Fish Hatchery Management - fisheries & aquaculture

BROODSTOCK, SPAWNINC, AND EGG HANDLING 165thousand. The vent must be covered to prevent egg loss. Fish will becomesufficiently relaxed for removal of eggs withinI to 2 minutes. Workersshould wear gloves to prevent injury from opercular and fin spines. Strippingfollows the procedure previously described in this chapter.Because the broodfish of anadromous species of Pacific salmon die afterspawning, no advantage is obtained by stripping the female. Females arekilled and bled. Bleeding can be accomplished by either making an incisionin the caudal peduncle or by cutting just below the isthmus andbetween the opercula to sever the large artery leading from the heart to thegills. The females are allowed to bleed for several minutes before beingspawned. A mechanical device is in common use that effectively kills andbleeds the fish by making a deep cut through the body behind the head.Bleeding reduces the chance of blood mixing with the eggs and reducingfertilization. The point of the spawning knife is placed in the vent toprevent the loss of eggs and the fish is lifted by the gill cavity and heldvertically over a bucket, such that the vent is j-l inch above the lip of thebucket. The fish can be held securely in this position by bracing the backof the fish between the spawner's knees. An incision is made from the ventto a point just below the ventral fin, around the ventral fin, back to thecenter line, and upward to a point just beneath the gill cavity. If the fish isripe, most of the eggs will flow freely into the bucket (Figure 56). Theremaining ripe eggs can be dislodged by gently shaking the viscera. If thefish is not ripe, gentle shaking will not dislodge the eggs and such femalesshould be discarded. Eggr that can only be dislodged by greater force willbe underdeveloped and infertile.The spawning knife needs a sharp blade, but should have a blunt tip toavoid damage to the eggs during the incision. Linoleum knives have beenused for this purpose, but personal preference usually determines thechoice of the knife.Male salmon also are killed prior to spawning. Milt is hand strippeddirectly onto the eggs in the bucket. The eggs and milt are gently mixedby hand.In the case of Atlantic salmon or steelhead, which may return to spawnmore than once, females should not be killed to obtain eggs. A female fishcan be spawned mechanically by placing her into a double walled, rubbersack with the tail and vent of the fish protruding. The sack can be adjustedto fit each fish. Water entering between the walls of the sack causes apressure against the entire fish, and will express the eggs if they are ripe.Female fish handled in this way seem to recover more rapidly than fromother methods of stripping. Milt is collected from the males and stored intest tubes. A male fish is held upside down and the milt is gently pressedout and drawn into a glass tube with suction.Reduction of damage to broodstock and increased efficiency are factorsof prime importance in any spawning operation. The use of air pressure