Many fishes when taken from the water have the entire body and gills covered with a coat of persistent mucus that can be removed only by determined effort. If you have any alum at hand, you can in a moment make up a pint or quart of alum water, which will cut the mucus instantly, and clear it off. Use a stiff brush--a large tooth-brush is the best thing--in cleaning off this mucus, and do not forget to cleanse the gills thoroughly.
Open the abdominal region of every fish by making a generous cut from the vent straight forward toward the ventral fins. Usually the length of the opening should be equal to about one-fifth of the entire length of the fish. If the fish be a large one, it has always been my practice to open the fleshy interior still farther by working through this cut, and detaching the skin from the flesh as far up each side as possible. This gives the spirits immediate access to the entire mass of flesh, and the result is very speedy and perfect preservation without any change whatever in the form or weight of the specimen.
Dr. Bean always directs that the viscera be preserved, to assist in identification, even though it becomes necessary to remove them from large fishes and preserve them in separate jars. When there is no particular reason for their preservation, it is a great advantage to remove them and throw them away. They are--unless of scientific value--an abominable nuisance, and do more to spoil good alcohol than all the rest of the fish.
Fishes that have begun to decompose, and have become offensive, yet are too valuable to throw away, may be disinfected by washing them inside and out with a moderately weak solution of pure carbolic acid and water, or with a solution made by dissolving a tablespoonful of chloride of soda in a pint of water.
For years a very common formula for preservative alcohol has been ninety-five per cent alcohol diluted with one-third of its bulk of water, or, in other words, three parts of alcohol and one of water. If there is any fault to be found with this solution, it is that it is stronger than is really necessary. I have preserved barrels of alcoholic specimens in a solution composed of two parts of proof spirits and one part water, and have never lost a specimen except through leakage. This solution is strong enough to stand considerable deterioration without the loss of its contents.