Atlantic tuna return thousands of miles to birthplace to spawn (10/3/2008)

Biologists are using fish's ear bone to identify individuals from different nurseries

The Atlantic bluefin tuna is the largest and most sought-after of all tunas, weighing as much as 1,400 pounds and capable of fetching as much as $50,000 or more in Asian markets where its meat is a prized commodity, one big reason why its numbers have declined precipitously since the 1970s. New research findings reported in Science have critical implications for how bluefin tuna are managed on both sides of the Atlantic Ocean.

A team of international researchers led by Dr. Jay Rooker of Texas A&M University at Galveston adds a new chapter to this emerging story, providing critical insights into the population structure and mixing of North American and Mediterranean populations of bluefin tuna. This comes at an important time as new assessments by international scientists suggest that both western and eastern fisheries are unsustainable at their current levels.

In the current study titled, "Natal Homing and Connectivity in Atlantic Bluefin Tuna Populations," Rooker and fellow researchers examine the chemical composition of the fish's ear bone - the otolith - to identify individuals from different nurseries. Chemical signatures in the form of stable carbon and oxygen isotope ratios served as a "birth certificate" and were used by the researchers to determine the origin of adolescent and adult bluefin tuna (2-20 years of age or more) on spawning and foraging grounds in the Atlantic Ocean.

The study shows that trans-Atlantic movement and mixing of populations was high with over half of the juveniles collected in North American waters being of Mediterranean origin. "North American fisheries for juvenile bluefin tuna appear to be supported to a large degree by the Mediterranean population, and thus the condition of this population will directly impact recreational fisheries for bluefin tuna in U.S. waters," according to Rooker.

"Our data coupled with archival tagging data clearly show that the migratory patterns of bluefin tuna are more complex than previously assumed and information on mixing must be included in future assessments to ensure that rebuilding efforts are successful."

Despite the high level of mixing, the team also observed that over 95 percent of adult bluefin tuna returned to their place of origin in either the Gulf of Mexico or Mediterranean Sea to spawn.

"Rates of homing reported here are extremely high and comparable to Pacific salmon, which are known to return to the streams in which they were initially spawned, with very high frequency," according to co-author Barbara Block from Stanford University.

"The new otolith chemistry findings fit perfectly with previous electronic tagging and recent genetic data, which show that distinct bluefin tuna populations mix across the foraging zones of the North Atlantic but separate into distinct spawning areas. This has important implications for the assessment of the western Atlantic bluefin tuna population. To know exactly how many of this smaller population remain, we have to account for this mixing."

Commercially harvested bluefin tuna (commonly called "giants" and weighing over 300 pounds) in New England and Canada were found to be nearly entirely of Gulf of Mexico origin. Results demonstrate that Northern Canadian waters may represent critical foraging habitat of the smaller, more vulnerable population that spawns in the Gulf of Mexico.

In addition to Rooker and Block, David Secor of the University of Maryland Center for Environmental Science; Gregoria DeMetrio of the University of Bari (Italy); Ryan Schlosser of Texas A&M University at Galveston; and John Neilson of the St. Andrews Biological Station co-authored the article. The research was supported by the National Oceanic and Atmospheric Administration's Southeast Fisheries Science Center, the Tag-A-Giant Foundation and the Monterey Bay Aquarium Foundation.

Note: This story has been adapted from a news release issued by the Texas A&M University

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