Distribution and population dynamics of the edible bivalve species Meretrix casta (Chemnitz) in the Dutch canal of Sri Lanka
E.M. JAYAWICKREMA AND M.J.S. WIJEYARATNE
Meretrix casta is a commercially important clam species in Dutch canal, a slow flowing brackish water body in the western coastal region of Sri Lanka. Its abundance at five sampling sites along a 50 km stretch of the canal, which ranged from 60 individuals m-2 to 950 individuals m-2, significantly increased with increasing sand, silt and organic matter content of the bottom sediments. Abundance was negatively correlated with the clay content of the sediments. No significant correlation was recorded between the abundance and environmental parameters such as temperature, salinity, pH and chlorophyll-a content of water. However, abundance was low at the sampling sites with extreme values of salinity (<3 ppt and >40 ppt) and pH (<8.0 and >8.5). Asymptotic length of the clams ranged from 34 mm to 43.1 mm and the growth coefficient ranged from 0.84 year-1 to 1.44 year-1. The ranges for total mortality, natural mortality and fishing mortality coefficients were 2.34-3.08 year-1, 0.84-1.44 year-1 and 1.16-2.24 year-1 respectively. The highest asymptotic length was recorded at the site where the abundance was the highest. The lowest natural mortality and the highest fishing mortality were also recorded at this site. Environmental parameters at this site were moderate and never reached extreme values indicating optimum conditions for growth and survival. The mean size at first capture (L50) ranged from 8.3 mm to 24.5 mm while the optimum value for L50 ranged from 13.6 mm to 23.3 mm. At four sampling sites, present L50 was smaller than the optimum value indicating the exploitation of small individuals. Exploitation rates at two sampling sites were higher than the optimum level indicating over-exploitation of the resource at some places along the canal.