Spatial analysis of seabed trawl marks, Frederick Patches, northern Great Barrier Reef
November 01, 2011

Authors: Robelly, M., Beaman, R.J.

Year: 2011

Publication: SEES Research Student Conference, 1 Nov 2011. School of Earth and Environmental Sciences (SEES), James Cook University, Townsville, Australia.


Trawling marks are a typical footprint of human activity, and more precisely of fishing activity, on the seabed. There are many consequences to the removal of habitat-forming benthic organisms, and so marine management agencies are concerned about the recovery of areas affected by bottom trawling. Here we conducted a spatial analysis of trawl marks identified in a broad-scale (43 km2) sidescan sonar mosaic in the vicinity of Frederick Patches, northern Great Barrier Reef.

This work was divided into three phases, starting with a process of improvement of mosaic image quality in ArcGIS, followed by the delineation and mapping of individual trawl scours and then classifying of the trawl marks into High Density and Low Density areas. The third phase consisted of an evaluation of the affected proportions within and outside of the Great Barrier Reef Marine Park Habitat Protection zones.

The results clearly show trawl marks, presumably due to the effect of otter boards used in holding the trawl net apart. High density scours occur inside an area of about 1.19 km2 and low density areas of about 1.87 km2. The combined affected area is 3.05 km2, or about 7.1% of the total mapped area. The affected areas can be divided into about 2 km2 inside the Habitat Protection (non-trawling) zone and 1.06 km2 inside the General Use (trawling) zone.

The implications of this study are the accurate positioning of trawl marks, which could be revisited for studying benthic substrate recovery. It is also clear evidence of earlier trawling inside Habitat Protection zones. This study therefore leads to greater understanding of the spatial nature of habitat impact and future resettlement of fishes or other species. Consequently, the improvement of the monitoring for fishing vessels operating within the Great Barrier Reef is essential to help curb this problem.

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