MIGRATORY DRIFT OF LARVAL FRESH-WATER SHRIMPS IN 2 TROPICAL STREAMS, PUERTO-RICO

1. Migratory shrimps are often major biotic components of tropical str eam communities, yet spatial and temporal patterns of their migration have yet to be described. This information is of increasing importance given the continued fragmentation of tropical streams by damming and water abstraction/diversion, which can disrupt migratory life cycles. 2. Larval amphidromous shrimps are released by adult females in freshw ater streams. They then drift passively to an estuarine habitat where they metamorphose before migrating back upstream. Drift of larval shri mps was sampled over two to five 24-h periods at each of three sites a long two rivers that drain the Luquillo Experimental Forest in Puerto Rico: the Espiritu Santo (10, 135 and 335 m a.s.l.) and the Mameyes (1 0, 90 and 290 m a.s.l.). A total of seventeen diel samplings were cond ucted. 3. Shrimp drift increased in the downstream direction in both c atchments, and had a significant positive exponential relationship wit h length of stream channel above each site. There was no significant d ifference between catchments with respect to mean daily drift rate per km of stream channel. Maximum observed larval shrimp density was 69 1 02 larvae 100 m(-3) (1.7 g dry mass 100 m(-3)), which is high relative to published invertebrate drift studies. 4. The pattern of shrimp dri ft agreed with the 'risk of predation hypothesis'. in stream reaches w ith predatory fish, drift of larval shrimps occurred at night and was slight during the day. A nocturnal peak in drift occurred between 19.0 0 and 22.00 h. At a high-altitude site, where predatory fish were abse nt, no diel pattern was discernible. 5. The present study provides inf ormation on the timing of migratory drift of larval shrimps, which can minimize the adverse effects of water abstraction from streams draini ng the Luquillo Experimental Forest. Elimination of water withdrawal d uring peak larval drift after dark will significantly reduce shrimp mo rtality.