Fish Tagging

Ecosystems have limits to the numbers of organisms and population sizes they can support. These limits are set by predation, competition, diseases and the physical habitat (reefs, artificial reefs, mud flats, sea grass, and marsh). Fish biologists are using two types of tagging to understand how the physical environment constrains the population dynamics of the red drum, North Carolina's state fish.

JACKSONVILLE - It’s a beautiful day on the New River, near Jacksonville, North Carolina. The water is brackish, which is a mix of salt and fresh water. The river is surrounded on all sides by Marine Corps Base Camp Lejeune, so there is little development. The shoreline is mostly trees and wetlands, which has helped keep pollution low and that has kept the water relatively clean and incredibly productive. Researchers with the University of North Carolina Chapel Hill Institute of Marine Sciences are standing on the front of a boat pullling in nets to see what they’ve caught. Catch and release studies like this tell researchers that flounder, crab and spot are just some of the aquatic species living beneath the surface of the water. But there’s a limit to what scientists can learn by simply looking at fish. 

“We can’t see through the ocean, the fish are invisible to us,” says Dr. Joel Fodrie, a biologist with the institute. “If I were studying trees and I tagged a tree and I came back the next day and the three was gone, I’d assume it was dead. With the fish, the fish may have just moved on.”

In short, scientists can learn about the fish tail, or the specifics of each fish and what it eats, and whether it is growing and doing well. However they can’t know the fish story, including where it lives and travels. And scientists believe that where fish live, which is known as their habitat, is very important. The salt grass marsh, the mud flats and oyster reefs are all habitats found in the estuary and some are under more stress than others. Scientists are hoping the tagging study they are working on can show the relationship between habitat health and fish populations.

“Managers, scientists and the general public all agree that habitat is good for fish, because without good habitat you lose fish,” says Fodrie. “But quantitatively, we can’t tell how much habitat translates in the number of fish living in an area or how much the population will grow. In other words, does decreasing habitat mean there will be less fish?”

So on this trip, Dr. Fodrie and his team are not only looking at what is caught in the nets they set the previous day. They are looking specifically for Red Drum. It’s North Carolina’s state fish and it’s also the subject of a unique tagging study in which scientists will track Red Drum to see where the fish travel and what types of habitats they live in.

The scientists discover that two Red Drum are caught in the nets and they each weight about 5 pounds. That makes the fish the perfect size for the experiment. The fish are placed in a tank with a mild anesthetic. It takes about a minute for the fish to roll belly up, which is usually a sign they are dead. In this case, they are just sleeping.

The fish are placed on a foam mat, and a tube with water running through it is placed in their mouth. That keeps water running over the fish’s gills so it can breathe.

Rachel Gittman, a graduate assistant, uses a scalpel to make a small incision on the underside of the fish. Then, she inserts a tracking device into the stomach cavity. The device is smooth, black, and about an inch long and a half inch wide. Once turned on, it sends an electronic signal, which is unique for each fish and can be recorded by listening stations set up around the estuary.

“It presses up against the organ but doesn’t do any damage,” says Matt Kenworthy, the lab manager, as he watches the procedure. “We’ve done studies to make sure the fish are fine and able to swim, eat and survive and they do fine.”

The incision is then sewn closed with surgical thread. A first aid cream for humans, normally found in the corner drug store, is applied to the wound.

“Yes, it wards off infections on fish too!” says Kenworthy, laughing.

The fish is then measured, weighed and a small piece of the tail is clipped off for DNA analysis. The fish is placed in a tank of fresh water and appears to be awake and swimming just fine in a few minutes. The fish is then released. The entire procedure, from sedation to recovery, takes about 15 minutes.

“We’re kind of on the cutting edge, because we are able to know what the fish are doing daily, even minute by minute,” adds Gittman, as she watches the fish swim away. “Before, if you put traps out and if you caught a fish, you assumed the fish was living in that habitat and feeding or seeking refuge. But now we can quantify how much time each fish is spending in each habitat. I think we are really moving forward in terms of seeing what fish use what habitat and what components of habitat are most important to them.”

There are about 100 Red Drum swimming around the New River Estuary. The fish appear as dots moving around on a video screen, which shows a map of the estuary. Besides the internal tracking devices, each fish also has a spaghetti tag attached. It’s a plastic tag, surgically implanted which pokes out the side of the fish, which has a tracking number unique to each fish as well as a number to call to report if the fish is caught. The tags are a way to track fish mortality so state fisheries managers can better understand the population and set fishing limits.

It’s too early to draw any conclusions about healthy habitats and fish populations from the study so far, but scientists are already fascinated by what they are discovering about fish behavior.

“We can already conclude that fish have as much personality has people, in terms of some like to travel and be on the go a lot and there are some that are homebodies,” says Fodrie.

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