Increasing awareness of mycotoxin-related issues in aquaculture is confirmed with the trend toward using plant-based ingredients in feeds. Future growth and sustainability of the industry depends on the ability of the sector to identify economically viable and environmentally friendly alternatives to marine-derived ingredients. The industry has been concentrating efforts on finding alternative sources of protein to substitute fishmeal in aquafeeds.

Consequently, many new alternatives are available, e.g. insect meal, macroalgae meal or single-cell protein.

However, high costs and limited availability are still challenges to overcome. Plant-based meals seem to be one of the most promising and viable solutions but a common problem that arises from the use of plant ingredients is the presence of mycotoxins.

Mycotoxin occurrence

The first big difference between livestock and aquaculture production is the level of knowledge about mycotoxin occurrence and co-occurrence in the plant feedstuffs used to make the diets. Only recently has interest about mycotoxin contamination in aquafeeds started to grow, so knowhow about mycotoxin occurrence in aquafeeds is still being accumulated.

In the past, small amounts of plant meals were included in the diets of carnivores and herbivores, which seems to have brought about some resistance to mycotoxins.

Due to the novelty of the topic, and contrary to the livestock industry, the contamination of aquaculture feedstuffs with mycotoxins is, in general, often neglected. There is a growing awareness of mycotoxin contamination in aquafeeds, however, we are still far from having solid knowledge of the mycotoxin contamination patterns in aquafeeds, and how the type of plant meal used influences it.

Tip #1: Survey your plant meals for mycotoxins to avoid any possible risk

Wrong information may lead to employing the wrong strategies

One of the main misconceptions deeply entrenched across the aquaculture industry is that the majority of mycotoxin issues result from poor storage conditions leading to aflatoxin contamination. It is true that poor storage conditions can lead to the growth of Aspergillus spp. and Penicillium spp., which can ultimately lead to the production of aflatoxins and ochratoxin A.

However, Biomin has observed that most of the mycotoxins found in aquaculture finished feeds are from Fusarium spp., i.e., resulting from field contamination of the raw materials used to produce aquafeeds. In this case, we are talking mainly about deoxynivalenol and fumonisins.

In some cases, aflatoxins continue to represent a challenge, especially in tropical countries and/or when storage conditions are inadequate.

Tip #2: Correctly identify the mycotoxin(s) in your diet or raw material in order to implement the correct management plan

How do I know if my fish/shrimp are being exposed to mycotoxins?

Mycotoxins are structurally very diverse. This characteristic generates a wide range of symptoms in mycotoxin-affected animals, ranging from decreases in production efficiency to increases in mortality. In aquaculture, symptoms are generally unspecific, which makes accurate diagnosis difficult. The diagnosis of mycotoxicoses in farm animals is further complicated two ways.

First, the synergistic effects of multiple mycotoxins in feeds creates a different pattern of symptoms.

Second, mycotoxins are responsible for suppressing the immune system, which allows opportunistic pathogens to colonize, prompting the display of secondary symptoms in the host. Sensitivity to mycotoxins varies greatly between species and is dependent on several factors that can modify the expression of toxicity including age, gender, nutritional and health status prior to exposure, and environmental conditions.

The situation is already very complex, but in addition to this we must consider the 138 different fish species and 38 shrimp species (FAO, 2011), with different feeding behaviours (herbivorous, omnivorous and carnivorous) and inhabiting different environments (freshwater, brackish water, marine). This high number of variables tends to dilute scientific output from all aquaculture research, not only in the field of mycotoxins. The low number of experts working with mycotoxins in their research compounds the problem, making it more difficult to have comprehensive diagnoses on the effects of mycotoxins in the main species.

Some reports describe clinical signs for the most common mycotoxins (Anater et al., (2016)), however, most of them are generalist parameters and can be attributed to any diverse pathologies or challenges e.g. anti-nutrition factors or lectins in the diet, or environmental changes (bacteria, environmental toxins). Some of the parameters referred to above include reduction in growth performance, alteration of blood parameters (erythrocyte/leucocyte count), changes in blood enzyme levels (Alanine Aminotransferase (ALT), Aspartate Transaminase (AST) or Alkaline Phosphatase (ALP)), alterations to the liver or the suppression of immune parameters.

Two notable exceptions are aflatoxicosis (yellowing of the body surface, (Deng et al., 2010) and ingestion of fumonisin (alteration of the sphinganine to sphingosine ratio, sa/so, (Tuan et al., 2003). Only aflatoxicosis can be visually identified so to correctly diagnose a change in the sa/so ratio, blood or haemolymph samples need to be collected and analysed.

Compared to livestock, there are a lack of any clear, clinical signs of mycotoxin ingestion in aquatic species (Figure 1i-v).

Tip #3: Keep a detailed and up-to-date record of your farm activities

The lack of any clear clinical signs of mycotoxicoses makes it very important to have a rigid mycotoxin management plan and a good record of farm activities. For example, an up-to-date record of environmental parameters (salinity, temperature, N-compounds, oxygen) and feed management (feed intake, identification of feed batches) could be fundamental to identifying the causes of a sudden decrease in feed intake or growth performance or an increase in mortality.

While analysing environmental and feed management parameters, you may also consider mycotoxin contamination depending on the success of your mycotoxin management plan.

Impact of mycotoxins: Are my contamination levels critical?

In aquaculture, it is common practice to study the impact of anti-nutritional factors (ANFs) present in the plant meals and try to overcome these limitations.

However, mycotoxins are often overlooked as ANFs. It is not yet common practice in academia to evaluate the raw materials used to formulate test diets for the presence of mycotoxins.

As a result, when comparing to livestock species, much less is known about the effects of mycotoxins in aquaculture species. The efforts of the aquaculture scientific community are even more diluted when taking into account the much higher number of aquaculture species compared to the number of livestock species. As reported previously, sensitivity to mycotoxins varies greatly between species and is dependent on several factors which can modify the expression of toxicity including age, gender, nutritional and health status prior to exposure and environmental conditions. However, for some species we can already provide some advice.

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