Fish Hatchery Management - fisheries & aquaculture

I82FISH HATCHERY MANAGEMENTA sample of eggs is placed in a single row in the trough until they fill itslength. The number of eggs per 12 inches is referred to Table 18, whichconverts this to number of eggs per liquid ounce or quart. All eggs thenare placed in a water-filled, 32-ounce (quart) graduated cylinder, the submergedeggs being leveled to the 32-ounce mark. The total number of eggsis the number per quart (or ounce) X the number of quarts (or ounces).The weight method is based on the average weight of eggs in a lot.Several 100-egg samples are drained and weighed to the nearest 0.1 gram.The average egg weight then is calculated. The entire lot of eggs is drainedin preweighed baskets and weighed on a balance sensitive to I gram. Divisionof the total weight of the eggs by the average weight of one egg determinesthe number of eggs in a lot. There are two sources of error in theweight method; variation in the amount of water retained on the eggs inthe total lot and variation in sample weights due to water retention. Differencesin surface tension prevent consistent removal of water from the eggs.Blotting pads of folded cloth or paper toweling should be used to removethe excess water from the eggs.In the displacement method, water displaced by the eggs is used to measurethe egg volume. This provides an easily read water level rather than anuneven egg level when volume is determined. Small quantities of eggs canbe measured in a standard 32-ounce graduated cylinder. For larger quantities,a container with a sight gauge for reading water levels is most convenient.A standard 25-milliliter burette calibrated in tenths of millilitersmakes an excellent sight gauge. A table, converting gauge readings to fluidounces, is prepared by adding known volumes of water to the containerand recording the gauge readings. The eggs are drained at least 30seconds in a frame net, and the underside of the net is wiped gently with asponge or cloth to remove excess water. The total volume of eggs then ismeasured by changes in gauge readings (converted to volume) when eggsare added to the container. The amount of water initially placed in thecontainer should be sufficient to provide a clearly defined water levelabove the eggs. The volume of water displaced by a known number of eggsis then determined by sample-counting; the more numerous and representativethe samples, the more accurate the total egg count will be. One ormore random samples should be prepared for each volume measurementand a minimumof five samples for the total lot of eggs. For sampling,count out 50 eggs into a burette containing exactly 25 milliliters of water.Determine the exact number of milliliters of water displaced. The numberof eggs per fluid ounce can then be determined from Table 19.The accuracy of these three methods has been compared, and only theVon Bayer technique showed a significant difference from actual eggcounts, with the displacement method being the most accurate. However,the weight technique is so much faster and efficient that it is considered