In this Adreno Tips video, Eckart talks about how to target pelagic fish when spearfishing. When most divers start spearfishing, they'll begin with targeting reef species as they're typically a little easier to hunt, but at some point, all divers will want to take at look hunting pelagic fish.
Before you do anything else, you need to ensure you have the right equipment. Depending on the size of the fish you're hunting, you'll be looking at a speargun between 110cm and 140cm with an adequate amount of rubber.
The second thing you'll need to ensure is that you're looking in the right places. If you're in the wrong location, no matter how much burley or how many flashers you have, you simply won't see any pelagics. Do some research on the local area and the fish species... from there, you'll be able to find locations that you know are likely to hold pelagic fish. Pelagic fish typically tend to live in deeper water areas, however, there are always exceptions.
Once you know the fish you're looking to target, work out what will bring those fish in. It's a big ocean, if you just jump in and hope to see a pelagic swimming past, you might be waiting a while. So use tools such as flashers, burley, baitfish and good body language to draw the fish into range.