A new satellite tagging technology has proven that it can help resolve the mysteries of tuna migration at a time when management strategies for these remarkable and commercially valuable fish are in dispute and their breeding population is in sharp decline.
The microprocessor tags, deployed in 1996 and 1997 by scientists from Stanford University, the Monterey Bay Aquarium and the National Marine Fisheries Service, revealed that tunas tagged off Cape Hatteras, N.C., were able to move as far as 1,670 nautical miles in 90 days -- and that some fish crossed the line separating eastern and western management zones for the bluefin fishery.
"The results of our work indicate that pop up technology works, and that survivorship is high. The fact that the bluefin spread out in 90 days across the western Atlantic and into the western margins of the eastern Atlantic management zone indicates these fish are on the move," said Dr. Barbara Block of the Tuna Research and Conservation Center (TRCC), a collaborative effort between Stanford University and the Monterey Bay Aquarium.
The findings appear in the Aug. 4 issue of the Proceedings of the National Academy of Sciences.
The new tags, called satellite pop-up tags, pop free of the fish at a preprogrammed time, float to the surface and beam their accumulated data via satellite to scientists in the lab, revealing where the fish moved and what ocean temperatures they favored.
Giant bluefin tuna, which can fetch as much as $80,000 apiece in the Tokyo seafood market, currently are managed as two separate Atlantic stocks with limited mixing between the western and eastern Atlantic. Using this management strategy, breeding stocks in the western Atlantic have declined by more than 80 percent over the past 22 years, and eastern Atlantic bluefin of similar ages have fallen by 50 percent during the same period. Western Atlantic tuna are managed under a strict annual quota; eastern Atlantic bluefin have been managed under catch quotas only since 1995, with a 25 percent reduction to be fully implemented this year.
"It's now possible -- and imperative -- to make use of satellite tag technology to determine the extent of transatlantic migrations," Block says. "In that way, management strategies can reflect fish behavior in the real world. With this technology it will be possible to manage giant bluefin tuna so mature breeding stocks could recover in our lifetime."
A 1994 report by the National Research Council recommended that fisheries scientists test the stock structure theories to confirm whether bluefin tuna are one or two stocks in the Atlantic Ocean.
"We believe that the advanced tagging technologies currently being applied to Atlantic bluefin tuna could provide essential information for resolving questions about the existence of one or two management units," added Eric Prince, chief of the migratory fishery biology division of the National Marine Fisheries Service in Miami, Florida.
According to Block, the satellite tags can be used to provide similar information about other pelagic fish species whose lives are equally mysterious. To date, she and her colleagues have put the tags on bluefin and yellowfin tuna, blue and striped marlins and salmon sharks.
What sets the satellite tags apart from other fish-tagging methods is the fact that scientists can recover the data without relying on tagged fish being caught and the tags voluntarily turned in to their labs by fishermen.
Historically, about 13 percent of conventional bluefin tags ;have been recovered. In the 1996 and 1997 experiments, data were recovered from 35 of 37 satellite tags. Importantly, this indicates high survivorship of the tag-and-release fish.
The tags were developed in collaboration with Paul Howey of Telemetry 2000, Inc. Tagging took place in a collaboration among the TRCC, the National Marine Fisheries Service and the recreational fishing community off Cape Hatteras. The research was financed by the National Marine Fisheries Service, the Packard Foundation, the MacArthur Foundation, and the National Science Foundation. Wild fish up to 234 kg were tagged at Cape Hatteras with techniques developed using captive yellowfin tuna at the TRCC. Extensive work with captive tunas demonstrated that the tags can be attached without harm to the fish, either in the lab or in the field.
Data from the 1997 study found that two out of the 37 bluefin tuna crossed between management zones and four additional bluefin were within 5 degrees longitude of the stock boundary meridian.
Researchers with the TRCC and NMFS plan to tag between 600 and 1,000 giant Atlantic bluefin tuna by the year 2000. This will enable them to rigorously test the stock structure hypothesis.
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