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Computer Technology Tracks Movement of Larval BillionsIt's difficult enough trying to track the movement of a fully grown fish in reef waters. But monitoring billions of fish larvae, each only millimetres long, from reefs over thousands of square kilometres in the Great Barrier Reef has been quite impossible - until now.
Years of painstaking research by Dr Lance Bode and co-workers at James Cook University, and optimum use the latest in computer technology, mean that reef managers now have a much better chance of knowing where tiny animals are transported throughout Barrier Reef waters. Computer simulations developed by Dr Bode and his colleague, research engineer Luciano Mason, illustrate only too well how complex the movement of fish larvae can be, and how risky it is to make general simplistic assumptions about what happens to these larvae. Individual larvae on reefs that are only a short distance apart can be carried varying distances, sometimes in opposite directions and at very different speeds. Mr Mason turns to his computer screen to illustrate this. The top half shows an outline of Green Island reef offshore Cairns, the bottom half depicts Scott Reef, 40km to the south. He presses a key and a red mass of larvae near Green Island starts to move north. A similar mass near Scott Reef moves only slightly at first, back and forth. A few minutes later, the animation, which represents 28 days in real time, ends. The first mass has moved a considerable distance north-west, parallel to the coast while the other has moved south. Each patch has remained largely intact, but has changed from a blob to a filament that stretches over a large distance. A few clicks of the computer keys, and Dr Bode and Mr Mason call up similar demonstrations for nearby Opal Reef, which results in a much more fragmented dispersal pattern by the end of the simulation. These animated demonstrations can provide reef managers with vital information about the movement of fish larvae and other small organisms. Scientists believe fish larvae are spawned on high tides and broadcast into currents but understanding the dispersal of larvae is crucial for the management of fish stocks. "In order to manage protected areas, we have to know whether the dispersal and subsequent recruitment of fish larvae occurs along specific pathways - the so-called 'source-sink hypothesis' ," Dr Bode explains.
The computer simulations provide this information, quickly and clearly. However, the simplicity of these short animated computer sequences belies the time and effort involved in getting to this stage. Reliable predictions of the movement of small animals or particles in Great Barrier Reef waters requires not only the computation of complicated current patterns generated by winds and tides, but also the even more complex interaction of these currents with individual reefs. "Modelling larval dispersal involves solving complicated sets of equations. In the Great Barrier Reef, the big problem is that the water has to flow through the reef matrix, which is an incredible maze-like structure, and hard to come to grips with in a computer model. We have had to work out how to describe the effects of the Reef in a mathematical sense. Although some water flows over reefs, most moves through the channels between reefs," says Dr Bode. In the model, each region of the Great Barrier Reef is divided into grids. The geometric outline of each reef in the grid, and the depth of water around it, have been incorporated into the mathematical equations, using new procedures that been developed to cope with the complex geometry of the Great Barrier Reef. The model can produce the current velocity and height of surrounding water, at any given time and place. "We see the effect of the reef matrix in the area south of Mackay," Dr Bode says. "With the rise and fall of the tides, water rushes into this region from opposite ends of the dense reef barriers. The result is the huge tidal variation observed in the Broad Sound region." Working out the height, speed and direction of tides around and over individual reefs and groups of reefs is difficult enough. But the tides are only one part of the problem. Since they are cyclic in nature, tides don't play a key role in carrying larvae from one area to another - they basically move things back and forth. The effects of winds and ocean currents contribute more significantly to net movement, and so must also be considered. The South Equatorial Current, that flows into Eastern Australia, bifurcates when it reaches the outer barrier reef, with one half moving north along the coast and the other south, to become the East Australian Current. The point of bifurcation can move a considerable distance over time. "The East Australian Current tends to affect the outer part of the Reef region, and there are inter-annual changes in the way this current operates. Currents closer to shore are dominated by south-east trade winds." Dr Bode first developed computer modelling techniques about 12 years ago when marine scientists and managers were mostly interested in working out how and where crown-of-thorns larvae were transported. "We have now been able to refine these techniques, including the impact of ocean currents, because computers are much faster. It is now feasible to look at areas like the whole Barrier Reef, rather than just small sub-regions." Although this stage of their research project has now been completed successfully, Dr Bode and Mr Mason are not resting on their laurels. "We feel that the challenge of getting to grips with the oceanography of the Great Barrier Reef is only the first step," Dr Bode says. "We're now investigating the role that larval behaviour can play in transport processes. Marine biologists at JCU have shown that these tiny larvae are capable of swimming quite enormous distances. In our latest investigations, we are examining the consequences for fish dispersal processes of incorporating directed larval movement. By Colleen Davis |
