The effects of a naturally-occurring parasite
in the Klamath River -- Ceratonova shasta – decline when juvenile salmon move
into areas of cooler water where exposure to the parasite is lower and where
the effects of the disease declines in severity, according to a recent study.
In effect, the cold water refuge also
functions as a disease refuge.
The same parasite is present throughout the
Columbia River basin, including in the Willamette River where high levels of C.
Shasta have been found in the past couple of years when mainstem river
(See CBB, July 10, 2015, “Bacteria Associated
With Warm Water Taking Toll On Salmon, Steelhead In Northwest,” http://www.cbbulletin.com/434489.aspx)
While the authors of the study surmise that
cold water refuges could offer the same protections in the Columbia basin, they
noted that the study was of just one river and that other rivers may differ,
according to co-author Dr. Sascha Hallett, senior research associate,
Department of Microbiology, Oregon State University.
C. Shasta causes enteronecrosis in salmon, a
disease that increases in severity as temperature and parasite doses increase.
Infections by this parasite cause severe intestinal disease, ceratomyxosis
(enteronecrosis), often resulting in death, the study says.
“The study investigated the abundance of the
salmon parasite C. shasta in a thermal refuge compared to the warmer mainstem
Klamath River during the summer, when high numbers of juvenile salmonids rely
on these cold water patches,” Hallett said.
Salmonids in the Klamath River mainstem must
contend with summer water temperatures that reach levels that stress the fish
and can sometimes be lethal. With climate change, the water temperature in the
Klamath River has been rising 0.5 degrees Celsius every decade since the early
1960s, the study says.
The final study was the result of a
combination of field work and lab studies. The field observations included
juvenile chinook salmon, coho salmon, and steelhead. The lab study tested
juvenile chinook and coho.
“In the field, we found that temperature was
2-8 (about 4 to 15 degrees Fahrenheit) degrees Celsius lower and that levels of
the parasite that cause the disease were lower in the refugium compared to the
mainstem, but the disparity in parasite load decreased later in the summer,”
she said. “Our lab studies showed that the fluctuating temperature that fish
would experience as they moved from the warm mainstem to the cooler refuge did
not impact disease severity, compared to a constant midrange temperature.”
The authors found that juvenile salmon move
between the main-stem river and thermal refuge habitats diurnally, seeking food
which is more plentiful in the main stem and seeking cooler water in the
refuge. In the process, fish infected with C. shasta during the summer
experience fluctuating thermal regimes rather than simply lower temperatures,
the study says.
Field work focused on one large,
“well-characterized refuge formed by Beaver Creek (rkm 261) that provides
thermal relief for juvenile salmonids during peak main-stem water
temperatures,” the study says. High densities of juvenile salmon have been
observed during snorkel surveys in this cool water refuge, particularly when
the Klamath River exceeds 22°C.
The study, “Klamath River Thermal Refuge
Provides Juvenile Salmon Reduced Exposure to the Parasite Ceratonova shasta” (http://dx.doi.org/10.1080/00028487.2016.1159612), was published online in June in Transactions of the American
Hallett’s co-authors are Luciano V.
Chiaramonte, senior microbiologist, Dr. Adam Ray, quantitative analyst, and Dr.
Jerri L. Bartholomew, professor, all with the Department of Fisheries and
Wildlife, OSU; and Alex Corum and Toz Soto, both fisheries biologists with the
Karuk Tribe in Orleans, Calif.
C. Shasta responds to water temperature,
according to Hallett, a “universal finding” that “applies to the parasite
throughout its range - all rivers in the Pacific Northwest, including the
Columbia River Basin.
Here’s how the parasite proliferates,
according to the study:
The parasite requires a salmonid host and the
benthic freshwater polychaete Manayunkia speciosa for completion of its life
cycle. An infected salmonid sheds C. shasta myxospores that are ingested by M.
speciosa. Infection in the polychaete host culminates in the release of
actinospores into the water column, where they encounter salmon. The outcome of
infection is primarily influenced by temperature and exposure dose, but is also
affected by host susceptibility and parasite genetics.
“Elevated water temperatures cause increased
and faster mortality in infected fish (e.g. 69 percent mortality and 31 mean
days to death (MDD) at 13°C, compared with 98 percent mortality and 16 d at
21°C for Chinook Salmon and a sufficient parasite dose (i.e. 10 spores/L)
exacerbates the effects of temperature,” the study says.
It also applies to all aspects of the
parasite’s life cycle, Hallett added.
“For example, the parasite proliferates more
quickly at higher temperatures in both its fish and its invertebrate
(polychaete worm) hosts,” she said. “Thus, disease is more severe in salmonids
at higher water temperature – a combination of higher proliferation as well as
fish being less capable of dealing with infection when they are stressed at
higher river temperatures. But higher temperatures also mean that polychaete
host populations increase more rapidly and that parasite release occurs earlier
in the year.”
Dr. Bartholomew is also studying areas and
timing of C. Shasta in the Willamette River through water sample monitoring at
specific sites. However, her studies have not compared levels of the parasite
in Willamette River refugia with the mainstem, nor investigated whether
refugia-usage in the Willamette River also decreases the severity of
enteronecrosis in salmonids, Hallett added.
As in the Klamath River, the parasite is
mainly present in the Willamette mainstem, but parasite levels in the
Willamette are lower and the pattern of distribution along the mainstem
gradient differs to the Klamath.
Thermal refugia likely play an important role
in salmonid health and thus preservation and restoration of these habitats is
important for salmon/trout populations, Hallett concluded.
“Both the fish and worm hosts of C. shasta as
well as the parasite are native to these rivers,” she said. “The high levels of
parasite and associated population-level impacts on wild and free-ranging salmonids
have only been recently documented and are highly variable between years. Thus,
the goal of our research is to better understand these shifts in host-parasite
dynamics and how disease effects can be lessened.”
Identification and preservation of these
cooler habitats is important for juvenile and adult salmonids during summer
(e.g. through preservation or restoration of riparian vegetation along river
margins and restoration/maintenance of snow-fed or spring fed streams that
provide refuges where they connect with the river mainstem), she said.
--CBB, December 19, 2014, “Inoculation For C.
Shasta, Salmon Parasite, Fails But Researchers Hope For Better Outcomes In
--CBB, October 10, 2014, “Stream Flows
Increased In Klamath River By 75 Percent To Fight Parasite Threatening Coho,
--CBB, August 22, 2014, “Parasite-Driven
Disease Hitting Klamath Salmon Hard Also Found To Lesser Degree In Columbia