Surprised no-one has commented on my earlier post - even if to rubbish it!
The leading support spine of a barbels dorsal fin has a serrated edge to it. I think these serrations probably help that rigid ray bond to the soft rays. If the line catches between this rigid ray and the soft rays it will be trapped and cut in.
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If you ever find a barbel with this damage test this area for yourself - you may be surprised. I certainly found out the hard way!
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I am sure we have all seen the occasional barbel with damage in this area. As they rummage along looking for your bait the line goes over their head and spooks the fish. Its dorsal fin pops up and the fish runs - trapping the line.......SNIP!
Seems a very likely scenario - any opinions on this?