Population Dynamics of Penaeus indicus (Crustacea: Penaeidae) in Rekawa Lagoon, Sri Lanka
Rekawa lagoon which is a small coastal lagoon (area 240 ha) in southern Sri Lanka supports a small-scale fishery. A major portion of its landings is comprised of penaeid shrimps. Peneaus indicus is the dominant species accounting for about 84% of the total shrimp catch. Population dynamics of P. indicus were studied using FiSAT software package. The analyses were based on data collected from catches of "fish kraals" in the lagoon from October 1994 to June 1995. Asymptotic total length (L∞) and growth constant (K) of P. indicus were determined as 20.3 cm and 1 year-1 respectively which were used to estimate mortality and exploitation rate (E). The total mortality rate (Z) estimated by means of length-converted catch curve method was 4.95. Using empirical equations, natural mortality (M) and theoretical age at length zero (to) were estimated as 2.02 and -0.18 years respectively. The mean selection length (Lc) was estimated by detailed analysis of the ascending part of the length-converted catch curve was 13.2 cm. Relative yield-per-recruit analyses performed by incorporating probabilities of capture have indicated that the P. indicus stock in the lagoon is presently exploited at its optimal level.
This study also indicated that the peak recruitment month in 1994 did not overlap with the spawning season of P. indicus in the sea resulting in low shrimp catches in the lagoon during the fishing season in 1994/1995. As evident from previous studies, high yields of shrimps were reported in Rekawa lagoon when the lagoon mouth opening coincided with the spawning season of P. indicus. Therefore establishment of a procedure is recommended for deliberate opening of lagoon mouth during June-July and October-November each year during which peak spawning of P. indicus takes place in the sea. These could be performed through active participation of fishing communities.