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Thursday, 1 March 2012

Marks seen on the echo sounder - part 1


Of all the instruments available to the skipper in the wheelhouse, the electronic echo-sounder still plays a major role in the detection of fish and reading of the seabed or 'botttom'. Early models, using sound waves beamed straight down from the hull of the boat, used a roll of paper that continually moved across to display the results received by the transducer from the bottom. Today's meters display in colour on a screen - pale blue being the weakest signal return all the way through to blood red indicating something much more solid.  A soft bottom like mud shows up as a thin red line with a fat orange/yellow section, a harder bottom loses the yellow and becomes a thick red as the hardness increases.


In this photo taken aboard the Ajax as she steamed for Newlyn two large marks of fish - thought to be herring - can be seen. The large white number indicates the depth of the water in fathoms - for those not familiar, a fathom is six feet or 1.82 metres. That means that the shoal of fish is around 15 times a fathom high - around 90 feet - that's a lot of fish! The screen is split with the lower half showing an expanded section from the bottom up to 3 fathoms - a typical working configuration for netters and trawlers wanting to interpret the water under the boat.