(Contributing authors: Walt Golet & Joe Dello Russo, University of Maine; Eric Orbesen & Derke Snodgrass, NOAA Fisheries Southeast Fisheries Science Center)
Go to most any marquee offshore fishing destination in the Atlantic Ocean—the northeast canyons, Outer Banks, Venice, Bahamas, Ascension Island, Cape Verde, Cape Town—and there’s a good chance one species is on the ‘available’ list for at least part of the year: yellowfin tuna. Like bluefin tuna, the yellowfin tuna is a highly migratory species that can travel thousands of miles across ocean basins and is heavily fished by both commercial and recreational fisheries wherever it roams. However, if you catch a yellowfin on a bucket-list trip to Ascension Island or in the Gulf of Mexico, Bahamas, or Caribbean, is it from the same group of fish that we routinely see off the Northeast coast of the United States? Or is it from a completely different group of fish that will never find their way into our waters? The answers to these questions are not only interesting to ponder, but in fact are critical to the successful management of yellowfin fisheries throughout the Atlantic Ocean.
In the Atlantic Ocean, yellowfin are managed by the International Commission for the Conservation of Atlantic Tunas (ICCAT), a regional fisheries management organization whose mission is to study the populations of tuna and tuna-like species (including pelagic sharks, marlins, and swordfish), assess population trends, and manage fisheries through regulations and quotas. As part of its responsibilities, ICCAT routinely conducts stock assessments, which can be thought of as a census of fish populations to evaluate their overall status—that is, whether they are overfished (we have removed too many of them overall) or are experiencing overfishing (the rates at which we are removing them is too fast).
In many instances, ICCAT conducts separate stock assessments for distinct groups of fish of a given species that are thought to be isolated from one another due to where they reproduce or spawn; for example, the eastern and western Atlantic bluefin tuna stocks, which are thought to primarily spawn in the Mediterranean Sea and Gulf of Mexico, respectively. However, even though yellowfin spawn in multiple locations throughout the equatorial waters of the Atlantic—including in the eastern Atlantic off Africa in the Gulf of Guinea and near Cape Verde and in the western Atlantic within the Caribbean Sea and Gulf of Mexico—the yellowfin stock assessment is conducted assuming that all yellowfin in the Atlantic Ocean, from South Africa to Brazil to the Caribbean to the Gulf of Mexico to Canada, are all part of one interconnected population. The question is, is this assumption accurate?
As it turns out, the answer to this question is not a simple yes or no, but is complicated and a subject of uncertainty among scientists and fishery managers. For example, recent genetic research indicates that yellowfin born in the eastern and western spawning areas are genetically distinct, which suggests that they return to their birthplace for spawning. However, conventional tagging data also demonstrates that yellowfin sometimes undertake trans-Atlantic migrations between eastern and western spawning areas, which suggests that individual fish may spawn in different regions throughout its lifetime.
In contrast, satellite tagging data has revealed that a portion of the yellowfin in some regions, including the Gulf of Mexico, exhibit only localized movements and may be resident there throughout their lifetime. To add to the complexity, otolith (ear bone) microchemistry data from yellowfin captured in the mid-Atlantic Bight and Gulf of Mexico shows that fish in these regions originate from all four major spawning areas, with the relative contribution of individual spawning areas being highly variable between years. Thus, some yellowfin fisheries, like the most popular ones in the US, appear to be highly reliant upon the success of spawning in all the major areas.
With all the mystery and complexity surrounding the dynamics of the yellowfin population throughout the Atlantic, it is perhaps no surprise that the results of the most recent yellowfin stock assessment (conducted in 2019) were shrouded in uncertainty, with near-equal probability that the stock was in good or bad shape. To complicate matters, trends in overall yellowfin biomass over the last two decades show a steady decline, and commercial and recreational fisheries operating throughout the Atlantic have exceeded the total allowable yellowfin catch annually since 2014.
With the yellowfin stock teetering on the edge of trouble and so many questions left to answer about their population structure, there has been a recent push from ICCAT and NOAA Fisheries to learn more about yellowfin population dynamics in the Atlantic Ocean, hoping to improve our ability to manage this valuable, international resource. As part of those efforts, a team of scientists from the NOAA Fisheries Southeast Fisheries Science Center, University of Maine, and the Anderson Cabot Center for Ocean Life at the New England Aquarium has been working to better understand how yellowfin caught from the Outer Banks of North Carolina to the Northeast Canyons, in the Gulf of Mexico, and in the Caribbean are connected to one another as well as to spawning areas throughout the Atlantic Ocean.
Since 2010, the team has deployed over 100 pop-up satellite tags in popular yellowfin fishing locations like the Northeast Canyons, Outer Banks, Veracruz, and Curaçao, and tracked the movements of individual fish for periods up to 319 days, or just over 10 months. In addition to the pop-up satellite-tag deployments, the team has worked to deploy conventional spaghetti tags on yellowfin throughout each of these regions as part of ICCAT’s Atlantic Ocean Tropical Tunas Tagging Program (AOTTP) which is a large, cooperative project that deployed over 100,000 tags on tropical tunas, such as bigeye, skipjack, and yellowfin, throughout the Atlantic from 2015 to 2021.
So, what have we learned after nearly 15 years of intensive tagging efforts? Well, it’s a tale of two tags. Looking only at the pop-up satellite tag data, we’ve learned that yellowfin tagged in the northwest Atlantic don’t seem to be buzzing between regions all that much, at least not on an annual basis. In fact, estimated tracks from over 90 pop-up satellite tags revealed high levels of residency off the Northeast US, within the Gulf of Mexico, and in the Caribbean. For example, none of the 26 yellowfin tagged off the Northeast US, from the Outer Banks to the northeast canyons, moved into the Gulf of Mexico or Caribbean Sea—the western Atlantic spawning areas. Instead, these fish remained in offshore waters north of the Sargasso Sea and in association with the Gulf Stream for up to 10 months after release. Similarly, fish tagged in the Caribbean Sea exhibited extensive movements throughout the region for up to 6 months after release but also did not move toward the Gulf of Mexico or Northeast US. The majority of the 50 yellowfin tagged in the Gulf of Mexico also remained in that region in the 6 months after release, with only 3 fish traveling out of the Gulf through the Florida Straits and into the Bahamas during the fall and spring. These movements represented the highest degree of regional mixing seen in the satellite tag data and were one directional (Gulf to greater Atlantic).
While geographical movements recreated from pop-up satellite-tag data appear disconnected, where yellowfin like to spend their time in the water column is consistent across tagging regions. Looking at the vertical distribution of yellowfin allows us to relate yellowfin movements to oceanographic conditions that may be favorable, such as warm surface currents like the Gulf Stream or Gulf of Mexico Loop Current. If you’ve seen cinematic-quality surface feeds with a popper in hand or quadrupled up while surface trolling for yellowfin, you know these fish like to feed at the surface. Indeed, pop-up satellite-tag data shows that yellowfin spend 97% of their time in the upper 200 meters of the water column, and 58% of their time in the upper 50 meters. True to their tropical tuna designation, tagged yellowfin also spent 82% of their time in waters warmer than 70F, and 66% of that time was in waters approaching 80F. While warm surface waters are preferred, yellowfin are fully capable of going deep and into cold water; the deepest a yellowfin went in our dataset was 2,923 feet and the coldest temps experienced were below 40F.
Collectively, the pop-up satellite-tag data paint a picture of resident yellowfin with consistent local behaviors but no strong connectivity between regions. However, data from recaptured yellowfin tagged as part of the AOTTP paint a somewhat different picture. Since 2018, five yellowfin tagged off the Northeast US—two in Veatch Canyon, two in the Hudson Canyon, and one from the Point off Oregon Inlet—have been recaptured off the coast of Africa after being at liberty for 532 to 967 days. These recaptures indicate that at least some of the yellowfin we see off the Northeast US migrate back to the coast of Africa to spawn. But, when considered along with the pop-up satellite-tag data, they also tell us that this long migration is perhaps not undertaken annually. Instead, it may be that adult yellowfin make the long trek across the pond to spawn only every year or two, remaining resident in the Northeast US during years when they aren’t quite ready to make the swim. Yellowfin tagged as part of the AOTTP have also yet to be recaptured in the Gulf of Mexico or Caribbean Sea, but two yellowfin tagged off North Carolina and recaptured down in Venezuela during the mid-90s provide evidence that there is at least some connection to the Caribbean Sea spawning area. Curiously, even though otolith chemistry data has shown that some yellowfin caught off the Northeast US were spawned in the Gulf of Mexico, no tagging data has shown direct movements between these two regions in either direction. Thus, the level of connection between the two most popular yellowfin fishing areas off the US coast remains somewhat of a mystery.
Despite all we’ve learned about Atlantic yellowfin tuna over the past decade, perhaps the biggest thing is that we need to do more research to truly understand Atlantic yellowfin population dynamics. Do yellowfin really return back to their birthplace every time they spawn? How often do they spawn? Are they spawning every time a tag shows them being present in a spawning area? These are among the many questions that scientists hope to answer with additional tagging research, and also through the power of ever-advancing chemical and genetic techniques. So, the next time you put a stud yellow on deck and think about how great that first steak is going to taste, also think about the tens of thousands of miles of ocean that fish swam through, where that fish came from, and what migration paths might have lay ahead. The answers to those questions are key to ensuring that more yellowfin can hit the deck in years to come.
» For more information on the AOTTP results and to monitor Atlantic yellowfin tuna tag recaptures, check out this link.
