Do shrimp hold the key to a clearer Lake Tahoe? Researchers plan to find out

A mysis shrimp. A mysis shrimp. Photo: Tahoe Environmental Research Center Photo: Tahoe Environmental Research Center Image 1 of / 49 Caption Close Do shrimp hold the key to a clearer Lake Tahoe? Researchers plan to find out 1 / 49 Back to Gallery

In recent decades, Lake Tahoe has grown murkier and murkier, with people quick to blame obvious culprits: a rise in tourism and development, along with fluctuations in drought conditions and rainfall.

But an unlikely crustacean culprit may also play a role in the story of the lake's decreasing clarity, some researchers now believe, according to the annual State of the Lake report from the Tahoe Environmental Research Center at UC Davis.

Mysis shrimp were introduced to Lake Tahoe in the mid-20th century by what was then called the California Department of Fish and Game, with the hope they would provide a food source for the mid-sized fish that lake goers enjoyed catching on fishing trips, like the Tahoe angler.

"It was a deliberate introduction," said Professor Geoffrey Schladow, director of the Tahoe Environmental Research Center.

But things didn't go according to plan once the shrimp, which measure about a half inch-long, were in the water.

"It turned out to be an ill-conceived idea," Schladow said, "because the shrimp have this daily vertical migration. During the day, they migrate down into the dark, and because the fish are sight feeders they can't see them. Then at night, they come up and it's dark, and because the fish are sight feeders they still can't see them."

When the shrimp come up at night, they eat the zooplankton that the fish also feed on, making things worse for the fish.

"So the net result was that the food supply for the fish went down, the fish got smaller and Tahoe was left with an invasive species," Schladow explained.

Changes in the water at Emerald Bay, about 12 miles north of South Lake Tahoe, were a surprising clue that the shrimp might have an effect not just on the fish, but also on the clarity of the water. For some reason that researchers don't fully understand, several years ago the Mysis shrimp disappeared from Emerald Bay. In the time that followed, populations of bosmina and daphnia zooplankton started to come back.

"What took us by surprise is once we had the daphnia back there, the clarity of Emerald Bay improved by something like 40 feet in a year, which is a huge change," said Schladow.

"The hypothesis this brought to mind for us is that maybe these native zooplankton that had been removed by the Mysis had been serving this role of keeping Tahoe's water clean by the way they feed," he said.

Not unlike this reporter, the zooplankton are "very indiscriminate eaters."

"They literally shovel anything into their mouths," ingesting water-clouding fine particles and phytoplankton and discharging them as feces that sinks to the bottom of the lake, he explained.

After a couple years, the shrimp returned to Emerald Bay, and clarity returned to where it had been before.

"It was a perfect experiment, in our minds anyway," Schladow said, "but it was an experiment that nature performed that we had no control over."

But it made researchers wonder if the results could be recreated by trawling for the shrimp — and if some of the declining clarity that was attributed to tourists flocking to Tahoe in the mid-20th century was actually caused by shrimp's changes to the lake's ecology.

Now, a new pilot program funded by the Nevada Division of Environmental Protection and the California Tahoe Conservancy has started to trawl for the shrimp in Emerald Bay with the hope that clarity might increase as the zooplankton populations bounce back — and that successful results could be scaled up and applied to Lake Tahoe.

The trawling is occurring now and is expected to take about 3 months, after which the clarity and chemistry of the water will continue to be monitored.

The trawling must be done at night because of the shrimp's daily migration to the bottom of the lake during the daylight hours. At night, they hover 50 to 60 feet below the surface of the water, according to Schladow.

"It's a big boat with an even bigger net, but it has to happen at night," he said. "We have an echosounder to tell exactly what depth they're at and adjust the net to be at that height."

"If our hypothesis is correct and if we can develop a way to remove sufficient Mysis, we'll have two ways of working on Tahoe's clarity — controls on land use and this new, ecology-based approach," he said.

Researchers are excited by the possibilities. Added Schladow, "It's wonderful to be surprised."

Filipa Ioannou is an SFGATE staff writer. Email her at fioannou@sfchronicle.com and follow her on Twitter