Abstract: Alevins or sac fry can be efficiently incubated under crowded conditions such that the biomass-to-water weight ratio is between 48:1 and 100:1; provided that oxygenated water--rather than aerated water--is circulated through the incubator. Preferably, the water, when leaving the incubator contains approximately seven to fifty milligrams per liter of dissolved oxygen.

Abstract: A cylindrical fish culture tank adapted for a self-cleaning operation. Oxygenated water, whether fresh, recycled, or a mixture of fresh and recycled, enters a tangential inlet near the bottom of the tank and develops an upwardly flowing spiral action. The spiral action, assisted by swimming movements of fish, causes solids to gravitate towards the center of the tank. The bottom of the tank is shaped as an inverted obtuse cone with a screened, normally closed drain opening in the center. At the top of the tank is a screened overflow drain, from which water is normally withdrawn for discharge or for cleaning and filtering. About once a day, the fish culture tank may be cleaned by removing the solids through the screened drain. An airlift assembly may be connected to the drain for recovery of live fish when the screen is taken off the drain. At transfer or harvest time, the live fish may be drained from the bottom of the tank and recovered.