Combining a Trawl Camera and Hydroacoustics for Verification of Species

We present a simple and portable self-contained underwater video camera system that can be used during trawl surveys to obtain non-destructive species composition information when the cod-end of the trawl is removed. This system allows commercially valued species such as hake to be assessed without capturing species of concern such as salmon, rock fish and marine mammals. The video camera records the layering of organisms in the water column and when combined with depth information from a trawl mounted CTD the images can be correlated with acoustic echograms to corroborate acoustic signatures. Experimental trawls with closed cod-end have shown good agreement between camera-based estimates of abundance and catches for species large enough to be easily observed. Parallel green lasers are used to estimate species size and these estimates compare favourably to catch information. Digital image recording allows extended or quickly repeated operation of the trawl nets. We recognise the value of developing real-time video imaging for trawling activities in the future, as currently the viewing of our images is delayed.