Amphidromy and Migrations of Freshwater Shrimps. Ii. Delivery of Hatching Larvae to the Sea, Return Juvenile Upstream Migration, and Human Impacts

Hatching (Stage-1) larvae of amphidromous shrimps do not feed and must reach salt water within a few days to molt to Stage 2, the first feeding instar. Stage-1 larvae are transported from to the sea after upstream hatching by drifting in stream flow or are carried to estuaries for hatching by females migrating downstream. Hatching usually occurs during seasons or periods of high stream flow. After development in the sea, newly metamorphosed benthic postlarvae (juveniles) must find stream mouths and migrate upstream to the adult freshwater habitat. Such migrations are striking, occurring during periods of low but continuous flow, with many juveniles walking or swimming alongside the shore at night. The migratory behavior is a positive rheotaxis, with downstream river flow the directional cue. Juveniles are capable of climbing over or around low obstacles in their path provided that there is some downstream flow. Both larval drift and juvenile migrations are blocked by high dams without passageways and by the reservoirs behind them. Water extraction from streams is a significant source of larval mortality. Human impacts can be mitigated by appropriate conservation measures, e.g., restriction of water extraction during periods of larval abundance, and construction of passageways up and around dams and reservoirs to allow juvenile migration to upstream habitats.