Production of fish generally involves 4 distinct phases: spawning of broodstock, egg hatching to fry phase, fry to fingerling phase, and a grow-out or production phase. Broodstock of some species of fish are allowed to spawn naturally in ponds (eg, catfish, perch), while other species are allowed to spawn seasonally in tanks by taking advantage of the natural environmental cues such as changes in temperature and photoperiod (eg, salmonids, striped bass, tilapia). Most species of fish have well-defined spawning seasons, but most can be induced to spawn artificially by adjusting the environmental cues to the appropriate season for spawning or by administering hormones. In catfish and perch, the female lays eggs in the environment, and males are allowed to fertilize the eggs naturally before the eggs are collected and hatched in a controlled environment. Eggs of salmonids and striped bass, however, are usually manually stripped from the females and collected before fertilization. Milt (ie, sperm) that has also been physically stripped from the males is then mixed with the eggs before the resulting fertilized eggs are incubated in a controlled environment. While most foodfish species in aquaculture use external fertilization to produce viable eggs, a few commercially important groups of tropical fish use internal fertilization to produce live young.
Fish eggs come in a variety of sizes and forms. For example, catfish eggs are adhesive and are deposited in a gelatinous mass, yellow perch eggs are laid as sticky gelatinous strands, and eggs of salmonids and striped bass are distributed over the substrate. Developing eggs have an extremely high metabolic rate and therefore need to be maintained in optimal water quality. Nitrogenous waste levels must be kept as low as possible, and dissolved oxygen levels must be appropriate for the species. Thus, flow-through systems or systems with suitable filtration are required to maintain eggs. Once the eggs hatch, the fry of most species initially derive their nutrition from the attached yolk sac. The sac-fry of some species of fish are unable to swim upon hatching, but as they mature and use up the yolk material, they gain the ability to swim. At this time, the fry swim to the surface, inflate their swim bladders, and usually start feeding on natural or artificial diets. Depending on the species, fry can be stocked directly into ponds that have been fertilized to stimulate growth of phytoplankton and zooplankton as a food source or stocked into raceways or tanks and started on artificial diets.
Some species of fry are maintained this way for 5–8 mo until they become fingerlings (3–5 in.) and large enough to stock into ponds, tanks, or cages. During this time, fry that were initially fed on natural diets can be acclimated to artificial diets. As the fingerlings grow, feed particle size and amounts are increased to produce rapid growth.
The final stage is the grow-out production phase in which fingerlings are stocked in various aquatic systems (eg, ponds, raceways, tanks) and fed at accelerated rates to produce optimal growth to market size. The size of fingerlings to stock in the various aquaculture systems depends on a combination of factors, including time of year, availability of fingerlings, desired stocking density, and aquatic system being used. Larger fingerlings will reach market size faster than smaller fingerlings but are more expensive because they require more time and resources to produce. In addition, larger fingerlings may be more difficult and more expensive to transport. Depending on the species of fish, water temperature, and feeding rate, the time required to produce commercial market-sized fish from fingerlings may be as short as 6 mo (eg, tilapia) to as long as 12–15 mo (eg, salmonids).
Last full review/revision July 2011 by Stephen A. Smith, DVM, PhD