Influence of Migration Barriers on Tropical Diadromous Fishes

Recent research demonstrated impacts of large dams on migrations and life cycles of native amphidromous fishes in Puerto Rico.  However, little is known on impacts of other artificial barriers to fish passage (small dams and road crossing) in tropical island river and stream corridors.  With no database of artificial stream barriers, we set out to find and document these barriers to assess their influence on distribution and abundance of native fishes.  We surveyed a total of 311 manmade barriers to native fish migration, comprised of 36 high dams greater than 17 m above the downstream water surface (height for fish to traverse), 190 low dams less than 17 m in height, and 85 road crossings.  In total, 74% (6,773 river km) of riverine habitat is impeded by artificial barriers in Puerto Rico, creating a disconnect of upstream habitat from the ocean.  The remaining 26% (2,414 river km) of riverine habitat is almost exclusively at lower elevations, limiting higher altitude stream habitat availability.  We sampled fish across the island from 123 locations, incorporating all 46 major river drainages in Puerto Rico.  From these samples, we confirmed that while high dams completely exclude all native fish species from accessing upstream habitat, some low barriers appear to allow passage of several native fish species.  Understanding the influence of all artificial stream barriers on native fish passage will aid in increasing stream habitat availability and enhancing freshwater fish communities in Puerto Rico.