Exploring the Possibilities of Mariculture for Promoting Blue Economy of the St. Martin's Island, Bangladesh

Abstract

Bangladesh, a nation situated on the northern coast of the Bay of Bengal, has been endowed with a wide range of natural resources. The ocean has a crucial role in driving socio-economic advancement through stimulating economic activities throughout the country, particularly in coastal regions. The notion of the ocean’s economy, commonly known as the blue economy, is a multifaceted approach that aims to foster economic expansion, ensure environmental sustainability, promote social inclusivity, and enhance the resilience of marine ecosystems. St. Martin's Island, the sole island that bears coral in Bangladesh, is unquestionably among the most cherished possessions and has been chosen as the study area due to its unique geography, size and location within the coastal zone. Furthermore, this region serves as an appropriate pilot model location due to its abundance of readily accessible data resources. This study emphasizes the potential of mariculture in promoting the growth of the blue economy on the island. The objectives of the study were to determine the best site, season, and species of fish and seaweed to be cultivated in the coastal waters adjacent to the Island. In order to accomplish the study's goal, the present research work examined the composition, abundance, and distribution of physiochemical properties, plankton, nutrients and heavy metals in the coastal waters of the Bay of Bengal in the vicinity of St. Martin's Island, Bangladesh. Twelve distinct sampling stations were visited at various seasons between 2019 and 2021 to collect samples. Physicochemical properties were quantified to analyze water quality fluctuation by season. Three seasons specifically pre-monsoon, monsoon and cool dry winter season’s temperature, salinity, pH, DO, EC, TDS, and transparency were measured. The average temperature from the stations ranged from 24.16ºC to 27.4ºC. Mean seasonal temperatures for the pre-monsoon, monsoon and dry cool winter seasons are 26.83±0.21ºC, 27.06±0.22ºC, and 24.61±0.39ºC respectively. The salinity ranged from 24.42 ppt to 33.76 ppt, with the highest salinity (32.91±0.51) being measured in the pre-monsoon season. The value of pH was rather consistent throughout the year, ranging from 7.71 to 8.22, with the highest value (8.18±0.02) being measured in the pre-monsoon season. The amount of dissolved oxygen ranged from 4.82 to 6.74 mg/L, with the highest in the dry cool winter season (6.47±0.21 ppm) while the minimum dissolved oxygen was measured in the monsoon season (4.95±0.09 ppm). The value of electric conductivity ranged from 39.0 mS/cm to 53.46 mS/cm, with the highest value (52.06±0.90) being measured in the pre-monsoon season. The value of total dissolved solids ranged from 20.35 to 27.9 g/l, with the highest value (27.51±0.23) being measured in the pre-monsoon season while the lowest (20.98±0.35) was in the rainy monsoon season. The water around the island was clearer in the cool dry winter season with a range of 4.14±02 m. Furthermore, the water is least (1.45± 0.03) clear in the rainy season. A thorough examination of the season resulted in the identification of 55 unique phytoplankton species. The overall number of phytoplankton ranged from 75,000 to 450,000 individuals per cubic meter, with the maximum abundance observed at station 10. Coscinodiscus sp. dominates all stations with cell densities of 12,500–87,500 ind/m 3 and other dominant species were Chaetoceros, Thalassiosira, and Thalassionema. Another investigation disclosed the presence of 34 distinct species of zooplankton. Most species were copepods, and station 8 had the most species. The observed range for the standing crop of zooplankton was between 55,000 and 125,000 individuals per cubic meter. Additionally, the community included Polychaeta and Cirripedia. The species of utmost significance encompassed Oithona, Canthocalanus, Balanus, Euterpina, and Microsetella sp. The levels of plankton abundance were found to be maximum during the pre-monsoon season, whereas moderately lower abundance was seen during the cool dry winter season. All the nutrient concentrations exhibited highest mean values during the cool winter season (Nitrate: 0.673±0.074; Nitrite: 0.139±0.015; Silicate: 8.66±0.253; Ammonium: 0.275±0.046; and Phosphate: 0.165±0.029 mg/L) while during the pre-monsoon season the values were slightly lower (Nitrate: 0.336±0.041; Nitrite: 0.012±0.010; Silicate: 7.757±0.389; Ammonium: 0.163±0.046; and Phosphate: 0.120 ± 0.019 mg/L). During both seasons silicate and nitrate are the prime contributor of the nutrients followed by ammonium and phosphate, respectively. The study found that heavy metal concentrations (μg/L) for Lead (Pb) 76.825±37.91, Copper (Cu) 27.478±2.78, Arsenic (As) 0.990±0.19, Chromium (Cr) 3.475±1.92, Cadmium (Cd) 6.365±4.08 and Zinc (Zn) 44.534±12.09 in pre-monsoon hot season and for Lead (Pb) 24.909±9.28, Copper (Cu) 23.987±1.71, Arsenic (As) 1.0591±0.31, Chromium (Cr) 3.033±2.23, Cadmium (Cd) 3.727±1.26, and Zinc (Zn) 21.097±11.44 in cool dry winter season are still safe for any living organism. Based on physiochemical qualities, plankton distribution, nutrients and heavy metal distribution, and pollution indexes (HPI, HEI, NPI, TERI), site, season and suitable species of fish and seaweed were chosen. As a result of the collective findings, it could be stated that the pre-monsoon season was strongly suitable for cage culture and cool dry winter season was strongly suitable for seaweed culture in the coastal waters of St. Martin’s Island. For cage culture, stations 10, 8 and 9 were selected to cultivate herbivore, omnivore and carnivore fish respectively, based on physiochemical attributes, nutrients, heavy metals and plankton abundance and distribution. On the other hand, based on physiochemical attributes, nutrient and heavy metal distribution station 1 was chosen for seaweed culture. However, it is also plausible to consider stations 10 and 12. The suitability of mariculture in a coastal region is contingent upon a comprehensive assessment that considers various aspects, including environmental, economic, social, and regulatory concerns. In this study the benefit cost ratio (BCR) was taken into account as the basis of suitability of mariculture (BCR > 1 suitable, BCR < 1 unsuitable, BCR =1 marginally suitable). According to the results of a thorough investigation of all of the studies, John’s Snapper (Lutjanus johnii), a carnivorous fish species was chosen as a representative species for cultivation and the monoculture technique of fish farming was used in this study. Lutjanus johnii, had a Benefit cost ratio (BCR) value of 1.1651 after being caged in coastal waters. BCR value showed that cage-growing of John’s Snapper at St. Martin's Island is profitable. The study suggests that the large- scale commercial cage culture of L. johnii has the potential to yield extremely significant profit. Based on the comprehensive analysis of all the studies, Padina gymnospora (Brown algae) was selected as a commercially important algae to be cultivated in the coastal waters adjacent to St. Martin’s Island. BCR for Padina gymnospora was 2.75, using the long line method and 3.364 using the floating net method respectively which indicates cultivation of this species using both methods are profitable in the Island. BCR analysis also suggests that floating net culture is superior to long line cultivation for the purpose of growing seaweed. Nonetheless, the study suggests that cage and seaweed farming in St. Martin’s Island and adjacent coastal areas can promote the country's blue economy by increasing fish and seaweed production. Therefore, the cultivation of fish and seaweed in the coastal waters of the island has the potential to enhance the socio-economic well-being of the island's residents and contribute to the overall economic growth of the country. Water quality, diseases outbreak, habitat alternation, genetic homogenization, feed dependency, market volatility, technology and lack of regulations are some of the key limitations associated with mariculture research and practice. These limitations must be carefully considered and addressed to ensure the success of research efforts.