Fish Hatchery Management - fisheries & aquaculture

BROODSTOCK. SPAWNING, AND EGG HANDLING 187use is a rubber bulb fitted to a short length of glass tubing. The diameterof the tubing is large enough to allow single eggs to pass through it anddead eggs are removed by sucking them up into the tube. A more elaborateegg picker can be constructed of glass and rubber tubing and deadeggs are siphoned off into an attached glass jar (Figure 65).A flotation method of separating dead from live eggs still is used inmany hatcheries, and particularly in salmon hatcheries. Eggs are placed ina container of salt or sugar solution of the proper specific gravity, so thatlive 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 islonger. The container is filled with water, and common table salt or sugaris 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 anddevelopmental stages of the eggs. Floating dead eggs are then skimmed offwith a net. Best results are obtained if the eggs are well eyed because themore developed the embryo, the more readily the eggs will settle.Several electronic egg sorters are commercially available that seParatethe opaque or dead eggs from the live ones. Manufacturers of thesemachines claim a sorting rate of 100,000 eggs per hour. Another commercialsorter works on the principle that live eggs have a greater resiliencyand will bounce (whereas dead eggs will not) and drop into a collectingtray. This sorter has no electrical or moving parts.Enumeration and transfer of fry are important facets of warmwater fishculture, because the eggs cannot be counted in many instances. The fry ofmany species, such as largemouth bass, smallmouth bass, and catfish, arespawned naturally in ponds, and then transferred to a rearing pond. To assurethe proper stocking density, fry must be counted or their numbers estimatedaccurately. Many methods are used, and vary in comPlexity andstyle.The simplest, but least accurate, is the comparisoz method. A sample offry are counted into a pan or other similar container. The remaining fry arethen distributed into identical containers until they apPear to have thesame density of fry as the sample container. The sample count is then usedto estimate the total number of fry in all the containers. Other methods involvethe determination of weight or volume of counted samples and thenestimating the number of fry from the total weight or volume of the group.The most accurate methods require greater handling of the fry but, whenthey are small, handling should be kept to a minimum to reduce mortality.In catfish culture, a combination of methods is used. The number ofeggs can be estimated by weight or from records on the Parent fish. Thegelatinous matrix in which catfish eggs are spawned makes the volumetricmethod of egg counting impractical. There are approximately 3,000 to5,000 catfish eggs per pound of matrix, and the number of eggs can beestimated from the weight of the mass of eggs. After the eggs hatch, fry are