Designing an animal-friendly fin tag

For over two decades, Monterey Bay Aquarium and Stanford University have partnered to study some of the world’s most mysterious ocean predators at the Tuna Research and Conservation Center (TRCC). Some of the latest work to come from the TRCC include an innovative tuna tag design, and a paper recently published in the journal Science detailing the discovery of a hydraulic mechanism in tuna dorsal fins, which helps them swim with speed and precision.

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In his office at Stanford University’s Hopkins Marine Station in Pacific Grove, California, Dr. Vadim Pavlov holds a pale, sleeve-like device. Its smooth lines and soft edges make it seem more like a child’s toy than a high-tech scientific product. He slips the device over a model of a dolphin dorsal fin and “swims” it around his office, mimicking a dolphin’s movements as it leaps and twists out of the water.

The device is a prototype of a new tag design intended to track top ocean predators, such as sharks and tunas, without using pins and bolts that penetrate the fin.

“Even when the dolphin leaps, the tag stays on,” Vadim says. “But, how did we do it?”

Form and function

Vadim is one of the world’s top experts in biomimetics: the science of translating natural phenomena, such as the flow of water over a dolphin’s dorsal fin, into useful technology.

For years, he’s been tackling the challenge of tagging and tracking wildlife in the open ocean. He wanted to provide “animal-friendly” tags as an alternative to the invasive bolt tags anchored into the fins of apex marine predators such as sharks, dolphins and tunas. For Vadim, that’s not just a scientific goal; it’s personal, inspired by his experience as a free diver. “I don’t like swimming with lots of gear, so I don’t think [animals] do either,” he says. “They are very sensitive to anything on their bodies.”

A traditional tag can cause drag on an animal as it swims through the water.

Traditional bolt tags, a key tool in marine animal field studies for the last half century, are kind of like an ear piercing. Researchers punch through the cartilage and collagen in the dorsal fin and attach tags that can help track the animals, or collect environmental data such as salinity, temperature, and depth.

“But over time, these bolt tags do not move with the animals,” Vadim explains. “They can alter the flow of water around the animal’s bodies, and can even cause animals to turn more in one direction over time,” he says. “The faster the animal swims, the greater the energy needed to override the drag.”

Smaller animals, such as harbor porpoises and juvenile dolphins and sharks, are especially susceptible to the pitfalls of traditional bolt tags. “There’s a conflict between the animal’s biology and the technological requirements of the tag,” says Vadim. “So my job became how to reconcile that disconnect.”

Be like water

Before he could design a new concept for a non-invasive tag, Vadim studied how dorsal fins on different species interact with their liquid water environment.

Vadim set about 3-D scanning and measuring cetaceans such as the harbor porpoise, common dolphin, bottlenose dolphin, white-beaked dolphin, the Atlantic white-sided dolphin and pilot whale.

He needed to measure variables like flow pattern around the dorsal fin, the fin’s natural range of motion and the biomechanics of movements such as breaching and sharp turns. “The dorsal fin of marine animals serves as a vertical keel, preventing rolling and yawing during swimming, as well as side-slip during maneuvers,” he says.

With a series of computer-assisted designs (CAD), Vadim and his student, Fritz Mayr-Melnhof, created models of cetaceans and ran them through the computerized flow simulation to show the additional drag when a tag is attached.

A reduction in drag

These models, appearing on the screen as cascading streams of animated rainbows flowing around the bodies of tunas and dolphins, show that Vadim’s prototype tags increase drag by just 1-2 percent—compared with traditional bolt tags, which can increase drag by 10 percent.

“We want the animals to feel like they can move freely so we can get the most accurate representation of where they go and what they do,” says Vadim. “Ideally, the tags shouldn’t feel any more invasive to the animals than a wrist watch on our own bodies.”

But without any bolts or pins, how do the tags actually stay in place?

“It’s the same aerodynamics that work for Formula 1 race cars,” Vadim explains. “As air flows over the car, it hits the spoiler and presses the car with downward force against the ground. That’s called inverted lift force. In the case of marine animals, it’s the flow of water that presses the tags into place.”

The design has promise for dolphins and sharks, which have similar rigid dorsal fins. But tunas presented a new set of challenges.

“One strange fish”

Vadim came to the Tuna Research and Conservation Center (TRCC) in 2015 to design a tag prototype specifically tailored for tunas. The project was part of his postdoctoral research with Stanford professor Dr. Barbara Block.

First, he worked with TRCC Research Technician George Parish to capture video footage of tunas swimming. During routine examinations, Vadim noticed a small, black, fluid-filled chamber, or “vascular sinus,” at the base of the fin. The chamber was connected to a larger network of vessels that intertwined with the channels running between the bones of the fin (also known as bony fin rays).

Nate Hansen, co-author on the paper recently published in Science, was baffled. “The scientific literature offered no clues as to what it could be. So, by the second week into the investigation, we were thinking, This is one strange fish.”

So the team decided to perform a test. By injecting this chamber with a saline solution, they discovered, they could manipulate the fin’s movement. Eventually they could fine-tune the pressure and volume of liquid needed to articulate the fin up or down at a range of angles.

The team checked the video cameras to decipher when and how the tunas used this anatomical feature to move their dorsal fins.

“And there it was,” says Vadim. “The fins moving just like this—” he waves his fingers back and forth like a fan opening and closing. “After viewing the footage, we could measure the exact angle of the fins and how that angle changed based on which behavior the tunas exhibited.”

Hydraulic forces behind fin movement

Between video footage of tunas in the TRCC and in Monterey Bay Aquarium’s Open Sea Exhibit, along with other tests, Vadim and the team determined that the tuna’s lymphatic system serves as a hydraulic mechanism for maneuvering the dorsal fins. The movement of the fins increases during, and directly after, feeding—moving back and forth along finely tuned angles to aid in quick swimming and sharp turns to catch their prey.

Vadim invited Benyamin Rosental, postdoctoral researcher at Hopkins and expert in immunology, to help him investigate the origins of this unique anatomical feature. Benyamin’s sophisticated tests revealed the vessels and channels belonged to the tuna’s lymphatic system. The lymphatic system is critical for immune function, but scientists hadn’t known lymphatic fluid could act as a hydraulic fluid in locomotion.

This fascinating discovery marks the first time the lymphatic complex has been shown controlling a combination of muscles, vascular vessels and fin rays in a vertebrate. It demonstrates what the team has been calling the fine “tunability” of tuna dorsal fins for fast and precise ocean maneuvering.

Now that Vadim has an intricate understanding of tuna dorsal fins, he’s been able to design and test a number of his non-invasive tag prototypes on TRCC tunas.

“Next,” he says, “we need to get the tags onto animals in the wild.”

With time and field studies, these tags may help researchers gather the best possible data on vulnerable predators, such as Pacific bluefin tunas, that play an essential role in maintaining the health of our global ocean.

 

— Athena Copenhaver

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Featured photo: A tuna at the Tuna Research Conservation Center tests out a non-invasive tag prototype. Photo courtesy TRCC.

Learn more about bluefin tuna research at Monterey Bay Aquarium.