Influence of marine and freshwater connectivity on the dynamics of subtropical estuarine wetland fish metapopulations

Estuarine wetlands have received little study in dry tropical and subtropical areas despite the fact that unpredictable rainfall makes their temporal continuity tenuous. In fact, many of these wetlands exist for much of the time as isolated pools that are remnants of extensive areas of shallow water that occur during infrequent flooding. Among the most important functional components of estuarine wetlands are the multifaceted connections between wetland units, and between wetlands and adjacent habitats, that maintain wetland integrity and allow fish to access them as nurseries. As the principal determinants of the likelihood of recolonisation, these connections are critical to metapopulation dynamics. To understand the ways in which the unique patterns of connectivity in these wetlands influence metapopulation dynamics and the compositions of fish fauna, we investigated patterns of physical and biological connectivity among estuarine wetland pools in the Fitzroy River delta of Australia’s dry subtropics. Ten wetland pools ranging in salinity from fresh to hyperhaline, and differing in periodicity of connection from weeks, through months, to years were sampled regularly from February 2004 to May 2005, using cast nets. There were 4 distinct categories of pools; isolated freshwater pools, in-stream freshwater pools, regularly connected poikilohaline pools (pools where salinity varied over time from close to fresh to hyperhaline) and infrequently connected poikilohaline pools. The catch per unit effort and size structures of fish fauna of the different pool categories reflected connectivity, which determined the extent to which both marineand freshwaterspawned species could re-colonise pools following local extinction. Overall, factors related to recolonisation were more important than those related to extinction in determining fish assemblage structures. Moreover, in contrast to many other metapopulation situations many extinction factors were heavily influenced by connectivity rather than being independent of it.