2017 Mahi-mahi Tagging Expedition

Ten miles off the coast of Miami, a 96-foot research vessel drifts along the deep-blue waters of the Gulf Stream. A fishing rod bends dramatically followed immediately by the scream of drag. Martin Grosell tightens his grip and holds on.

“He’s a big one!” he exclaims, as a 30lb bull mahi-mahi erupts from the water in a flash of gold and green. The rest of the crew readily looking on from their assigned posts. One is standing on the bottom of a narrow set of dive-stairs at the ships stern, holding an outstretched sling like a catcher waiting for a pitch. Another, scribbles down the fish’s activity level as “excellent” on her clipboard.

The large fish is gently guided into the awaiting sling along the water’s surface after a lengthy battle.

“Get the tail up! Get the tail up!” someone yells. As the fish tussles and desperately tries to escape.

With the quick a tug of a rope the sling closes just in time.

Dr. Grosell and the others are part of a group of scientists from the RECOVER consortium at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science investigating the impacts of oil spills on mahi-mahi.

Mahi-mahi are a popular commercial and sportfish throughout the world – often targeted by recreational fishermen for their strength and acrobatics on the line. After the Deepwater Horizon disaster in 2010, scientists were concerned how mahi-mahi and other economically important fish – like tuna – would respond to the oil.

“Our previous studies have shown oil exposure negatively impacts these fish in a variety of ways from heart function to vision to swim performance,” says PhD student Lela Schlenker, the experiment’s lead scientist. “Those tests have all been done in a lab setting. What we’re trying to do here is see how oil impacts their survival out in the wild, but first we need to better understand how they behave in the wild under control conditions.”

To answer these questions Lela and the RECOVER team are off the coast of Miami outfitting wild mahi-mahi with pop-up satellite archival tags or PSATs.

“These tags will collect important temperature, depth, and migration data over 96 days,” she says.

Additionally, the PSATs will collect acceleration information which is going to tell her whether these fish are spawning. Lela and the team will remain at sea for the next three days aboard the University of Miami’s research ship the R/V Walton Smith conducting the largest mahi-mahi tagging experiment to date.

In the past, mahi-mahi have been tagged with some success, but most tags have not stayed attached for longer than 10 days. The team hopes to increase the tag retention and survivability of these fish by trying something new – holding them for 24 hours in 1,320-gallon recovery tanks aboard the ship to recover from angling and handling stress before they are released.

A task easier said than done.

Accompanying the Walton Smith on this expedition is an experienced local fishing charter out of Miami – the Miss Britt. The Miss Britt is acting as the research teams eyes and ears while at sea; using their extensive fishing knowledge and speed to track down schools of mahi-mahi and then radio a rendezvous location to the Walton Smith.

By the time Walton Smith arrives, the crew of this Miss Britt is already hooked up on a few mahi-mahi. The two crews work together using the appropriately named “tennis ball method” to transfer lines – and fish – between vessels.

A tennis ball whizzes by overhead (an unusual sight ten miles out at sea) from the stern of the Walton Smith and lands into the outstretched hands of the Miss Britt’s deckhand.

A perfect cast.

The deckhand unclips the ball and attaches the line to their leader with the hooked-up mahi-mahi. The fish – now on the Walton Smith’s line – is reeled in, but not without a fight. Once close enough, the fish is gently guided into the awaiting sling.

Here the clock starts and time is of the essence.

The mahi-mahi is quickly carried up the stairs in the sling and lowered into a bin of oxygenated water. This is to keep the fish as healthy as possible throughout the procedure. The entire team works in unison to quickly de-hook, measure, tag, sample, and record. The fish is then immediately placed in one of the onboard recovery tanks. The whole process is very fast – from sling to tank in about 1:40 seconds – and is reminiscent of a NASCAR pit crew.

Over the next 24 hours, the fish are closely monitored, ensuring the water chemistry and temperature are at the ideal levels to help the fish recover from the stressful catch and tagging procedure. When time is up, the mahi-mahi is corralled into the sling one more time and gently lowered down into the sea.

“This is the first time an experiment like this has been done, and it’s incredible to see this fish swim off healthy at the end of it,” says Schlenker. “They have a ton of spirt and they’re usually very happy get away from us, which is a good thing.”

At the end of the three-day cruise, nine fish were successfully tagged and released.

The next step for Lela and the RECOVER team is to patiently wait until the tags detach from the fish, float to the surface, and send the data back to the lab.

“This’ll be exciting to see where these fish go, how they travel, and what sort of environments they’re in. This is really important information for managing mahi-mahi and learning more about their ecology”.


Watch the trailer for an upcoming documentary about the experiment here:



This research was made possible by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the RECOVER Consortium.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

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