Effect of Salinity on Survival and Settlement of Larvae of the Freshwater Shrimp, Macrobrachium nipponense, inhabiting Lake Kojima

Abstract

Freshwater shrimp, Macrobrachium nipponense, were collected in summer from an artificial brackish-water lake, Lake Koj ima, in Okayama Prefecture, western Japan. Their hatched larvae were reared in the laboratory at 25°C in different salinities ranging from 0 to 32 ‰. The pelagic larvae successfully metamorphosed to the 1st postlarva only in salinities of 4 and 16 ‰ with survival ratios of 15 to 20 %. In the laboratory, an increase in salinity from 4 to 16 ‰ accelerated the settlement of middle-stage pelagic larvae. This, however, resulted in a shorter larval period and smaller 1st postlarvae than those obtained in 4 ‰. A field survey in late summer showed an aggregation of 7th-to 9th-stage zoeae and 1st post larvae in more saline (1-20 ‰) deep water, while 1st- to 3rd-stage zoeae were distributed in the less saline (0-1 ‰) surface water. Thus, both the laboratory experiment and field observation indicate that high salinity water induces larval settlement of M. nipponense in this brackish-water lake.