Characterization of Spatial and Temporal Variability of Crassostrea spp. Spat Settlement to Ascertain Allocable Sites for Spat Collection at Cox’s Bazar, Bangladesh

Abstract

Spat settlement (spat density, recruitment, and mortality) of edible oysters Crassostrea spp. was estimated for three sites (Nunia Chara [NC], Chowfoldandy [CD], and Sonadia Island [SI]) located along the Cox’s Bazar coast of Bangladesh from February 2019 to January 2020 by deploying shellstrings. Field observations were made every 15 d to understand the monthly variation of oyster spat settlement more precisely. Environmental variables varied significantly with the study site × month interaction or across study sites and months. Gonadal maturation and spawning of oysters may depend on certain environmental conditions, which were greatly influenced by seasonality, especially the monsoon. Spat density, recruitment, and mortality rates were significantly different with the study site × month interaction. The NC site offered higher spat density and recruitment than CD and SI during most months; spat density was lower due to increased mortality during the monsoon (late May to mid‐October) at all sites. Spat density was positively related to salinity, pH, and total suspended solids but negatively related to temperature and high‐ and low‐tide water depths. Recruitment was positively related to salinity and negatively related to low‐tide water depth. Spat mortality was positively related to high‐tide water depth but negatively related to low‐tide water depth. Eight different groups of fouling organisms were observed during the study, which could affect spat settlement. Most interestingly, oyster drills at NC and SI and marine macrophytes and bushlike organisms at CD and SI could contribute to increased mortality during the monsoon. This study showed that none of the three sites would be potential locations for spat collection on shellstrings during the monsoon; however, if avoiding the monsoon, NC has a higher potentiality of allocation for spat collection both for commercial farming and oyster restoration efforts.