What’s Going on With Bluefin Tuna?

There have been a lot of rumors swirling about lately when it comes to Atlantic bluefin tuna. Many U.S. fishermen are seeing more bluefin, both large and small, than they ever have in their lifetimes. Scientists, meanwhile, recently concluded a stock assessment suggesting that the status of western Atlantic bluefin tuna is not as rosy as it might seem. In this blog, we’ll try to explain the current state of the science, discuss how it might possibly be reconciled with what folks are seeing on the water, and lay out next steps for both science and management.

Some Brief Background

Atlantic bluefin tuna are assessed and managed as two separate stocks: A western stock that spawns in the Gulf of Mexico, and a much larger (about tenfold) eastern stock that spawns in the Mediterranean Sea. We know that, in any given year, a lot of “our” fish along the east coast—in some cases over 75% depending on time of year and size class—are Mediterranean-origin fish that migrate to our waters to feed.^[1] However, fisheries scientists have not yet incorporated such mixing into the stock assessment process, so the two stocks are managed separately, divided by a vertical line drawn through the central Atlantic.

The member nations of the International Commission for the Conservation of Atlantic Tunas (ICCAT) are responsible for managing Atlantic bluefin tuna. The U.S. receives a little over half of the western stock’s Total Allowable Catch (TAC) and allocates about 20% of its quota to the recreational sector.

State of the Science

Both the eastern and western stocks were assessed by ICCAT’s Bluefin Tuna Species Working Group this year. While the assessments won’t be final until they are adopted by ICCAT’s Standing Committee on Research and Statistics (SCRS) in a few weeks, anyone interested in learning about the assessment in greater detail should definitely take a look at the Working Group Report. We’re grateful to Dr. John Walter of NOAA Fisheries, who serves as the Chair of ICCAT’s Western Bluefin Tuna Species Group, for clearly summarizing the report’s findings during a NOAA Fisheries-hosted public webinar on September 3. You can view a PDF of Dr. Walter’s presentation here. Below are some key takeaways.

For years, the western bluefin fishery was dominated by the historically large 2003 year class. Folks who spend a lot of time on the water have likely seen this “marching cohort” grow from footballs and mediums in the late 2000s/early 2010s into the jumbo 105-110-inch fish they are today.

In 2017, after stock assessment scientists determined that the western stock was not experiencing overfishing, ICCAT set an annual TAC of 2,350 metric tons (mt) for 2018-2020, an increase from previous years. This bump in TAC was enacted to let fishermen take advantage of the big 2003 year class as it “peaked” and passed through the fishery. Even at that time, scientists predicted that any harvest of more than 1,000 mt would reduce western bluefin biomass as fishermen cropped down the 2003 year class—which may sound worse than it is, as that decline wasn’t a result of overfishing but instead a decline in 2003 year class fish. In any event, the current 2,350 mt TAC was projected to reduce western bluefin biomass by about 7.5%, and a reduction in the TAC for 2021 and beyond was expected as the 2003 fish faded out.

Fast forward to this year, when scientists updated the assessment with new data from the past couple of years. Key inputs to the assessment process are indices of abundance (including from commercial and recreational fisheries), which are used to estimate the numbers of larval, juvenile and adult bluefin tuna that are in the western Atlantic. The juvenile abundance estimates, based on the Large Pelagics Survey of recreational anglers that many of you are familiar with, can be used to estimate recent recruitment (e.g., a lot of 35-inch, three-year-old fish could indicate a successful recruitment event three years before).

In the three years since the last assessment, with three more years of data of hand, the recruitment story has changed, suggesting that 5-10 years ago, there were fewer baby bluefin than scientists had thought in 2017. Those fish are now on the cusp of entering our commercial fishery (minimum size of 73 inches), meaning that, according to the data, there will be fewer of them than predicted in the last assessment. As a result, scientists now predict that the 2,350 mt TAC over the past three years will have reduced bluefin tuna biomass by 12% instead of 7.5%, as predicted in 2017, because fewer fish are coming in behind the 2003 year class.

So, to sum up, we have a fishery that a) was already expected to have a decline in TAC due to the peaking of the 2003 year class and b) has new evidence of lower-than expected recruitment. ICCAT’s Standing Committee on Research and Statistics (SCRS), has not yet released its recommendations for the western bluefin TAC, but will do so in the next few weeks. While we can’t predict what those recommendations will be, it’s likely the SCRS will recommend a TAC reduction, especially since the current TAC is projected to have a 94% probability of leading to overfishing, as you can see below.

Square Peg, Round Hole: Scientific Findings Versus On-The-Water Observations

Okay, time to address the elephant in the room: How can a reduction in harvest be on the table when many folks along the east coast (including us) are seeing more fish, of a number of different size classes, than they ever have? We understand the frustration, particularly among those who have been in the fishery for a long time and have made sacrifices over the years to help improve it. We don’t claim to have all of the answers but want to give a few potential explanations.

One potential answer to the apparent disconnect is the concept of localized abundance. We’ve got to remember that the coastal region from New Jersey to Maine is one tiny sliver of the bluefin’s range. And it’s a region that’s likely to concentrate big numbers of fish given the huge amount of food available. But what we’re seeing doesn’t necessarily translate into a fishery that’s in good shape across the board. And while fisheries scientists have worked to incorporate how environmental factors could affect large fish distribution (and thus accessibility to the fishery) into their assessments, they haven’t yet done so for small fish.

Many recreational folks have been seeing enormous catches of schoolie bluefin (27-47 inches), particularly off NJ and NY. Don’t those mean good recruitment? The answer is that yes, they do—however, those fish are only three years old, from the 2016 year class. They won’t be entering the commercial fishery for another four or five years, and the current management advice will only be for the next three years and will focus on adult, commercial-sized fish (whether that should be the case, given the big numbers of small fish the recreational sector is capable of harvesting, is a story for another blog…). So while having all of those small fish around is a great sign and will inform management advice in a few years, it doesn’t have a huge impact on this year’s assessment.

Lastly, we want to emphasize that the middle word in the term “best available science” is there for a reason. The stock assessment scientists in the Bluefin Species Working Group do the best they can with the information they have, and there’s always room for improvement when it comes to refining stock assessment approaches, improving existing indices (such as our recreational rod-and-reel indices), and establishing new ones (for example, aerial surveys). As a science-based group, ASGA’s goal is always to ensure use of the best available information—even if its imperfect—to inform management advice, as well as to help improve the state of the science where we can. And some big changes to bluefin science are coming our way that could help narrow the gap between what fishermen see and what scientists estimate.

Next Steps: Management

As mentioned above, ICCAT’s SCRS will be releasing its management recommendations soon. In a “normal” year, ICCAT would hold its annual Commission meeting in November to decide on management measures, taking into account the SCRS findings and advice—but, of course, this is not a normal year. Without getting into the rigamarole of ICCAT procedures, it sounds like ICCAT will work to roll over all management measures by one year unless it’s absolutely necessary to take action, in which case countries will try to work together through “correspondence” (email) over about a month-long period this fall. We expect that the western bluefin issue will fall into the “action necessary” category, but how the discussions will evolve is anyone’s guess.

Next Steps: Science

There’s always more going on in the world of bluefin tuna science, but there are two ongoing projects that could have huge implications for how we assess and manage bluefin tuna.

As mentioned earlier, there’s a disconnect between the way we assess and manage bluefin and what we know about their biology. We manage them as separate eastern and western stocks, but know that a lot of fish regularly cross the Atlantic, meaning that there isn’t always a clear A-to-B relationship between juveniles and adults—what if, for example, a fish is of eastern origin but doesn’t travel west until it’s a giant?

The issue of mixing has been a thorn in the side of both bluefin scientists and managers for a long time, but change is afoot. Scientists are currently working to develop a new Atlantic-wide management approach for bluefin called Management Strategy Evaluation (MSE), which entails simulating the fishery to determine what management actions should automatically be taken as stock status changes. MSE will explicitly incorporate stock mixing, thereby linking the eastern and western stocks—which is a good thing for us given that the eastern stock is ten times larger than the western stock and appears to be doing quite well. While COVID has thrown a wrench into MSE planning, scientists hope it will be ready for action for 2023, meaning that the management impacts of the current assessment may be short-lived.

Another bigtime source of uncertainty and bone of contention among many is the degree to which existing indices are reflective of true bluefin abundance. Scientists are currently trying to address this shortcoming through a process known as “close-kin mark-recapture analysis,” which genetically links larval bluefin with adults and can provide a census estimate of adult bluefin rather than relying on indices. This is the method that folks in Australia use to assess southern bluefin tuna, and the same scientists who developed it there are helping ICCAT’s scientists. We’re hopeful to see this effort further evolve in the next several years.

We hope you found this update helpful, and yes, we know it is a lot of information. We’ll be keeping a close eye on both domestic and international bluefin science and management efforts, and encourage you to reach out to us if you have any questions.

1. Kerr, L.A., et al. 2020. Mixed stock origin of Atlantic bluefin tuna in the U.S. rod and reel fishery (Gulf of Maine) and implications for fisheries management. Fisheries Research 224. https://doi.org/10.1016/j.fishres.2019.105461. ↑