Living in two worlds: Diadromous fishes, and factors affecting population connectivity between tropical rivers and coasts

Abstract

Among the large range of life history patterns of tropical fishes, about 200 species from 20 fish families undertake diadromous migrations. In most diadromous fish species, only a proportion of the population undertakes migrations and this proportion varies widely between species and families. Three different types of migration are anadromy, catadromy, and amphidromy. Tropical anadromous species are mostly clupeoids, including several shads and herrings. These species spawn in freshwater and migrate to the sea as juveniles, and most of the population matures there before returning to breed in freshwater. Catadromous species have the opposite behavior-they spawn in the sea before migrating to freshwater where they mature. Anguillid eels, mullets, and many centropomids from tropical regions are catadromous. The most common form of diadromy in the tropics is amphidromy. The largest groups of amphidromous fishes are the gobies and gudgeons. Amphidromous fishes spawn in freshwater and the larvae migrate to the sea before migrating back to freshwater, and are common on many islands of all the major oceans. The freshwater and marine components of diadromous fish populations rely on freshwater flows to maintain their connectivity. Most tropical diadromous fishes migrate between habitats during seasonal monsoonal floods. The construction of dams, and shifts in the intensity and reductions in the quantity of rainfall from changing climate are two of the major threats to maintaining connectivity between freshwater and marine populations. The examples of these effects presented here suggest that tropical diadromous fishes will face increasing challenges in maintaining their populations unless greater effort is made to facilitate their migrations. © 2009 Springer Science+Business Media B.V.