Hand stripping is commonly used for taking the spawn of many species of fish. Brood fish are separated by sex prior to hormone injection to prevent spawning in the holding-tank. It is important to determine the exact time of ovulation when hand stripping.
However, the eggs of cold-water species may remain viable for several days after ovulation; for example, in trout, eggs are usually stripped within 3 to 4 days. The eggs of some species such as striped bass and white bass progressively clear or become transparent as they near ovulation.
This visual cue is used by hatchery workers to estimate the approximate time of ovulation. An egg sample is taken by carefully inserting a tube (catheter) into the urogenital opening and examining the sample under a microscope.
Eggs taken more than 15 hours before ovulation cannot be accurately staged using this method. For most species, ovulation can best be verified by checking the female to determine when eggs flow freely from the vent.
At least one hour prior to the earliest anticipated time of spawning, female fish are captured and the process of checking to verify ovulation is initiated. Tropical species are usually checked every 45 minutes until ovulation is verified, temperate water species are usually checked every hour.
It is not necessary to take the fish out of the water to verify ovulation. The fish is turned belly up and gentle finger pressure is applied to the abdomen starting at the pectoral fins, moving slowly toward the vent. Do not try to squeeze or force the eggs from the fish; this will only injure the female.
Frequent or rough handling of females retards ovulation, reduces spawning success and increases fish mortality. If only a few eggs flow from the vent when slight pressure is applied, partial ovulation has occurred; the fish should be released and checked again later.
Attempting to hand strip a female fish that has only partially ovulated will result in few mature eggs and physical damage to the ovaries, preventing a complete spawn. When eggs flow freely from the vent, complete ovulation has occurred.
The hatchery worker quickly plugs the flow of eggs by placing a thumb over the vent. Brood fish may be anesthetized with MS-222 for stripping, if necessary, after ovulation is verified. It is important to insure that no water comes in contact with the eggs until after the milt is added and mixed.
Water and slime from the vent and tail area of the female fish are dried with a towel. Water activates the sperm and also causes the opening through which the sperm enters the egg (micropyle) to close. For many fish, this closure takes place within only 45 to 60 seconds.
To strip the eggs, the fish is held slightly on her side, tail down; gentle hand pressure is applied to the abdomen, moving toward the vent (Figure 2). The stream of eggs is directed into a clean, dry bowl positioned so that water from the fish does not drip onto the eggs.
The head of small fish can be held by one hand while the eggs are stripped with the other. A cloth glove may be worn to help hold the fish while stripping. Larger species are either wrapped in a towel and held by one or more hatchery workers while another strips the eggs, or the fish maybe restrained on a padded table or stretcher for stripping.
Good quality eggs usually flow readily from the genital opening of the female and have little ovarian fluid. If the ovarian fluid is watery or milky and many of the eggs are cloudy white, this indicates poor quality eggs.
Authors:
R.W. Rottmann, J.V. Shireman, and F.A. Chapman
