rainbow trout eyed egg

Hatching rate depends on water temperature, but usually will be completed within 2 to 4 days after commencing. Empty shells should not be allowed to accumulate in the incubating units. If the eggs are incubated separately from the rearing troughs, the sac fry should be transferred into troughs shortly after hatching is complete. Up to 30,000 fry can be stocked into a standard fry trough 10 feet long and 18 inches wide. The water flow rate should be 8 to 10 gallons per minute for most facilities. Keep the water level in the trough fairly shallow (3 to 4 inches) and the flow reduced until fry swim up, approximately 2 weeks after hatching at 55 oF. Any dead fry, egg shells or deformed fish should be removed regularly.

When about 50 percent of fry swim up, begin feeding with a small amount of starter mash on the surface three to four times daily. Continue until most fish are actively feeding. Feed every 15 minutes if possible, but not less than hourly at this stage. A large kitchen strainer makes an excellent tool for distributing the starter feeds throughout the tank. Automatic feeders are certainly more convenient than feeding by hand, but many are not well suited to distributing the smallest feed sizes.

Feed approximately 10 percent of the fish weight per day for 2 to 3 weeks, or until fry are about 1 inch long (approximately 1,000 per pound); then feed according to a published feeding chart. Fry feed should be formulated to contain approximately 50 percent protein and 12 to 15 percent fat. Excess feed and fish waste must be removed from the troughs at least daily. Small paintbrushes or feathers work well for cleaning the rearing troughs.

Siphons also can be used if care is taken to avoid the fish. After the fry have been actively feeding for 2 weeks, sample count the fish every week and adjust the feeding rate and feed size accordingly (Table 2). Adjust the fish densities in the troughs as necessary to prevent overcrowding. In the standard fry tank described previously, fish are typically kept below 1 pound of fish per cubic foot of water volume. Monitoring dissolved oxygen levels will help determine when fry density should be reduced. Ideally, the dissolved oxygen level should not be lower than 6 ppm. The fry will be ready to move into larger tanks in the hatchery when they grow to 1 inch in length. In areas where Yersinia ruckeri, the causative agent of enteric redmouth disease (ERM), has been detected, the fish should be vaccinated 2 weeks before moving them to a production facility. The recommended minimum size for immersion vaccination of trout against ERM is 4.5 grams, or approximately 100 fish per pound.

Upwelling incubators maintain adequate circulation by using the water flow to partially suspend the eggs. These incubators should contain no more than two-thirds of the total incubator volume in eggs when operating. The flow rate in upwelling units should be adjusted so that eggs are lifted approximately 50 percent of their static depth (i.e., if eggs are 6 inches deep with water off, they should be approximately 9 inches deep with water on). Upwelling incubators are available in several different models, or can easily be constructed from PVC, fiberglass or other materials.

All types of egg incubators should be covered to protect developing embryos from direct light. To accomplish this when using vertical trays, do not put eggs in the top tray. Eyed trout eggs purchased from a commercial supplier will usually contain a very small percentage of dead eggs, and may not require treatment for fungus. However, if the eggs are more than 3 days from hatching, dead eggs should be removed regularly to limit fungal infections. Removing dead eggs is more effective than chemical treatment at controlling fungus, but can be very time consuming.