This article includes the lecture given by Dr. Julia Pié, head of the technical team of Biovet S.A., at the 31st Biovet’s International Symposium 2018 in Tarragona. It covers the first session focused on aquaculture, which took place on May 28th. By the title, “Mycotoxicosis in aquaculture and other digestive problems. Introduction of plant protein source to diets” is a review of the most important mycotoxins in animal feed, its classification, symptoms and associated injuries. Biovet’s veterinarian Dr. Maria Soriano has made the adaptation of the paper.

Feed for carnivorous fish farming generally contain between 40-50% of animal protein, usually supplied by fish oils or fish meal. Although nowadays, proportions of plant protein sources included in diet are increasing and reaching a 50-70% of vegetable matter in the diet.

This fact allows to lower the cost of aquaculture diets and ensure the supply of high quality aquaculture feed to the producers, slightly reducing the exposure to raw materials’ availability and price fluctuation of fish meal.

The most used plant species in aquaculture are: soy, canola, corn, cotton seeds, peas/sweet lupins, rice, yucca and wheat.

Some factors could be named as disadvantages of plant protein ingredients:

• Presence of antinutritional factors (ANF), which affect digestibility of other nutrients and require the application of thermic treatments to destroy them (Chart 1). Also, a high carbohydrate content that fish -especially carnivorous species- are not able to digest due to the lack of enzymes, making the addition of enzymes in feed necessary to maximize digestion and minimize residues.
• Its amino acids profile, since no vegetable ingredient have a balanced aminoacids composition, unlike fish meal.
• Presence of mycotoxins in feed and its negative effects on productivity and product quality, which will be addressed later.

For all these reasons,success reached with these ingredients is variable, and only some cases have achieved fish meal high replacement rate through the use of vegetable ingredients without adverse effects on growth.

Down below, we deepen into the third disadvantage mentioned before. That is quantifying theoretical economical losses caused by mycotoxins on an increase of 5% of the conversion rate and the cost of the feed (Tacon et al., 2011). In quantifying it by 2020, it would be 341 million dollars in salmonids, 375 in marine fish, and 914 in fresh water fish, respectively.

Such increase in the cost is due to the presence of mycotoxins in feed, which could cause health problems, damage in the tissues or immuno-suppression and lead to an increase of mortality (Chart 2).

For what is mentioned before, it is necessary to review on the most relevant mycotoxin types in animal nutrition, by making a classification and a brief description of them, together with their symptomatology and associated lesions.

Mycotoxins

Mycotoxins are secondary metabolites, generally toxic, produced by some fungal genera in sporulation. They are biologically active and are not essential for fungal growth, but enhance their survival of next generation by inhibiting the growth of possible rivals. Mycotoxins are extremely stable, remain active in feed during long periods of time and easily resist feed processing conditions.

A classification of mycotoxins depending on their chemical composition has been made (Chart 3), since it guides us to the mechanism of action and symptomatology that they could cause.

Aflatoxins

Aflatoxins are the most famous mycotoxins of coumarinic group, and they are produced by fungi Aspergillus. Sensitivity to them depends on age and species (Chart 4), although it must be highlighted that young animals are more susceptible than adults. Visible symptoms include a reduction in growth rate. Its main action is in the liver, in chronic cases it can be observed the development of hepatic tumours.

Ochratoxins

Most representative mycotoxin from lactinic group is ochratoxin, being type A the most pathogenic. Even though there is not much data about its toxicity in aquaculture species, its effects are generally associated to a decrease in weight gain.

Fumonisins

In the case of fumonisins, belonging to amininoalcohol group, there are some available trials showing its negative effect on tilapias, observing a reduced growth in juveniles but less evident than in moniliformin (MON).

Trichothecenes

Trichothecenes should be also highlighted because of their frequent onset, and the worsening in growth rate that they produce (Chart 5).

As said before, there is not much information about the effect of some mycotoxins, because their detection is relatively recent. Besides, there is not a correct classification to work on them, also because there is not a general agreement between all clinicians about the real dimensions of the problem. All these issues and the fact that it is impossible to prevent mycotoxins from reaching the food chain, have lead to the increase in the use of mycotoxin binders in recent years.

Mycotoxin binders block the absorption of mycotoxins, thus, reducing their negative effects on health and productive parameters and its presence in meat.

Conclusions on the presence of mycotoxins in aquaculture feed made of vegetable protein

In conclusion, from this review about the presence of mycotoxins in aquaculture feed, it can be noted that:

• The use of fish meals as the main protein source in diets for fish farming is no longer sustainable (economically or environmentally). For this reason, the use of vegetable protein sources as partial replacement has been  increasing.
• Ingredients of a plant origin have components that can affect digestion, such as carbohydrates and ANFs, or may contain toxic substances such as mycotoxins.
• Heat treatments and the use of enzymes help to diminish their negative effects.
• Mycotoxin binders can avoid the negative consequences caused of mycotoxins ingestion.

Bibliography:

• Chubb, L.G. (1982). Anti-nutritive factors in animal feedstuffs. Recent Adv. Am. Nutr. vol. 38: 21-37.
• A. I. Mehrim (2013). Medicinal herbs against aflatoxicosis in Nile tilapia (Oreochromis niloticus): Clinical signs, postmortem lesions and liver histopathological changes.
• FAO (2016). Estado mundial de la pesca y la acuicultura. Contribución a la seguridad alimentaria y la nutrición para todos. Roma. 224 pp. (www.fao.org)
• L. Manyes et Al. (2015). Mycotoxins and their consequences in Aquaculture: A Review. Aquaculture – September 2015
• Tacon et Al. (2011); Peer–review literature on mycotoxins consequences in fish growth performance.