Where western Atlantic bluefin tuna spawn, as well as the size and age when they do so, has been an oft-contested subject in how it relates to the management and rebuilding of this severely depleted stock. A new paper published in the Proceedings of the Natural Academy of Sciences is pouring more fuel on the fire: It reports on the discovery of a new spawning location and a smaller size of maturity for fish that are spawning there.
ICCAT manages Atlantic bluefin under two separate stocks: western Atlantic and eastern Atlantic. The assessments for eastern Atlantic fish assume spawning occurs primarily in the Mediterranean, with fish becoming reproductively mature somewhere around age four. The assessments for western Atlantic fish are based on spawning occurring in the Gulf of Mexico at around roughly nine years of age. Still, evidence from tagging studies indicates maturation might occur even later in both.
The article throws another wrench in the bluefin reproductive riddle: the discovery of 67 bluefin larvae found in an area between the northeastern U.S. continental shelf and Gulf Stream, called the Slope Sea. According to the study, it is unlikely these fish were spawned in the Gulf of Mexico based on their size and when they were found. The authors also cite data from an exploratory longline cruise from 1957 in this area that reported bluefin tuna in reproductive condition; however, some in the scientific community question this.
Finding bluefin larvae — albeit a very small number that were, in all likelihood, not spawned in the Gulf of Mexico — is indeed a great discovery. Where things get contentious is the authors’ corresponding interpretations: namely, that this is “unequivocal evidence that bluefin spawn in the Slope Sea,” and that they estimate only 32 percent of the western Atlantic bluefin spawning now occurs in the Gulf of Mexico. They also indicate the age of reproductive maturity has been overestimated, with western fish likely maturing around age four or five. As such, the authors state that western bluefin could actually sustain higher fishing mortality rates, which is a pretty tough pill to swallow considering how much the western stock has declined since the 1960s.
Understandably, interpretations like these, based on a sample size of 67 larvae and data from a historical longline cruise — findings that might or might not be verifiable — have generated, well, let’s say some spirited discussions in both the scientific and management communities. But even if this is true, where does it leave us from a management perspective? Stock-assessment biologists from NMFS say not any better and potentially even worse. Think about it this way: We’ve been operating under the assumption that we are dealing with a very specific spawning location and fish maturing at a relatively old age, and we’re still a long way from rebuilding the western stock with the management measures we currently have in place. If we do indeed have multiple spawning locations and fish that are maturing earlier, we are, in effect, accomplishing less with more.
No doubt the subject of spawning location and age of maturity will continue to be a hot topic for some time to come. Still, the point remains: We’ve got a long way to go until the stock of this iconic species is back to where it needs to be.