Results of Pop-Up Satellite Tagging of Atlantic Bluefin Tuna Yield Surprises
Dr. Molly Lutcavage1, Dr. Rich Brill2, Dr. Julie Porter3, Greg Skomal and Brad Chase4, Anne Everly1 and U.S. tuna fishermen Ed Murray Jr., Mike Genovese, Bill Chaprales, Anthony Mendillo5
1New England Aquarium, Edgerton Lab, Central Wharf, Boston MA 02110
2Honolulu Lab, NMFS, 2570 Dole Street, Honolulu, HI 96822
3Department of Fisheries and Oceans, Biological Station, St. Andrews, New Brunswick E5B 2L9, Canada
4Massachusetts. Division of Marine Fisheries. PO Box 68, Vineyard have, MA 02568; Division of Marine Fisheries, 30 Emerson Ave., Gloucester, MA 01930
5Palm Beach Gardens, FL; Cape May Courthouse, NJ; Marstons Mills, MA; Isla Mujeres, Mexico
Beginning in 1997, in a collaboration between scientists and fishermen, our research group conducted satellite tagging of giant Atlantic bluefin tuna, Thunnus thynnus, in New England and Canada. Our goals were to determine the long-term movements, origins and behavior of bluefin tuna schools found on the New England shelf in summer and fall. We targeted adult fish comprising spawning-size classes (>80 inches SFL) and programmed the majority of the satellite tags to detach from the fish over their presumed spawning period (April to July).
Figure 1. Estimated migration paths of two
bluefin tuna released from a purse seine on
October 8, 1999. Their pop-up archival tags
reported on December 25, 1999.
From 1997 to 1999, all successfully released single point satellite tags reported from the central Atlantic roughly between Bermuda and the Azores. Each year, about 30 percent of tags on New England fish reported from east of the 45º W stock-division line, and none of the giant bluefin were in or near their only known spawning grounds in the Gulf of Mexico or Mediterranean Sea. Since none of the giant bluefin tagged in the Gulf of Maine were located in either spawning ground when their tags reported, it seemed possible that a previously unknown spawning area might exist in the Central North Atlantic. The alternative hypothesis, that bluefin tuna do not spawn annually, is also an astonishing, although remote, possibility.
Another possible explanation of why giant bluefin were not found in either known spawning area is that the timing of tag releases was insufficient to detect spawning. In 1999, we deployed 21 of Microwave Telemetry's newly developed pop-up archival tags (PTT-100) on New England giant bluefin for attachments of up to one year.
These pop-up archival tags are pressure tested to 3,300 psi, record ambient light levels at two minute intervals, and temperature (+ 0.2º C) and depth once an hour and at sunrise and sunset. As a fail-safe, the tag can be programmed to detach at a preset depth (e.g., 1,000 m), or when the tag stays at a user-defined constant depth for a predetermined interval.The tag transmits raw data defining temperatures, pressures and estimates of sunrise and sunset time. The data is processed by the manufacturer, and longitude and latitude estimates are generated with proprietary software; estimated errors are given as about one degree of longitude and several degrees of latitude.
In February 2000, at the PFRP Symposium "Tagging and Tracking Marine Fish with Electronic Devices," we presented estimated migration paths from the first two pop-up satellite archival tags. The two 400 pound fish, schoolmates released from a purse seine set in October 1999, had crossed the Gulf Stream and taken similar routes to the southeast, but were thousands of miles apart when their tags jettisoned on Christmas day (fig. 1).
We obtained high reporting rates with these new tags (17 out of 21 tags, or 81 percent) and they returned data capable of depicting daily geolocation estimates and ambient temperatures (80 - 340 days) of 12 fish. Without exception, these migration paths overlap reporting locations of single-point tags from previous years, and stretch to the Azores. It was also apparent that not all Gulf of Maine giant bluefin return annually to the New England shelf. Furthermore, migration paths deduced from pop-up archival tags showed that fish did not interrupt their residencies in the Central North Atlantic to visit either known spawning area.
The economic and conservation implications of possible spawning of bluefin tuna in the Central Atlantic are enormous. Since 1981, Atlantic bluefin tuna have been managed by ICCAT as two exclusive biological units separated by a management line at 45º W. The biological rationale of this management division is based partly on the presumed separate and exclusive spawning grounds (in the Gulf of Mexico and the Mediterranean Sea), and very low mixing rates (<4-7 percent) across the line. An important result of the satellite tagging is the recent organization of an exploratory research cruise, scheduled for June, 2001, to locate and determine the reproductive status of giant bluefin tuna in the North Central Atlantic. In the summer of 2000, we deployed ten pop-up archival tags on giant bluefin tuna in New England; these tags are due to report in September, 2001. This summer and fall, we plan to release 80 giant bluefin tuna outfitted with these high tech, fishery-independent tags. Hopefully, this will yield the most extensive portrayal of their migration paths in the North Atlantic to date.
Researchers and fisheries managers alike are faced with the thorny question: are the warm waters of the Central North Atlantic home to spawning bluefin tuna? If not, then what is the role of this poorly-studied region in the bluefin tuna’s life cycle? The initial longline cruise planned for bluefin research in summer 2001 will journey to these distant regions. It carries a high risk of failure, but with luck, we may soon have the answer to these critical management questions. Even then, the fact remains that Gulf of Maine bluefin comprise only a fraction of the combined Atlantic and Mediterranean population. High-tech satellite tags, remote sensing tools, extensive financial resources, and an expert, international scientific team will be needed to clarify the migration paths and spawning habits of this long-lived, highly migratory species. A better understanding of the bluefin tuna’s movements, behavior and spawning areas is vitally needed to improve international management and conservation of this valuable marine resource.
© Copyright 2001-2006 Microwave Telemetry, Inc. All rights reserved.
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