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Trawling for a Better Way to Assess Fish Health

On a warm morning last August, Ryan Schloesser and his labmate, student Lauren Nys, trawled off Oyster, VA. After a summer filled with collecting fish, they worked with experienced ease, throwing around jokes as smoothly as they tossed their nets behind the boat. What they pull up in their nets should help fisheries managers better predict the size of fish populations.

The crew of the R/V Shearwater let out a net to trawl for flounder. ©Janet Krenn/VASG

The crew of the R/V Shearwater let out a net to trawl for flounder. ©Janet Krenn/VASG

By Janet Krenn

On a warm morning last August, Ryan Schloesser and his labmate, student Lauren Nys, trawled off Oyster, VA. After a summer filled with collecting fish, they worked with experienced ease, throwing around jokes as smoothly as they tossed their nets behind the boat. What they pull up in their nets should help fisheries managers better predict the size of fish populations.

But predicting population size is more complicated than simply going out and counting fish, says Schloesser, a Virginia Institute of Marine Science Ph.D. candidate and Virginia Sea Grant-funded research fellow. “There are some years when we see a high abundance of young-of-year fish, but then we don’t get a high adult abundance. We don’t always know why that is.”

Schloesser suspects that some young fish don’t make it to adulthood because they don’t survive the winter, when waters are colder and prey is less available. Assuming that fat young fish are better equipped to withstand cold temperatures and intermittent food supply, Schloesser hopes to demonstrate a non-lethal method for measuring fat content in popular fish species including summer flounder, striped bass, and Atlantic croaker.

On this day Schloesser was hunting for small flounder on which to test his method using an aptly named device he calls the fatmeter. The fatmeter is a little handheld box that uses radiowaves to assess fat content. To work the fatmeter, Schloesser lays the fish on a flat surface and holds the device along its belly. The process takes about a minute aboard the boat, and Schloesser says it’s relatively easy and harmless for the fish compared to other methods: “As opposed to taking out the fish’s liver,” Schloesser chuckles. “They’re not swimming away after that.”

Schloesser, a Wisconsin native, says he always knew he wanted to be a marine biologist. As an undergrad at Texas A&M, he studied marine biology and got his first experiences working on the water: otter trawling for juvenile red snapper and later researching blue fin tuna.

“I’m excited by questions,” he says. Adding with a laugh, “Rhetorical questions.” Questions like, how does the environment affect the number of young fish that survive to become citation-sized seafood? What do we need to know to predict how many fish we should take next year? All joking aside, these big picture issues have real implications for the way fisheries are managed and for our ability to ensure that we have fish to catch in coming years.

These complicated questions all fall under ecosystem-based management, which Schloesser adds, “is a data-hungry approach.” Ecosystem-based management takes into account all of the environmental factors that may affect fish populations, including weather, the availability of food, or the number of predators.

Understanding the health of young fish is one more piece of the puzzle, a piece that needs more investigation. In working up last summer’s data and comparing the fatmeter to other methods for measuring fish health, Schloesser has found that the fatmeter doesn’t measure up to other methods for flounder, which are typically a lean fish. However he found promising results when he used it on fattier species, such as striped bass and croaker.

With the fatmeter’s reasonable success, Schloesser is motivated to answer those complicated questions.

“It all comes down to sustainability. The oceans are becoming more and more important as a source of food,” he says. “It comes down to flounder, striped bass, and croaker being around for hundreds and hundreds of years.”