Researchers from UMass Boston and the National Oceanic and Atmospheric Administration (NOAA) have published a paper announcing the discovery of a new Atlantic bluefin tuna spawning ground in the Atlantic Ocean. The spawning ground is located in a broad area of the open ocean called the Slope Sea, south of New England and east of the Mid-Atlantic states. Prior to this work, most scientists had thought that Atlantic bluefin tuna spawn exclusively in the Gulf of Mexico and the Mediterranean Sea.
The new study, published in the Proceedings of the National Academy of Sciences, has important implications for how bluefin tuna stocks are managed in the Atlantic. In particular, this discovery reveals that Atlantic bluefin tuna in the western Atlantic are less vulnerable to fishing and other stressors than was previously assumed.
“Our observations indicate that the number of spawning bluefin in the western Atlantic is likely to be 50 percent greater than that in the Gulf of Mexico.” said Molly Lutcavage, co-author of the study and director of the Large Pelagics Research Center at UMass Boston.
Atlantic bluefin tuna returns some of the highest price per pound to fishermen, and has a unique physiology that allows it to range from the sub-arctic to the tropics. After years of overfishing, Atlantic bluefin tuna have made a comeback. Recently, the International Commission for the Conservation of Atlantic Tunas declared that overfishing of the western stock, targeted by U.S. fishermen, has ended.
The evidence for a new Atlantic bluefin tuna spawning ground came from a pair of NOAA Northeast Fisheries Science Center cruises into the Slope Sea during the summer of 2013.
“We collected 67 larval bluefin tuna during these two cruises, and the catch rates were higher than is typical in the Gulf of Mexico,” said David Richardson, lead author of the study and a fisheries biologist at the Northeast Fisheries Science Center. “Most of these larvae were spawned within a week of us collecting them. These small larvae could not possibly have been transported into this area from the Gulf of Mexico spawning ground. Instead they provided clear evidence for spawning in this region.”
The discovery of a new spawning ground resolves a longstanding debate in Atlantic bluefin tuna science. For years, scientists only recognized two spawning sites, one in the Gulf of Mexico, and one in the Mediterranean Sea. But because tagging studies showed that some bluefin tuna did not visit the known spawning sites at the expected age of maturity of 8 to 9 years, scientists long assumed that western Atlantic bluefin tuna must need more years to reach maturity. Thus, fisheries management organizations reduced fishing quotas to allow more bluefin tuna to mature.
Lutcavage was an early and consistent supporter of an alternate hypothesis—fish that did not visit the Gulf of Mexico and Mediterranean Sea were spawning elsewhere.
“The discovery of a spawning in the Slope Sea, the analysis of the tagging data, and the documented younger age of maturity, provide confirmation that western Atlantic bluefin tuna are spawning at a younger age, and over a much larger space and time period than recognized. These population insights should be used in the stock assessment,” Lutcavage said.
The researchers also indicate that a reevaluation of the population structure of bluefin tuna is needed. These migratory patterns observed by Lutcavage and her colleagues raise the possibility that some level of reproductive mixing between the eastern and western stock may be occurring in the Slope Sea spawning ground.
“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. With the confirmation of spawning in the Slope Sea, we will need to revisit much of what we thought we knew about this species,” Richardson said.