HIGH DAMS AND MARINE-FRESH-WATER LINKAGES - EFFECTS ON NATIVE AND INTRODUCED FAUNA IN THE CARIBBEAN

Caribbean steams are dominated by a shrimp and a fish assemblage for w hich amphidromy (eggs or larvae carried to the ocean followed by migra tion of juveniles upriver) is suspected. Effects of of dams on this as semblage 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 per meability with respect to migration, (2) the extent to which permeabil ity is a function of spillway discharge, (3) the relative roles of nat ive 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 a nd juveniles of shrimps and fishes in Puerto Rican streams via electro fishing and sampled shrimp larvae with drift nets. Replicate stream re aches 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 permea ble, dams with spillway discharge has smaller populations of native sp ecies above these structures than below the dams or on undammed stream s. Our data on adult and larval distributions, combined with the absen ce 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 e xotic abundance was consistently inversely related to abundance and sp ecies richness of native fauna across all sampling categories. The pre valent amphidromy in these streams provides a tight marine-upland link age that is disrupted by dams in several ways. We recommend adding shr imp and fishways to dams on these tropical streams.