A
Right: Two identical visual outputs from the DIDSON with the static background (substrate) present (A) and the background removed (B). The output provides a ‘bird’s-eye view’ of the fan shaped sector illuminated by the DIDSON, scale is in metres horizontally away from the DIDSON. A school of mugilids (between 200 and 300 mm) is highlighted, their acoustic ‘shadow’ can be clearly seen in image (A). A group of small shoaling species (< 100 mm) can also be seen in image (B) once the background has been removed.
B
DIDSON in operation in the East Kleinemonde Estuary.
Image: Alistair Becker
The DIDSON The DIDSON was developed by the University of Washington’s Applied Physics Laboratory and was designed to be used by the US Navy for underwater surveillance and uses lenses that focus returning sound beams to create high resolution images. It became available to the scientific community around 2003. It operates in two frequency modes: a low frequency mode (1.0 MHz) which can be used at ranges of 50 -70 m, but produces poor resolution images. In high-frequency mode (1.8 MHz) the range is reduced to about 10 m. However, the images are at a much higher resolution. The DIDSON ‘looks at’ a fan-shaped arc across a 29º horizontal plane and 14° vertical plane. The 29° arc is broken up into 96 separate horizontal beams, 0.3° in width and 14° high. The beams themselves are composed of 512 longitudinal ‘bins’. So each image is formed from 96 x 512 ‘data values’. The longer the range the more stretched each of the 512 bins will be, resulting in lower resolution. The DIDSON works best when it is positioned still and aimed so the sound beams just skim across the bottom which allows fish swimming in the water to be easily seen. Scientists have used the DIDSON to count salmon migrating up rivers in the northern hemisphere, to check the effectiveness of by-catchreducing devices on commercial fishing gear and to study fish under ice. This is the first time one has been used in South Africa and also the first time one has been used to study fish in small closed estuaries which are common across the southern hemisphere.
For more information: Becker, A, Cowley, PD and Whitfield, AK. Use of remote underwater video to record littoral habitat use by fish within a temporarily closed South African estuary. Journal of Experimental Marine Biology and Ecology 2010: 391: 161-168.
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