Predation might cause the disparate distributions of fish and shrimp in Puerto Rican streams. Adult shrimp (Atya lanipes) are present only above waterfalls that pose barriers to predatory fishes. Because we have not found Atya in fish stomachs, we think that these amphidromous shrimp avoid predation by migrating to locations above waterfalls as post-larvae. We designed experiments in an artificial y-maze and in natural streams to test whether Atya avoid predatory fishes. In artificial streams, adult Atya avoided the scent of 2 fishes (Gobiomorus dormitor and Agonostomus monticola) regardless of whether the fish had recently been fed Atya, but they avoided the scent of Anguilla rostrata only when they had recently been fed Atya. Postlarval Atya avoided the scent of G. dormitor fed Atya and of crushed conspecifics. We then tested whether adult Atya in 4 natural headwater streams would redistribute themselves if exposed to the chemical scent or physical presence (visual, mechanical, and chemical cues) of G. dormitor. We expected Atya to move upstream from the manipulated pools, resulting in increased emigration from and lower Atya abundance in the pool. Atya relative abundance did not decline significantly in the pools when fish scent was added, indicating that chemical cues alone were not threatening enough to alter adult behavior in natural streams. The relative abundance of adults declined significantly in the pools when fish were added to in-stream cages, a result suggesting that addition of mechanical or visual cues to chemical cues signaled imminent danger and caused Atya to alter their behavior. Emigration increased, but lower activity levels also could have reduced abundances in pools. Our study provides mechanistic evidence that predatory fishes alter the behavior of postlarval and adult A. lanipes, and might explain why adult Atya are absent in reaches downstream of waterfalls.
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