Fish Hatchery Management - fisheries & aquaculture

BROODSTOCK, SPAWNING, AND EGG IIANDLINGr59water is added to wash the eggs for incubation. In the wet method, a pan rspartially filled with water before the eggs are expressed from the femalefish. The milt from a male is then added. Because the sperm will live lessthan 2 minutes in water after being activated, considerable speed is necessaryby the spawn takers. The dry method generally is accepted as the bestprocedure.Eggs are washed or rinsed thoroughly after they have been fertilized andbefore they are placed in the incubator. In some species, the eggs are allowedto water-harden before being placed in an incubator. Water-hardening is theprocess by which water is absorbed by the eggs and fills the perivitellinespace between the shell and yoke, causing the egg to become turgid. Precautionsshould be taken to protect eggs from exposure to direct rays of brightlight, because both sunlight and artificial light are detrimental.Some species, such as walleye and northern pike, have eggs that are extremelyadhesive. Often during the water-hardening process of adhesiveeggs, an inert substance is added to prevent the eggs from sticking together.Starch, black muck, clay, bentonite clay, and tannin have been used asseparating agents. Starch, because it is finely ground, does not have to bespecially prepared, but muck and regular clay must be dried and siftedthrough a fine screen to remove all coarse particles and then sterilized beforethey can be used. Starch or clay first must be mixed with water to theconsistency of thick cream. One or two tablespoons of this mixture is addedto each pan of eggs after fertilization is completed. When the separatingagent has been mixed thoroughly with the eggs, the pan is allowed tostand for a minute. Water is then added, the separating agent is washedfrom the eggs, and the eggs placed in a tub of water to harden. Constantstirring during water hardening helps prevent clumping. The water shouldbe changed at least once an hour until the eggs are placed in the hatchery.Striped bass also may be hand-stripped as an alternative to tank slrawning.Both males and females of this species usually are injected with hormones,as described in a later section of this chapter. An egg sampleshould be taken and examined between 20 and 28 hours after a hormoneinjection. Egg examination and staging requires microscopic examination.The catheter used for extraction of the egg sample is made of glass tubing,3 millimeter O.D., with fire-polished ends. The catheter is inserted approximately2 inches into the vent and removed with a finger covering theend of the tube to create a vacuum that holds any eggs in place in thetube. Extreme care is needed while the catheter is inserted into the ovary.The catheter should be instantly removed if the fish suddenly thrashes;such thrashing usually is immediately preceded by a flexing of the gill covers.Careful manipulation will permit the catheter to be inserted into thevent with a minimum of force, preventing damage to the sphincter muscles.If these muscles are torn, eggs at the posterior end of the ovary will waterharden.The plug thus formed will prevent the flow of eggs.