Representatives of the tuna industry have expressed a clear need to understand the oceanography of the Port Lincoln tuna farming region so that they could predict likely trajectories and effects of phytoplankton blooms and any instances of dirty water on tuna farming, enabling them to develop emergency management protocols for such events. They also need to identify the range of variation in environmental conditions in both the water column and sediments within the farming zone and how this might impact on farming practices, particularly if fish are to be kept for longer periods and if stocking numbers increase. In the event of an emergency (e.g. algal bloom, oil spill, disease outbreak), the industry needs to identify areas likely to be impacted and safer areas where pontoons might be relocated. Industry would also benefit from real-time weather and oceanographic information being available at the farm site to optimize visits to the farms for feeding, maintenance etc.

To meet this need, we propose to develop an integrated hydrodynamic and biogeochemical model of the tuna farming area, based on that which CSIRO have developed for the Huon Estuary. Based on the knowledge that CSIRO have gained from this previous study, and our knowledge of the data available for the Port Lincoln area, a number of knowledge gaps have been identified that need to be addressed before we can have confidence in the outputs of any model that is developed. These gaps include the extent and nature of benthic-pelagic coupling in the area (ie how do nutrients move between the water column and the sediments), the nature and role of phytoplankton in the system, the nature of the hydrodynamic regime in the area, and how prone the sediments are to resuspension under different wind and wave regimes. While it will still be possible to develop a basic modeling framework without addressing these knowledge gaps, we feel that doing this will result in the development of a poorly calibrated and inadequately verified model that could provide misleading results, possibly resulting in decisions by farmers and managers that are detrimental to the industry.

The industry depends on a healthy marine environment to maintain production and profitability. Tuna farms are a significant point source of nutrients into the marine environment (most of the nitrogen in the feed is excreted as ammonia), but this needs to be put into the context of natural nutrient levels and their seasonal variations. It is important to establish the carrying capacity of the farming region which both maintains a healthy and productive industry and protects marine environmental values. Phytoplankton blooms, including potentially harmful species such as Chatonella, are known in the Port Lincoln area but the causative factors are poorly understood and the influence of advection from regions further off-shore (e.g. upwelling cells) is unknown. Thus the risk they pose to the industry is unknown, and needs to be assessed.

Sediments are a key site for regeneration of nutrients back into the water column, but their quantitative importance in nutrient recycling in the Port Lincoln area is presently poorly known. Also, sediments are resuspended by waves and tide presenting an additional threat to penned tuna through gill damage, reduced feeding and as a possible source of harmful algal species from released cysts. The consequences of sediment resuspension can be dramatic as seen in the high mortality associated with the April 1996 event when tuna were held in pens located in shallow water inside Boston Island. While this is less likely with current farming practices, it would still be wise to properly asses the threat of a repeat event. Alternatively, during milder resuspension events, organic wastes may be dispersed over a larger area, allowing them to be assimilated more rapidly. The dropping of the nets to the seafloor at the end of the farming season may also act to disperse wastes and increase their assimilation.

This combination of issues and risks constitutes a significant scientific challenge. While we must continue to guard against any major environmental changes due to eutrophication, it is clear that more subtle environmental effects (phytoplankton bloom frequency and composition, zooplankton and jellyfish swarms, sediment resuspension) may affect industry production and profitability. Equally, regulators need assurance that marine ecosystems will not undergo unacceptable environmental change as industry develops. Industry and managers not only require knowledge of environmental changes, but a capability to resolve and predict the environmental response to changes in the offshore ocean regime, changes in catchment loads, and effects of the industry itself. Advances in observation technologies, in scientific understanding, and in modelling capability, together make it possible for scientists to develop the knowledge, understanding and prediction needed to underpin both long-term planning and short-term operational decisions. The goal here is for the CRC to work with industry and managers to provide an environmental information and prediction system which allows industry and regulators to manage environmental risk.