Evaluating baseline conditions and resulting changes in demersal fish communities of South East Australia

Knowledge of the full extent and severity of ecological changes in human-influenced ecological systems is needed to identify today's management priorities and to set realistic restoration objectives for the future. However, reconstructing ecosystem baselines and understanding the causes and rates of ecological changes is often hampered by the scarcity of data about population and community status before exploitation, and by the challenges of fitting these (historical) data into modern analytical methods. This study aimed to identify the main sources of fishing impacts on marine communities of South East Australia and to gather and examine historical data available for the region that can provide information on baseline (pre-fishing) conditions to compare changes in fish communities as the fishing industry developed. South East Australia provides an ideal case study as its history of exploitation is relatively recent and there were surveys undertaken prior to exploitation. While a range of commercial fisheries have evolved in South East Australia since European colonization, during the last century bottom trawling has been the major fishing activity in the region. Bottom trawl surveys that included information on demersal species abundance and community structure were carried out both before and at different stages since trawling exploitation begun, and thus provide insights into the full extent of fishing impacts on South East Australian demersal fish communities. These surveys, covering the period 1898-1997, were performed by various research agencies, which collected and organized catch and effort data in different formats Additionally, the detail of the information reported changes across surveys and over the years. Hence, the initial need was to collect, digitalize and standardize all the information available. The bottom trawl survey dataset resulting from this step contained a total of 3,083 tows sampling 574 species among chondrichthyes and osteichthyes. It spans the entire history of trawling exploitation and is analyzed in this study as an entire dataset for the first time. A comparison of pre- with post-trawling exploitation data (1898-1910 and 1980s- 2010s, respectively) revealed marked changes in the structure of demersal fish communities of South East Australia. These included shifts in the catch composition of the main families, as well as sharp declines in the total and individual family catch rate, most likely related to the effect of fishing. Among the steepest declines were those of key commercial families, such as flatheads and morwongs, on the continental shelf of Tasmania. The effect of trawling on demersal fish communities of South East Australia was also revealed by the application of species accumulation curves to the survey dataset. Specifically, the rate of species accumulation with area decreased as trawling intensity increased, suggesting that trawling modified community structure through the removal of particular species and through changes in the abundance and spatial distribution of the remaining species. This study's findings have direct application to management and monitoring of the natural resources in South East Australia, and important implications for sustainable use and conservation prioritization. The study also provides a framework and approach that can be of guidance for the collection, standardization and analysis of analogous, patchy, unbalanced and overlooked historical datasets around the world.