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An Estimation of Phosphorus Loading in Aquatic Environment Due to Aquaculture Practices in Bangladesh

Rabeya Rahmatullah
Department of Aquaculture, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh.

S.M. Rahmatullah
Department of Aquaculture, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh.

Abstract/ Executive Summary:

Culture of mono-sex GIFT tilapia (Oreochromis niloticus, L.) was carried out in a recirculating system for 15 weeks. Tilapia fry of uniform size of 0.76 g were stocked at a density of 142 fish/m³. Fish were fed with commercial feed containing 25% protein. Considering fish feed as the sole source of total amount of phosphorus applied, the total amount of phosphorus input (dry basis) was 3380.83 g of which fish retained 16.89% and the rest remained unutilized which remained confined within the system. This result may lead to an assumption that to achieve a total aquaculture production of 1,859,808 t in 2013-2014 in Bangladesh, minimum input of phosphorus in the form of feed was 3,719,616 t of which 18,226.12 t remained unutilized and hence loaded to the environment. This huge amount of phosphorus loading causes environmental hazards and economic loss.

Keywords: Aquaculture, Phosphorus, Tilapia, Environmental hazard.

Location: Bangladesh Agricultural University

Start Date: 00-00-2014

End Date: 00-00-2014

Program Area: Animal Health and Management

Commodity: Aquatic animal

Objectives:

To determine the amount of phosphorus (% of input via feed) harvested by fish
To make a prediction about the total phosphorus loading from aquaculture practices in Bangladesh.

Methodology:

Site and Duration of Experiment: The experiment was carried out in an existing recirculating aquaculture system of the Faculty building of Fisheries, Bangladesh Agricultural University, Mymensingh for 15 weeks. Layout of the System: The system consisted of 12 equal sized cemented cisterns and 2 smaller ones. Each cistern had inlet and outlet facilities connected to the bio-filter system which was located along the east and south sides of the system. The substrates of the bio-filter consisted of styrofoam, gravels, PVC nets and shells of mussels. The pump house was located at the southeastern side of the system. The pumping unit consisted of two water pumps which ran alternatively for six hours to ensure continuous water flow and aeration. Nine cisterns of uniform size were selected for conducting the experiment. The cisterns had a length, width and depth of 2.06 m, 1.8 m and 0.94 m respectively. So each cistern had an area of 3.708 m²and a volume of 3.486 m³. Water level in the cisterns was adjusted at a depth of 0.76 m to maintain the water volume of 2.82 m³. The desired level of water in the system was maintained by continuous recirculation of water at a rate of 15 l/min. In each cistern, water level was controlled by a PVC pipe placed at the opening of the outlet located at the middle of the cistern. The pipe was covered by a larger pipe (in terms of diameter and height) with four notches at the bottom which facilitated easy removal of waste materials from the cisterns. Water loss from the cisterns due to evaporation was adjusted every week by adding groundwater. System Preparation: The cemented cisterns were cleaned thoroughly with washing powder and washed with freshwater. After complete drying they were filled with groundwater upto a level of 0.76 m which was maintained throughout the experiment. The supplied water had a pH of 8.07, dissolved oxygen content of 6.2 ppm, NO₃ content of 0.03 ppm, NO₂content of 0.01 ppm, alkalinity of 70 ppm and total phosphorus content of 0.3 ppm. Polyester net having a mesh size of 6 cm was placed over each cistern to prevent fish from predatory birds. Fish Culture and Sampling: Fry of mono-sex GIFT tilapia (Oreochromis niloticus, L.) belonging to the same age group were collected from Reliance Aquafarms, Boilor, Trishal, Mymensingh. The fry were evenly distributed in the extra cisterns of the system and kept for seven days so that they can adjust to the environment of the experimental setup. Mono-sex GIFT tilapia fry of uniform size (average weight of 0.76 g) were stocked at a density of 142 fish/m³. Fish were supplied with commercial feed having an average protein content of 25%. Starter feed was fed the first ten days, then nursery feed for twenty days and finally grower feed for rest of the days. At the beginning of the culture period the feeding rate applied was 15% of body weight per day then gradually reduced to 6% of body weight towards the end of the culture period. Feed was administered three times a day apportioned as follows, one-third at 9.00 AM, one-third at 1.00 PM and the rest at 5.00 PM. Sampling of fish was done to adjust the feeding rate. Ten fish from each cistern were randomly selected. Fish were caught by a big scoop net and weighed in grams by a sensitive electronic balance. The health condition of fish was visually examined. Sampling was done at 7 days interval at the beginning of the experiment and at 15 days interval towards the end of the experiment. At the end of the experiment the fish were harvested by draining the cisterns. Determination of Total Phosphorus Content of Samples: The total phosphorus content (%) of fish and feed was determined at the beginning and end of the experiment. The total phosphorus contents of these samples were determined. Dry samples are digested by a mixture of nitric acid and perchloric acid (2:1). The digests are mixed with certain amounts of ammonium molybdate and stannous chloride and prepared for determining their transmittancies or absorbancies by spectophotometry at 660 nm. Finally results are obtained from calibration graph.

Source of Information: Environmental Research International | Year-2015 | Volume 1 | Pages 06-09

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

This experiment clearly demonstrates that a huge amount of phosphorus is being wasted every year. This wastage not only contributes to environmental pollution but also reflects huge resource and economic loss. In Bangladesh the demand for TSP (Triple Super Phosphate) fertilizer in 2013-2014 was 675,000 t of which a small portion is produced and the rest amount is imported (Fertilizer Association of Bangladesh, 2015). If managed properly, the phosphorus loss from aquaculture can contribute to the fertilizer sector of Bangladesh and save millions of money invested in this sector; and at the same time will contribute to the alleviation of environmental hazards caused by phosphorus loading.

Knowledge Management: Journal