brown in herbivores, depends on the eating habits in nature. However, in animals of intensive aquaculture fat accumulation can determine a whitish or yellowish (fatty liver) coloring.

The liver secretes bile, which is stored in a gallbladder before discharging the intestine, where it makes lipid digestion function, stores glycogen and fat and synthesizes proteins.

The operation of these histological structures is effective upon hatching.

The pancreas has nodular form and placed in different positions depending on the species: (1) diffuse around the pyloric caeca (2) around the pyloric caeca (3) around the portal vein and integrating with the liver forming a macroscopically organ known as hepatopancreas.

Histologically it consists in an endocrine and an exocrine fraction which flows through the pancreatic duct into the bile duct and finally in the intestine. The exocrine function is the secretion of proteolytic enzymes associated with the digestion of proteins.

The operation of these anatomical structures is effective upon hatching.

The study of the anatomical basis – physiological nutrition of fish and alterations caused by industrialization and the production of profitable growth performance shows the need to incorporate to the standard formulation, as appropriate to the genetic lines in production, several additives which support the digestive physiology of fish such as flavoring agents, acidifiers, digestive enzymes, probiotics, liver regenerators and intestinal conditioners along with general additives such as mycotoxin binders, antioxidants and preservative

III. Aquaculture: Scientific Bases for the nutrition of fish

III-STANDARD NUTRITIONAL INFORMATION 

As described above  fish are not a homogeneous zoological group from a nutritional standpoint. Therefore in this section we will present the general and basic information for each of the following food groups (herbivores, omnivores carnivores and also the list of technological additives to be used as a result of industrial practices in aquaculture nutrition.

III.1 major ingredients commonly used 

The ingredients used in the preparation of the diet for fish and crustaceans are contained in the following Table No. 1:

As indicated in the introduction the use of fishmeal and fish oil derived from marine mining is being reduced for political reasons. For this reason some allowance will be made with processing aids, which allow use of terrestrial plant origin products as substitutes for animal proteins and fats of marine extraction.

III.2 basic formulation for herbivorous fish 

We consider Ctenopharingodon idella (com. Amur grass carp) and Colossoma macropomum(com. black Pacu) as models. A master formulation is included with chemical evaluation and expected feed conversion rate in Table 2:

III.3 basic formulation for omnivorous fish 

We consider Piaractus mesopotamicus (com. Pacu) and Pligioscion squamosissimus (com. River Corvina) as models.

A master formulation is included with  chemical evaluation and expected feed conversion rate in Table 3:

The composition corresponds to 35% and the expected conversion rate of 1.65

III.4 basic formulation for carnivorous fish

We consider Arapaima gigas (com.Paiche) and Scianus ocellatus (com. Red Drum) as a model. Table 4:

Sparus aurata formulation:

Traditionally feed for carnivorous fish based  their protein content on fish meal and oil derived from fisheries with low commercial interest for human consumption but are now being replaced by raw materials of plant origin. So then the main ingredients were oils and fishmeal, soybean and canola, corn, beans, sunflower meal and vegetable oils.

Currently the development of aquaculture sea bass, sea bream and sea bass has increased the use of pea protein and rice, as well as the direct replacement of fish meal protein is feasible up to 30 to 40 percent with synthetic amino acids for bass and corvina and up to 60% in gold.

The chemical composition corresponds to 40% protein and an expected conversion rate of 1.35 to 1.91 as a function of the percentage of non-animal products included and their heat treatment.

III.5 basic formulation for crustaceans 

We consider Cherax quadricarinatus (com.Red clampps Lobster ) and Penaeus setiferus (com. White shrimp) and Penaeus duorarum (com. Pink Shrimp) as models.

Table 5:

* 2L of water for cassava starch preparation

The chemical composition corresponds to 37% protein and an expected conversion rate of

1.25

III-6 technological Additives 

(1) The inclusion of cereals and legumes in  food for herbivorous fish improves the nutritional quality but requires the contribution of protease and carbohydrases with the inclusion of acidifyings, complementary to the production of gastric hydrochloric acid.

(2) The use of prestarter food in omnivorous fish accelerates  gastric and intestinal function , but requires supplementation with protease, carbohydrases and acidifying in the early stages of breeding and intestinal conditioners and hepatic regenerative in the second phase of breeding.

(3) The use of legumes to reduce marine protein in feed for carnivorous fish and crustaceans requires the incorporation of carbohydrases and cellulases at all stages as well as hepatic intestinal conditioners and regenerators.

(4) Finally general technological additives are also necessary on account of the prevention of stress (immunostimulatory), prevention of mycotoxins (binders), prevention of microbiological contamination of food (preservatives), prevention of intestinal protozoa from water (antiprotozoal) and improvement of water quality (saponins).

The following sections will develop the information to use  each group of technological additives.

IV Aquaculture: technological additives, Acidifiers

IV.1 Definition

Acidifiers are technological additives  with the function  to replace the hydrolitic function of clorhydre acide produced by the stomach in the early stages of life of the animals or if food intake with high binding capacity of acid (usually related to the quantity and protein quality which forms part of the feed)

IV.2 Anatomic and physiologic basis for the use of acidifiers in aquaculture.

The cranial part of the digestive system is made up of esophagus and stomach.

In fish the esophagus connects the pharynx to the stomach, having generally thick walls, allowing  to relax for the passage of food, but has no digestive function.

In nowadays fish the basic stomach has a sigmoidal shape (more curved in carnivores) with a blind pouch, different size according to the species, directed caudally. The interior can be divided into three regions (cardia, fundus and pyloric portion). The gastric mucosa is arranged in numerous folds and contains glands secreting hydrochloric acid and pepsin.

But the stomach of the ancestors of fish was not like this and so during the embryonic period and early weeks of age, the stomach of the fish is not functional because evolutionarily fish stomach was developed as a dilatation of the primitive gut (which consisted of an uniform tube as it exists in Ciona intestinalis an ancestor of modern animals)

Even animals lately evolved from fish have also this feature.For example larval forms of amphibians have stomach, with no digestive function but only as a storage area.

So,anatomically and physiologically, the production of hydrochloric acid required for the hydrolysis of food is not available in the early life stages of fish or may be insufficient if the acid binding capacity of a balanced food is very high.

The lack of production or the reduction of free hydrochloric for anatomical and physiological reasons are the justification for the use of acidifiers in the nutrition of the early life stages of fish.

IV.3 Main acidifiers used in aquaculture.

In animal feeding two types of acids are used: Weak organic acids and strong mineral acids.

The organic acids are not, strictly speaking,acidifiers, because its use does not produce a significant decrease in pH. Instead operating on the food as preservatives they inhibit microbial growth temporarily thus behaving as bacteriostatic and fungistatic. They are made ​​of a chain 1C (formic) 2C (acetic); 3C (propionic and lactic); 4C (butyric); 6C (citrus).

Strong mineral acids have hydrolytic effects of food and replace the effect of gastric hydrochloric acid.

In aquaculture phosphoric acid, sulfuric and hydrochloric are used.

The phosphoric acid can be used in food as an acidifier or in water ponds as a fertilizer.

Sulfuric and hydrochloric acids, bearing no solid form, are used as the acidulant in water, but not in food.

IV.4 Results due to the inclusion of Alquerfeed acidificante.
We performed a study of the use of A.acidificante (combination of phosphoric acid, fumaric acid and citric acid) at doses of 15 g / kg of food in Sparus aurata from 29 g to 100 g of body weight.

The fish were fed with a feed of composition: Crude Protein 42%

Crude fat 17.5%; Nitrogen free extractive matter 25.68%; Fiber 0.95%

Ash 6.28% and 7.59% moisture

The control plot had not any incorporated acidifying and test batch had a 1.5% of Alquerfeed acidificante, being obtained the following results:

In this study we have not included the parameters related to the acid binding capacity of food before being treated and the acid binding capacity of food in the stomach of the treated food. This data would be relevant if to assess the effect on the pH of the intestinal contents.
IV.5 Conclusions.
1) The use of A.acidificante improved growth in 8%.

2) The use of  A.acidificante index conversion in 3%.

3) The use of  A.acidificante improved daily food intake.

4) A. acidificante acts at the gastric level  increasing the level of pepsin in the gastric wall in 0.43% and the level of pepsin in the gastric content of 100%. This effect is related to the hydrolytic action of strong inorganic acids on food by eliminating the acid fixing capacity. The acid pH and the presence of hydrolyzable proteins stimulate pepsin secretion and its conversion into pepsinogen.
5) A.acidificante has no effect on the intestine as seen by comparing the histological structure of the intestinal mucosa, in treated and untreated fish, seen in the following images.

6) The use of inorganic acids allows to start feeding with feed at early ages and improves all production parameters for their activity at gastric level as substitutes of hydrochloric acid which is  naturally produced in more advanced stages of development.

V.TECHNOLOGIC ADITIVES: MYCOTOXIN BINDERS

V.1 MYCOTOXICOSIS IN AQUACULTURE 

The introduction of feed in aquaculture has been the occasion for the arrival of mycotoxin contaminants of cereals to the aquaculture food chain causing several changes that will be described together with the most appropriate solutions.