Tuna are very sensitive to variations in water temperature. They congregate along surface temperature breaks which form at the edge of thermal eddies that spin off from the main current. The importance of temperature breaks is critical particularly yellowfin and albacore so it is important that anglers understand them.
In a nut shell plankton and other microscopic organisms congregate along temperature breaks. This in turn attracts bait species like slimies, sauries, redbait and squid who feed on the prolific life. That in turn attracts the big boys, thus completing a basic form of the food chain.
Tuna have the remarkable ability to conserve heat that is produced while they swim. This makes them a highly adaptable and efficient predator who hunt the temperature breaks making them ideal targets for the lure troller.
Underwater structure such as canyons, seamounts and reefs are also good areas to focus your efforts. Sydney’s Browns Mountain, a small seamount a few miles over the shelf is a prime example of fish holding ground. Depending on what the ocean currents are doing, underwater structures like this create upwellings, which in turn cause temperature breaks. Use the nautical maps or your GPS chart plotter to make yourself more accustomed with the bottom features of your area. Alternately the big bluefin tuna in Tasmania are often caught close to islands especially Tasman Island and Pedra Blanca.
Modern technology is great but the old fashioned technique of spotting and identifying birds is also important. Like ‘sign posts in the sky’ sea birds can often guide you directly to the fish. Different birds will tell you different things and just by watching their behavior you can learn a lot about what is going on beneath the surface.
Tuna are very messy eaters and leave a lot of scraps when they feed. Seabirds are well aware of this fact and are always ready to take advantage of an easy meal. Tuna push bait to the surface, trapping it and making it easier to feed on. Somehow the birds are always there ready and waiting.
Two of my favourites are gannets and terns. Whenever I encounter these two I nearly always find tuna. Gannets are easily visible when they circle about high up making them true sign posts in the sky. When they start diving in then it is on for young and old!