Chesapeake Biological Laboratory

Using sound, scientists search murky Bay waters

Chesapeake Biological Laboratory researchers seek to learn more about elusive mysid shrimp

Ryan Woodland

A favorite meal for some of the Chesapeake Bay’s most economically important fish may also be the Bay’s best-kept secret.

While juveniles of summer flounder, white perch, and weakfish heavily dine on mysid shrimp, scientists on shore know very little about this tiny but critical part of the food web.

“Because we know so little about mysids, they are really a black box. We know they’re important, but we don’t know much of anything about them,” said Ryan Woodland, an assistant professor at the University of Maryland Center for Environmental Science’s Chesapeake Biological Laboratory.

Woodland has been studying forage fish since graduate school and knew even then that he wanted to know more about mysid shrimp. Now, with funding help from Maryland Sea Grant for a two-year study, he hopes to answer some of his outstanding questions about the local abundance and ecological role of mysid shrimp.

The results of this study could help fishery managers when they set harvesting regulations, Woodland said.

You would think by now the Chesapeake Bay has been studied for so many years we should know, but clearly we don’t have an answer. It does require a lot of coordinated effort to answer those questions.

Hongsheng Bi
Associate professor, Chesapeake Biological Laboratory

 

Traditionally, managers would take use scientific data about, for example, striped bass to determine how many could be harvested without affecting the population.

Now, however, management is moving toward an ecosystem-based approach. That approach would consider not only how a population responds to a harvest, but also how that population might respond to the availability of food in a given year, how the environment has changed, or if threats from larger fish have increased, he said.

Adaptive Resolution Imaging Sonar, or ARIS, works like a camera, but relies on sound rather than light, making it ideal for searching even the Bay’s most turbid water.

“We want to be able to eventually manage for fisheries so that our harvest accounts for other factors that can be influencing a given species,” Woodland said. “Then we can say if it’s a poor year for forage, maybe we don’t want to harvest as much this year, or maybe we need to think about harvesting less in certain areas, because there’s less food available and it might be a poor year for growth.”

Working alongside Woodland as co-principal investigators are Elizabeth North, an associate professor at UMCES’ Horn Point Laboratory and Hongsheng Bi, an associate professor at the Chesapeake Biological Laboratory. Danielle Quill, a Maryland Sea Grant Fellow, will also assist Woodland on the project.

North will help Woodland interpret the data that Bi helps him collect.

Bi has spent the last few years using Adaptive Resolution Imaging Sonar, or ARIS, to understand jellyfish distribution in the Bay, but it can detect anything in its path in the water column.

ARIS works like a camera, but relies on sound rather than light, making it ideal for searching even the Bay’s most turbid water, Bi said.

It also offers an advantage over traditional sampling methods.

“If you think about traditional net sampling, you can’t drag your net everywhere. You go to your station, you drop your net, then you’re done,” Bi said. “But in this particular case, while your boat is going, you are recording. We can cover a lot more ground than traditional net sampling and at the same time, the data we get is high spatial resolution.”

Hongsheng Bi

As the boat moves, the device will constantly search with sound and the sound bounces back to the device, ultimately building an image.

“With sea nettles, we can even see the tentacles,” Bi said.  

Essentially, ARIS can give scientists a similar ability as dolphins have with echolocation without having to enter the water.

Even with ARIS, finding an organism that can be less than a half-inch long would pose a problem with the shrimp if not for one well-known habit—it swims in dense swarms during the daytime.

“It’s like a swarm of bees,” Woodland said.

A swarm could measure up to 30 feet long, he estimated, but added it won’t be clear until further into this study how the size or concentration of a swarm may change based on water quality or depth.

The scientists were successfully searching for these swarms as part of a pilot program. That initial study also gave them a starting point now that they are fully funded. They will start surveying the Patuxent and Choptank rivers in southern Maryland and on the Eastern Shore  in May and conduct monthly samplings through September.

During the daytime, they will use the sonar to locate the swarms. Then at night, when the shrimp typically disperse and swim toward the water’s surface, the scientists will skim the water’s surface with plankton nets to capture them.

Using stable isotopes, the scientists can learn about the shrimp’s eating habits and determine if their habits vary depending on water quality.

“We want to figure out, relatively speaking, where are they along the tributary, when they are the most abundant, and does their abundance change in response to water quality, to temperature, salinity, and hypoxia,” Woodland said.

What they are able to learn from this study could create more opportunities for future studies, including understanding more about the communities that make up the mysid shrimps’ swarms. For now, all the scientists know about the shrimp in the Bay is from the little work they have done to prepare for this study and some historical survey data.

“You would think by now the Chesapeake Bay has been studied for so many years we should know, but clearly we don’t have an answer,” Bi said. “It does require a lot of coordinated effort to answer those questions.”

See ARIS in action

Mysid swarms via radar

A swarm of mysid shrimp can be seen in this video captured using the ARIS device.