Shallow water seagrass fish communities of Intermittently Closed and Open Lakes and Lagoons (ICOLLs) of south-eastern Australia

The structure and dynamics of fish communities associated with shallow water seagrass habitats were investigated in estuaries along the south-eastem coast of Australia. The majority of estuarine systems in this region of southeast Australia are intermittently closed and open lakes and lagoons (ICOLLs). The fish communities within these coastal lakes have not been previously studied in detail. The sampling method employed in this study was aimed at the capture of new recmits, mainly small juveniles of marine and estuarine fish species using the seagrass beds as a nursery habitat, as well as species that were permanent residents of the seagrass meadows. Research was conducted over a three-year period in eight estuaries spanning approximately 500km of southem New South Wales (NSW). These estuaries are Lake Illawarra, St Georges Basin, Lake Conjola, Burrill Lake, Durras Lake, Coila Lake, Wallaga Lake and Merimbula Lake. Within seven of these estuaries, three locations representative of the entrance, central and upper locations were sampled quarterly for three years. Fish sampling was undertaken using a seine net (6mm stretch mesh, 25m long), primarily in Zostera capricomi seagrass beds. Environmental data was also recorded during each sampling event, and included salinity, conductivity, temperature, pH and turbidity. Overall, 101 fish species were found within the region. Within individual estuaries species diversity ranged from 41 to 61 species. The most specious families were the Gobiidae, Syngnathidae and Monacanthidae. The catch was dominated by large abundances of a few species, particularly from the Ambassidae, Atherinidae and Gobiidae families. There were significant spatial and temporal differences in the species diversity of seagrass fish fauna for each individual estuary. With the exception with Merimbula Lake, spatial and temporal differences in tiie abundance of fish were also evident. However, there was little consistency to these pattems, either within estuaries or between them. The location factor had little influence on fish diversity and abundance pattems, and there were no decline in species diversity and abundance with increasing distance from the estuary mouth. The largest percentage of variation was due to either lower species diversity or abundance found during the winter months, or large single catches of schooling fish species. Fish community dynamics were investigated by multivariate analyses. The results indicated that the fish species compositions within each estuary were highly similar on a spatial scale. There was little compositional change in the fish assemblages with increasing distance from the estuary mouth. Yearly changes in fish community composition were more noticeable than seasonal changes. It was found that the south coast fish fauna was composed of a "core" group of species that were caught consistently throughout time and were found at all locations. There were no 'restricted' species that had sfrong or distinct location-associations within these estuaries. Rather, some species may have had preferences for certain regions but were not restricted to them. Commercial fish species diversity and abundance pattems were investigated for six of the estuaries, and the recmitment dynamics of five major species were also examined for seven of the estuaries. The major findings were that there was a substantial increase in the strength of recruitment in 1998, and this was evident across all of the selected species and across all estuaries. This indicated that for these commercially important species, processes responsible for widespread and substantial increase in recruitment must be large-scale, and not restrictive processes confined to an individual species life-history characteristics or to a particular local area. Rainfall data from the south coast of NSW, and data relating to El Nino phenomena were discussed as the probable tiigger for the high recmitinent that occurred in the spring/summer of 1998. Overall, there were no consistent sites of recmitinent in these