TAHOE: Outsider shrimp an issue

will return to what it was a few years ago,” he said.

As Schladow expected, the Daphnia and Bosmina, the two types of zooplankton, were present in the lake, and the clarity was still very high during their most recent venture out on Lake Tahoe last week.

The sampling crew, consisting of Brant Allen (who has been working on Lake Tahoe for 34 years), Katie Senft, Brandon Berry (a Davis native), and Erik Young, collected the data covered by the Lake Tahoe Clarity Report 2022.

Clarity is measured as the depth to which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water.

According to a press release from the UCD Tahoe Environmental Research Center, in 2022, Lake Tahoe’s average annual clarity was 71.7 feet compared to 61 feet in 2021. The key finding from 2022, according to the release, was the great improvement in lake clarity from August through December, when the average Secchi depth was 80.6 feet. This coincided with the highest numbers of the zooplankton Daphnia and Bosmina. The states of California and Nevada, which share a border at Lake Tahoe, are actively working to restore lake clarity to its historic 97.4 feet.

Because the lake is very dynamic and can have an exceptionally high or low reading anytime, Schladow said a single day’s increase in Secchi depth is insignificant. “What is significant is that for five months, for over 12 sampling days, we have seen consistently high clarity and the presence of the native zooplankton. That has never happened before in over 54 years of measurements.

During those 54 years, there have been droughts, floods, and wildfires. So what is happening now is significant and instructive.”

Schladow explained the Mysis shrimp were deliberately introduced to Lake Tahoe in the 1960s as it was believed they would be helpful for game-fish growth. It didn’t work out that way, and the average fish size declined. They were also introduced to many other lakes across the West, and they rarely had a positive influence.

In Flathead Lake, Montana; and Lake Washington, Washington, they changed the food web drastically and negatively. At Tahoe, it was recognized a long time ago that they were impacting the food web.

Two other UC Davis scientists (Bob Richards and Charles Goldman) showed that in the late 1960s, Schladow said.

“What is new now is that for the first time, we have established the connection between the Mysis and clarity. Mysis removed the native zooplankton that had naturally been feeding on the ver y tiny algae and clay particles that can cause clarity decline. Once the zooplankton were removed, there was nothing to remove the algae and clay particles, and clarity declined. Because of all the uncontrolled development at Lake Tahoe in the 1960s and 1970s, nutrients (that cause algae growth) and clay were being washed in faster than ever,” Schladow said.

In late 2021, the Mysis population unexpectedly crashed, and it took 12 months for the Daphnia and Bosmina to build up their numbers and start their natural cleansing.

A different zooplankton species (copepods) that do not impact clarity, had been the main food for Mysis in the last 50 or so years. Early last year, we noticed that they seemed to be developing a fungal infection (we are currently having specimens examined), and very soon after that, they largely disappeared. With little left to eat, we believe the Mysis simply starved to death.

The very small numbers of Daphnia and Bosmina in the lake now had no predators, but it takes time for them to reproduce and build up their numbers, Schladow said. “That is why there was a time gap.”

Knowing that the Mysis shrimp have harmed water clarity, TERC’s scientists have worked in the past with students from the Graduate School of Management and most recently with a nonprofit called Shrimply Blue. They have shown that if Mysis were harvested from Lake Tahoe, then the cost of removing them would largely be offset by the cost of marketing the shrimp. They would not be used for shrimp cocktails but instead used as the key ingredient in dog treats. The Mysis, because of the great diet they have in Lake Tahoe, are very high in Omega-3 fatty acids and proteins. Past research has shown us the level to which the Mysis have to be brought down in order to allow Daphnia to coexist with Mysis. That is about 20% of what their normal population is.

Schladow said a future restoration plan could include the harvesting of Mysis, the production of highly nutritious dog treats, and the proceeds from that going back to pay for the harvesting and to help continue the research at Lake Tahoe. As all of that work would be happening at Lake Tahoe, it would be creating new employment opportunities there.

Schladow said when scientists saw Daphnia return to Emerald Bay 10 years ago, kokanee salmon were much larger. The State Record kokanee was caught that year adjacent to Emerald Bay. The previous state record was established in the later 1960s, right when Mysis were first introduced. “Is that coincidence? I think not,” he said. Do the Mysis shrimp do anything good for the balance of the lake?

“Not really,” Schladow said. “They have changed the balance of the lake, so in a way they are part of the new balance. But it is not a good balance. If Mysis were controlled, then the lake would be a lot closer to its native state, and the new balance would have higher clarity and quite possibly larger game fish. That is another aspect we are currently studying.”