Factors influencing migration of Penaeus indicus in the Negombo lagoon on the west coast of Sri Lanka

Juveniles of Penaeus indicus (Milne Edwards) occur in the Negombo lagoon Sri Lanka and on maturing they return to the sea for reproduction. Cohort analysis indicated pulses of immigration to the nursery areas in February and June. The growth rates for juveniles were relatively high compared with that of adults. Three groups of prawns were identified in the lagoon: small quick movers, larger long stay individuals and an intermediate group. There were few identifiable emigration pulses from the lagoon towards the offshore areas. It appears that osmotic stress caused by low salinity waters in the lagoon during the periods of high precipitation may be the strongest influence triggering the onset of emigration.