Aquaculture is a rapidly growing industry in Australia, and as such there are substantial resource allocation issues. South Australia is at the forefront of this development with a range of innovative aquaculture industries, an active group in PIRSA Aquaculture addressing policy and management issues, and another in SARDI Aquatic Sciences providing the scientific and technical background information for such matters through targeted research and development (R&D). As such, South Australia provides an ideal model for other States.
While a reasonable level of information exists and, through the Aquafin CRC, continues to grow for tuna farming, this is not the case for most of the other marine aquaculture industry sectors. A fundamental concern in managing these industry sectors is determining the level of production that a given area can sustain without undue effects on the environment. This can be done in two broad ways, the first is by experimentally increasing production and assessing the response of the environment through an environmental monitoring program. The second is through the use of comprehensive models to determine the expected nutrient inputs, under a given level of production in combination with pre-defined trigger points for the nutrients which we believe represent levels above which an environmental impact will occur. The second method is the focus of this project and its advantage is that it allows us to predict the optimal level of production in relation to the principles of ecologically sustainable development. The development and refinement of such models will provide tools to assess the consequences of management responses, allowing a more considered approach to the expansion of the aquaculture industry. Another result will be greater certainty of resource access for industry, which should encourage investment in South Australian aquaculture.
This project will build upon and support the project “Innovative solutions for aquaculture planning and management – Project 5, Environmental audit of aquaculture developments in South Australia”. Both projects will provide much of the scientific and technical data for input into the project “Innovative solutions for aquaculture planning and management – Project 1, Decision support system for aquaculture development”, where “Decision support system” is defined as a computer based, integrated method for supporting management decisions. Decision support systems must incorporate rigorous and scientifically sound decision criteria and, as such they require a good understanding of the potential environmental impacts that may result from aquaculture, as well as the characteristics of existing or future farm sites and the ecosystem in which they exist.
Final report
As aquaculture continues to grow, both in South Australia and elsewhere, it is becoming increasingly necessary to understand how wastes are circulated through the environment, both to minimize environmental impacts, and to minimize feedbacks that may reduce production. This information will also allow direct comparisons of nutrient inputs to the marine environment between aquaculture and other industries. In this project, we develop budgets for both nitrogen and phosphorus derived from feed inputs into a yellowtail kingfish pen in Fitzgerald Bay. These are then used as the basis for a ‘carrying capacity’ model, which can be used to predict the extent of increased nutrient loadings that will be observed with increases in production. In addition, we refine a model of carbon deposition, and apply it to the same location, to examine likely patterns of aquaculture-derived sediment on the seafloor. To underpin both the nutrient budgets and the models, we also conducted a series of laboratory experiments on fish metabolism (focusing on yellowtail kingfish, but with some work on mulloway), and undertook field investigations of nutrient cycling at Fitzgerald Bay.
The physiological work focused on determining the oxygen consumption of both yellowtail kingfish and mulloway under a variety of environmental conditions. To do this, an existing flume tank was modified into a flume respirometer, that allowed fish of up to 3 kg in weight to be swum against a constant current in an airtight environment, allowing decreases in water oxygen concentrations to be measured. This drop in oxygen then allowed the amount of energy the fish was using to swim at a given speed to be calculated. Using this information, it was also possible to determine how much energy the fish needed simply to maintain itself in a resting state. This information is important for the modeling, as it allows us to estimate how much of the feed inputs are actually metabolized by the fish and released into the water as carbon dioxide, versus how much is released as either dissolved nutrients or solid wastes. Because of problems with the original flume, a second smaller flume was obtained to conduct more detailed studies on YTK. As well as oxygen consumption, these experiments allowed the maximum sustainable swimming speed of both species to be calculated, and for mulloway, the response of fish to lowered oxygen levels was assessed. This experiment showed that mulloway could survive at very low oxyen levels (<20% of saturation), although their metabolic performance suffered when saturation levels dropped below 50%.
The nutrient budget work showed that an annual production of 2,000 tonnes of YTK in Fitzgerald Bay will lead to the release of ~ 400 tonnes of nitrogen and 100 tonnes of phosphorus into the environment. Most of the nitrogen released is in the dissolved form, while most phosphorus is released in particulate form. The nitrogen figure compares to a discharge of ~1,100 tonnes of N from southern bluefin tuna farming off Port Lincoln, 48 tonnes from the Whyalla wastewater treatment plant, and 210 tonnes from the Whyalla steelworks. The carrying capacity model suggests that an additional 1463 tonnes of YTK can be produced in Fitzgerald Bay annually, on top of current production levels of ~2,000 tonnes, before existing water quality guidelines are breached.
The carbon deposition model predicts that areas of high sedimentation are very localized around individual pens. The majority of wastes are dispersed in a north-south direction, with southward dispersal being predominant. There is very little dispersal in an east-west direction. As a result of the tight deposition patterns, increased sedimentation rates outside of leases would only be appreciable if pens are located very close to the lease boundary.
Keywords: Aquaculture, carrying capacity, nitrogen budget, phophorus budget, sedimentation, oxygen consumption, yellowtail kingfish, mulloway.
