Hossain, M. Shahadat, et al. “Oyster aquaculture for coastal defense with food production in Bangladesh.” Aquaculture Asia (2013).
ISSN: 0859-600x Vol XV111-1 January-March 2013
Bangladesh is home to 35 million people who heavily rely on its 166,000 km2 of maritime territory as a source of natural resources. Geographically, Bangladesh is vulnerable to coastal erosion and other damages from natural weather patterns such as monsoons, store surges, and cyclones. Current means of reducing coastal erosion are primarily short term barrier solutions, such as concrete blocks, earthen dikes, and sand filled tubes. However, with projections of rising sea level and intensified storms in the future, there is a need for solutions that can help to mitigate coastal erosion issues in Bangladesh on more of a long-term basis. .
‘Ecosystem engineering’ could be the solution. One method proposed in Bangladesh is the use of oyster aquaculture for development of artificial bivalve reefs. The hope is that the oyster beds will reduce ocean-related stress on coastal Bangladesh; the reefs will theoretically redirect sediment deposition and increase shoreline stability, with the potential of also supporting local fisheries by providing habitat for fish and crustaceans.
Research areas were identified based on where natural oyster beds are already found, which is generally where there are artificial structures such as pylons from jetties and bridges. Two sites were chosen for this project, the first off the southeast side of Moheshkhali Island in a mudflat with a mangrove forest, and the second off the southeast side of Kutubdia Island, also in a mudflat, but this time protected by an earthen barrier coated with brick cement. Salt extraction, agriculture, and fisheries are the main activities on both islands.
Investigators were interested in determining which substrate is most suitable for the reefs: dead oyster shells, live oysters, windowpane shells, or stone/rubble. Substrates were put on bamboo woven mattresses designed with four compartments; one for each substrate so the effectiveness of each could be compared and contrasted against the other substrates.
Water samples and estimates of ‘spat’ density on each substrate was collected every two weeks for 24 weeks from the beginning of May to mid-October at the full and new moon. ‘Spat’ is oyster larvae attached to shells. It is used as an indicator of growth and survival rate.
The greatest density of spat was observed during the second week in the windowpane substrate at both sites. The live oyster substrate hosted spat at both sites, but the dead oyster shells and stone/rubble substrates had inconsistent spat occurrences between the two sites. Substrate performance was compared by principle component analysis which confirmed that the windowpane shell substrate was the most successful.
Factors limiting spat survival
The establishment of the artificial reefs relies on the success of natural spatfall and settlement. In addition to the role of the substrate in spat survival, other factors such as water quality, resources availability, and habitat hardiness, must be considered
Spat survival is in part a function of salinity and siltation in the water. A greater degree of spat mortality is observed in all substrates during the monsoon months when siltation is high and salinity is low. Survival and settlement can also be inhibited by competition for space with barnacles, snails, and sea anemones. In addition, burial from increased river discharge during monsoons and the destruction and loss of bamboo mattresses from water forces limited the availability of space for settlement. The bamboo mattresses, which were used for their rugged quality and economic feasibility, did not stand up to their reputation and were easily damaged from the high stresses of the coastal ocean.
The southeast coasts of Moheshkhali Island and Kutubdia Island proved to be suitable habitats for artificial oyster reefs. Their construction has the potential to enhance coastal protection and provide both food resources and employment opportunities in the coastal soft substrate regions of Bangladesh. Oyster reefs also improve water quality from filtration and naturally regulate faunal populations and ecosystem dynamics. The concept of a ‘living shoreline’ could be a long term solution to sustaining the coastal regions of Bangladesh.