Fish Hatchery Management - fisheries & aquaculture

BROODSTOCK, SPAWNING, AND EGG HANDLINGI{J5the best of the methods evaluated. The displacement method takes twicethe time required by either of the other methods. The weight method isrecommended when large lots of eggs must be enumerated, while the displacementmethod is more desirable with small lots of eggs.Another method of egg inventory, which differs from other volumetricmethods basically in egg measuring technique, sometimes is used by fishculturists. Eggs are measured in a container, such as a cup or strainer filledto the top, and an equal number of containerfuls of eggs are put in eachegg incubator tray or jar. Sample counting consists of counting all the eggsheld in one measuring container. To get accurate egg inventories, the samemeasuring unit must be used for the sample counts as for measuring theeggs into the incubator. Measurement by filling the container to the topeliminates errors in judgment. This method gives a good estimate of the:"",:1":"-O"rof eggs, but does not estimate the number of eggs per fluidSeveral methods have been used for the estimating number of stripedbass eggs. Estimates can be made by weighing the eggs from each femaleand calculating the number of eggs on the basis of 25,000 per ounce. Theeggs can also be estimated volumetrically on the basis of Von Bayer's table.Largemouth bass and catfish eggs are measured by weight or volumetricdisplacement.Various mechanical egg counting devices have been developed that usephotoelectric counters (Figure 63). The eggs are counted as they pass alight source. Velocities producing count rates of up to 1,400 eggs perminute have proven to be accurate. Air bubbles, dirt, and other matter willinterfere with accurate counting and must be avoided.Salmonid eggs should be physically shocked before egg picking (removalof dead eggs) commences, after the eggs have developed to the eyed stage.Undeveloped or infertile eggs remain tender and they will rupture whenshocked. Water enters the egg and coagulates the yolk, turning the eggwhite; these eggs then are readily picked out. Shocking may be done bystriking the trays sharply, siphoning the eggs from one container toanother, or by pouring the eggs from the incubator trays into a tub ofwater from a height of 2 or 3 feet. Care should be taken to make sure thatthe eggs are not shocked too severely or normally developing eggs also maybe damaged. (Figure 6+).Numerous methods for removing dead eggs have been in use in fish culturefor many years. Before the introduction of satisfactory chemical fungicides,it was necessary to frequently remove (pick) att dead eggs to avoidthe spread of fungus. In some instances where exposure to chemical treatmentsis undesirable, it still is necessary to pick the dead eggs.One of the earliest and most common methods of egg picking was with alarge pair of tweezers made either of metal or wood. If only small numbersof eggs are picked, forceps or tweezers work very well. Another device in