Spatial and temporal variability of seagrass fishes in intermittently closed and open coastal lakes in southeastern Australia

A large-scale survey of seagrass fish communities was carried out quarterly over a three-year period in six coastal lakes with entrances that were intermittently closed and open to the ocean. These coastal lakes, located along ∼400 km of the southeastern Australian coastline, ranged from 4.2 km2 to 38.9 km2 in water area, but experienced a similar range of salinities, temperatures, pH and turbidities over the study period. A total of 203 936 individual fish, belonging to 46 families and 93 species, were caught and released, with the most diverse families being the Gobiidae (12 species), Monacanthidae (9 species) and Syngnathidae (7 species). There were major changes in numbers of fish caught between years, accompanied by changes in the compositions of the species assemblages. These changes were associated with heavy rainfall and flooding in the region during the second year of the study, which caused the opening of all lakes to the sea, and allowed recruitment of marine spawning species. This inter-annual variability was much larger than any of the seasonal or spatial differences observed, which were inconsistent within and between the studied lakes. The salinity regime of individual lakes played a role in structuring the fish assemblages on a regional scale, rather than any obvious biogeographic factors. Artificially opening the entrance of one lake, to alleviate flooding, resulted in the loss of a large seagrass bed and a subsequent decline in recruitment of economically important fish species to that area. The research suggests that, for these small estuaries, conservation of fish diversity might best be achieved on a “whole-of-estuary” basis, rather than by zoning of areas within individual lakes. It also indicated that artificially opening the lakes, or carrying out other entrance works, should be done with great caution as the impact that these activities may have on their fish communities remains largely unpredictable.