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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 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.
The University of the Virgin Islands (UVI) in St. Croix uses a variation on the multiple rearing unit system. They operate four fish tanks of the same size, with same-age fish in each, stocked in time increments. Fish grow from fingerling to market size in one tank, with no movement until harvest. In this scenario, there is always a tank that is either ready for or nearing harvest. While tank volume is not utilized efficiently, fish stress and labor costs are decreased, while knowledge of stock inventory is increased (Rackocy et al. 2006).
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.