UCSC researchers study effects of wildfire runoff on steelhead sense of smell

UC Santa Cruz scientists are testing whether wildfires at the Landels-Hill Big Creek Reserve and other coastal watersheds have harmed the ability of endangered steelhead to smell their environment. These fish rely on olfaction to detect prey, sense predators, and return to their natal creek to spawn. (Photo by Adam Welz)
The mobile fish behavior lab ready for business at the Landels-Hill Big Creek Reserve. (Photo by Morgan Abbott)
The lushness of the creek corridor belies the fact that much of Landels-Hill Big Creek Reserve burned in an autumn 2020 wildfire. (Photo by Lance Takata)
A juvenile steelhead acclimating to its new surroundings in the holding portion of the Y-maze. (Photo by Morgan Abbott)
UCSC undergrad Litzia Galvan has been an integral part of the salmonid olfaction project, helping to set up the mobile fish behavior lab, handle captured fish, and analyze samples for heavy metals. A first generation college student, she is a participant in the CAMINO program, which helps pay and mentor UCSC undergraduates as they get involved in research. (Photo courtesy of Litzia Galvan)

In 2020, a massive lightning storm ignited wildfires across much of Northern California. Among the thousands of acres it charred were multiple coastal watersheds from San Mateo County to Big Sur.

For UC Santa Cruz fish ecologist Eric Palkovacs, the burns offered a chance to study how the runoff after wildfire affects one of the region’s most iconic fish: steelhead trout. Unlike rainbow trout, which technically belong to the same species (Onchorhyncus mykiss), endangered steelhead hatch in in freshwater streams, migrate to the ocean to mature, and finally return to their natal stream to spawn.

Palkovacs worries that chemicals in runoff from burned landscapes are causing the trout’s sense of smell to go haywire. That could mean disaster for an entire generation of the federally threatened fish.

“Salmonids rely on their sense of smell to create an imprint on their home watershed when they’re juveniles. If that imprinting process is interfered with, they may not be able to make the return migration back to their home watershed as an adult,” said Palkovacs, a professor of ecology and evolutionary biology.

His concerns are based on the findings of previous research by UC Santa Cruz environmental toxicologist Peter Weiss-Penzias, who has found elevated levels of mercury in even pristine coastal areas. Together with NOAA fisheries biologist Dave Rundio, Weiss-Penzias reported that steelhead from the Landels-Hill Big Creek Reserve in Big Sur contain more mercury than trout from most other West Coast locations. One fish caught in the reserve contained as much as one part per million of mercury in its tissues, making it decidedly unsafe for human consumption. Yet the reserve watershed drains wilderness untouched by mines, factories, or other mercury-emitting activities.

“It’s one of the cleanest places in the northern hemisphere,” Weiss-Penzias says. The mercury itself, the product of industrial pollution and natural emissions, was likely deposited from the atmosphere.

Palkovacs is particularly concerned about the juvenile trout rearing in coastal streams during blazes. “Wildfires may create this release of mercury that may be particularly harmful because it’s emitted in one big pulse,” Palkovacs says. “Steelhead use smell to sense predators, detect other fish, and find prey. Is that going to interfere with their life in the stream?”

To find out, Palkovacs has joined forces with both Weiss-Penzias and fish biologist Maryam Kamran of Virginia Tech. The three are principal investigators on a National Science Foundation RAPID grant to understand the impact of wildfires on salmonid olfaction.

“We joke that we’re on a fishing expedition, because we’re not exactly sure what we’re going to see. But that’s the point of these RAPID grants, is to assess the effects of catastrophic events, like wildfires,” Weiss-Penzias says.

Natural experiment

The aftermath of the 2020 burns laid the groundwork for a natural experiment. The researchers found four coastal watersheds where the mosaic pattern of the fire burned one drainage, but left an adjacent one unscathed: Pescadero Creek along the San Mateo coast; the San Lorenzo River in Santa Cruz; the Carmel River near Monterey; and the Landels-Hill Big Creek Reserve (part of the UC Natural Reserve System), which they paired with unburned Willow Creek just to the south.

At each of the four pairs of sites, NOAA Fisheries Santa Cruz laboratory scientists caught up to a dozen live steelhead. The researchers then tested each fish’s ability to smell via what they call a Y-maze trial. The watery maze itself is as basic as can be: a holding area that branches into two arms. The test involves dripping the scent of a predator in one arm, and plain creek water in the other, then tracking which arm the fish prefers.

They compared fish from burned and unburned watersheds to see if the release of toxins from wildfires had impacted their ability to smell the predator cue. In fish from the unburned watersheds, the olfactory systems should have developed normally and they should exhibit strong avoidance of the predator cue, Palkovacs said.

While the maze itself was simple, the effort required to run the trials outdoors was immense. Testing the fish in the field both enables the animals to be tested in familiar waters and reduces the stresses and potential deaths of being transported to a lab.

Lab technician Morgan Abbott fabricated the team’s mobile fish behavior lab, the first ever to test fish outdoors in their home environments. The elaborate contraption features lights to illuminate the fish, web cameras to track their movements, pumps and hoses to supply water from the creek, and covers for the mazes to ensure the fish are influenced by smell and not sight.

“It took a lot to set up the whole thing. We had to make sure the mazes were clean, make sure that the footage device was connected to cables, laid out a long water hose to get enough creek water to supply the experiment,” says Litzia Galvan, a fourth-year marine biology major working in Palkovacs’s lab. The project represents the first and only opportunity she’s had to do field work in college.

Interpreting the behavior of the fish is surprisingly complex. “We use auto-tracking software to determine how much time the fish spent moving versus nonmoving. How much time was it spending on one arm of the Y versus the other side? Was it moving in quick, jerky movements or was it smoothly swimming around? These are indicators of whether the fish was feeling comfortable or exhibiting a response that would indicate stress,” Palkovacs says.

After the maze test, each fish was euthanized. The scientists then dissected out the organs responsible for smell, known as the olfactory rosettes, as well as samples of muscle tissue and creek water.

To determine how well these olfactory cells are functioning, Palkovacs plans to analyze which of their genes are active. “If the gene expression profile of normal salmonid olfactory cells is different in burned and unburned watersheds, that would give us a clue as to whether they are imprinting as they normally would, or whether there was some interference with that process,” he says.

Heavy metals

On the chemistry side, Weiss-Penzias is in charge of analyzing whether heavy metals such as mercury, copper, lead, and manganese are present in the water and tissue samples, and at what levels.

Galvan has been helping to prepare the fish muscle tissue for analysis. “[We] add chemicals to the muscle tissues so they can dissolve. Once they do, we’re going to put them into a spectrophotometer and that will let us know the chemical composition of each sample. Then we want to compare how many chemicals are inside, and what’s the most dominant chemical,” Galvan says.

At present, Weiss-Penzias is in the midst of analyzing samples from the rest of the study sites.

So far, samples taken from San Lorenzo River after the major storm of October 25 show far more mercury than samples taken earlier that autumn, when water in the river originated from groundwater. “This makes sense because it’s washing everything off and putting it in the river,” Weiss-Penzias says. Fire would easily have volatilized whatever was present in vegetation or soil, as mercury evaporates at temperatures as low as 85 degrees Fahrenheit, potentially moving it into streams and, ultimately, into the fish.

These findings are supported by the results of another analysis of mercury at the NRS’s Blue Oak Ranch Reserve east of San Jose on Mount Hamilton. As at Landels-Hill Big Creek, large swaths of Blue Oak also burned during the autumn 2020 wildfires.

For a National Science Foundation GEOPATHS internship with Weiss-Penzias, UCSC undergraduate Ariel Santero took soil samples from both burned and unburned areas of the reserve. “The general trend was that the burn samples have less mercury because all that mercury had volatilized into the atmosphere, where it may have got into local waterways, maybe traveled even further than that. And the unburned samples still had their mercury because it was not volatilized,” says Santero, who presented her results at the December 2021 meeting of the American Geophysical Union.

Of most biological concern is the amount of mercury that is actually dissolved in the water and moves into stream food webs. In its ionic form, solubilized mercury is more reactive. It can more easily pass through cell walls and interfere with the biochemical processes of fish and other organisms. To distinguish between soluble and particle-bound mercury, Weiss-Penzias will test both filtered and unfiltered samples.

Weiss-Penzias is equally intrigued by other toxins he might find in the samples. “What will be new for me is quantifying the other metals besides mercury. It will provide an assessment of variability across watersheds and conditions where there haven’t been many measurements done in the past,” Weiss-Penzias says.

The research team hopes their work will yield more definitive results later in the year. Either way, the project will shed light on how wildfire, an increasingly common component of California life, affects the water and the organisms that call this state home.