High dams and marine-freshwater linkages: Effects on native and introduced fauna in the Caribbean

Caribbean steams are dominated by a shrimp and a fish assemblage for which amphidromy (eggs or larvae carried to the ocean followed by migration of juveniles upriver) is suspected. Effects of of dams on this assemblage are likely to demonstrate complex interactions as a function of reproductive strategy and type of dam structure. Our goals were to determine (1) whether high dams reduce or eliminate steam corridor permeability with respect to migration, (2) the extent to which permeability is a function of spillway discharge, (3) the relative roles of native fauna and disturbance suppression (in this case, river regulation) as predictors of success by exotic fishes, and (4) the uniformity and extent of obligate amphidromy in this assemblage. We sampled adults and juveniles of shrimps and fishes in Puerto Rican streams via electrofishing and sampled shrimp larvae with drift nets. Replicate stream reaches were assigned to the following five categories: undammed, above or below dams, and with or without water released over spillways. Dams without such discharge were impermeable barriers that eliminated all native fish and shrimp fauna from upstream reaches. Though more permeable, dams with spillway discharge has smaller populations of native species above these structures than below the dams or on undammed streams. Our data on adult and larval distributions, combined with the absence of first-stage shrimp larvae, indicate that amphidromy is obligate for most of the native fauna. Disturbance regime appeared to be a poor predictor of successful invasion by exotics in this system, whereas exotic abundance was consistently inversely related to abundance and species richness of native fauna across all sampling categories. The prevalent amphidromy in these streams provides a tight marine-upland linkage that is disrupted by dams in several ways. We recommend adding shrimp and fishways to dams on these tropical streams.