November 15, 2018
By Jeff Weakley
Florida scientists unlock the blackfin tuna code.
A flash of gold in the depths is often the angler's first sight. Dark fins indicate a blackfin tuna, not the similar—and potentially much larger—yellowfin. The species overlap in some waters off Florida.
Over the years, Florida Sportsman has covered the ins and outs of the blackfin tuna fishery around the Florida coastline: Places these fish are historically abundant, such as the famous Humps in the Florida Keys; the shrimp fleet in the Gulf of Mexico; bluewater ledges off Northeast Florida. We've researched baits that are productive—from live sardines to ballyhoo, small trolling plugs to streamer flies. We've covered the advent of fluorocarbon leader material, its low refractive index apparently conducive to fooling the sharp-eyed blackfins.
And yet, like many of the anglers and captains we've spoken with, we've always wondered about the basic biology of our feisty little blackfin. What conditions do they favor? How fast do they grow? When do they spawn? What means might be used to study them, to discern their migratory or foraging cycles?
It's the brutally ironic thing about fisheries science. The fish that we know the most of, tend to be those which have been most heavily exploited for the commercial market. It's only when a fish, such as the red drum, bluefin tuna or swordfish, reaches a crisis point that effort and funding align to document critical details of life history, range and reproduction. In the case of blackfin tuna, it has taken basic curiosity and academic initiative to spur this kind of attention.
There's never been much of a market in the southeastern U.S. for blackfin tuna. Not like the giant bluefin, whose Western Atlantic populations crashed after the sushi market exploded in the 1970s. And yet, the blackfin, a smaller member of the tuna clan, has long been a tremendously popular catch for recreational anglers. They are hard fighters for their size, and their deep red flesh is delicious when seared over a hot grill. At times they seem to be wary feeders, goading us to do what we as anglers love to do: Figure out better ways to catch them.
Quietly, in the last few years, scientists have been doing some figuring of their own.
Right here in Florida, the fisheries lab at Nova Southeastern University (NSU), in Fort Lauderdale, has emerged as a fountain of useful research into blackfin tuna. Under the direction of Assistant Professor Dr. David Kerstetter, three research papers have contributed much to the scientific and common understanding of the species' habitat, growth and reproduction.
Habitat: Ups and Downs
For starters, a tagging study using sophisticated instruments has yielded interesting results as to blackfin tuna habitat utilization. Nova graduate student Jenny Fenton is listed as the principal author of Habitat utilization of blackfin tuna in the north-central Gulf of Mexico, published in October 2014.
In April 2012, the NSU researchers on Fenton's team fished off the coast of Louisiana. They captured 10 blackfin tunas using hook-and-line gear, 7/0 circle hooks and chunks of little tunny. The blackfins ranged from 27 to 34 inches—fairly husky, for the species. A 4 ½-inch pop-up satellite archival tag (PSAT) was planted in each fish before release. Every 90 seconds, the X-Tag model PSAT tag records temperature, pressure (converted into depth) and light levels. After a preprogrammed number of days at sea (in this study, 10, 19 or 28 days), each tag released and floated to the surface, transmitting its data archive via satellite.
The researchers found that the blackfins spent 90 percent of their time within the first 187 feet of the surface, but dived periodically to depths as great as 650 feet or more. Also, the fish favored water temps between 71 and 80 degrees F—which certainly dovetails with what most Florida anglers observe.
Most interestingly, the tagging results verified what fishermen have long noticed: blackfins stay deeper during daytime, and ascended to shallower depths at night. The researchers didn't suggest a motive for the behavior, but a fisherman looking at bits of squid in a blackfin's belly might infer the diving has to do with foraging.
Blackfins may also exhibit site-fidelity, another way of saying, they find a place they like, they stick around. Among the 10 tags planted in the Habitat utilization project, six popped off within 20 miles of the original point of capture. Fishermen who've noticed that schools will mob up on a particular wreck or deepwater ledge wouldn't be at all surprised to learn that a blackfin might stay in the area for 28 days or longer, but it's interesting to see confirmation through technology.
On the subject of site fidelity, Kerstetter, when I interviewed him for this article, expressed curiosity about the blackfin's range. Unlike the trans-Atlantic bluefin, yellowfin and bigeye, blackfins are strictly limited to the Western Atlantic. “Blackfins don't make the big circle migration,” he said. “And we're not sure why.”
The researchers concluded that further studies should be explored using this technology. There had been some preliminary concern as to whether the weight and drag of a satellite tag would overburden the blackfin, a smallish creature by comparison to the bluefin tuna, tarpon and large sharks that had been previous carriers of PSAT devices. “External tags with fishery-independent reporting capabilities are an available option for smaller tuna species,” the researchers happily concluded. Only one of the 10 blackfins died during the study period, 5 hours after release. Researchers did note that it was the smallest of the fish, by a small margin. It had been hooked in the jaw, and the hook removed, as was the case with most of the fish. Interestingly, one blackfin had been gut-hooked and the hook was left in the fish; it ranged 28 days and 97 miles, apparently undeterred by the hook.
Age and Growth Rates
Another NSU graduate student, Jessica Adams of the Florida Fish and Wildlife Conservation Commission's Cedar Key lab, co-authored with Kerstetter a pioneering age/growth study of blackfins and some other small tunas.
Adams' study involved establishing a counting protocol for the layers of material in a tuna's ear bone, or otolith, in order to calculate a reliable means of aging a fish. Then, comparing this to lengths of sampled fish, the researchers produced a growth curve, yielding important insights into how quickly these fish grow. The largest blackfin sampled was 31 inches and 8 years of age. Generally, at one year, most blackfins are 18 inches, according to the curve. A 4-year-old fish may be about 27 inches.
When, and at what size, blackfin tuna begin to spawn were questions tackled by NSU student Sonia Ahrabi-Nejad for her master's thesis published in 2014.
Analyzing the reproductive organs of blackfin tuna taken in the Florida Straits from 2010 through 2014, Nejad determined that blackfins in this region spawn May through June. Some of the offspring will reach sexual maturity within one year, at around 16 inches. But full recruitment into the spawning stock doesn't occur until the males reach about 20 inches (year 1 ½) and females, 24 inches (year 2).
Anglers downstream of the north-flowing current in the Florida Straits—specifically, along the southeast and east central Florida coast, out of ports such as Palm Beach and Stuart—might find the timing of all this quite canny. Might the small, sub-20-inch blackfins caught during October be the product of that spring/early summer spawning period? Based on the age/growth studies, some of these fish are mature; many are not.
For his part, Kerstetter—who not only teaches about fish, but enjoys fishing in South Florida—hasn't vocalized specific recommendations for blackfin management. Whereas bluefin, yellowfin, big-eye, and skipjack fisheries are governed by federal Highly Migratory Species restrictions, at present there aren't any regulations for blackfin. Funding for research has been thin. Kerstetter said he was proud of his team for establishing baseline data. At the same time, he seemed deflated by the ongoing lack of consideration from the fisheries management community.
“We've gotten a couple of small awards here-and-there for our blackfin work—including a couple of internal NSU Presidential Faculty Research and Development Grants—but nothing at the federal or state level,” Kerstetter said. “Since it's not legally an HMS species, NOAA doesn't want to fund anything for it, and the state of Florida doesn't seem to want to. Thus, we've mostly self-funded the research, with my graduate students Jenny, Jess, and Sonia doing most of the work for their master's thesis projects. ”
Despite the challenges, Kerstetter said he was heartened by the pure nature of the work. “As scientists, we live to do this stuff,” he said. He also expressed delight in the specimen contributions and support given by recreational tournaments in South Florida, describing the relationships as “a win-win.”
Views from the Flybridge: Blackfin Tuna
Dennis Forgione, veteran captain of the Free Spool out of Haulover Inlet in north Miami-Dade County, indicated that most of his blackfin fishing was focused during April through June, when spawning-size blackfins into the 30-pound range work their way into the reef edge waters. “Usually they're in 80 to 120 feet of water,” Forgione said. “We catch them chumming and fishing live pilchards. The bite's definitely best at low light—early morning, late afternoon. But we had a cloudy, rainy day this summer when we caught them at noon and all day long.” He noted that these fish seem to be distinct from the “winter” blackfins. “In winter, we get little blackfins schooling farther offshore. You'll hear on the radio that a guy caught 10 tunas in February—and then you find out later that collectively they didn't weigh 15 pounds.”
Three hundred miles north of Miami, Robert Johns, captain of the Jodie Lynn out of St. Augustine, also finds blackfins schooling offshore. But, they are a larger grade than the ones encountered at the same time off South Florida:
“Our best period is Christmas to Easter, when some days we'll see them busting baits as far in any direction you can see. I'd say they average 10 to 14 pounds, with some over 20. That's out 25 or 30 miles, on the shelf edge. But we catch some year-round. In summer, sometimes we get big blackfins right on the beach, in the pogy pods.”
On Florida's Gulf Coast, Mike Graef, captain of the Destin charterboat Huntress, says the fall blackfin fishery is dependable. Graef slow-trolls live herring or runners, but also catches them on cut baits. “Usually we catch them on inshore breaks, in about 240 feet of water, but sometimes they'll push within 10 miles of the pass,” he said. “Here it's all about the bait—you find the bait balls, the tuna will be there. Generally our fishery is over by Thanksgiving, but if the water doesn't cool below 74, they might hang around.”
Destin's fall blackfin bite adds color to the town's month-long, multi-species Rodeo held each year in October. At last year's event, on day one, Marty Esposito of Santa Rosa Beach caught the Charter Division-winning blackfin on Graef's Huntress.
Farther south on Florida's peninsular Gulf Coast, it's a much longer run to the blackfin grounds, but they're definitely out there. Andrew Purcell, who runs Siesta Key Fishing Adventures out of Sarasota, says that on a lot of grouper and snapper trips, 15- to 25-pound blackfins make fine additions to the fish box. “We always stop a few miles short of our bottom fishing spots, because we're looking for even more spots,” explained Purcell. “We'll put out a 5- or 7-line spread and troll a mix of lures—X-Rap 30s, cedar plugs, feathers.
“We start finding blackfins here at about 30 miles, which is 100 feet of water, and on out 95 miles or more.”
Right around the first of November, blackfin tunas make their way into the waters off Key West. Captain Robert “R.T.” Trosset has had the fishery dialed in for years. On the Atlantic side, Trosset says the blackfin bite is best in 180 to 260 feet of water, on offshore wrecks and reefs. A typical fish is 10 to 30 pounds. “We catch them anchoring up and fishing live pilchards,” he said.
In March, there'll be a push of blackfins on the Gulf side of Key West, too. These fish are evidently attracted to the bycatch shoveled overboard daily by the commercial shrimp fleet that works the area. “We catch them using chum from the shrimpboats. When chum is hard to come by, some guys will using flutter jigs and other lures. The key,” said Trosset, “is to get the jig down below the bonitos. The blackfins hang deeper.” FS
First published Florida Sportsman October 2015