ON THE BIOLOGY AND FISHERY OF THE GIANT FRESHWATER PRAWN MACROBRACHWM ROSENBERGU (DE MAN) IN THE NORTHERN BOLGODA LAKE, SRI LANKA

Abstract

In the northern Bolgoda lake, Sri Lanka, the salinity and the temperature
of the bottom water are consistently higher than those of the surface. Both the
salinity and temperature are influenced by the local monsoon rains.
The gaint freshwater prawn (Macrobrachium rosenbergii) occurs in the lake
throughout the year except March and September. Considerable numbers of them
are caught from April to June and again from November to January. The carapace
length of male prawns range from 5 cm to 12 cm (mean 8.2 + 1.4 cm)
and that of females from 5 cm to 11 cm (mean 7.2 + 0.9 cm). The mean length
of the prawns caught increases from 6.6 cm in April to 7.9 cm in June and from
7.1 cm in November to 7.6 cm in January. The observations indicate that M.
rosenbergii migrates into the lake, obviously from the nearby freshwater streams,
twice during the year.
The seasonal distribution of the berried females shows two peaks corresponding
to the above two periods, indicating that there are two spawning seasons
for M. rosenbergii in the lake during a year.
The fecundity of the berried females varies from 82,000 to 170,000 (mean
113,300 + 30,985). The males in, general weigh more than the females. The
catch per unit effort varies from 0 to 2.25 prawns per net per day. The fishery
for M. rosenbergii in the lake coincides with its two spawning immigrations