Commonly called just ‘Density’, this metric can indicate to a producer relative risk levels to a stock’s production efficiency as low density generally carries lower risk while high density represents a risky situation (relative to that producer’s context).
Stocking density is a metric often used by producers to relate the proportion of biomass in a production unit relative to the total tank, pond or cage volume. Commonly called just ‘Density’, this metric can indicate to a producer relative risk levels to a stock’s production efficiency as low density generally carries lower risk while high density represents a risky situation (relative to that producer’s context). Regarding stocking density in this way allows the producer to stock units appropriately to avoid mortality, reduced growth rates, and other obstacles to good production performance over time.
A common way to express stocking density is in kilograms of biomass per cubic meter of rearing space. For instance, if a 10 cubic meter unit holds 100 kilograms of biomass then the stocking density would be 100 kilograms per 10 cubic meters = 10 kg/m3. Each type of production environment will have different limits for maximum safe density in each rearing unit, depending on several factors.
Average weight of the stock being raised, temperature and dissolved oxygen, and the water exchange rate will all affect the maximum permissible density. As fish grow, the oxygen required for optimal growth per kilogram of body weight decreases. In other words, one kilogram of trout with an average weight of 2 grams will consume more oxygen during a given period of time than one kilogram of trout weighing 500 grams on average. This is a natural biological effect, and as nothing can be done to remediate this condition the producer needs to take this effect into account when planning densities in production units.
Temperature and dissolved oxygen will in most cases work hand in hand toward limiting or increasing maximum permissible stocking density in any rearing unit. Since trout prefer cooler waters between 10-15 degrees Celsius, they will not survive at similar densities when reared in water that is 20 degrees
Celsius. Not only does increasing temperature increase metabolism, thereby increasing oxygen demand, but the saturation point of dissolved oxygen decreases as temperature rises, limiting the total amount of oxygen available to the fish. Production facilities that experience wide temperature fluctuations through the year will need to take this into account and adjust stocking densities according to the season to avoid exceeding maximum permissible density during the warmer seasons.
Water exchange rate is a factor of total influent flow into a rearing unit and the volume of the unit. Dividing the volume of the unit by the influent flow rate (keeping the volume units the same – if flow is measured in liters, the volume of the unit should be expressed in liters as well, etc.). A higher exchange rate means that low oxygen water is replaced by fresh influent water more frequently, meaning more oxygen is available for respiration and a higher stocking density is possible. For farms that experience variable seasonal flow, care should be taken to monitor stocking density relative to water exchange rate and standards should be established relating maximum stocking density to exchange rate throughout the year.
In addition to concerns related to diminishing water quality, producers may encounter other issues when stocking densities are high. Increasing the number of animals in a given rearing unit increases the ability of pathogens to multiply quickly, should they enter the farm. High stocking densities may also lead to aggressive interactions between animals, such as tail-nipping or other signs of violent behavior. With these considerations in mind, producers should seek to establish maximum stocking densities in all of their rearing units based on experience and knowledge of the changing environment on their farms.
