Importance of marine-estuarine-riverine connectivity to larvae and early juveniles of estuary-associated fish taxa

The dependence on connectivity and use of estuaries by two major groups of fishes, namely estuary-associated marine and diadromous species, are reviewed. The former group comprises marine estuarine–opportunists and marine estuarine–dependents, and the latter anadromous, catadromous, and amphidromous species. Examples of ingress to estuaries by larvae and juveniles of species from each group illustrate the importance of freshwater-estuarine-marine connectivity to the life cycles of those species. Factors that threaten estuarine connectivity, including the potential/possible consequences of global climate change on the contribution of these key taxa to coastal fish assemblages at pristine or recent historical levels, are highlighted. The implications of reduced connectivity on the current and future status of these major groups in estuaries are also discussed. Finally, it is noted that the abundance of fishes in the above guilds has already declined substantially and that there are no clear prospects for a reversal of this trend. Possible future research on fishes and coastal connectivity include applications of environmental DNA and otolith microchemistry, and the assessment of fish responses to the removal of dams to restore connectivity in catchment rivers.