Year

2003

Degree Name

Doctor of Philosophy

Department

Faculty of Science

Abstract

The structure and dynamics of fish communities associated with shallow water seagrass habitats were investigated in estuaries along the south-eastern 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 recruits, 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 southern 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 the abundance of fish were also evident. However, there was little consistency to these patterns, either within estuaries or between them. The location factor had little influence on fish diversity and abundance patterns, 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 strong 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 patterns were investigated for six of the estuaries, and the recruitment 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 trigger for the high recruitment that occurred in the spring/summer of 1998. Overall, there were no consistent sites of recruitment in these ICOLLs, and patterns of settlement for all species at the three locations within each estuary were not consistent through time. However, there were small regional differences in the timing and magnitude of recruitment in the seven estuaries along a latitudinal scale.

The results from the large-scale sampling program indicated that local processes within individual ICOLLs may be a much more important factor in the structuring of these fish communities than differences in geomorphology or biogeography for these ICOLLs. To investigate these local processes in detail, a sampling program was undertaken in two estuaries. Lake Illawarra and Durras Lake. These two estuaries were chosen, as they are very different systems in terms of shape and size, entrance conditions and degree of catchment and lake usage. Sampling of the shallow water seagrass fish communities was conducted at thirty-two sites within each estuary every six weeks for one year. The high degree of similarity in the shallow water fish community around each estuary that was found in the larger sampling program was still evident at a finer scale of sampling, and there were still inconsistent patterns of settlement through time within each estuary. This intensive sampling program provided a clearer understanding of the effects of individual lake morphology and hydrology on fish community patterns.

Consideration of the information and conclusions from the two sampling programs have led to a number of hypotheses. It is proposed that lake morphology, individual hydrological characteristics and the local climatic regime are important determinants in the structuring of ICOLL fish assemblages in southeast Australia. The patterns in fish diversity and abundance, and in community dynamics were contrary to traditional concepts of estuarine fish patterns, which have been largely developed from research carried out in large riverine and permanently open estuaries, or in intermittently open estuaries where there are distinct seasonal rainfall patterns that influence the salinity gradients within the estuary. It would appear that the small size of ICOLLs in southern NSW, lack of seasonal rainfall patterns, the dominance of wind-induced transport rather than tidal-induced water transport, and the relatively stable salinity around the perimeter of the ICOLLs allows the widespread distribution of marine spawning species within these systems.

This research has direct implications for the sustainable management and conservation of ICOLL environments and their fish communities. These coastal lakes were found to be important fish habitats, particularly in terms of their role as recruitment and nursery areas for a wide range of marine and estuarine species. However there were few, if any, consistent patterns in species recruitment, diversity and abundance, either between locations within estuaries, between estuaries within the region, or between seasons and years. Local hydrology, morphology and climate affecting individual ICOLLs are the most important factors structuring fish communities. Currently these ICOLLs are managed using a generic approach, based on the assumption that different ICOLLs have similar ecological characteristics. Based on the results from this research, the most suitable approach for management purposes is to consider each estuary as a unique situation requiring a detailed and localised approach.

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