The artisanal, stake net fishery for shrimp in the Negombo lagoon, Sri Lanka, was studied over 2 years. There are around 22 sites, at which 57–63 stake nets can be fixed, at sites close to the sea mouth of the Negombo lagoon. The estimated mean catch rates of the fishery were 23.21 and 26.19 kg per operation in 1998 and 1999, respectively. The estimated annual fish productivity from the fishery reduced from 240 t in 1998 to 215 t in 1999. Subadults of penaeid shrimps formed 82% of the stake net catch. The net monthly income per fishermen varied from Rs. 5000 to 33 200 (mean = 13 667). In the Negombo lagoon stake net fishery, a mechanism for equity sharing was formulated by the fishing community.

The population dynamics of Metapenaeus dobsoni (Miers) from the west coast of Sri Lanka was investigated based on catch/effort and length frequency data, using FiSAT software. Annual yield of M. dobsoni was estimated at 296 t in 1998 and 362 t in 1999. This species formed 54% of the total shrimp catch from the system. The asymptotic length (L) and the growth rate constant (K) were estimated to be 11.49 cm and 1.02 yr−1 for males and 12.8 cm and 1.73 yr−1 for females. The fishing mortality coefficients were 2.42 and 1.85 yr−1 for males and females, respectively. The natural mortality was estimated at 1.72 yr−1 for males and 2.51 yr−1 for females. The recruitment for the estuarine environment and the offshore areas was continuous but peaked twice per year. The maximum sustainable yield (MSY) was estimated at 294.65 t, around the current catch estimates. To obtain maximum sustainable economic yield (MSE = 293.56 t) the present effort should be reduced by 10%.

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.

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