Photo courtesy USFWS
In fish hatcheries one of the first steps in stable, predictable, production is elimination of mortality in eggs and early life history stages of development. Clean water, sufficient dissolved oxygen, and elimination of disease agents are paramount for the fertilized eggs to survive and hatch. The means to accomplish these goals are quite well developed for salmonids, but less so for other species.
Online literature reveals very few papers dealing with disinfection of trout and salmon eggs that have been published in the last thirty years. Research on treatments for other species, such as catfish, is an ongoing effort, probably because these species have not been propagated as long as trout and salmon. The OIE (Office International des Epizooties) publication, Disinfection in Aquaculture, produced by Y. Torgersen and T. Hastein in 1995 remains a very useful summary of egg treatments. The 2011 List of Drugs Approved for Aquaculture produced by the U.S Food and Drug Administration is another good summary.
The use of any chemical to treat a specific condition in target animals, including fish eggs, requires consideration for several factors. First, the treatment must accomplish the desired result; i.e., it must be efficacious. Most nations require scientifically valid data that demonstrate the efficacy of the treatment.
A second consideration is that the treatment should not harm the target animal. This seems obvious, but it has often been overlooked.
Safety to workers using the chemicals, and safety to the environment are additional factors that must be demonstrated before a specific substance and treatment technique can be approved by regulatory agencies. These agencies – agencies that are charged with protecting public health and environmental quality – must receive documentation that the substances used are not toxic, allergenic, or carcinogenic.
Reasons for treatment
The primary reasons for disinfecting eggs are to remove fungus and other disease agents that can affect successful hatching and to clean the eggs for transport to other locations. Fungi, such as, Saprolegnia sp., and many bacteria can affect hatching success and are ubiquitous, even in clean waters.
Most nations, plus most states and provinces, have regulations concerning the potential importation of disease agents by way of egg surfaces. Surface disinfection has proven successful in diminishing the negative effects of these fungi and bacteria.
A few disease agents can occur inside eggs, which is a different problem and is essentially impossible to treat by disinfecting the surface of the egg. Disinfectants that would kill/neutralize disease agents inside the egg typically would kill the developing embryo also.
In addition to disease problems, eggs may also be treated to counteract the effects of characteristics that are valuable to wild fishes, such as, adhesiveness (e.g. walleye), or to break up gelatinous egg masses (e.g. channel catfish).
Water quality factors, such as hardness of the water, or the concentrations of specific ions, can affect the effectiveness of various treatments. Hatchery managers have to be aware of the characteristics of their water supply and be prepared to make appropriate adjustments. In general, clean, well-oxygenated water of the same temperature that will be used for incubation helps avoid unwelcome “surprises.”
Baths, in which the eggs are submerged for a pre-determined time in a carefully measured concentration of the treatment solution, are the most common technique for disinfection eggs.
In situations where a higher concentration of the treatment chemical is preferred, a dip of short duration may provide similar results, and require less time.
Flow-through systems allow for longer exposure times and lower concentrations of treatment chemicals, but are more difficult to control. The key consideration always is obtaining sufficient concentration and exposure time to kill the disease agent on the surface, without killing the embryo inside the egg.
Treatments designed to counteract adhesiveness of eggs, or to break up gelatinous egg masses, generally do not use toxic chemicals and, therefore, are typically of less concern to regulatory agencies. For example, clay or Fuller’s earth, used to counteract adhesiveness on walleye eggs are generally regarded as safe and not subject to regulation.
Breaking up the mucilaginous matrix of catfish egg masses presents a special case. Incubating these egg masses in warm water obtained from surface sources leads frequently to disastrous fungal and/or bacterial infections. When the infection starts in the center of the egg mass it can go undetected until most of the eggs are dead. Dissolving the mucilaginous matrix allows the eggs to be incubated as individual units similar to procedures used for salmonids.
Chemicals in common use
The list of chemicals that have been used to treat fish eggs is long, but the list of approved chemicals is much shorter. Readers interested in a more complete list are advised to do an online search of “chemicals and treatments used in aquarium husbandry.” The UN Food and Agriculture Organization has also published a list of chemicals used in coastal aquaculture, most of which have been used for disinfection and/or treatment procedures.
The most commonly used treatments for trout and salmon eggs are variations on iodine compounds that are sold under a variety of trade names. The same is true for formalin, weak solutions of which have several trade names. Hydrogen peroxide is also sold under several trade names. The following is just a short list of substances that have been used to disinfect fish eggs or to treat them for other purposes (Several substances listed below are not approved by regulatory agencies and should not be used without specific approval):
Commonly used: Formalin; hydrogen peroxide; Chloramine T; Iodine/Iodophore compounds; ozone; copper sulfate; potassium permanganate; quaternary ammonium compounds.
Less Commonly used and/or banned in some countries: Sodium carbonate peroxyhydrate; Penicillin G; streptomycin; glutaraldehyde; sodium hypochlorite; sodium hydroxide; sodium sulfite; malachite green; mercury compounds.
- John G. Nickum
Photo courtesy USFWS