Environmental Capacity Management of Shrimp Culture-Based Aquatic System in the Southwest Coastal Region of Bangladesh

Abstract

It is commonly assumed that expansion of coastal aquaculture in Bangladesh poses risks of exceeding environmental capacity. Therefore, it is very essential to estimate the environmental capacity of an aquatic system as well as to develop guidelines for the management of shrimp aquaculture to stay within the environmental capacity. A study was conducted under a DFID-funded research project involving Nautilus Consultants Ltd. (UK) to investigate this issue. The study was conducted in an aquatic system of Dumuria upazila under Khulna district, comprising a river, a canal and 10 shrimp ponds; the results were also verified by a wider-area survey. Shrimp culture practices were mostly extensive to improved extensive, without proper pond preparation, fry nursing, stocking or feed management. Consequently, the production rate was low, averaging only 191 kg ha⁻¹. High mortality of shrimp was reported immediately after heavy rainfall in some of the shallow ponds. The study encouraged farmers to report conditions leading to a variety of potential indicators being tested to indicate poor water and soil quality prior to shrimp mortality events. Estimation of the overall nutrient mass balance per hectare indicates that fertilizer was the highest nutrient contributor for both TN and TP, giving rates of 60.2% and 82.4%, respectively. Feed was the second-highest contributor, estimated at 21.1% N and 9.3% P. The total inputs of nitrogen and phosphorus were 48.7 kg ha⁻¹ cycle⁻¹ and 28.95 kg ha⁻¹ cycle⁻¹, respectively. Among the inputs, only 33.4% of nitrogen and 6.1% of phosphorus were removed in harvested form. A large portion, 39.1% N and 92% P, remained in the sediments and was unaccounted for. According to the estimated nutrient dynamics, N and P were not exceeding the environmental capacity standard. Year-round water quality observations indicated that there were no major differences among the river, canal and pond water, which might be due to the water-flushing rate of 468% per cycle. The water quality in shrimp ponds during the grow-out period in this experiment was within an acceptable range. It may be concluded that the aquatic environment is able to accommodate the present level of aquaculture practice. However, the assimilative power of the aquatic environment is in danger of being degraded with the expansion of shrimp culture areas. Given the poor yields, investment in more intensive culture has been slow to materialize. On the other hand, saline waterlogging for long periods in shrimp farms resulted in salinization, scarcity of freshwater and destruction of various freshwater species. Such cumulative impacts must be considered in planning aquaculture and other development if environmental capacity is not to be exceeded.