I'm looking forward to the video!! Your mate was lucky to have it on for so long, mine have always dropped off much sooner!
I've had two encounters with porbeagles foul hooked on mackerel feathers. The first was in August 2005 and the shark dived straightening one of the feather hooks - 20 minutes later one of the shark rods went off, which was my first shark at 6' 10" long and an estimated 248 lbs. - I would guess this was the same fish I'd encountered just before on the feathers.
The second encounter was on the 29th July this year, shark fishing on the 9 mile mark. We'd been at anchor for about 5 hours when my friend was at the stern of the boat and suddenly recoiled - WHOA! A porgie had come up the dubby trail right to the back of the boat and all but made him crap himself!. I was mackerelling at the time and wound the rod in, so I could work one of the closer shark rods to see if I could get it to take. No sooner had I started to wind the mackerel rod in, the rod buckled over and I found myself hooked onto the porgie on 20lb class gear. This one stayed on for a couple of minutes before the offending hook pulled out. Unfortunately it didn't come back and take a shark bait - so it was a blank on the sharking front that particular day.
On both occasions, I definitely had no mackerel on the line, but I can easily imagine a shark having a go at a struggling mackerel. Porgies can be attracted to the boat by the vibration of hooked fish and often lurk beneath it for some time.
