Fishermen catching yellowfin tuna by pole and line fishing. They are standing in the racks on the Bureau of Commercial Fisheries Vessel HUGH M. SMITH.

Fishermen catching yellowfin tuna by pole and line fishing. They are standing in the racks on the Bureau of Commercial Fisheries Vessel HUGH M. SMITH.

Measuring the length of a bluefin tuna.

Pole and line tuna fishing boat from Fuenterrabia, Northern Spain.

Pole and line tuna fishing boat from Fuenterrabia, Northern Spain.

Scientists tagging bluefin tuna in the Bay of Biscay with traditional tagging method.

After tagging, the bluefin tuna are returned to the sea. The tag is visible on the back of the tuna.

Man on left is holding tuna while man on right prepares to insert tag.

Fisherman bringing tuna to tagging table while pole and line fishing for additional tuna continues. Scientists were conducting the tagging operation on a chartered commercial fishing vessel. All tuna caught were tagged and returned to the sea.

After tagging, the bluefin tuna are returned to the sea.

Pole and line fishing on a chartered vessel to catch tuna for tagging. Scientists tagged each fish caught and returned it to the sea.

After tagging, the bluefin tuna are returned to the sea.

Scientist preparing to tag albacore tuna - Thunnus alalunga.

Pole and line fishing on a chartered vessel to catch tuna for tagging. Scientists tagged each fish caught and returned it to the sea.

Scientist displaying pop-off electronic tag. This tag is inserted into a fish's flesh and pops off when it has been programmed to do so. It transmits a signal to an ARGOS satellite for tracking large pelagic fish species.

Scientists measuring a bluefin tuna - Thunnus thynnus.

A bluefin tuna caught in the trap at Stintino.

Bluefin tuna male sexual glands.

Bluefin tuna female sexual glands. Also, note heart in upper right of container .

A bluefin tuna attacked by a shark while being brought aboard during pole and line fishing operations. This tuna was attacked during a scientific tagging cruise on a chartered vessel.

Photo #1.Cross-section of first spine of dorsal fin of young bluefin tuna showing annual growth rings. These rings are similar to tree growth rings. The wide (brown) areas represent summer growth during periods of high food intake. The narrow (white) bands represent periods of less fast growth during the winter when the fish's metabolism slows.

Photo #2. Compare this cross-section to the previous image. Notice the greater width of the outer band. This is because the first cross-section was from a bluefin tuna caught in June while the second was from a tuna caught in November. The wider area represents a long period of high food intake during summer/ autumn months. These images are greatly magnified.

Cross-section of first spine of dorsal fin of a one-year old bluefin tuna. This image is greatly magnified.

A fin whale - Balaenoptera physalus -on the Bay of Biscay.

A pod of killer whales on the Bay of Biscay.

Artist's conception of school of tuna pursuing a school of bait fish while pole and line fishing is being conducted from fishing vessel.

Tuna hooked by pole and line fishing makes easy prey for an opportunistic shark.

Besides man, killer whales are major predators pursuing tuna.

Artist's conception of pole and line fishing. When one fish is hooked, the rest of the school tends to follow the hooked fish to the side of the fishing vessel .

Scientist tagging a juvenile tuna.

Preparing to tag a large yellowfin tuna caught on longline gear by the Bureau of Commercial Fisheries Ship OREGON in the Caribbean Sea.

Diagram illustrating concept of selectivity of mesh size on size of fish captured during trawling operations.

Scientists measuring dimensions of tuna caught in Hawaii area of Pacific.

Modern tuna purse seiners in port.

French tuna purse seiner F/V KERGUELEN at Mahe, Seychelles. Note helicopter on flying bridge. It is used to visually search for schools of tuna.

The Spanish tuna purse seiner F/V TXORI-EDER in the western Indian Ocean. Smaller vessel on the stern is secured to purse seine and when a school of tuna is encountered, the small boat is launched and it helps ship encircle it.

Tuna purse seiner fishing in the eastern Atlantic Ocean.

Observation helicopter operating off tuna purse seiner.

Spotters looking for signs of tuna such as logs on surface or agitated water. Tuna tend to congregate around floating logs or other floating objects on the surface.

Spotter using large binoculars to look for signs of tuna.

School of tuna agitating the surface while pursuing smaller fish.

Tuna purse seiner is headed towards school of tuna to encircle with net. Workboat that helps hold net during deployment is still on starboard stern. Smaller vessel in water is used to make noise to scare tuna into staying within confines of net prior to ship and workboat deploying and closing the net.

Tuna purse seiner is deploying net with assistance of workboat which is seen at far end of net. The workboat will pull the net in a circle to capture the tuna.

Tuna purse seiner is deploying net with assistance of workboat which is seen at far end of net near horizon on upper right of picture. The boat appears as a small rectangle behind a vertical rope.

Workboat holding net prior to ship encircling school of tuna.

Helicopter eye view of ship pulling net in circle to capture tuna. Workboat is in lower center of photo and is holding the end of the net.

Ship is finishing deployment of purse seine net. Almost all of the net is now in the water and the ship will continue to encircle the school of tuna while the workboat (not shown) holds the other end of the net.

Ship has finished deploying net which is still attached by a cable. Prior to net closing, the small boat in photo makes noise to keep tuna herded into net.

Encircling the school of tuna is now complete. The workboat is bringing the end of the net back to the ship so that hauling in can proceed.

The net is now closing in a nearly perfect circle. The purse seiner is now starting to pull the net back on board. The workboat is seen to the left and is secured to the ship. It is now acting like a tugboat and is pulling the purse seiner away from the net so that it does not become entangled.