Primary productivity connects hilsa fishery in the Bay of Bengal

Abstract

tropical hilsa shad (Tenualosa ilisha) contributes significantly to the society and economy of Bangladesh, India and Myanmar, but little is known about their habitats across the life cycle and their relationship with environmental drivers. this study describes spatial and temporal variability of productivity in the Bay of Bengal (BoB) relating to hilsa fishery. Decadal data on net primary productivity, nutrients (i.e. nitrate, phosphate and silicate) and zooplankton were collected from Aqua MODIS, world ocean database and COPEPOD respectively with spatial resolution 1°×1°. Moreover, monthly abundance of phytoplankton, hilsa catch and long-term catch dynamics were analyzed to determine the associations between variables. The present study was extended over 3.568 million km 2 area, of which 0.131-0.213 million km 2 area characterized as the most productive with net primary production of >2,000 mg C/m 2 /day, 0.373-0.861 million km 2 area as moderately productive with 500-2,000 mg C/m 2 /day, and 2.517-3.040 million km 2 area as the least productive with <500 mg C/m 2 / day which were consistent with field verification data. In case of nutrients, the Ganges-Brahmaputra-Meghna (GBM) delta was rich in nitrate and phosphate than that of the Ayeyarwady delta, while silicate concentration persisted high all over the northern BoB including the deltas. A peak abundance of phytoplankton was observed in GBM delta during the months of August-November, when ~80% of total hilsa are harvested in Bangladesh annually. Variations in seasonal productivity linked with nutrients and phytoplankton abundance are important factors for predicting hilsa habitat and their migration patterns in the deltaic regions and shelf waters of BoB. These results can be useful in forecasting potential responses of the hilsa in BoB ecosystem to changing global ocean productivity. The Bay of Bengal (BoB), which shares many characteristics of the Indian Ocean, is a distinctive system characterized by shallow oceanic arm, deposition of sediment, freshwater plume, seasonal reversal of ocean currents, semidiurnal tides, oxygen-rich surface waters, and abundant biodiversity and fisheries resources 1. The northern BoB has the widest shallow shelf region, extending more than 100 nautical miles (=185 km), which is 3-4 times wider in Bangladesh than that in the Myanmar, eastern coast of India and a global average of 65 km. Nutrient is the major driver of primary productivity in BoB 2,3. The coupled climate-oceanographic processes (e.g. atmospheric depressions and tropical cyclone, storm surge, internal wave, eddy pumping, and river inputs) are injecting nutrients to the shallow zone and thereby promoting the primary production in upper layers of coastal waters 4-6 , although productivity of an area or ecosystem is dependent on many other bio-physico-chemical parameters such as light 7,8. Phytoplankton is the base of marine food web and thus primary productivity is a key driver of zooplankton and ichthyoplankton dynamics influencing the planktivorous and predatory...

Remote sensing of fish-processing in the Sundarbans Reserve Forest, Bangladesh: an insight into the modern slavery-environment nexus in the coastal fringe

“…Camp operation fluctuates seasonally (Fig. 4) to reflect the productivity of the water and the availability of fish (which increases from August, peaking between December and February) (Kay et al 2018;Hossain et al 2020). Evidence of modern slavery activity shifting to proximal warehouses and ports has been noted previously (Jensen 2013;Brown et al 2019) as an adaptive response to changing environmental conditions, conditions that in some cases may result in increased vulnerability to modern slavery (Fig.…”

Life at the borderline: Responses of Ganges river dolphins to dry‐season flow regulation of river and canal habitats by the Farakka barrage

“…The increase in fish abundance during late winter was mainly the result of the anadromous hilsa migrating upstream along the river and the feeder canal (Sajina et al, 2019), but was decoupled from primary autotrophic productivity of the system. Decoupling of hilsa–phytoplankton dynamics has been documented in the Bay of Bengal (Hossain et al, 2020), but the underlying mechanisms and effects of decoupling may be different for river systems. Migration of hilsa further upstream has been blocked by the barrage, leading to differential seasonal fish catch dynamics above and below the barrage.…”

Spatial variability of nutrient sources determining phytoplankton Chlorophyll-a concentrations in the Bay of Bengal

“…Previous reports have identified a strong relationship between phytoplankton biomass or primary production and fisheries production in various coastal and pelagic ecosystems (Nixon & Thomas, 2001;Nixon & Buckley, 2002). Particularly in the Bay of Bengal, the production of the tropical hilsa fishery, which contributes to the economies of India, Bangladesh, and Myanmar, is controlled by primary production (Hossain, Sarker, Sharifuzzaman, & Chowdhury, 2020).…”