Those 600lb fish are a bit difficult to land. There are a lot around Madeira judging by how many are being sold in the fish market there.
You could be just Mackerel fishing and hook a tuna.
Looking at the garfish, they used to be rare and always chasing mackerel shoals. I sometimes caught the odd garfish when out in the boat but its not a fish to eat, Nasty if it comes off the hook and starts snapping at your feet in the boat. I like watching Barracuda when out snorkeling, a menacing looking fish but it just stays still when humans are swimming nearby.