You'd know about it if one of these picked up your mackerel tail...
And one who knows more about them than most is pike angler Dennis Smith - who believe it or not has also landed great white sharks weighing up to 3,500lbs.
Thames Gateway region are offering a rare chance to enjoy a talk by Dennis, covering his experiences of fishing for these incredible creatures and some incredible underwater video.
It's at the Papermakers Arms at Hawley, near Dartford, Kent, on Tuesday night (7pm). Admission is £3 PAC members, £4 non-members.
More on the Thames Gateway blog here.