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Monday, May 27, 2024 | Back issues
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Slope Sea Discovery May |Solve Bluefin Mystery

WASHINGTON (CN) - Evidence of a previously unknown spawning ground for Atlantic bluefin tuna could resolve longstanding uncertainties regarding the species' life-cycle and vulnerability, according to a new study. The study, announced by the National Oceanic and Atmospheric Administration Monday, suggests that scientists have been overestimating age-at-maturity, an important fishery management marker, and the species may be less vulnerable to fishing pressure than they had believed, the agency said.

Understanding the complex life-cycle and migratory patterns of this "high value" species is vital for rebuilding stocks that have been depleted by global overfishing. "Recent international cooperation in managing catches has contributed to increasing trends in the abundance of both the eastern and western management stocks," NOAA said. Atlantic bluefin tuna is harvested by fisheries in more than 20 countries, the study noted.

It had been thought that the eastern stock spawned in the Mediterranean Sea at age 4, and the western stock spawned in the Gulf of Mexico at age 9. Through electronic tagging, researchers learned some fish did not visit either spawning ground, which brought their suppositions into question, leading some researchers to postulate that the age-at-maturity was even higher, perhaps 12-16 years, according to the announcement.

Study co-author Molly Lutcavage, with the Large Pelagics Research Center of the University of Massachusetts Boston, supported the alternative hypothesis that the fish were spawning elsewhere. Samples taken during two research cruises east of the mid-Atlantic states in the summer of 2013 appear to back up her theory.

"We collected 67 larval bluefin tuna during these two cruises, and the catch rates were comparable to the number collected during the annual bluefin tuna larval survey in the Gulf of Mexico," study lead author David Richardson of NOAA's Northeast Fisheries Science Center, said. "Most of these larvae were small, less than 5 millimeters, and were estimated to be less than one week old. Drifting buoy data confirmed that these small larvae could not possibly have been transported into this area from the Gulf of Mexico spawning ground."

The postulated new spawning ground is in an area of the Atlantic Ocean south of New England, known as the Slope Sea, between the Gulf Stream and the northeast U.S. continental shelf. The researchers now hypothesize that only the largest tuna, those over 500 pounds, migrate to the Gulf of Mexico to spawn, while smaller fish make a shorter migration to the Slope Sea, NOAA said.

"Last year, we demonstrated using endocrine measurements that bluefin tuna in the western Atlantic mature at around 5 years of age. That study, and ones before it, predicted that these smaller fish would spawn in a more northerly area closer to the summertime foraging grounds in the Gulf of Maine and Canadian waters," Lutcavage said. "The evidence of spawning in the Slope Sea, and the analysis of the tagging data, suggests that western Atlantic bluefin tuna are partitioning spawning areas by size, and that a younger age at maturity should be used in the stock assessment."

The change in the age-at-maturity management assessment suggested by the study is important because late-maturing populations that spawn in restricted areas are considered to be more vulnerable to overfishing and environmental change. A third major spawning area, coupled with the younger maturation rate suggests that these tuna may be able to sustain higher fishing rates and be more resilient to climate change and ecological disasters such as the Deepwater Horizon oil spill, according to the study. The findings also challenge the idea that the western and eastern stocks are isolated from each other.

"Past analyses of Atlantic bluefin tuna population structure and mixing between the western and eastern Atlantic stocks may need to be revisited because they do not account for the full spatial extent of western Atlantic spawning," Richardson said. "So much of the science and sampling for Atlantic bluefin tuna has been built around the assumption that the Gulf of Mexico and Mediterranean Sea are the only spawning grounds. This new research underscores the complexity of stock structure for this species and identifies important areas for future research." The study, published in the Proceedings of the National Academy of Sciences of the United States of America, was conducted by researchers from NOAA's Northeast Fisheries Science Center and Alaska Fisheries Science Center, the Large Pelagics Research Center at the University of Massachusetts Boston, the School of Marine Science and Technology at the University of Massachusetts Dartmouth, and NOAA's Greater Atlantic Regional Fisheries Office. Sampling was carried out by NOAA, the Bureau of Ocean Energy Management, and the U.S. Navy through interagency agreements for the Atlantic Marine Assessment Program for Protected Species.

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