Pacific Southwest RegionCalifornia, Nevada and Klamath Basin
A worm, a parasite and a salmon
U.S. Fish and Wildlife statistician, Dr. Nicholas Som, a career mathematician and former school teacher in Colorado, draws the life cycle of Ceratonova shasta on a chartboard, showing alternation between salmonid fish and polychaete worm hosts, "There was a thought that if we could disrupt the transmission of the spores between the salmon and the worms, or vice versa, or disrupt the habitat that the worms depend on, that might be an effective management tool for reducing impacts of the disease,” he said. Credit: John Heil/USFWS
Arcata Fish and Wildlife Office employs statistician to understand relationship of fish to diseases and parasites
By John Heil
November 29, 2018
A human eyelash. That’s the size of a polychaete worm.
A recent U.S. Fish and Wildlife Service study on the polychaete worm has identified the importance of it to the survival of juvenile salmon in the Klamath River.
Chinook salmon migrate up a Northern California river to spawn. Service biologists from the Arcata fish and Wildlife Office and California Nevada Fish Health Center worked closely with Oregon State University, and Klamath Basin tribes to initiate several fish disease-related studies to understand the impact the worms have had on Chinook salmon infection and mortality in the Klamath River. Credit: USFWS
Understanding the worm’s lifecycle is critical in understanding their contribution to the infection and mortality of the Chinook salmon in the Klamath River since the early 2000s; an infection rate that reached 100 percent of fish in some samples since 2005.
Service fish biologist Nicholas Hetrick recognized early on the need to develop a prevalence of infection study on the parasite (Ceratonova shasta), its impact on the juvenile salmon population levels. Credit: John Heil/USFWS
To unravel the mystery, Nicholas Hetrick, the Fish and Aquatic Conservation lead for the Arcata Fish and Wildlife Office since 2004, recognized early on the need to develop a prevalence of infection study on the parasite (Ceratonova shasta), its impact on juvenile salmon population levels, and how the distribution of polychaete worms factored in.
Hetrick worked closely with the Service’s California Nevada Fish Health Center, Oregon State University, and Klamath Basin tribes to initiate several fish disease related studies which generated an enormous amount of complex data to analyze.
“As juvenile salmon migrate down the Klamath River, we would expect the population to build as fish enter the river from tributaries and emerge from main-stem spawning sites, but instead we were seeing the population disappear the further down river you went,” said Hetrick. “There was obviously a big issue, but we just didn’t know at the time what the cause was.”
“It was like looking for a needle in a haystack,” said Dr. Nicholas Som, a career mathematician and former school teacher in Colorado. “You can’t see them from the surface sitting in a raft. You have to be in the water on the bottom of the river looking for them.” Credit: John Heil/USFWS
Enter Dr. Nicholas Som, a career mathematician and former school teacher in Colorado who studied ecological statistics at Oregon State University.
Som, who has always been drawn to rivers since childhood, made a conscious decision to take his passion for statistics and pair it with his love for moving water.
“Nick’s desire to work with rivers and fish is also a self-serving interest because he likes to fish and he wants to catch more fish,” said Hetrick with a smile. When interviewing for this position what stood about Nick was his interest in applied management and not just wanting to do research or apply statistics – but wanting to make a difference on the ground.”
Discovery of the parasite as a cause of salmon mortality by Oregon State University’s Dr. Jerri Bartholomew of the C. shasta life cycle involved transmission of the parasite between the salmon and the worms, and different types of parasite spores that infected the salmon.
“There was a thought that if we could disrupt the transmission of the spores between the salmon and the worms, or vice versa, or disrupt the habitat that the worms depend on, that might be an effective management tool for reducing impacts of the disease,” said Som. “Only, we didn’t know which habitats the worms were using.”
That was the start – to determine what kind of habitats the worms use and how that relates to where the infected worms were.
Hanging on a wall near Nicholas Som's office is the answer to the question. The whiteboard shown here displays the formula he developed, enabling the agency to predict which sections of the river are potentially habitable by the polychaetes. Credit: John Heil/USFWS
“We had to start over – begin to quantify which habitats the worms use and knowing where they are,” said Som. “A lot of people had their face in the water and thought they knew where to find these worms, and we went there and didn’t find them. We had to rethink our sampling process for the habitat study.”
Essentially, Som’s job was to translate how likely a spot is to be suitable habitat for polychaete worms into a mathematical formula. This formula needed to incorporate the habitat as well as water flow and speed of flow.
The formula he developed consisted of three main components: density of substrate (sand and silt at the small end of the spectrum, up to bedrock), depth of the water, and water velocity. The idea was to quantify habitat conditions required for polychaetes to persist, and then create conditions that fall outside of that suitable range through management actions. “Think of it ‘like changing the setting on a treadmill to 30 miles per hour,’ you wouldn’t last either,” said Hetrick.
Eventually they found the worms. “It was like looking for a needle in a haystack,” said Som. “They have a patchy distribution. You can’t see them from the surface sitting in a raft. You have to be in the water on the bottom of the river looking for them. The densest colonies look like 60s shag carpet.”
This image shows the comparison from 2014 to 2016 of polychaete worms found in the Klamath River. “The densest colonies look like 60s shag carpet,” Nicholas Som, a statistician with the Service said. “Before the 2016 photo was taken, there was a flow event large enough that our model predicted reduced habitat for polychaetes relative to 2014, and this photo, taken at the same location in both years, demonstrates that it was true.” Credit: Julie Alexander/Oregon State University
With their formula in hand, Som is able to predict which sections of river are potentially habitable by the polychaetes. He followed that up by looking at water management alternatives – “What happens if we send water from the dam down in differing patterns?” For example, flat-line base flows that don’t change at all, versus unstable flows that have short spikes or tall spikes.
“There are all sorts of ways to shape water releases, but it’s a precious resource that has a limited volume. We need to devise the most efficient way to send water,” said Som. “We take a couple of options and apply our formula and see which one of those creates the lowest amounts of potential habitat for polychaete worms, and we can apply the formula in the areas where they historically have been the most abundant.”
Som’s research on the impact of water deliveries in the Klamath to the survival of the polychaete worm has since been confirmed. The percentage of sites showing decreases in the amount of polychaetes was about seven times greater for the years with high peak discharge (11,200 cubic feet per second) compared with the years with very low peak discharge (1,890 cubic feet per second).
A Chinook salmon showing signs of disease with a distended stomach – a visual clue of a sick fish. Credit: USFWS
In 2016, the Service’s California Nevada Fish Health Center and the Arcata office found the amount of fish disease was dramatically lower than the previous year, and when they went back to look at polychaete habitats - they had been reduced – mirroring what Som’s formula had predicted.
“We went back after a high flow event and found significant changes that matched what our predictions were,” said Som. “It feels fantastic - incredibly satisfying. With this information we can make changes that can benefit fisherman, tribes, and the ecosystem.”
“We want to make sure managers are making decisions using the best available, peer-reviewed science,” said Hetrick. “We’ve gained a lot of ground over the years and will continue to do so in the future.”
Critical partners involved in understanding the salmon disease dynamics in the Klamath River include: Dr. Scott Foott and Kim True of the Service’s California/Nevada Fish Health Center, Humboldt State University and Oregon State University’s Dr. Julie Alexander and Dr. Jerri Bartholomew.
“We’ve been incredibly lucky to overlap with both Nicks,” said Alexander. “Nick Hetrick has been so instrumental in directing and supporting disease research on Klamath River salmonids. His fundamental knowledge of aquatic ecosystems is extensive and his interpersonal skills are admirable. He manages to pull together really great teams and people listen to him.
“Nicholas Som is one of the most versatile statisticians I have ever met or had the pleasure to work with. Most of the research on the Klamath has involved him in some capacity. His skill set is really unique - he effortlessly explains math to the layperson and cranks out outstanding science. I think Nick Hetrick summarized him well - he is passionate about salmon, and it makes such a big difference.”
About the writer...
John Heil serves as the deputy assistant regional director for external affairs in the Pacific Southwest Region headquarters in Sacramento, California.
An avid sports fan and boogie boarder he also enjoys the outdoors and all that wildlife has to offer. John is also an U.S. Air Force veteran and spent part of his career overseas as a television and radio broadcaster.
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