Population dynamics of Indian mackerel Rastrelliger kanagurta in northwestern coastal waters of Sri Lanka

The population dynamics of Rastrelliger kanagurta from the northwestern coastal waters of Sri Lanka were studied based on the length frequency data collected from October 1993 to September 1994. The resource is mainly exploited by gillnets and beach seines. The length frequency distributions ofRastreIliger kanagurta for the two gear types were similar with two prominent peaks at 13.5-15.5 cm and 20.524.5 cm (total length) length ranges. In addition, smaller individuals with total lengths varying from 7.5-10.5 cm were also observed among the beach seine catches.

Population parameters were estimated using the FiSAT software package. The asymptotic length, growth constant and the growth performance index were estimated to be 28.0 cm (total length), 1.5 year -1 and 3.07 respectively. The estimated value for the natural mortality coefficient using the Pauly's empirical formula was 2.37 year-1. The fishing mortality coefficient for the combined fishery was estimated at 2.62 year-1 while the estimated values for the total mortality coefficient and the exploitation rates were 4.99 year-1 and 0.53 respectively. The recruitment was continuous with two peaks per year.

The length based Thompson and Bell model was used in the bio­economic prediction. Of the possible scenarios, increasing the gillnet effort while maintaining that of the beach seines at the present level appeared to be the most realistic in accordance with the existing social and legal framework. The maximum sustainable yield and maximum sustainable economic yield for the combined fishery were estimated at 207 MT and 200 MT respectively. As the corresponding effort levels were 3 and 1.6 times the present effort, it is reasonable to conclude that the stocks are not over­exploited at present.