Fish
Feed Materials
Aquaculture production is still growing and the growth expectation of the industry has demanded significant reformulation of aquaculture diets. This has resulted in an increased use of plant proteins and energy sources from oilseed meals and cereal grains replacing the traditional fish meal and fish oils in diet formulations. The increased reliance on commercially prepared feed formulated with higher levels of grain material means that fish have the same risk of potential exposure to mycotoxins as terrestrial agricultural species. In many commercial fish diets, less than 5% of feed materials are from animal origin. This is due to the cost and safety issues related to using animal derivatives in feed (e.g. BSE scare). In carp and warm water species, such as channel catfish and Nile tilapia, diets are predominantly formulated using high levels of grain and plant protein, whereas cold water species, such as salmon, still require some levels of animal protein to maintain growth and performance (Abbas, 2005). As the reliance on plant-derived feed materials increases in commercial fish formulations, so does the risk of exposure to mycotoxins with feed manufacturers and producers beginning to realise their potential to impact stock health and performance as well as product quality. The table below details those mycotoxins associated with feed materials commonly used in the preparation of commercial fish diets.
Link: Aquaculture feed materials and their related mycotoxins (Abdelhamid, 2007)
NB Soymeal data should be included as a major component of fish feeds
Feeds |
Mycotoxins |
Bone meal |
Vomitoxin and Zearalenone |
Cottonseed meal, bran |
Aflatoxin-B1, Citrinin, Ochratoxin-A, Vomitoxin, and Zearalenone |
Grains |
Aflatoxin-B1 & G1, Citrinin and Ochratoxin-A |
Maize |
Aflatoxin-B1, Fumonisins, Ochratoxin-A and Vomitoxin |
Maize flour, beans |
Aflatoxins, Cyclopiazonic acid, Patulin and Griseofulvin |
Maize, peanut meal, sunflower meal, sorghum, wheat |
Aflatoxins, Cyclopiazonic acid, Ochratoxin-A, and Zearlenone |
Maize, Peanut oil |
Aflatoxin-B1 |
Milk products |
Aflatoxins-B1, B2. M1 and Patulin |
Peanut, rice |
Cyclopiazonic acid |
Rice bran |
Aflatoxin-B1, Ochratoxin-A, Citrinin, Vomitoxin, Cyclopiazonic acid and Moniliformine |
The dangers of mycotoxins to fish has been known since the 1960’s, when trout hepatoma (liver tumour) problems were initially investigated (Halver, 1969). Certain species are known to be more susceptible to toxicoses than others, rainbow trout being notably sensitive to the carcinogenic effects of aflatoxin, for example. Although several hundred mycotoxins are known, the mycotoxins of most concern to the aquaculture industry based on their toxicity and occurrence are aflatoxin, ochratoxin A, trichothecenes (Don, T-2 toxin), zearalenone, fumonisin and moniliformin.
Aflatoxin
Aflatoxin-contaminated cottonseed meal was identified as the causative agent in the outbreak and high mortality seen in trout in the USA as early as 1960 (Goldblatt, 1976). Although concerns have been raised regarding the potential transmission of mycotoxins from affected fish into human food, investigations have shown that this is unlikely.
Symptoms of aflatoxicosis in fish:
- poor growth and feed efficiency
- cancerous tumours
- reduced feed consumption
- pale gills
- kidney abnormalities
- gastric gland damage
- anaemia (low red blood cell counts)
- impaired blood clotting
- damage to liver
- immune suppression
- high or spiking mortality
- high leukocyte counts
- regurgitation
The benchmark species for sensitivity to aflatoxins is the rainbow trout, which has an LD50 of 0.5-1 ppm in feed. Other species, including the warmwater channel catfish , can tolerate increased levels up to around 10 ppm (LD50 11.5 ppm), although losses in performance are evident at these levels in some species. Aflatoxin metabolism appears to differ between species, hence the variability to severity and types of symptoms exhibited. Nile tilapia is also sensitive to this toxin, showing high mortality, reduced muscle mass and major organ damage when fed contaminated feed (Marzouk et al., 1994; Abdelhamid et al., 2002b&c). The sensitivity threshold appears to be approximately 0.25 ppm fortilapia.
Sarcione and Black (1994) investigated the use of serum alpha fetoprotein as an indicator of hepatocellular carcinomas in fish and have verified its usefulness as a suitable test. Other assays are also available (Abd-Allah et al., 1999) for determining organ specific effects of mycotoxins.
The precursor aflatoxin compound, sterigmatocystin (STC), has also been shown to be detrimental to fish health and production performance (Abdelhamid, 1988). Trials conducted by this researcher have shown that catfish and common carp have poorer growth, lower muscle protein and fat levels and higher mortality when exposed to this compound.
Trials using mycotoxins adsorbents in Thailand-raised tilapia and diets contaminated with aflatoxin, have shown that toxicity symptoms can be ameliorated by the use of a binding agent (Tenjgjaroenkul, 2008). The results (table below) showed that adding Mycosorb™ to the contaminated feed significantly improved growth performance and moderated the increase in liver enzymes (alanine aminotransferase) seen with increasing levels of exposure to aflatoxin.
Effect of aflatoxin on tilapia final body weight
Aflatoxin level (ppm) |
Control |
Contaminated |
+ Mycosorb |
0 |
49.1a |
|
|
1 |
|
26.8d |
43.5b |
10 |
|
21.2e |
37.0c |
50 |
|
13.2f |
22.7e |
Other research (Staykov et al., 2008), using naturally contaminated maize diets containing high levels of aflatoxin, showed decreased average weight of common carp (loss of 20 g), which was restored by the use of 2 kg/t Mycosorb™. FCR increased from 1.68 in the uncontaminated control group to 1.90 for the fish fed the contaminated feed, but levels of 1.75 were achieved when the contaminated feed was supplemented with the toxin adsorbent. Examination of the liver revealed that those fed the diets containing the toxin had enlarged livers relative to their body size, which was not evident in fish fed contaminated diet supplemented with the adsorbent.
Ochratoxin
Limited information is available on the effects of ochratoxin in aquaculture, however reduced growth and feed efficiency and high mortalities have been reported in several species when exposed to contaminated feed. The LD50 in trout has been reported as 4.7 ppm in feed (Lovell, 1992). Ochratoxin-A was reported to cause major abnormalities in eggs of zebra fish leading to high mortality in hatchlings. (Abdelhamid, 2007).
Symptoms of ochratoxicosis in fish:
- poor weight gain
- necrosis of liver and kidney
- higher fat levels in carcass
- lower haematocrit
- pale kidney
- pale and swollen liver
- high mortality
Cyclopiazonic acid
Research by Lovell (1992) showed that CPA was more toxic to channel catfish than aflatoxin. Channel catfish fed various levels of CPA (0.1 – 10 ppm) in feed demonstrated toxicity effects including reduced growth rate, necrosis of the gastric glands and neural damage (manifested as convulsions). The LD50 was determined to be 2.82 ppm. Further work, feeding catfish for 10 weeks on a diet containing 0.1 ppm CPA in feed, resulted in significantly poorer growth, and feeding 10 ppm caused the appearance of protein granules in kidney epithelium and necrosis of gastric glands (Jantrarotai and Lovell, 1990b).
T-2 toxin
The effects of T2 toxin were first studied in rainbow trout and most recently in channel catfish. It appears that younger fish are more sensitive to exposure than older ones, with the difference in LD50 ranging from 6-8 ppm in feed. Symptoms typically involve major damage to the gut and mucosa accompanied by severe oedema. This has a major negative impact on the growth performance and feeding efficiency of growing fish.
Symptoms of T2 toxicity in fish
- intestinal tract damage
- severe oedema and fluid accumulation in the body cavity and behind the eyes
- increased activity of lysozyme and alkaline phosphatase enzymes
- depressed growth
- poor feed conversion
- poor hematocrit levels
- increased mortality
- abnormalities in the stomach and kidneys in juvenile channel catfish (Manning et al., 2003a).
Zearalenone
Zearalenone acts as a hormone analogue (mimic) in terrestrial animals, and appears to have the same effect in fish. Research conducted by Vanji et al. (1974) using carp fed maize-based diets contaminated with high levels of zearalenone (1000 ppm) reported testicular damage in the fish, which was reversible when the toxin was removed from the diet.
Fumonisin
Fumonisin is of concern to the aquaculture industry as it is considered a major problem in maize. The toxic effects of Fumonisin FB1 in fish have been mainly investigated in channel catfish, which are found tolerate up to 20 ppm in commercial feed without performance loss or ill effects (Li et al., 1994). The biggest threat that fumonisin poses to fish may be its ability to alter the immune system. Levels of 20ppm have been reported to cause immunological changes (Pepeljnjak et al 2002).
Clinical symptoms of exposure to this toxin include:
- poor growth and performance
- reduced hematocrit values
- increased liver glycogen
- high mortality
- immunosuppression via reduced antibody production
For other fish species, Petrinec et al. (2004) found that feeding diets contaminated with 100 or 10 ppm FB1 to carp caused damage to blood vessels, liver, pancreas, kidney, heart and brain tissue.
Multiple Mycotoxin Exposure
An increasing number of research papers are showing that synergism exists between a number of mycotoxins. A mycotoxin may be present at a “safe” level and if an other mycotoxin is present also at a safe level then the two together due to this synergistic effect can become harmful. The combined effect of aflatoxin B1 and T-2 toxin was demonstrated by McKean et al (2006) in mosquitofish. The study indicated that the LD50 for aflatoxin was 681 ug/l and the LD50 for T-2 was 147 ug/l. When these two mycotoxins were combined the expected LD50 was 414ug/k however in reality it was significantly lower at 234 ug/l.
As with other vertebrates, fish are more sensitive to the effects of mycotoxins when multiple forms are present within the feed. Various investigations have shown that fish feed is as much at risk of multiple mycotoxin contamination as other types of animal feed.
Toxicity of Mycotoxins in Fish
Toxin |
LD50 mg/ml water |
Aflatoxin-B1 |
0.5 mg/kg BW |
Aspertoxin |
6.6 mg/kg BW |
Grusiofolvin |
0.28 mg/ml |
Ochratoxin-A |
1.7 (mg/ml) |
Patulin |
18.0mg/kg BW |
Stemfon |
1.2 mg/ml |
Sterigmatocystin |
0.24 mg/ml |
T-2 toxin (trout) |
6.5 mg/kg body weight) |
References – these should be included
Abbas 2005
Abdelhamid 2007
Goldblatt 1976
Halver 1969 etc etc
