Authors: Maxine Canvin and Finley Forwood are students in the MSc Sustainable Aquaculture programme in the School of Biological and Marine Sciences at the University of Plymouth. They are also winners of the 2021 Mycotoxin Aquaculture Challenge supported by BIOMIN.

In the US, aflatoxin alone is predicted to cost from US$52.1 million to US$1.66 billion annually in losses to the food and feed corn industry. These costs have also a high impact in the aquaculture industry, from crop producers, to aquaculturists and potentially to consumers. Over 700 mycotoxins have been identified globally, yet many of their impacts on aquatic species are still being discovered.

Mycotoxins in Aquaculture

Aflatoxins, zearalenone, fumonisins and richothecenes and ochratoxin are the mycotoxins that globally are most significantly present. Mycotoxins are secondary metabolites produced by a variety of fungi, contaminating plant-based ingredients of aquafeeds. The primary fungi posing a threat to aquaculture include:

  • Aspergillus sp.
  • Fusarium sp.
  • Penicillium sp.

Since 1995 the percentage of fishmeal and fish oil in feeds has fallen, with plant-based ingredients becoming increasingly popular. Plant-based ingredients are a more sustainable and affordable alternative to fish meal and fish oil, which, although providing desirable nutritional benefits, are lacklustre in the face of their environmental impacts and financial costs.

With this shift, plant-based feeds are now the primary sources of protein for species including tilapia, salmon and shrimp. Even during growth, plants are susceptible to contamination, especially under extreme conditions, such as severe bouts of hot, humid weather and insect swarms.

Furthermore, with the future implications of global warming, mycotoxins will become more prevalent. Moreover, when feed materials are stored under poor conditions, particularly the combination of heat and humidity, the risks of mycotoxin contamination increase.

Mycotoxins and their impacts

The symptoms of mycotoxin contamination, broadly termed mycotoxicosis, that fish encounter differ greatly. From reductions in feed conversion ratios (FCRs) resulting in issues that range from weight loss and reductions in reproductive fitness to cancerous effects and compromised immune systems, whilst also increasing the risk of viral and bacterial infections.

A consequence of these effects, which may act synergistically, is increased mortality. In general, juvenile fish are more adversely affected than their mature counterparts and effects across species differ significantly.

Additionally, the probability of mycotoxin co-occurrence in feedstuffs is high, causing synergistic or additive effects of mycotoxins. These impacts are currently largely unknown. However, exposure to multiple mycotoxins is thought to exacerbate pre-existing mycotoxin induced symptoms.

Furthermore, bioaccumulation of mycotoxins has been observed in shellfish, such as mussels, posing a threat to human health, with aflatoxin B1 recognised as the most naturally occurring carcinogen known, as well as the transfer of mycotoxins up food chains.

There is a significant variation in the susceptibility of cultured species to mycotoxins. Identification of the most impactful mycotoxins for each species will reveal the best methods to combat them at production and market levels, reducing risks to both animals and consumers.

Detecting, preventing and combating mycotoxins

As the old adage goes, 'prevention is the best cure', therefore employing the correct strategies reduces the risk of mycotoxins from proliferating during pre- and post-harvest. Pre-harvest procedures encompass the 'good agriculture practice' (GAP) and 'good manufacturing practice' (GMP), reducing the likelihood of fungal growths and mycotoxin production.

Mycotoxins are difficult to completely remove from grains, therefore the first step in feed production should be the correct identification of the prevalence and levels of contamination by mycotoxins. Tandem mass spectrometry (HPLC MS/MS) is considered the most reliable methodology known as 'Gold Standard' for mycotoxin identification, as it best suits for industry-grade aquafeed businesses. In circumstances where rapid identification is required immunoassay-based strategies (ELISA) can also be used due to its practicality.

But what are the accepted levels of mycotoxins in aquafeeds? EU authorities have established guidance levels for mycotoxins; however, these levels underestimate the biggest challenge the aquafeed industry faces. Even sub-clinical effects caused by mycotoxins lead to significant production and economic losses. Therefore, vigilance is key in preventing and managing contamination.

If mycotoxicosis is detected in fish, treatment is often unsuccessful. Instead, the prevention of mycotoxin contamination of feeds should be prioritised. This can be achieved by ensuring feed is stored appropriately, under cool, dry conditions, the addition of mould inhibitors and toxin binders to feeds. For those that do not produce their own feed and rely on imports, the latter is a must.

An alternative source of protein to plant-based ingredients are insect meals, which do not appear to concentrate mycotoxins. The aquafeed industry has shown significant interest in insect meals as there is potential to supply aquafeeds. Particular interest from the scientific community has been shown towards black soldier fly larvae, common housefly larvae and yellow mealworm.

Significant research is being undertaken to enhance existing and develop novel methods to combat mycotoxins in the event that samples return positive. While chemical procedures are deemed unsafe for human consumption and reduce the ingredient's nutritional value, physical and biological methods are viable options.

In this role, the use of feed additives is essential for preventing mycotoxicosis in aquaculture species. Different strategies can be used for each mycotoxin profile, there are 3 main ones: adsorption, biotransformation and bioprotection. Adsorption is used for the removal of adsorbable mycotoxins, such as aflatoxin (AFLA), which can be bound to the surface of mineral binders.

Biotransformation is successfully used to detoxify non-adsorbable mycotoxins, including deoxynivalenol (DON), zearalenone (ZEN) and fumonisins (FUM) into less toxic compounds. The process occurs using microorganisms or specialised enzymes.

Finally, bioprotection provides support to liver and gut health, which are the main organs affected by mycotoxins. Novel methods of removal show significant potential to contribute to the arsenal of preventative measures available.

Mycotoxins: an unavoidable threat to cultured species

Mycotoxins are currently an unavoidable threat to cultured species fed plant-based ingredients. This threat is exacerbated by global warming, as rising temperatures continue to generate extreme environmental conditions, promoting mycotoxin prevalence and co-occurrence in feeds.

As stakeholders, whether you are veterinarians, nutritionists, feed formulators or business owners, understanding mycotoxins and their threats is fundamentally important for the efficient, safe and profitable running of your business. If you produce fish for human consumption, then you are also responsible for providing fish that meet food safety standards.

In Mycotoxins in Aquaculture, Biomin provides a comprehensive guide to mycotoxins for producers and scientists. The continued research and investment into mycotoxins and their solutions will contribute to provide solutions to manage their growing threat to the aquaculture industry.

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