Milt can be collected from males and stored up to three weeks prior to stripping eggs. Males are captured and anesthetized, if necessary. The fish is then turned belly up, and the vent area dried by blotting with a towel.
The area just behind the pelvic fins is gently massaged toward the vent to extrude the milt. The first few drops of milt are wiped away. The milt is collected by inserting a plastic tube attached to a syringe into the urogenital opening. Suction is applied while stripping to draw the milt into the syringe.
Care must be taken to insure that water, urine, intestinal contents, slime, or blood is not mixed with the milt. It is best to collect and store milt separately from each male to avoid contamination.
The milt is expelled into a sterile plastic bag, and antibiotic (e.g., 50 micrograms of dry streptomycin sulfate per milliliter of milt) may be added to control bacteria. The bag is filled with oxygen, sealed with a rubber band, and gently swirled to mix the antibiotic.
Rough handling and shaking of milt may be detrimental to the sperm; mixing should be done slowly and gently. Bags of milt should be laid flat to maximize the surface area of milt exposed to the oxygen and immediately stored on ice in a cooler or refrigerator.
Do not freeze the milt as this will kill the sperm. Oxygen may be replaced and the milt should be gently swirled in the bag periodically to insure maximum aeration. Milt that has been contaminated with blood, slime, etc., will appear to have congealed and should be discarded.
Milt has been held in this manner for up to three weeks; however, the quality of the stored milt may deteriorate with time. The motility of stored sperm should be checked before it is used to fertilize eggs. Milt with no or low motility should be discarded.
When using stored milt to fertilize the spawn, it should be mixed with water first and then gently shaken for five seconds before being added to the bowl with the eggs. Remember, the sperm remain active in water for a very short period of time, so this must be done quickly.
Eliminating the stickiness from eggs:
Ovulated eggs of many species such as white bass, sturgeon, paddlefish, common carp, and channel catfish become sticky after coming in contact with water. During natural spawning, this stickiness causes the eggs to become attached to rocks, sticks, or aquatic plants. Catfish eggs are connected by a sticky matrix that holds the eggs together in a mass in the spawning cavern or container.
In the hatchery, this stickiness causes problems during incubation. Silt-clay, bentonite and Fuller’s earth have been used to remove the stickiness from the eggs of many species of fish. Do not use diatomaceous earth because the sharp edges of the diatoms will damage the eggs.
The dried material is added to hatchery water until a suspension is formed and a residue accumulates on the bottom of the container. Paddlefish and sturgeon eggs are commonly treated with silt-clay as soon as the first few sticky eggs are noticed after fertilization, usually 1 to 4 minutes.
The silt-clay suspension is added to the fertilized eggs at a ratio of 2 to 4 parts suspension to 1 part fertilized eggs. The mixture is gently stirred by hand. Any clumps of eggs on the side of the container are gently broken up. The suspension is poured off the eggs, and fresh suspension should be added every 10 minutes to maintain proper temperature and dissolved oxygen.
Continue the process until the eggs do not stick to fingers or each other when removed from the suspension (minimum of 20 minutes). Urea and salt solution has been used to remove the stickiness from common carp eggs.
The addition of water to common carp eggs and milt will result in their sticking together in a clump within a few seconds. By using urea-salt solution instead of water, the spawn can be fertilized without the eggs sticking.
A commonly used solution is prepared by dissolving 30 grams of urea and 40 grams of salt in 10 liters of hatchery water. The solution is added to the eggs and sperm. The initial volume of solution added is approximately 25 percent of the volume of eggs.
The mixture is gently stirred continuously with a feather, plastic spatula, or by hand. It has been observed that the motility of common carp sperm lasts much longer in the urea-salt solution (20 to 25 minutes) than in water (1 to 2 minutes). As the eggs water harden, additional solution is added.
A portion of the solution with the dissolved sticky material is poured off at intervals and replaced. After about 1 to 1.5 hours the water hardening process is completed. The eggs are then transferred to a tannic acid solution (750 mg/L) for 5 seconds to eliminate any remaining stickiness.
To remove the tannic acid, the eggs are thoroughly rinsed with fresh water. The urea and salt solution has also been used to remove the stickiness from white bass eggs for the production of hybrid striped bass.
A solution is prepared in a McDonald jar with 5 liters of hatchery water, 15 grams of urea and 20 grams of salt. The solution is aerated with a weighted air stone at the bottom of the jar until the chemicals are dissolved. A small amount of the solution is added to cover the eggs and sperm, fertilizing the spawn.
The mixture is gently stirred. Four minutes after the solution is added to the eggsperm mixture, 400 milliliters or less of fertilized eggs are placed in each jar with the solution. Air flow is adjusted to keep the eggs in suspension without rupturing them.
After 6 minutes, the urea and salt solution is poured off the eggs, and 0.75 grams of tannic acid mixed in 5 liters of hatchery water (150 milligrams/liter) is added and aerated for an additional 6 minutes to eliminate any remaining stickiness. The water inlet valve to the jar is opened, flushing the tannic acid.
Tannic acid alone has also been used to remove the stickiness from white bass, sturgeon, and paddlefish eggs. A tannic acid solution of 150 milligrams/liter is often used for this purpose. This solution is prepared by adding 0.75 grams of tannic acid to 5 liters of hatchery water in a McDonald jar just prior to adding the eggs.
The solution is aerated with a weighted air stone at the bottom of the jar. The fertilized eggs (1 minute after the water is added to activate the sperm) are placed in the jar. Aeration is adjusted to just keep the eggs in suspension.
The air stone is removed after 10 to 12 minutes, and the water inlet valve is opened to the jar. Although not absolutely necessary, an excess quantity of milt appears to help reduce stickiness of the eggs.
When the alkalinity of the water is above 200 milligrams/ liter, additional tannic acid maybe needed. However, an excessive amount of tannic acid can strengthen the egg shell, resulting in difficulties at hatch. For white bass eggs, an additional disadvantage of this procedure over the use of urea-salt is that some batches of eggs are extremely sticky, especially if only a limited quantity of milt is available.
In addition, the resulting egg shell is opaque rather than clear, preventing microscopic examination of the developing embryo. A sodium sulfite (Na2SO3) solution can be used to dissolve the gelatinous matrix of catfish egg masses so they may be incubated in hatching jars, eliminating many problems associated with traditional paddle-wheel-trough incubators.
The solution is prepared by mixing 15 grams of sodium sulfite with 1 liter of hatchery water. If the water supply has low alkalinity, the pH must be adjusted back to that of the hatchery water supply using 10 percent hydrochloric acid (HCl).
The egg mass is removed from the spawning container at least 24 hours after spawning and placed in a plastic pan. One liter of the sodium sulfite solution is added per 500 grams of eggs. The egg mass is gently kneaded and stirred until the gelatinous matrix is completely dissolved.
The entire contents of the pan is poured into a hatching jar and the water flow is adjusted to gently tumble the eggs without washing them from the jar.
No more than 1400 grams of eggs should be incubated in a sevenliter hatching jar. Dead eggs, white in color, float near the top of the egg mass and should be removed by siphon to prevent fungus problems.
Authors:
R.W. Rottmann, J.V. Shireman, and F.A. Chapman
