Development of Sustainable Integrated Aquaculture Systems with Assessment of Environmental, Social, and Economic Implications

Current methods of large-scale aquaculture systems are not sustainable, and pose a threat not only to wild fish stocks but also to the environment. Intensive production of shrimp, for example, has been linked to the degradation of the environment as a result of habitat modification and/or effluent discharge. Although aquaculture effluent is regulated under the Clean Water Act, Section 104, 33 U.S.C., even the discharge of treated aquaculture effluent into open waters may impose negative environmental impacts in the long term. To minimize the negative impacts on the environment innovative methods of aquaculture are now being explored to promote sustainable aquaculture production. One such method is the re-utilization of aquaculture effluent in value-added commercial applications.

Untreated aquaculture effluent contains high levels of nitrogenous (i.e. ammonia and nitrates), phosphorous, and potassium nutrients resulting from uneaten fish feed and fecal material. Nutrient concentrations are also proportional to the quantity of fish in the production system. Hence, the negative environmental impacts resulting from the discharge of aquaculture effluent from intensive systems will continue to intensify without the adaptation of sustainable practices. As the aquaculture industry continues to expand globally, cost-effective sustainable methods of water quality management must be implemented both in the United States and internationally.

To address the management of nutrient laden aquaculture effluent, an experiment was performed at the Environmental Research Laboratory, in Tucson, Arizona to evaluate the potential environmental and economic benefits of using aquaculture effluent from the fish Tilapia (Oreochromis niloticus) to grow several food crops including Barley (Hordeum vulgarein), Basil (Ocimum basilicum), and soybean (Glycine max), in a soil based medium. Re-circulating integrated aquaculture-agriculture (RIAA) systems were established by irrigating individually potted plants with effluent from 5000 L pools using an above surface drip system. Integrated systems combine aquaculture and agriculture practices in which the nutrient rich aquaculture effluent is used to irrigate plants. This is particularly advantageous in terms of nutrient utilization as nitrogenous wastes contained in the aquaculture effluent are utilized as a form of fertilizer by the plants.

Several successive experiments were run in which a different food crop was grown in each experiment each lasting between 10 and 14 weeks. In each experiment, three RIAA systems referred to as Trts. 1, 2, and 3 were simultaneously run with densities of Tilapia at 3, 6, and 9 kg m^-3 respectively. Water from each 5000 L pool was distributed to a total of 300 plants divided equally between one and ½ gallon pots filled with 1:3 soil (Sunshine mix #1 ) to sand mixture. Pots were set up in a common pot array and placed in irrigation channels built at a slight negative slope (-2°) to initiate re-circulation. Plant and animal growth rates as well as water quality were monitored over time to evaluate the conditions which generated the highest economic and environmental benefits. Animal and plant growth were determined in each system as (g w^-1) and (cm w^-1) respectively. Water quality was assessed by measuring total nitrogen TN, NO3, NH4, P04 and orthophosphate every other in both in the aquaculture effluent and influent to elucidate plant and nutrient interactions.

Fish growth rates ranged between the three systems and between experiments did not significantly vary (Multiple regression analysis, p = 0.43). Plants irrigated with water from treatments with increasing densities of fish grew consecutively larger and plants grown in larger pots grew to larger sizes than those in smaller pots within the same treatment. Total nitrogen and total phosphorous concentrations remained consistent over time within treatments (Duncans Multiple Range Test, a = 0.05) but significantly increased from trts 1 to 2 to 3 (DMR, a = 0.05 in relation to increasing densities of fish. Ratios of inorganic to organic nutrient loads for both nitrogenous and phosphorous compounds increased over time and approximately 90% of all nutrients were consistently removed from the effluent as the aquaculture effluent migrated through the potted soil. Approximately 10% water was lost per week as a result of evaporation and plant transpiration (i.e. 90% of the water was re-captured and re-utilized).

Results from this experiment demonstrate that integrative production systems, combining aquaculture with soil based agriculture, is a sustainable and robust means of production. Numerous plant species grew readily when supplied with aquaculture effluent suggesting that RIAA technology can be used to grow both ornamental and food production plant species. The materials necessary to develop RIAA systems are relatively inexpensive, easy to obtain, and easy to maintain. As a result, implementation of this technology in both the United States and lesser developed countries may provide a rapid payback period and high return on investment while minimizing the negative impacts on the environment.