What are the different kinds of tuna, and how are they caught?
There are five primary species of fish that are used in canning. They are caught using a variety of methods. 62% of the fish are seined with nets, 14% are fished with longlines, and the remainder are caught with other methods.

Purse Seine
This method of fishing involves a net that can be up to 2000 meters in length and up to 250 meters deep. The net is carried by a large vessel as they drive around searching for fish. When a tuna school is located, the vessel deploys a smaller boat called a skiff which pulls the net off the primary vessel as the primary vessel drives around to encircle the tuna in the net. When the tuna have been encircled in the net, both vessels will come together and the skiff will give the end of the net back to the main vessel. The skiff will then hook up to the main vessel to pull on it so that it will not drift back over the net. The net has floats or corks at the top and rings at the bottom along with a weighted line used to sink the bottom of the net into the water. A rope called a purse line is pulled in through the rings, causing the bottom of the net to close and keep the fish from escaping. The net is pulled in aboard the boat until eventually it is very small, at which point the fish are removed from the net.

Longline
In longline fishing, a boat sets out a long rope called a main line that will runs for miles beneath the surface. Attached to this main line will be several shorter lines called gangions (pronounced gan-yuns) or snoods. The gangions will have baited hooks for catching tuna that will have diffrering lengths. The long mainline will be supported by multiple floats and lines that will keep the main line at the depth of the fish. After deploying a longline, the fishing vessel will typically deploy another line, then return to an earlier set line. When retreiving the line, fish are removed from the hooks and the main line is re-spooled aboard the vessel.

Surface Troll
These vessels use lures that are dragged across the surface behind a boat that is moving at about 5-6 miles per hour. The lures typically look like a squid and are made out of rubber or some other durable material. The hook is a heavy duty double hook, usually without a barb which allows the fisherman to quickly release the fish.
The boat uses outriggers to allow them to fish more lures (also called jigs). The lines used are heavy braided cord called tuna cord which run from the poles. There is typically a shock reducing device called a rubber snubber on every line which reduces the chance of losing fish that get hooked. The cord will be connected to a monofiliment leader that is usually about 2-3 fathoms in length.
Tuna swim to the surface and bite these jigs. The boat continues moving, sometimes in a circle, and the crew pull the fish in one at a time. The hook is removed and quickly returned to the water to catch another fish.

Pole and Line
In commercial tuna pole and line fishing, hand-held long heavy-duty poles without reels are used. A line made of steel cable or monofilament that is shorter than the fishing pole is tied to the end of the pole. At the end there will be a hook which will either be baited or have an artificial jig. After the boat locates fish, they will approach the school and start throwing chum (small live bait fish used to attract the tuna). The tuna will swim to the boat and eat the chum, at which time the crew members will get in a rack that is mounted outside the boat with their poles. The artificial jigs or baited hooks are dropped into the water, and when the tuna bite, the fisherman manually lifts the tuna out of the water with the pole, flipping the fish into the boat. The special hook design of the artificial jig quickly falls out of the mouth of the fish, allowing the fisherman to quickly put it back into the water without having to even touch the fish.

Out of the fishing methods mentioned above, the most ocean-friendly methods with the least bycatch of other species and impact to the environment are surface troll and pole and line. Other species will frequently bite longlines or be caught in the seine, where the other two fishing methods almost exclusively catch their target species.