Fishing through history: Tahoe’s ecosystem ever-changing as native numbers decline
Lake Tahoe has changed over the years and that change becomes obvious when looking at the lake’s food web.
Due to overfishing, introduction of non-native species (both on purpose and accidental) and elevated water temperatures, the lake basically has a new ecosystem. And while all but one native species still exist in the lake, their numbers are dwindling.
“Before stuff was introduced, Lake Tahoe had a very simple food web,” said Dr. Sudeep Chandra, a professor of biology at the University of Nevada, Reno. “It had about 12 types of invertebrate orders … maybe up to 18 but not a lot.”
A look at the 1800’s food web shows the Lahontan Cutthroat Trout at the top of the chain of offshore fish — fish that hang out away from the shoreline. Under it is the tui chub and mountain whitehead.
These offshore fish feed on zooplankton and benthic macroinvertebrates, or bottom dwellers, which feed on phytoplankton, near-shore plants and algae.
Feeding on the zooplankton and benthic macroinvertebrates are the near-shore and bottom fish which consist of speckled dace, Paiute sculpin, Lahontan redside and Tahoe sucker.
Chandra, who specializes in restoring native species and managing non-native species, said there were about two to six native plants, so overall, not a complex food web.
However, over the years, the cutthroat population began to decline.
“It was essentially commercial fisheries of cutthroat trout in the late 1800s, they were harvesting more fish than the population could sustain,” said Brant Allen, biologist with UC Davis, Tahoe Environmental Research Center. “So, with the decrease in the cutthroat trout, they started looking for other species of game fish that could replace them.”
Between the 1800s and 1900s rainbow, brook, brown and lake trout were all introduced into the lake.
The introduction of lake trout, also known as mackinaw, marked the end for the cutthroat trout. The mackinaw had established themselves in the lake by the late 1910s and by 1940 the cutthroat trout was gone from Lake Tahoe.
Sarah Hockensmith, the outreach director for the Tahoe Institute for Natural Science has studied natural resource management. She is an avid angler and has caught many of Tahoe’s non-native fish but still, she’d rather those species not have been introduced.
“Never in the history of anything has introducing a non-native animal to a habitat ever been a good thing,” Hockensmith said. “So, it’s really important to remember that sometimes people have good intentions and that’s why it is so good to rely on data and science and what’s natural to make good decisions, because if you just are trying to fix a problem or make something beneficial to humans by introducing a non-native species or trying to correct it with another non-native species, it’s typically going to lead to a failure. Especially in that habitat or within that ecosystem.”
Allen said there have been efforts in lakes around the country to find a way for mackinaw and cutthroat to live in harmony and there has yet to be success.
“There have been some improvements but I wouldn’t say that it has yet been successful,” Allen said in regards to an effort in Yellowstone Lake. “And the reason they’re spending so much time and energy and money trying to remove lake trout is because they are the predator of cutthroat, when they were put in the Cutthroat disappeared essentially. And so that story’s been told in multiple places, not just Tahoe.”
There have been efforts to reintroduce cutthroat into the watershed, and while it hasn’t been successful in the lake, there has been success in alpine lakes around the basin.
Between 1888 and 1936 crayfish were introduced in the watershed surrounding Tahoe, to be a food source for humans. However, they found their way into the Tahoe and without any real predators, they’ve settled in and grown to large sizes.
“Crayfish numbers have more than doubled since they were first estimated in the 1960s when the shrimp was first being introduced and we think they’re doubling because now the trout no longer feed on them and they’re basically able to grow,” Chandra said.
During the winter, they migrate from the shores down to deeper waters, eating all the native near-shore plant species along the way. In addition, Chandra said the crayfish are likely eating the eggs of native near-shore fish.
In addition to the various trout that were introduced, Kokanee salmon were accidentally introduced in the 1940s. A salmon fish hatchery in Tahoe City spilled, allowing Kokanee to enter the lake.
“They’ve been kept in the lake by artificial spawning and rearing of eggs in Taylor Creek,” Chandra said. “They wouldn’t survive in high numbers if we didn’t have the Taylor Creek fall fish run.”
In the 1960s, mysis shrimp were introduced into the lake as a food source for the lake trout. Even though it was done intentionally, it was a mistake the basin is still dealing with today.
Mysis shrimp have large, sensitive eyes so during the day, they migrate down to the depths of Tahoe, out of reach for the trout. Not only are they not a good food source, but they also feed on native zooplankton, Daphnia and Bosmina, which are two species that feed on algae.
In addition, those two species of zooplankton are great sources of food for Kokanee.
“Early research from UC Davis in the 70s shows that the spawning numbers of Kokanee crashes once the shrimp are introduced because the food is gone and the sizes of the Kokanee went from about 50 centimeters to about 2 feet down to 35 centimeters so just above a foot,” Chandra said.
While all of these are examples of species being intentionally introduced, starting in the 1980s, several species were accidentally introduced, including water warm fish like bass and bluegill.
Goldfish have also been introduced from people dumping aquariums in the lake and species like Curlyleaf pondweed, Asian clam and Eurasian watermilfoil have hitched rides on vessels and established themselves in the lake.
There have been several efforts to eradicate those species and recent efforts, like the bubble-curtain and ultraviolet boats, have had some success. The Tahoe Regional Planning Agency has implemented boat inspections to prevent other non-natives from hitching rides.
But when it comes to the bigger species, like lake trout, Hockensmith said it’s impossible to eradicate those.
Allen said there is a philosophical debate about even if they could eradicate those species, should they.
“There’s been the cold water game fish species that were put in like trout and Kokanee, rainbows and browns; those were put in as sanctioned introductions by the resource agencies, California Department of Fish and Game and Wildlife,” Allen said. “And they provide a great recreational aspect to the lake and there’s a huge economic component to it.”
There are businesses such as sporting good shops, and fishing tours that have built their business around those fish. In turn, those companies bring in a lot of tourism dollars to the basin and contribute greatly to the local economy.
And there are other positive benefits. Hockensmith said Osprey populations have been able to nest in Tahoe for the first time because they can easily catch rainbow trout or warm water fish.
“It’s interesting to see the balances of these non-native fish and how other opportunists like these other native animals, like bears and raccoons, are able to capitalize on that,” Hockensmith said. However, she added in the scheme of things, it’s still bad.
Chandra said of all the native species present in the 1800s’ food web, the only thing missing today is the cutthroat.
“The natives fishes are still in the lake, still in the creeks but they’ve had a tenfold decline since the 60s,” Chandra said. “They’re hurting.”
Scientists are studying the problem and find solutions and more awareness can help prevent people from making the problem worse.
Chandra said, “It’s an absolute critical time to be telling people the story of, ‘your native species are still here but we’re not protecting them like we should be,’ and then we need to understand what’s causing the decline.”
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