BROODSTOCK, SPAWNING, AND EGG HANDLING 187 



use is a rubber bulb fitted to a short length of glass tubing. The diameter 

 of the tubing is large enough to allow single eggs to pass through it and 

 dead eggs are removed by sucking them up into the tube. A more ela- 

 borate egg picker can be constructed of glass and rubber tubing and dead 

 eggs are siphoned off into an attached glass jar (Figure 65). 



A flotation method of separating dead from live eggs still is used in 

 many hatcheries, and particularly in salmon hatcheries. Eggs are placed in 

 a container of salt or sugar solution of the proper specific gravity, so that 

 live eggs will sink and dead eggs will float because of their lower density. 

 A sugar solution is more efficient than salt because the flotation period is 

 longer. The container is filled with water, and common table salt or sugar 

 is added until the dead eggs float and live eggs slowly sink to the bottom. 

 The optimum concentration of the solution may vary with the size and 

 developmental stages of the eggs. Floating dead eggs are then skimmed off 

 with a net. Best results are obtained if the eggs are well eyed because the 

 more developed the embryo, the more readily the eggs will settle. 



Several electronic egg sorters are commercially available that separate 

 the opaque or dead eggs from the live ones. Manufacturers of these 

 machines claim a sorting rate of 100,000 eggs per hour. Another commer- 

 cial sorter works on the principle that live eggs have a greater resiliency 

 and will bounce (whereas dead eggs will not) and drop into a collecting 

 tray. This sorter has no electrical or moving parts. 



Enumeration and transfer of fry are important facets of warmwater fish 

 culture, because the eggs cannot be counted in many instances. The fry of 

 many species, such as largemouth bass, smallmouth bass, and catfish, are 

 spawned naturally in ponds, and then transferred to a rearing pond. To as- 

 sure the proper stocking density, fry must be counted or their numbers es- 

 timated accurately. Many methods are used, and vary in complexity and 

 style. 



The simplest, but least accurate, is the comparison method. A sample of 

 fry are counted into a pan or other similar container. The remaining fry are 

 then distributed into identical containers until they appear to have the 

 same density of fry as the sample container. The sample count is then used 

 to estimate the total number of fry in all the containers. Other methods in- 

 volve the determination of weight or volume of counted samples and then 

 estimating the number of fry from the total weight or volume of the group. 

 The most accurate methods require greater handling of the fry but, when 

 they are small, handling should be kept to a minimum to reduce mortality. 



In catfish culture, a combination of methods is used. The number of 

 eggs can be estimated by weight or from records on the parent fish. The 

 gelatinous matrix in which catfish eggs are spawned makes the volumetric 

 method of egg counting impractical. There are approximately 3,000 to 

 5,000 catfish eggs per pound of matrix, and the number of eggs can be 

 estimated from the weight of the mass of eggs. After the eggs hatch, fry are 



