Production of the freshwater prawn Macrobrachium rosenbergii in Israel. II. Selective stocking of size subpopulations

At the end of a temperate zone growout phase which lasts from May until mid-November, approximately 25% of the prawns would remain under market size. Two methods were employed to reduce this loss: overwinter storage in heated facilities with continual growth in production ponds in the following season, and selective stocking of separated juvenile size subpopulations into production ponds in the first growout season. Small (c. 10 g) prawns stocked in vacant nurseries at high densities at the end of the growout season reached the market size of > 25 g within 87 days. Small prawns were also released into production ponds in early spring after overwinter storage and reached the large market size after an additional growout period of 54 days. Nursed juveniles, undergoing secondary nursery, demonstrate a wide size distribution which allowed separation into two distinct size subpopulations. The large juveniles group, averaging 7.8 g, was transferred into intensive prawn monoculture ponds, while the small juveniles group, averaging 5 g, was stocked at low densities in a fish polyculture system. Stocking separated large juveniles, as opposed to stocking a whole mixed population in intensive monoculture, improved yields (1367 and 1127 kg·ha^-1 respectively); average weight (47 and 40 g respectively); and the percentage of large market size prawns (61 and 48% respectively). Almost all of the small juveniles stocked in polyculture reached the large market size (54 g average) with a survival rate of 86%. The two methods of selective stocking of size subpopulations provide the means to maximize the percentage of marketable prawns within a single growout season, under the constraints of the temperate zone winter.