1. INTRODUCTION
2. CHEMICAL CONTAMINANTS
3. BIOLOGICAL TOXINS
G. L. Rumsey
Tunison Laboratory of Fish Nutrition
Cortland, New York
Toxic materials affecting fish feed ingredients are my cotoxins, toxic products in plant materials, bacteria, residues of pesticides, drugs, and agricultural chemicals. Since my cotoxins are covered in another chapter, the discussion here will revolve around the other adventitious toxins.
2.1 Pesticides and Chlorinated Hydrocarbons
2.2 Seed Dressings
2.3 Cross Contaminants
2.4 Other Sources of Chemical Contaminants
Work at the Columbia Pesticide Laboratory of the U.S. Fish and Wildlife Service shows that the most common dietary contaminants have been the organo-chlorine pesticides DDT, DDE, dieldrin, endrin, and industrial chemicals such as polychlorinated biphenyls (PCB), phthalate esters, and hexachlorobenzene (HCB). Most, but not all, residues are due to the presence of contaminated fish oils and/or meal in the finished diets.
The effects of the chlorinated hydrocarbons have been reasonably well documented in the published literature. Detrimental effects on egg hatchability and survival of young fish are quite striking. Toxaphene has been shown to interfere with vitamin C metabolism which affects collagen synthesis and ultimately causes skeletal problems. Dieldrin affects the phenylalanine hydroxylase enzyme which is involved in amino acid metabolism and glutamine synthetase which is involved in protein metabolism. The inhibition of phenylalanine hydroxylase increased the level of blood phenylketo acids, a condition similar to the genetic metabolic disorder phenylketonuria (PKU) that is associated with mental retardation in humans; inhibition of glutamine synthetase might interfere with ammonia detoxification which would be physiologically deleterious. A stimulation of thyroid activity has been associated with DDT and its analogues and endrin has been implicated in the inhibited mobilization of liver glycogen. It would appear that serious attention should be focused on pesticides and other industrial contaminants in dietary feedstuffs and finished fish feeds.
The Columbia National Fishery Laboratory has made recommendations that fish oils used at 3-5% of the diet should contain no more than 2 ppm and fish meals no more than 0.1 ppm of organochlorine contaminants, including PCBs. In addition, the U.S. Food and Drug Administration (FDA) regulations state that the maximum amount of PCBs permitted in animal feed for food producing animals is 0.2 ppm, and the amount permitted in animal feed components is 2.0 ppm.
Another serious group of contaminants of the ration that may gain entry anywhere from the ingredient supplier to the fish feeder include the newer seed disinfectants and fungicides, and certain drugs and chemicals added to feeds. Most seed treatments include organic mercury compounds which are very chemically reactive and readily absorbed. The compounds are highly toxic and the amounts absorbed in light, accidental, contacts accumulate and build up their effects on fish.
Cross contamination occurs when trace amounts of a drug or chemical used as a feed additive are incorporated into a feed which is not supposed to contain the drug. The feed usually affected is the batch immediately following a medicated feed which legally contains the drug in question.
Measurable amounts of the drug-containing feed may remain in the production system and become intermingled with other feed being manufactured. It may occur in one piece of equipment or it may be a combination of residues throughout the entire system. To discover the source, all equipment from the point of addition of the drug to the point of shipping must be checked.
Although there are some partial remedies that can be taken in scheduling feed production or with special procedures between feed mixings, experience has shown they are less than adequate answers to the problem. The safest procedures would involve having a completely separate production line or, better yet, a feed manufacturing facility devoted exclusively to fish feed.
Boiler water additives, hydraulic fluids, lubricants (on pellet mill rolls and other processing equipment), fumigants, defoaming agents, and coating materials are examples of materials that could become feed additives during feed manufacture. Misuse of these materials could conceivably lead to human loss as well as fish mortality.
A very serious source of contamination usually overlooked is that of rodent contamination of stored ingredients. There are approximately 1200 serotypes of salmonella organisms that inhabit a great variety of warm and cold blooded animals, including fish. Of these, approximately 80 serotypes have been definitely associated with diseases. Animal by-product derivatives are the worst offenders and the primary source of salmonella contamination in mixed feeds. Fish meals represent the next highest source of salmonella contamination. Attention has thus far focused on feeds being the means of perpetuating the salmonella cycle (rat to faeces to by-products to feed to fish). Attention also should be given to the role of soil, water, contaminated feeding equipment, and wild birds and terrestial animals as contributing to the presence of salmonella organisms in a given environment.
Any discussion on adventitious toxins has to include the antinutritional factors in plant proteins. Nearly all sources of plant protein possess associated factors which must be eliminated by special processing techniques to make them of maximum nutritional value. For example, for over fifty years, soybeans have been known to be improved by heat treatment.
3.2.1 Soybeans
Factors present in raw soybeans can markedly affect the intestinal tract of animals and influence the digestion and utilization of many nutrients. Inclusion of small amounts of soybean meal will depress growth rate, increase pancreas size, decrease fat absorption, and metabolizable energy of the diet of fish. In addition, it has also been found that the feeding of raw soybeans causes the gall-bladder to contract, increases excretion of bile acids, lowers intestinal proteolytic activity, and affects methionine metabolism. Raw soybeans also contain protein which cause agglutination of red blood cells in vitro. It appears that the haemaglutinins (lectins) of raw soybeans are responsible for some of the decrease in efficiency of feed utilization. Apparently these lectins are all capable of causing release of intestinal membrane-bound lipases and amylases, causing these digestive enzymes to be eliminated in the faeces, thereby reducing their digestive capabilities. Fortunately, all of these factors can be destroyed or reduced to minimal levels by proper heat treatment.
3.2.2 Other vegetable protein sources
Groundnuts contain trypsin inhibitor activity which is not destroyed by heat treatment. This is thought to be due to the fact that the inhibitor activity resides in the tannins which are a part of the groundnut skin. cottonseed contains gossypol and fatty acids with a cyclopropene ring which causes nutritional difficulties. Linseed meal contains an antipyridoxine factor and a cyanogenic glycoside. Rapeseed meal contains glucosides which when hydrolyzed yield a goitrogen. Most all plant protein sources contain phytic acid which can interfere with both mineral and protein availability. Particular attention must be paid to mineral supplementation when formulating fish diets containing fairly high amounts of plant protein. Even though the temperature which destroys heat labile growth inhibitors is high, minimum protein damage is involved at these high temperatures provided the optimum peak of temperature is reached for a very brief period of time. The brevity of the period at peak temperature is more important than any other factor in obtaining! optimum protein quality.
