Mycotoxins in aquafeed
A hidden threat to fish and shrimp farming
The occurrence of mycotoxins in aquatic feeds and their effects on target species are topics that continue to gain attention due to the general trend of replacing expensive animal protein sources such as fishmeal with cheaper plant-derived proteins.
Symptoms in aquatic species
Many scientific publications have reported on the effects of mycotoxins in fish and shrimp at different contamination levels, enabling a better understanding of mycotoxin-related ailments. However, there are still only few validated clinical symptoms for mycotoxin exposure in fish and shrimps.
The majority of the described effects of mycotoxins in fish and shrimp are general symptoms and could be attributed to diverse pathologies or challenges, for example, anti-nutritional factors or lectins in the diet. Aflatoxins and fumonisins are two notable exceptions:
- Aflatoxin ingestion (aflatoxicosis) causes a yellowing of the body surface
- Ingestion of fumonisins increases the sphinganine to sphingosine ratio
The most frequently reported clinical manifestations of mycotoxin ingestion are 1) a reduction in growth performance and 2) alteration of hematological parameters.
Effects of mycotoxins in fish
Aflatoxins are highly carcinogenic and their concentration is strictly regulated in several markets worldwide. Several aquatic species are sensitive to aflatoxins. In the case of sea bream (Sparus aurata), aflatoxin B1 (AFB1) shows negative effects on the hepatocytes already after 24 of exposure.
The feed conversion ratio (FCR) and weight gain of Beluga (Huso huso) are negatively affected by concentrations of AFB1 ranging from 20 to 100 ppb. Other symptoms such as liver necrosis can be observed as well. Similar effects might occur in other species such as red drum (Sciaenops occellatus) and rohu (Labeo rohita).
In tilapia, AFB1 affects the growth rate and FCR at concentrations ranging from 100 to 2500 ppb, however some studies showed how already 50 ppb were enough to cause liver necrosis.
Some aquatic species are sensitive to deoxynivalenol (DON), especially salmonids such as rainbow trout and Atlantic salmon. Both species show sensitivity at low levels (300-500 ppb). The main symptoms observed in several studies were significant decreases in:
- Feed intake
- Feed efficiency
- Energy utilization
Studies on the effects of zearalenone (ZEN) in farm animals have mainly focused on dysfunction or structural disorders of the reproductive tract. Several studies have confirmed that ZEN modulates estrogen receptor-dependent gene expression in aquatic species, thus affecting the reproduction of fish.
This has been shown in zebrafish (Danio rerio), where the exposure to ZEN reduced spawning frequency, or changed the relative fecundity from one generation to the next. Together with affecting reproduction, ZEN was reported to show genotoxic effects such as defects in heart and eye development and upward curvature of the body axis of zebrafish.
In aquaculture species, fumonisin B1 (FB1) has been generally associated with reduced growth rate, feed consumption and feed efficiency ratio, as well as impaired sphingolipid metabolism. In rainbow trout, FB1 has been shown to induce changes in the liver’s sphingolipid metabolism at levels lower than 100 ppb and was able to induce liver cancer in 1-month-old trout.
According to the literature, ingestion of FB1 by carp resulted in lesions in liver and pancreas already at concentrations as low as 500 ppb. Performance parameters such as average weight gain and body weight dropped after dietary administration of low doses of FB1. FB1 affected the performance of Nile tilapia as well. Reduced average weight gain and increased Sa/So ratio have been reported in the literature.
Studies on the toxicity of ochratoxin A (OTA) in aquatic animals are very scarce. The few available studies reported effects such as severe degeneration and necrosis of kidney and liver leading to inferior weight gain, poorer FCR, lower survival rates and hematocrit levels. OTA is also immunosuppressive and one study conducted on catfish showed how animals exposed to this mycotoxin become more susceptible to pathogenic infections.
Mycotoxins in shrimp
Impact of mycotoxins in shrimp are less investigated than for fish. However, there is some evidence in the literature. Black tiger shrimp (Penaeus monodon Fabricius) was reported to be susceptible to levels of AFB1 as low as 20 ppb. The main effects are reduction on weight gain up to 50% compared to the control group.
In Pacific white shrimp (Litopenaeus vannamei) exposure to AFB1 is usually associated with increased mortality, damages to hepatopancreas, immune suppression and ultimately, drop in performance parameters.
Regarding the impact of DON on shrimp, those are mostly related to performance, where concentrations as little as 200 ppb have been associated with reduced body weight and/or growth rate.
The effects of FUM in shrimp have not been extensively investigated. However, there is some evidence suggesting that low doses of these mycotoxins are capable of affecting the texture of the flesh, with repercussions on the quality of the product during ice storage.
Synergistic effects of mycotoxins in aquatic species
There are a limited number of studies where the issue of synergistic interactions between mycotoxins is addressed. FB1 was observed to produce synergistic effects with AFB1 in trout, as it was able to promote the onset of aflatoxin-initiated liver tumor. The combined effects of AFB1 and T-2 toxin were studied in Gambusia affinis.
Effects of AFB1 and deoxynivalenol were studied on carp (Cyprinius carpio) and it was shown that the negative effects of the two mycotoxins taken together were greater than their effects individually.
Mycotoxin occurrence in aquafeeds
The inclusion of plant materials contaminated with mycotoxins in compound aquafeeds will increase the risk of mycotoxin contamination in aquaculture feeds. Gonçalves et al., (2016) compared the mycotoxin occurrence levels from 41 samples of finish aquaculture feed, both shrimp and fish, in Asia and Europe, with the available literature on fish/shrimp mycotoxicoses.
The authors found that levels found for the samples analyzed during 2014 were within the sensitivity level of several important species in aquaculture. Gonçalves et al. (2016) highlighted the fact that the mycotoxins levels found can compromise aquaculture species, even just taking into account single mycotoxins levels.
According to Gonçalves et al. (2016) the number of species affected by mycotoxins would be even higher than stated in the study due to the lack of research in some important species and the existence of mycotoxins synergisms not taken into account on that study.