Trawling For Clarity

It’s exactly the things that make the lake special — its depth and clear blue hue — that have helped the shrimp interfere with the lake’s ecosystem. Because of how deep and clear the lake is, the things that wildlife officials hoped would happen when they introduced the shrimp backfired.

The biologists who introduced the shrimp didn’t know what the consequences would be, Nevada Department of Wildlife Fisheries Supervisor Kim Tisdale said.

“Obviously, if they could go back in time and reevaluate that, they definitely wouldn’t have made that same decision,” Tisdale said. “But we have the luxury of knowing the results today. They didn’t have that luxury then.”

Before the shrimp were introduced into Tahoe, zooplankton in the lake ate algae and sediment, clearing the water by doing so. Some fish also ate the zooplankton. Then the states introduced the shrimp, and everything changed.

The shrimp hate light, and because Lake Tahoe is so clear, they have to retreat to the depths of the lake during the day to avoid it. Then, each night, they undertake a round-trip migration as long as 2,000 feet to the top of the water to feed. Once the sun rises again, they swim back down.

While officials put the shrimp in the lake in the hopes of fish eating them, that hasn’t been the case because of this nightly vertical migration.

Some fish in the lake do eat the mysis, but others, like the kokanee salmon, never get the chance to. Kokanee and other fish in the lake are sight feeders, which means they search out food by eye. These fish live in the upper parts of the lake, so when the fish can see, the shrimp are down below. When the shrimp are near the surface, it’s too dark for the fish to see and eat them.

Not only did the shrimp not fulfill their intended role, but they also brought a devastating and unintended consequence.

After the shrimp migrate each night, they’re starving from the long journey. Because of this, they’ve managed to decimate their main food source in Lake Tahoe: native zooplankton.

These zooplankton were an important food source for the kokanee, and the size of those fish in Lake Tahoe has dropped dramatically since the shrimp were introduced. But these tiny creatures that the shrimp were devouring, called Daphnia, were also keeping the lake clean, researchers found out.

“As far as zooplankton in Lake Tahoe go, Daphnia is responsible for removing about 85% of the particles and algae from the water column,” said Katie Senft, one of the researchers on Allen’s team.

Daphnia feed on the lake’s algae and sediment. They use the algae as fuel and process the rest into pellets, which they excrete. The pellets sink to the bottom of the lake, where they settle at the bottom and don’t dissolve back into the water, removing this sediment from the lake.

Other zooplankton in the lake are like rabbits, Senft said. They’re fast, and they don’t eat much. Meanwhile, Daphnia are more like cows: They’re slow and incredibly efficient at eating up all of the algae and sediment in the lake.

“So if you’re a mysis shrimp swimming through at night, you’re going to eat the cow,” Senft said. “But then you have these inefficient grazers, the rabbits that are left and the field gets overgrown, and you lose your clarity.”

Allen and his team’s goal is to have about 5,000 Daphnia per cubic meter in part of the lake. When they last measured the Daphnia in 2018 in Emerald Bay, the part of the lake they’re studying, they couldn’t find any at all.

But a bizarre turn of events about 10 years ago in Emerald Bay, where Allen and his team are studying the shrimp, gave them hope that this could be undone.

Emerald Bay is a small inlet on the southern shore of Lake Tahoe that tends to be murkier than the rest of the lake. In winter 2011, Allen’s team went out on the bay to check on the mysis, only to discover that they had all but disappeared.

Allen to this day doesn’t know why the shrimp disappeared, but as they monitored the bay over the next couple of years, the team noticed another major change.

About two years after the shrimp disappeared, the bay’s Daphnia population returned to normal and the water clarity doubled. The bay’s clarity that year even exceeded Lake Tahoe as a whole, which is very unusual.

But the shrimp eventually came back. And when they returned, they had a massive food resource thanks to the Daphnia resurgence that took place in their absence.

As the shrimp fed on this rebuilt Daphnia population, their birth rates went up, Allen said. Their population boomed, and they began to graze down the Daphnia once again.

“And so our shrimp came back and the clarity went back to what it was prior to the incredible clarity we’d seen,” Allen said.

People have long blamed urbanization and tourism for the lake’s waning clarity, Schladow said. But this occurrence made him rethink that.

“I suddenly realized, maybe the urbanization was not the only thing happening,” he said. “Maybe our screwing around with the ecosystem had maybe an equally big impact on the clarity.”

The disappearance of the shrimp gave the researchers an idea: Could they replicate the increase in clarity in Emerald Bay by removing the shrimp?

That’s the question Allen and his team at the Tahoe Environmental Research Center are trying to answer. After a few years of sporadic funding, the center finally got a grant in 2018 to test their plan.

A few days a week Allen, Senft and fellow researcher Brandon Berry leave the lab after a full day’s work, have a burger at the Bridgetender Tavern in Tahoe City, change into warm clothes and head out on the lake. They drive their boat, the Research Vessel John Le Conte, across the lake to Emerald Bay to trawl for mysis shrimp. It takes more than two hours for the crew to get from Tahoe City to the bay because the large boat can only go about 8 mph.

Because the shrimp are so small, the trawl they use to remove the shrimp looks more like a tube sock floating through the water than a deep-sea net. They pull the trawl across the lake twice before hauling it back in and dumping clumps of oily shrimp into large buckets.

During this two-year pilot project, they want to find a way that humans or machines could remove enough shrimp from the lake to make a lasting impact.

“What makes this project unique is that we’re looking at ways to improve Tahoe’s water clarity by looking at the aquatic ecosystem and trying to adjust the aquatic ecosystem to its historic state,” Allen said, “because our preliminary data is showing that that could have big implications for water clarity.”

The shrimp are here to stay in one way or another, Allen said, and there’s no point trying to eliminate them completely. But if he and his team can reduce the number of shrimp in the lake by about 75%, he says, the lake’s Daphnia population could return to normal.

But achieving this would mean a huge reduction in the number of shrimp in the lake. In December 2018, Allen’s team counted more than 100 shrimp per square meter in Emerald Bay. Figuring out how to bring the shrimp population down to the level Allen hopes for is a huge challenge.

Even if the center’s efforts now prove successful, the current method is not sustainable, he told us. Since the shrimp are only near the surface after dark, the team currently works full days in the UC Davis lab before heading out to trawl all night.

They’re doing this grueling night shift in the hope that, at the end of their pilot project, they can pass a plan on to someone else to remove the shrimp more efficiently.

“What we’re doing is we’re trying to figure out, if someone was hired to do this work, how would they go about it?” Senft said.

But for now, they stay awake during their trawls with the help of peanut M&Ms, the many spiders living on the boat and occasional mysis shrimp cookout experiments. The team has yet to find the right seasoning to make them bearable.

“They’re not very tasty,” Allen said. “We initially just tried eating them raw … wasn’t very good. We boiled them — that was only slightly worse.”

Whether it’s self-driving boats or selling shrimp to the supplement industry, Allen is hopeful that an efficient way to remove the shrimp from the lake is out there. Right now, the team is looking at building a smaller trawl so they can scoop up juvenile mysis shrimp along the shore before they get the chance to reproduce.

“My vision of success is developing year-round techniques for removing shrimp,” Allen said. “You’re not going to remove enough shrimp in a single season to get to this target level that allows native zooplankton to come back.”

For Schladow, it’s easy to acknowledge this is an odd way of approaching lake clarity. But it’s mitigating the damage humans have done to the lake that he thinks could be a missing piece of the puzzle to help keep Tahoe blue in the face of a changing climate.

“The lake was doing perfectly well before we came here,” he said. “Everything we can do to help it get back to how it was operating then can only be good.”