STUDY CASTS LIGHT ON ORIGINS OF EAST COAST'S YELLOWFIN TUNA

 

When I began running my own boat offshore forty years ago, the first tuna I caught was a yellowfin.

We were trolling well south of Fire Island Inlet—I can’t say just how far, as the boat didn’t have a LORAN and GPS was not yet available, but it was certainly quite a bit farther from shore than the folks who designed the 20-foot Sea Ox intended—when one of the rods bent for a second, straightened, then curved over again as line raced off the reel.  A moment later, a second rod went down and I snatched it out of the holder.

Mike Mucha was on the first fish, and was getting it under control.  Mine was still taking line, and given that we didn’t yet have much real tuna gear—I was fighting my fish with a 3/0 Penn Senator and an Ugly Stick better used for trolling for bluefish—I was starting to wonder whether it was going to stop before I ran out of line.  Instincts honed on stripers and blues kicked in as I thumbed the reel’s spool to slow down the tuna’s run, only to have it erase my thumbprint and raise a blister as soon as I touched the line.  But it wasn’t too much longer before Mike put a 53-pound yellowfin in the boat; a while after that, my 67-pound  fish joined it in the cooler.

Throughout the 1980s, yellowfin were a big part of Long Island’s offshore fishing scene.  Anglers slayed them in the canyons, but even closer inshore—sometimes inside of 20 fathoms—they were reliable targets for blue water fishermen.  On a couple of occasions, surprised surfcasters even caught yellowfin from the beach.

Abundance seemed to decline through the ‘90s, and by the early 2000s, yellowfin tuna could be hard to find.  Recent years have seen that trend reverse, with fish again becoming abundant for much of the season.  But the fluctuations in yellowfin numbers caused many anglers to wonder why the number of fish changed from year to year, and to ask where Long Island’s yellowfin came from.

A recent paper has cast some real light on the question, and it seems that a lot of our yellowfin originate farther away than we might have thought.

That paper, titled “Nursery origin of yellowfin tuna in the western Atlantic Ocean:  significance of Caribbean Sea and trans-Atlantic migrants,” appeared in the September 28 edition of the journal Scientific Reports.  The lead author, Dr. Jay R. Rooker, performed similar research on the origins of bluefin tuna about a decade ago.

The researchers found that some chemicals found in the otoliths—so-called “ear bones” located in the heads of the tuna—were unique to certain areas of the sea.  Because the otoliths grow as the tuna ages, recording each year in a similar manner as a tree records each year in discreet rings, the researchers could analyze the chemical composition of that portion of the otolith laid down in a yellowfin’s natal year, and from that determine the fish’s origin.

Scientists already knew that yellowfin tuna in the Gulf of Mexico and in the Mid-Atlantic Bight could trace their origins from one of four regions:  the Gulf of Mexico, the Caribbean Sea, the ocean around the Cape Verde Islands, and the Gulf of Guinea; the former two regions are, of course, in the western hemisphere, while the latter two regions are located off the Atlantic coast of Africa.  Of the four spawning areas, the Gulf of Guinea produces the greatest number of juvenile yellowfin.

Given the multiple spawning areas, the scientists’ first task was to analyze the chemical composition of Year 0—that is, less than one-year-old—yellowfin tuna caught in each of the regions.  With that knowledge in hand, they could look at the chemical signatures of otoliths from yellowfin caught off the coast of the eastern United States, to determine those tuna’s origins.

The specific chemical signatures examined were the ratios of calcium to five other elements—lithium, magnesium, manganese, strontium, and barium—as well as the levels of two stable isotopes, carbon 13 and oxygen 18.  Because the chemical signatures from each region varied from year to year, the researchers had to age the adult and sub-adult fish sampled, and only compare the chemicals found in their otoliths to those in otoliths of juveniles spawned in the same year.

The initial analysis informed researchers that yellowfin from the Gulf of Mexico and Caribbean Sea typically had calcium/manganese ratios that were significantly lower than that found in fish spawned off the African coast.  Yellowfin of Gulf of Mexico origin also had lower calcium/strontium ratios and carbon 13 values than those spawned elsewhere, while sharing low calcium/barium rations with yellowfin originating in the Gulf of Guinea.

The analysis revealed that the origin of the yellowfin sampled varied widely from year class to year class, particularly in the Mid-Atlantic Bight where, depending on the statistical approach used, between 39 and 48 percent of the 2013 year class originated in the Gulf of Mexico and between 43 and 48 percent originated in the Gulf of Guinea.  

The mix for the 2014 year class was very different, with the Gulf of Mexico making only a minor contribution while the Caribbean Sea contributed between 44 and 59 percent of the yellowfin, and the two eastern Atlantic nursery areas—the Cape Verde region and the Gulf of Guinea—together providing between 41 and 54 percent.  A similar pattern emerged in 2015, where Caribbean fish accounted for between 33 and 51 percent of the adult tuna, with the eastern Atlantic contributing most of the rest, somewhere between 41 and 44 percent.

The percentages were somewhat different for adults caught in the Gulf of Mexico, as might be expected given the Gulf’s status as a spawning area.  However, yellowfin originating elsewhere still made significant contributions to the 2014 and 2015 adult year classes.  The Gulf of Mexico only contributed a small minority of fish to the 2014 year class; between 62 and 72 percent originated in the eastern Atlantic, while about 30 percent originated in the Caribbean Sea.  The 2015 year class was dominated by yellowfin spawned in the Caribbean, which made up as much as 82 percent of the total sampled.

The importance of yellowfin spawned outside of United States waters to the western Atlantic fishery shouldn’t come as a surprise, for there are many records of yellowfin tagged in U.S. waters being recaptured in the Gulf of Guinea, which can reasonably be interpreted as adults returning to their natal waters in order to spawn. 

The implications of such movements for yellowfin tuna management are very clear.

Neither the United States nor any other nation can successfully manage the yellowfin tuna resource on its own.  Measures adopted by the International Commission for the Conservation of Atlantic Tunas, binding on all member nations, represent the only viable way to effectively manage the resource.

The yellowfin’s multiple nursery areas can potentially contribute to the resilience of the stock; if oceanographic conditions are unfavorable for spawning in one region, good conditions in another region may well make up for any decreased production.  At the same time, harmful fishing practices thousands of miles from the United States can harm the East and Gulf Coast yellowfin fisheries.

As is often the case with pelagic fisheries, the abundance of yellowfin on the Gulf and East Coasts is contingent on maintaining prudent management practices throughout the entire North Atlantic basin.