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Every morning, as the summer sun begins to warm the surface of Upper Klamath Lake, a group of fish biologists drop their boats into the water.

On Wednesday, they revved up their motors and sped out of Shoalwater Bay, heading east around Eagle Ridge. This time of year, millions of slender, green Aphanizomenon flos-aquae cells make it look like tons of grass clippings have been dumped into the lake. The research boats skidded over the algal bloom, making wakes in the lawn-like water. They were headed to Rattlesnake Point—to check on some suckers.

The U.S. Geological Survey has operated mesocosm experiments in Upper Klamath Lake each summer since 2014, placing groups of juvenile suckers in netted cages dotted throughout the lake. The mesocosms allow them to study the fish in their natural habitat (instead of an overly sterile lab) without having to track them throughout the lake.

The goal is to figure out what’s killing the young suckers before they’re able to reach sexual maturity and support their species’ populations. Fish biologist Summer Burdick said the current theory is that there’s not one silver bullet that causes mass juvenile die-offs every year. Poor water quality, whether it’s high pH levels, high temperatures or low dissolved oxygen, can stress a fish out to the point where it becomes more susceptible to parasites or disease outbreaks.

“It’s death by a thousand cuts,” Burdick said.

The mesocosm experiment has used a similar study design for different purposes each year: 2014 was a pilot year to see if the experiment was viable. In 2016 and 2017, USGS set up three mesocosms throughout the lake to see what the last straw was that killed the fish—and how many died. In 2018, they tested out which areas of the lake would have the best environment for the Fish and Wildlife Service to eventually place rearing pens for their sucker recovery efforts. In 2019, scientists studied whether injecting the mesocosms with pure dissolved oxygen would increase survival rates.

It’s a 20-minute ride from Shoalwater Bay to Rattlesnake Point, where the mesocosm and floating platform that surrounds it emerge from the middle of the lake. The PVC pipe structure is 100 square feet in area and about 15 feet high, most of which is below the water’s surface. Nets surround the prism, keeping fish contained while still in their natural habitat.

About 80 juvenile suckers of varying sizes, courtesy of FWS’s Gone Fishing facility, populate the mesocosm. Each fish has a PIT tag installed on its abdomen, allowing three antennae spread equally between the lake’s surface and bottom to track where in the water column they tend to congregate in response to environmental conditions. A water quality monitoring instrument records the temperature, pH, dissolved oxygen content and conductivity of the water every hour.

In addition to downloading the water quality and antenna data and performing maintenance on the structure, the researchers also did sucker checkups. They dipped nets into the mesocosm to catch a few suckers and transported them into a bucket on the platform. A few tiny marbled sculpins, also native to the Klamath watershed, were small enough to slip through the mesocosm nets and are a harmless bycatch.

After tossing the sculpins back to the water, biologists pulled each sucker out, measured its length, weighed it, scanned its PIT tag and examined it for ailments like parasites and injuries. Sometimes, when temperatures are especially balmy, they may add ice to the bucket to keep the cold-water fish more comfortable during the evaluation.

Though they’re believed to result in more holistic studies on suckers than lab experiments, mesocosms are still considered an artificial environment because they don’t allow the suckers to freely move throughout Upper Klamath Lake. Food wise, however, gut studies have shown that the fish are still able to continue their regular diets during their stay in the aquatic cages. But mesocosms add real value to sucker research by removing the need for scientists to attempt to replicate lake water quality conditions.

Burdick said that’s important, because the conditions the suckers endure constantly in their natural habitat are likely more of a factor in their overall survival than whatever parasite or bacterial infection ultimately kills them each year. Identifying what those constant, underlying stressors are is key to reviving their populations.

“The thing that actually kills the fish could be different, but the thing that stresses them is probably similar,” she said.