Mapping seagrass in the Swan-Canning Estuary using sidescan sonar

Seagrass distribution and other measures of seagrass condition have been used as broad scale biological indicators of marine and estuarine health. To date the distribution of seagrass in the Swan-Canning Estuary has been assessed based on aerial photographs, which is ineffective in deep, turbid waters. Sidescan sonar systems (SSS) provide acceptable results in a much wider range of environmental conditions, particularly as they are not affected by the turbidity or water depth of the river. A SSS was successfully used to collect backscatter data from 10 survey areas in the Swan-Canning Estuary ranging from 0.5 – 15m in depth. The SSS was able to distinguish areas of seagrass from bare sediment. A simple threshold classification method was used to produce geo-referenced habitat maps from each of the 10 areas showing the predicted distribution of low and high density seagrass, sand, mud and rock. The SSS is considered cost-effective and efficient for similar studies in other shallow water estuaries provided it is conducted in conjunction with ground-truthing at selected reference points.