Opto-electronic counters for juveniles of macrobrachium rosenbergii (de Man): Supporting equipment and use of counter B under commercial hatchery conditions

Abstract

To count postlarvae or juveniles of M. rosenbergii with the high speed opto-electronic Counter B, a clean water supply, a system for delivery of juveniles into the counting device, the collection of counted juveniles and partition of these into containers for delivery to ponds, are required. Juveniles are collected from rearing tanks without contamination by particles of detritus, which would otherwise be counted. The infrastructure, counter performance, juvenile handling procedures and survival tests done on machine-counted juveniles are described. Counting speed (juveniles per hour), counting errors and labour needs (men) are: 200 000, <1% and two for machine counting; 10 000, >2% and six for hand counting; a supervisor is required in both cases. Survival of machine-counted juveniles is the same as that of hand counted ones, indicating that the electronic counter and methods of using it have no adverse effects on juvenile prawns of this species. It is believed that this counter is unique, and that it has wide applicability to counting various aquatic organisms.