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Knowing when fingerlings are available for purchase will help ensure quality fingerlings. Certain species such as bass, bluegill, and yellow perch fingerlings are considered seasonal and are easiest to find during the summer months after they have been feed-trained. Small fish that are available off-season will likely be stunted and would not achieve optimal growth rates. Species such as tilapia and koi can be bought consistently year-round.
Regardless of the supplier, anytime fish are purchased they should be handled properly, acclimated, and added into a quarantine system for 1-2 weeks to help prevent any disease/parasitic outbreaks within the main production system. If the fish are healthy at the end of the quarantine period, then they should be size-graded and distributed into the main system. Addition of salt to the water during transportation and holding can prevent disease issues by reducing stress on the fish and resulting in a higher survival rate. Information on salting for transport and holding can be found in SRAC Publication No. 390 (Wynne and Wurts 2011).
Production: If producing fingerlings in-house, the producer will need to determine the amount of fish needed to meet production demands. Typically, oversizing fingerling production is done to maintain maximum production capacity. Fingerling production will need to be done in a separate system to limit the spread of disease and to ensure optimal conditions for growth. The producer will also need additional tanks for broodstock, which should be of known lineage, age, and proper size (Egna and Boyd 1997). Spawning can be natural or artificial but is typically natural in a commercial setting (Egna and Boyd 1997). The benefits of producing fingerlings in-house include cutting out the fingerling supplier, ensuring quality fingerlings, getting a quick supply of fingerlings, and potentially earning additional revenue from fingerling sales. Some downsides include the need for more space, need for quality broodstock, need for fingerling production expertise, and a higher initial investment.
Fish culture should be well planned, as mismanagement of densities within the system can lead to issues with nutrient build-up/deficiencies, solids accumulation, water quality concerns, and poor fish health. Consider that aquaponic systems typically do not operate with a fish density exceeding 0.5 pounds/gallon. Three of the most common fish production plans are sequential rearing, stock splitting, and multiple rearing units.
Sequential Rearing: Sequential rearing involves one tank, containing multiple age-groups of fish (Rackocy et al. 2006), where the market-sized population is selectively harvested, and fingerlings are restocked in equal number. While this seems manageable, the continuous grading required can be stressful on the remaining stock, leading to increased risk of disease and death. In addition, stunted fish remain in the system, consuming feed that will not yield any return for operation costs. Carnivorous fish are not well suited for this management strategy, as younger fish are susceptible to predation.
Stock Splitting: Stock splitting requires accession of fingerlings at a high rate, followed by halving the population when tank biomass capacity is reached (Rackocy et al. 2006). Benefits include the ability to remove stunted fish and better control over inventory. However, moving the fish increases the risk of disease and fish loss. Swim ways, a permanent or temporary channel connecting tanks, have been successfully installed to limit stress on fish but accurate counts and weights of the fish are hard to ascertain.
Multiple Rearing Units: Operating multiple rearing units is the most popular method of fish stocking and management. This method utilizes several tanks connected by a common filtration system (Rackocy et al. 2006). When maximum biomass in one tank is achieved, the entire population is moved to a larger tank, typically connected via a hatch or swim way.