Hook, line and sonar: finding fish is high-tech Fish finder: Master's student Matthew Holland checks the sonar. Forget the hook and line, one Sydney scientist is using sonar to track fish. At Narrabeen Lagoon on the northern beaches, ecologist Alistair Becker lowers his $120,000 sonar in shallow water at the mouth of the lagoon. "It's like a really high-tech fish finder," he says. The long cylinder-shaped device allows the University of NSW researcher to watch individual fish as they move, often against the tides, between the open ocean and the estuary. The world-first research project offers a unique insight into the daily behaviour of the lake's fish, a task previously impossible without high-resolution sonar technology. "We can collect all this information about the fish, their size and their abundance, without harming them," Dr Becker, an associate lecturer, says. At present scientists only know broad patterns of fish behaviour - many marine fish spawn in the ocean and then their larvae move into estuaries - but little is known about their daily movements between the two vastly different habitats. "It's really important to see how much movement there is between them ... We can use that information for managing commercial fisheries and for conservation," he says. The sonar can detect fish bigger than 10 centimetres and those swimming up to 20 metres in front of the sonar's beams. On a laptop, the data displays as images that look like an ultrasound. Since June Dr Becker and his master's student, Matthew Holland, have collected about 36 hours of footage at seven frames a second. "Video allows us to view how fast they're swimming and their behaviour," he said. Unlike the sonars typically used by recreational fisherman, which have only one sonar beam, Dr Becker's device has 96 beams and meant the pair could see fish school, feed and sometimes attack each other. "And because it is using sound to create videos not light, we can obtain footage in the middle of the night," he says. Narrabeen is a popular fishing location and the insights from Dr Becker's experiments there will give fisheries managers more detailed information about the area's population dynamics. Its narrow opening that intermittently closes also provides the perfect trial location for the study as about 60 per cent of estuaries in south-east Australia are intermittently closed.