Bluewater trolling is not a matter of just chucking a few lures out the back and trolling around till you get a bite. Instead it is a deliberate search pattern to locate fish. There is a lot of water between fish therefore anglers must take the time to learn where to look for the fish. The key is the ocean's currents.
The currents govern all pelagic species, both bait and predators. Bottom structure such as reefs and canyons disrupt the current creating upwellings. Cooler water is pushed towards the surface where it mixes with the warmer surface layer and can result in substantial temperature variations. Baitfish congregate along these breaks to feast on plankton and other microscopic organisms. In turn, apex predators, like marlin and tuna, are drawn by the concentration of food, thus creating a food chain.
Reefs, canyons and even the continental shelf are prime examples of structures that cause upwellings. The development of high tech depth sounders and GPS units has made finding these formations an easy task. As has the introduction of satellites that can read the sea surface temperature.
Although structure is the obvious place to locate temperature breaks it is not the only option. Eddies of warm water regularly break free from the main currents (the East Australia Current and the Leeuwin Current in the West) and travel independently. At the front of these eddies the temperature can vary dramatically. These plankton rich waters draw baitfish and predators alike. In recent times I have been using sea surface temperature charts (www.seasurface.com) to help identify potential temperature breaks and have had huge success. Offshore fishing is becoming increasingly technical!
In tropical waters flotsam like a piece of rubbish or an old net are also a type of structure. In fact, longliners often report exceptional captures of immature tuna and blue marlin around flotsam in tropical waters.
The reason we look for temperature breaks, structure and flotsam is because it attracts bait and as all good offshore anglers know bait is the key. Although bait schools are often found in these areas it is still unpredictable and can turn up anywhere. Always keep your eyes open and watch the birds for signs of activity. Being observant will help you catch more fish.