182 FISH HATCHERY MANAGEMENT 



A sample of eggs is placed in a single row in the trough until they fill its 

 length. The number of eggs per 12 inches is referred to Table 18, which 

 converts this to number of eggs per liquid ounce or quart. All eggs then 

 are placed in a water-filled, 32-ounce (quart) graduated cylinder, the sub- 

 merged eggs being leveled to the 32-ounce mark. The total number of eggs 

 is 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 drained 

 in preweighed baskets and weighed on a balance sensitive to 1 gram. Divi- 

 sion of the total weight of the eggs by the average weight of one egg deter- 

 mines the number of eggs in a lot. There are two sources of error in the 

 weight method; variation in the amount of water retained on the eggs in 

 the total lot and variation in sample weights due to water retention. Differ- 

 ences in surface tension prevent consistent removal of water from the eggs. 

 Blotting pads of folded cloth or paper toweling should be used to remove 

 the excess water from the eggs. 



In the displacement method, water displaced by the eggs is used to meas- 

 ure the egg volume. This provides an easily read water level rather than an 

 uneven egg level when volume is determined. Small quantities of eggs can 

 be measured in a standard 32-ounce graduated cylinder. For larger quanti- 

 ties, a container with a sight gauge for reading water levels is most con- 

 venient. A standard 25-milliliter burette calibrated in tenths of milliliters 

 makes an excellent sight gauge. A table, converting gauge readings to fluid 

 ounces, is prepared by adding known volumes of water to the container 

 and recording the gauge readings. The eggs are drained at least 30 

 seconds in a frame net, and the underside of the net is wiped gently with a 

 sponge or cloth to remove excess water. The total volume of eggs then is 

 measured by changes in gauge readings (converted to volume) when eggs 

 are added to the container. The amount of water initially placed in the 

 container should be sufficient to provide a clearly defined water level 

 above the eggs. The volume of water displaced by a known number of eggs 

 is then determined by sample-counting; the more numerous and represen- 

 tative the samples, the more accurate the total egg count will be. One or 

 more random samples should be prepared for each volume measurement 

 and a minimum of 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 number 

 of eggs per fluid ounce can then be determined from Table 19. 



The accuracy of these three methods has been compared, and only the 

 Von Bayer technique showed a significant difference from actual egg 

 counts, with the displacement method being the most accurate. However, 

 the weight technique is so much faster and efficient that it is considered 



