Derivation of optimal fish stocking density via simulation of water quality model E2Algae

Ponds of various dimensions are often stocked with a variety of fish and some aquatic plants for aesthetic and recreation purpose. It is crucial to ensure that the water quality fulfils the criteria of National Water Quality Standard IIB, appropriate for recreational usage. A small fish pond located within Sunway University appears to have poor water quality. To evaluate the dynamics of dissolved oxygen (DO) in the pond, samplings of DO levels at hourly intervals at four locations and two depths over a period of 24 hours are conducted for several days. The pond has low DO of less than 1.5 mg/L throughout the 24-hour sampling period. This is far below the required level of 5.0 mg/L stipulated for Standard IIB. Fish respiration and biochemical oxygen demand (BOD) generation rates are estimated from laboratory experiments and literature review. These rate constants are then used to calibrate an in-house numerical water quality simulation model known as E2Algae to simulate DO dynamics subject to fish respiration, fish feed residuals, fish excretion and algae photosynthesis. The study concludes that fish stocking density must not exceed 0.25 kg/m3 and fish feeding rate must not exceed 2% total fish wet weight per day to maintain adequate DO level without the use of aerators. This simulation study also concludes that intensive aquaculture farms that utilize mechanical aerators operating at the rate of one horse power (HP) per 500 kg can potentially improve its energy efficiency to achieve one HP per 2000 kg fish by innovative design of aerator systems.