LDWF Tagging Program Getting Answers About Yellowfin Tuna

story by Trey Iles, LDWF Public Information

Yellowfin tuna is a highly migratory species that can be found in most tropical waters worldwide, including throughout the Atlantic Basin. But, like snowbirds from the north who flock to south Florida for the winter and end up staying, these free-moving fish appear to have an affinity for the Gulf of Mexico.

That’s just one nugget of information from the findings of a Louisiana Department of Wildlife and Fisheries research initiative on yellowfin tuna, which began three years ago and continues today.

The data the agency has collected in this study have yielded some interesting discoveries about yellowfin tuna and their use of Gulf habitat.

“This work began in 2012 when the agency realized how little work had been done on yellowfin tuna in the Gulf and how important this fishery is to Louisiana-based user groups,” said Brett Falterman, a Program Manager and Lab Director for LDWF who leads the research group.

Comparatively speaking, the catch of yellowfin tuna in the Gulf is a small fraction of what’s caught throughout the Atlantic Basin. In 2014, 101,305 tons of the fish were caught with only about 3 percent landed by U.S. vessels. And of that 3 percent, less than half was from the Gulf.

But in the northern Gulf, the Louisiana coast is the place to catch yellowfin tuna. More yellowfin are landed recreationally and commercially at Louisiana ports than in any other Gulf Coast state. That’s why LDWF biologists took an interest in this species.

“What we wanted to do, on one hand, is accentuate how important the regional fishery is, and at the same time, address the data gaps in this fishery,’’ Falterman said. “In 2012 and ’13 we started both an electronic tagging program as well as an intensive dock sampling effort. During the course of the study, we sampled almost 3,000 yellowfin tuna at Louisiana ports and sent samples to university partners at the University of Southern Mississippi’s Gulf Coast Research Lab and Texas A&M in Galveston.

“Using the expertise of our dock samplers, collaborations with university scientists, and this active tagging program, we were able to do six different studies in just a three-year period. That includes studies on ageing, diet, genetics, reproduction and the origin of yellowfin tuna in our fishery.”

Yellowfin tuna aren’t managed by individual states or countries. The International Commission for the Conservation of Atlantic Tuna (ICCAT), made up of 50 nations, coordinates stock assessments and makes management recommendations for tuna and other species throughout the Atlantic Basin.

“We’re very proud the results from the work that we’ve done were used in the yellowfin tuna stock assessment conducted by ICCAT this year,’’ Falterman said. “That makes it hard to discount the management impact of our work because our results from the Gulf are being considered when management recommendations for the entire Atlantic fishery are made by ICCAT.’’

LDWF’s initial research finds that tuna prefer the Gulf, particularly the northern Gulf off the Louisiana coast. And they’re here in droves.

“Anglers are always interested in the tagging results, and the surprising thing for us so far is that even of the yelllowfin tuna we’ve put long-term electronic tags on, none of them have left the Gulf,’’ said Falterman. “It implies this residency. But there are two questions to answer - where are they from and where do they stay.

“What we’ve found is that a portion of the Gulf yellowfin fishery was spawned in the Gulf, which has never been shown before. But a larger fraction of these fish actually come from distant nursery areas in the Atlantic.’’

Every life stage of yellowfin tuna has been found in the northern Gulf, including day-old larvae, juvenile tuna only months old, and mature adults in spawning condition. Some multi-year tag returns from adults show fish being caught in nearly the exact same area they were released.

“This is perhaps not the picture you might expect for a highly migratory species,’’ Falterman said.

So what keeps yellowfin tuna in the Gulf? It could be a combination of structure and other habitat. Yellowfin tuna in the Gulf gravitate toward oil drilling rigs and platforms.

“It’s known that yellowfin tuna have a high level of association with structure (oil rigs and platforms),’’ Falterman said. “A lot of the fish we tag are near structure and that’s what the recreational fishery focuses on. And some are recaptured near the same structure.

“While tuna being captured at the same rig where they were tagged might not seem that interesting, remember, many of the offshore oilfield structures in the Gulf are mobile. So when some of the tuna we tagged at one drillship were re-caught on the same drillship, after it had moved 50 nautical miles, we were left to wonder if it’s really the fish doing all the moving.’’

One of the surprises during the research has been the high recapture rate of the species. Conventional tagging with dart or spaghetti tags has been successfully used for studying fish movements for decades. Recapture is vital to the outcome of these projects. However, recapture rates are typically very low for pelagic fish like yellowfin tuna, largely because of their vast habitat and ability to travel long distances.’’

Pop-off satellite archival tags (PSATs) are also used to track yellowfin tuna. These tags literally pop off the fish. A small battery in the tag engages after a certain amount of time or at a preset date. When the current runs through the tag pin in saltwater, electrolysis quickly corrodes it, allowing it to come free from its tether and float to the surface.

These tags provide a way to track fish without relying on recapturing the fish or recovering the tag. But Falterman noted that with yellowfin tuna, it’s not foolproof. Yellowfin tuna are extremely active in the water column, cumulatively traveling tens of thousands of vertical feet per day, and they have delicate bodies, making attaching tags difficult.

“Because the goal of the LDWF project was to look at long-term movements, it made sense to do two things differently from the start,’’ Falterman said. “The first was to figure out a better way to attach PSATs to yellowfin tuna. The second was to take a chance on a different electronic tag type that had never been used on yellowfin tuna in the Atlantic before.’’

Biologists were able to better attach the PSATs to the fish and also used an internal archival tag, which must be surgically inserted. Internal archival tags use all their battery power to log data. Because they don’t use any of it for the pop-off mechanism or to upload the data through an antenna to a satellite system they have a much longer lifetime, some logging data for more than 10 years.

The catch for the internal archival tag, Falterman noted, is the recapture rate. Because the tags record data but stay with the fish, the only way to retrieve the data is if anglers who capture the fish return the tags. That has exceeded expectations, he said.

“Whether it’s the $200 reward for the internal tags or the inherent curiosity of fishermen that’s motivating participation, we are very pleased with the returns,’’ Falterman said.

To date, biologists have implanted 162 internal tags and attached 26 satellite tags to yellowfin tuna ranging from the 27-inch legal minimum fork length to fish more than six feet long, which can weigh more than 200 pounds. So far, 29 internally tagged fish have been recaptured for a 17.9 percent return rate, well beyond the 2.2 percent predicted from previous conventional tagging work.

One of the internally tagged fish was at-large for 1,017 days; the average days at-large for the internal tags to date is 225 days. The best so far for satellite tags is just over 300 days.

The upside with electronic tags is that, because of a process called geolocation, light levels measured by the tags can account for the position of the fish every day between release and recapture, not just the start and end points like with a conventional tag.

“The part that we’re most pleased with is that we’re getting a much higher return rate on the internal tags,’’ Falterman said. “It’s good for our study because the internal tags can stay on fish for a longer period of time. So we are now looking forward to some great multi-year returns.

“We’ve had Gulf-wide participation. We haven’t had a return from outside of the Gulf. But we’ve had recreational sector returns from Texas, Louisiana, Mississippi, Alabama and Florida. We’ve also had participation from the commercial sector, with returns from commercial fishermen based from Florida and Vera Cruz, Mexico.”

The internal tags also extend the life of the project. Falterman said a tag was returned in 2013 that was originally thought to have been part of this current project. However, it came from another project in which the tag was implanted in 2005.

“That’s a best-case scenario,’’ Falterman said. “You put out an internal tag you’re not sure what your horizon is. It’s kind of a function of the age of the fish that you tag. We can put a tag in a fish and we’re hoping the ones that haven’t been recaptured are still out there swimming. It’s reasonable to say the horizon for this current study is five to 10 years.’’

As time passes, Falterman said they expect to get fewer tags back. But the ones that do come back are the most valuable because they’ve been at large the longest, continuing to gather data.

“LDWF doesn’t manage this fishery,’’ Falterman said. “But we’ve helped to make sure that the best science is available to the managers and that our fishery is represented in the conversation on an international level.’’

Because of the expertise developed during this project, LDWF now receives federal funds for both the aging of yellowfin tuna from the Louisiana fishery and a continuation of the dockside work designed to tell from which nursery areas these fish come.

“I think this is a great example of proactive fisheries management because we do not want a problem to develop with this fishery,’’ Falterman said. “And this project serves as a great model for how a state agency can work together with university experts and federal fisheries managers for the benefit of stakeholders.’’



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