Summary

This report describes results from a modelling study of the NSW offshore marine ecosystem and its associated resources. It forms part of a broader collaborative study between CSIRO and the NSW Department of Primary Industries on ecologically sustainable development of the regional marine and estuarine resources of NSW.

This component of the study aimed to develop tools that could help address issues emerging along the NSW coast, particularly related to the ecological impacts of fisheries and the potential role of specific harvest regimes or conservation strategies. The tool that was adopted for this purpose was the Atlantis model incorporating:

• an ecosystem model based on physical, biogeochemical, and higher trophic level components and processes;
• a sector model that simulates the effects of fisheries and other human activities on the ecosystem;
• a sampling model that collects data from the ecosystem and sector models and calculates biodiversity indicators for the modelled components; and
• a management model that regulates the sector model.

The Atlantis model was implemented by representing the NSW coastline offshore to the upper continental slope as a series of interconnected polygonal boxes. The box structure was designed to reflect the biogeographical distributions of the region, as well as coastal characteristics such geomorphology, major rivers and bays, and human land-uses. The model was also resolved into a series of vertical layers to account for depth zonation across the continental slope. Physical exchanges between boxes were estimated from oceanographic model results.

The model calculated the distributions of various forms of nutrients and detritus over time, as well as the biomasses of 54 biological groups. These groups included plants, bacteria, invertebrates, finfish, sharks, marine mammals and birds. While most were broad functional groups, many commercial fish were represented at the level of individual species.

The main conclusions from the model runs were:

• The Ocean Trawl Fishery in NSW coastal waters is already fully exploited.
• Reduced fishing would allow a significant proportion of overfished predatory groups to recover with an accompanying decline in their prey (e.g. smaller fish species).
• Fishing levels seen in the 1970s and 1980s led to modelled biodiversity levels (as measured for the biological groups used in the model) that were consistent with very high fishing pressure.
• Marine Protected Areas potentially have a positive impact on sharks and rays and, by extension, a mainly negative impact on their prey groups. Shallow demersal fish and their prey can be impacted positively (if fishing effort previously in the reserve areas is removed, such as through buy-outs) or negatively (if fishing effort is displaced to neighboring areas).

The model will continue to be used to support sustainability of the NSW coastal and shelf system by running other scenarios (such as projections arising from climate change) and evaluating different management strategies.