Ive seen barbel roll several times on my local river. I find it happens a lot during flood conditions.

I thought barbel rolled so they could see who was fishing for them and bugger off when they see its me

Originally Posted by thesaint999

I thought barbel rolled so they could see who was fishing for them and bugger off when they see its me

ah yes you got that right !!!

Originally Posted by ChrisD

Another fish return technique I have seen used, I think by fishery biologists, was to actually puncture the swim bladder with a needle, so as to release pressure. I have seen deep sea anglers do this when returning fish caught from deep water wrecks as their swim bladders can blow when hauled up from depth.

Fish species that live in shallow water tend to have the channel linking the gut to the swim bladder. Deep water species tend not to have the link.
And there is an evolutionary reason for this:

A fish moving from ten feet deep to the surface, would see a decrease in its environmental pressure of about 1/3. From about 20 pounds per square inch to about 15. This is significant and would tend to make the bladder try to increase its volume by that same 1/3. Fish changing depths rapidly in shallow water need their bleed valve to minimise discomfort, by equalizing swim bladder pressure.
By comparison, a fish 1000 feet deep, changing its depth by the same 10 feet would only see about a 1% change in pressure, and therefore a negligible change in its bladder volume, so it does not need the air escape channel. Bring that same deep water fish to the surface and its environmental pressure decreases by a factor of over 30. Catching very deep water fish is therefore usually fatal for them. I have seen pictures where the swim bladder has expanded so much it was protruding out of the mouth.