Constanze Pietsch hadn't planned to study mycotoxins; her academic work focused on hormones and endocrine disruptions in fish. But then a colleague introduced her to zearalenone (ZEN) and its hormone-like effects on the endocrine system, and she found herself hooked.
She went on to study deoxynivalenol (DON) and ochratoxin in addition to ZEN, and quickly realized that a plethora of mycotoxins outside those you most often hear about — especially aflatoxin — were present in aquafeed, and had potentially negative effects on fish health. Fish may be even more sensitive to mycotoxins than terrestrial livestock, said Pietsch, who now works as the chief scientific officer at Pro Fish Care.
Though documentation of the negative effects of mycotoxins in aquafeed dates back to the 1970s, the fungal toxins often don't get the level of attention from the aquaculture industry that they see in livestock production. And on some level, that makes sense, according to Dr. Roy Rosen, DVM, a development scientist at DSM-Firmenich Animal Nutrition & Health. Traditionally, the greater inclusion of animal proteins meant aquafeed carried a lower risk of mycotoxin contamination than feeds for other animal species.
But that's changing with the increased use of plant-based ingredients and byproducts such as DDGS in aquafeed.
“The shift towards more plant-based ingredients in aquafeeds, driven by sustainability, cost and limited availability of fishmeal, has significant implications for mycotoxin contamination,” according to Vivi Koletsi, a technical support specialist at Alltech.
Because aquaculture involves a broader range of farmed species consuming a greater variety of feeds and feed ingredients, the research hasn't always kept pace with the need to know when mycotoxins occur in aquafeed, and what impacts they may have on fish health.
Global prevalence
Knowing when to screen for mycotoxins can be difficult for aquaculture producers, because the main visible symptom caused by mycotoxin exposure in most fish species is decreased growth — which could be caused by any number of problems, Koletsi said. Even within the mycotoxin category, specific toxins reduce growth performance through different mechanisms. DON, for example, typically causes fish to go off their feed, while aflatoxin damages the liver, and fumonisins (FUM) trigger lesions in the liver and kidneys.
But as the cost of mycotoxin testing has come down, it has allowed for more frequent screening, Pietsch said. When she conducted her own spot-tests of commercial aquafeeds, she found that mycotoxin levels in feeds from the largest brands were typically low — but that mycotoxins were indeed present.
That mirrors the findings of the January to June 2025 DSM-Firmenich World Mycotoxin Survey, which found ZEN, DON or FUM in one-fourth to half of sampled aquafeeds, and aflatoxin in less than a quarter.
The prevalence of specific mycotoxins within aquafeed also tends to vary according to geography, Koletsi said. DON and FUM tend to be more common in European aquafeeds, while aflatoxin and ochratoxin tend to be more common in the tropics.
Evolutionary weakness
Researchers first detected aflatoxin in trout feed in the early 1970s, when the contaminated feed triggered mass die-offs at a series of trout hatcheries in the U.S., Rosen said.
The basic assumption at the time, according to Pietsch, was that trout had a particular sensitivity to mycotoxins. But additional research, she said, has concluded that other aquatic species are just as sensitive — if not more sensitive — to mycotoxins as trout.
On a theoretical level, this makes perfect sense to Pietsch. Fish, unlike cattle, never evolved to eat grains. Ruminants developed a multi-chambered stomach and complex microbiome specifically to digest grass and grains and the mycotoxins that often come with them. Pigs, on the other hand, have a single stomach, and are much more sensitive to mycotoxins. Fish have an even shorter digestive system.
“When aquafeed contains a lot of grain and other stuff, then (fish) are exposed to toxins they have never seen in their lives or evolution before,” Pietsch said. “They didn't learn over the centuries to deal with them.”
Fish can also be exposed to mycotoxins through means beyond their feed, Pietsch said. Some mycotoxins have also been found in wastewater, which may trigger health effects in fish exposed to contaminated waterways.
But pinning down the effects of specific mycotoxins in specific aquatic species is a challenge, Rosen said, because there are more than 400 farmed aquatic species, and the existing research simply isn't as robust as in terrestrial livestock.
Mitigation strategies
In addition to their tendency to be more sensitive to mycotoxins, aquatic species may also require slightly different mitigation strategies than their terrestrial counterparts.
Mycotoxin binders appear to work in aquatic diets, Pietsch said, but feed formulators must also take care to ensure that the binders don't tie up essential nutrients like calcium. And she notes that here, too, the limited availability of research means that less common species could react differently to binders.
Including antioxidants and prebiotics to help offset the health effects that stem from mycotoxin contamination could also prove effective in aquatic species, Pietsch said.
But binders only work for certain mycotoxins. For others that don't respond to binders, Rosen said, finding aquatic solutions can be more difficult. Biological and enzymatic controls work, but these products need to go through additional steps to ensure they are heat-stable or applied post-pelleting, and that the delivery mechanism is effective, but won't allow the product to leach into waterways.
Alltech has seen success using yeast extracts, algae and, more recently, a novel bacterial ingredient to control mycotoxins in aquafeed, Koletsi said. But she warns that aquaculture producers shouldn't rely on binders as a sole mycotoxin solution. Screening ingredients to prevent mycotoxin contamination at the source remains key to preventing productivity losses, she said.
And research, Rosen said, will play a critical role in preventing feed contamination and identifying future means of mitigation. But with so many species and toxins to investigate — plus competing priorities such as disease outbreaks — he worries research funding may be stretched too thin. This could cause mycotoxin research to fall by the wayside.
His concerns aren't strictly hypothetical. Pietsch herself has shifted her research priorities away from mycotoxins and toward broader fish welfare concerns as a result of funding shortages.
“We still have a lot of work to do, especially in species that are not mainstream in Western countries,” Rosen said.
