Rice production, despite its important role in food security, could bring about environmental problems such as methane emissions and polluting water resources. To decrease such negative environmental impacts, the co-culture of rice with ecologically friendly aquatic animals such as crabs has shown promising results. However, there are still serious concerns about the proper implementation of rice co-culture systems. Having considered rice-crab systems, crab stock density and the amount of crab feed, among others, are two important factors which regulate the performance of the system and the associated environmental pollution. However, their optimal values and their underlying relationship with enviro-economic parameters (e.g. methane emissions, nitrous oxide emissions, ammonia volatilization, yield, N uptake, nitrate in drainage water, and profit) have not been scrutinized yet. Accordingly, a set of farm experiments has been performed to measure enviro-economic parameters under mono- and co-cultivation of rice. Moreover, the attempts were made to explore the underlying correlations between crab stock density and the amount of crab feed as two independent variables and measured parameters such as yield and greenhouse gas emissions. Furthermore, an appropriate optimization model was developed to find the optimal crab density and crab feed in order to minimize the environmental pollution and maximize crab and rice yield as well as net profit. At the end, a farm survey was also conducted to evaluate the shortages in co-culture systems. The results showed that, under optimal rice-crab co-culture system, the improvements in nutrient uptakes ranged from 5.2% to 23.3%, with the lowest for Zn uptake and the highest for N uptake. Under such circumstances, 355% lower global warming impact would be attained compared to rice mono-culture showing a significant contribution to greenhouse gas mitigation. Furthermore, farmers would benefit from 122% higher profit under co-culture systems. The results achieved herein also have policy implications because it would help to decrease national greenhouse gas emissions and avoid deterioration of water resources while help farmers to ensure earning a high profit.

An indoor aquaponic system (i.e., the integration of fish culture with hydroponic plant production in a recirculating setup) was operated for maximizing water reuse and year-round intensive food production (Nile tilapia, Oreochromis niloticus, and leaf lettuce) at different fish feed to plants ratios. The system consisted of a fish culture component, solid removal component, and hydroponic component comprising six long channels with floating styrofoam rafts for holding plants. Fish culture effluents flowed by gravity from the fish culture component to the solid removal component and then to the hydroponic component. Effluents were collected in a sump from which a 1-horsepower in-line pump recirculated the water back to the fish culture tanks at a rate of about 250 L/min. The hydroponic component performed as biofilter and effectively managed the water quality. Fish production was staggered to harvest one of the four fish tanks at regular intervals when fish attained a minimum weight of 250 g. Out of the total eight harvests in 13 mo, net fish production per harvest averaged 33.5 kg/m³ of water with an overall water consumption of 320 L/kg of fish produced along with the production of leaf lettuce at 42 heads/m² of hydroponic surface area. Only 1.4% of the total system water was added daily to compensate the evaporation and transpiration losses. A ratio of 56 g fish feed/m² of hydroponic surface effectively controlled nutrient buildup in the effluents. However, plant density could be decreased from 42 to 25-30 plants/m² to produce a better quality lettuce.