Because the internal anatomy of fish varies greatly, hand stripping may be difficult in some species. Sturgeon and paddlefish have no ovarian sac; the eggs are released into the abdominal cavity during ovulation. The best method for taking the spawn in many of these species is to surgically remove the eggs.
The first indication of ovulation for sturgeon and paddlefish is the appearance of several eggs stuck to the sides or bottom of the tank. The brood fish are usually left undisturbed for an additional 1 to 2 hours, depending on the size of the female (small females 1 hour and large females 2 hours), to insure complete ovulation.
If the female sturgeon or paddlefish is to be saved, it is first anesthetized. The fish is temporarily placed in an aerated holding tank with MS-222. When opercular movement slows and the fish is unable to right herself when turned over, she-is then placed belly-up on a stretcher.
Two hoses are used to ventilate the gills during surgery. One hose delivers aerated hatchery water, and the other delivers water from a recirculating tank containing aerated water with MS-222. During surgery, one of the two hoses is always in the fish’s mouth, flushing the gills providing dissolved oxygen.
The hoses are exchanged so that the fish remains anesthetized but opercular movement continues. Too much anesthetic will kill the fish. Before surgery, an antibiotic solution is applied to the abdomen. An incision is made along the midline of the abdomen, and the eggs are carefully removed with a spoon (Figure 3).
To avoid injuring the internal organs, only the eggs easily accessible are taken. The incision is closed with a half circle surgical needle and suture material; a length of surgical tubing, split length-ways, may be used to reinforce the abdominal tissue (Figure 4).
The incision area is treated with an antibiotic before the fish is returned to the holding tank. A high level of dissolved oxygen is crucial for rapid recovery of the fish. A greater quantity of eggs can be obtained by sacrificing the female sturgeon or paddlefish.
If the female is to be sacrificed, it is: 1) held in a net and killed with a blow to the head; 2) hung from a hook; 3) the tail is cut off to bleed the fish, minimizing the contamination of the eggs with blood; 4)an incision is made in the abdomen starting at the vent; and 5) a bowl is placed under the vent, directly below the incision.
The eggs flow quickly from the abdomen, pulled by the force of gravity. Not all the eggs will be free in the abdomen; some remain attached to the folds of the ovaries. These eggs are removed by hand and placed in separate bowls.
The proc&s of ovulation will be completed in the bowls. Within 15 to 20minutes these eggs can be fertilized. Visible amounts of blood, which can inhibit fertilization, may be present with the eggs.
After fertilization, these eggs are processed and incubated separately. For delicate species that seldom survive the rigors of hand stripping, humanely killing them and surgically removing the eggs may be the best option. In addition, more eggs can usually be obtained by this method than by hand stripping.
Once ovulation has been verified (see section on “Hand stripping”), the brood fish is held in a net and administered a blow to the head to kill it. The eggs can then be removed from the fish by carefully cutting open the abdomen, pinching off the oviduct, and removing the ovaries individually.
The total volume of eggs can then be gently squeezed out of the ovarian sac into a clean dry bowl to be fertilized.
Fertilizing the spawn:
Eggs are usually fertilized with fresh milt. Males are captured, wiped off, and held belly down over the bowl containing the eggs. The portion of the abdomen posterior to the pelvic fins is gently massaged to extrude the milt onto the eggs. The number of sperm in a volume of milt is extremely variable, ranging from millions to billions of sperm per milliliter.
Creamy-white milt contains more sperm per volume than grayish white milt. When available, milt from two or more males is used to insure fertilization of the spawn. Individual males can be used to fertilize more than one spawn.
The fresh milt is spread over the eggs and thoroughly mixed by hand, plastic spatula or feather. Only then is water added to activate the sperm. The sperm remain active in water for a very short time (less than 1 minute to 5 minutes), depending on the species of fish and the temperature of the water.
Water also results in closure of the micropyle of the egg in approximately the same amount of time. Water is added to only cover the eggs. Do not add too much water because the sperm may be too diluted to ensure fertilization of the eggs. To fertilize sticky eggs, see section on “Eliminating the stickiness from eggs. ”
The bowl containing the eggs and sperm should be gently rocked, swirled or stirred continuously for several minutes to insure uniform distribution of the sperm and to prevent the eggs from settling. Additional water is added as the eggs take up the water and enlarge (water harden).
After several minutes, the fertilized eggs are transferred to the hatching apparatus. Fertilized eggs should not be exposed to direct sunlight. Subdued lighting should be used in the spawning and incubation area.
Authors:
R.W. Rottmann, J.V. Shireman, and F.A. Chapman
