Subject: Whirling disease & Wild trout/fwd
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Essay. Whirling Disease and Wild Trout: The Montana Experience
By E. Richard Vincent
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Before 1991, the Madison River in Montana was a classic example of a
stable wild trout fishery. By continually monitoring fish
populations using mark-and-recapture population estimates in four
study sections (Pine Butte, Snoball, Varney, and Norris), Montana
Department of Fish, Wildlife, and Parks (MFWP) has effectively
protected existing wild trout habitat and changed angling
regulations to ensure a high-quality wild trout fishery (Figure 1).
In 1968, the MFWP negotiated a water flow agreement for the Madison
River from the Montana Power Company, which controls river flow via
the Hebgen and Ennis dams. This agreement ensures that annual
minimum flow does not go below 600 cubic feet per second (cfs) in
the upper river and 1,100 cfs in the lower (historic average winter
flows without reservoir influence).
Studies conducted in the early 1970s showed that the stocking of
catchable-size hatchery rainbow trout was detrimental to wild brown
and rainbow trout fisheries (Vincent 1987). As a result of this
study, the MFWP has managed most cold-water streams as wild trout
fisheries with no stocking of catchable-size hatchery rainbow trout.
In 1978, the MFWP enacted a catch-and-release-only angling
regulation for the upper 30 miles of the Madison River for brown and
rainbow trout. This was later extended for rainbow trout to the
entire upper 55 miles of the river. As a result, wild rainbow and
brown trout increased to more than 13 inches long (Vincent 1984).
>From 1978 through 1990, this wild-trout fishery flourished and was
stable both in size and age structure. However, in the fall of
1991, population estimates in the Pine Butte study section showed a
significant decline in the number of wild trout (Figure 2). The
decline was unusual because it only affected wild rainbow trout, not
brown trout. Younger age groups-yearlings and younger-were
affected. This decline, when first observed, appeared to be
isolated to the Pine Butte study section with no visible effect on
other downstream study sections. But in 1993, scientists noticed a
similar decline in wild rainbow trout numbers in the Varney study
section, almost 30 miles downstream. By the fall of 1994, rainbow
trout numbers had declined almost 90% in both study sections from
historic averages of the 1970s and 1980s. During this same period,
brown trout numbers were reasonably stable in both sections. Prior
to December 1994, we were uncertain about the cause of this decline.
We examined various factors that could contribute to significant
drops in wild trout populations: water flows, fish habitat changes
(spawning, immature and adult trout habitat), water temperatures,
angling pressure, and water quality). We eliminated each factor
either because it had not significantly changed during the last 20
to 25 years (water flows, water quality, water temperatures, and
fish habitat) and/or any significant changes would have affected
both wild trout species. For example, water flows never go below
agreed minimum levels, which acts to minimize the effects of
drought, and water temperatures seldom exceed 21C, even during
summer months, with mean daily July-August temperatures averaging
14C (Vincent 1977). While angling pressure can alter or depress
wild trout populations, the special restrictive
catch-and-release-only regulation installed in 1978 has averted this
problem. From 1978 to 1990, this regulation prevented any major
declines in larger sizes (age-3 and older) of both brown and rainbow
trout in all three study sections (Pine Butte, Snoball, and Varney),
where this regulation was applied.
Given that only wild rainbow trout exhibited large population
declines and that the rainbows showing highest mortalities were less
than 2 years old, primarily young-of-the-year (YOY), we began to
believe that the declines may be the result of an unknown fish
disease. In early December 1994, we collected YOY and yearling
rainbow and brown trout from various sites on the Madison River from
Quake Lake to Ennis Lake. These fish were tested for diseases, one
of which was "whirling disease." On 20 December 1994, we received
confirmation through histological examination of these fish by Beth
McConnell, a histopathologist at the U.S. Fish and Wildlife
Service's Fish Technology Center in Bozeman, Montana, that some of
the young trout examined were found to be positive for Myxobolus
cerebralis (MC) spores. Previously, the presence of whirling
disease in Montana had not been documented, either in the wild or in
any state, private, or federal fish hatchery. Since rainbow trout
are extremely susceptible to infections of MC spores, and brown
trout are relatively resistant, we suspected that this could be the
principal cause of measured decline in the wild rainbow trout
population.
In addition, young rainbow trout exhibited the largest population
decline. These young trout are known to suffer the most from
infections of Myxobolus cerebralis because of the predominance of
cartilage, especially in the cranial area. Samples taken within
this 55-mile reach of the river showed spores in up to 75% of the
young trout examined. Both young rainbow and brown trout were found
to be infected. Based on the following, we concluded that whirling
disease was the primary factor causing this population decline:
(1) Whirling disease was present in a large number of young trout;
(2) only rainbow trout showed large population declines;
(3) the rainbow population decline was due to the lack of young
rainbow being recruited into the population; and
(4) all other known factors that may have contributed to the
observed population decline had been eliminated as probable causes.
Although the evidence was compelling, no clinical signs of whirling
disease had been observed in the Madison River wild trout
populations prior to the 1995 field season. One goal for 1995,
beyond continuing annual fish population estimates, was to document
clinical signs in young wild rainbow trout.
To determine the possible effects of whirling disease on YOY brown
and rainbow trout, an electrofishing survey was conducted on the
species' abundance in the Pine Butte and Snoball study sections in
1995. At the start of the survey in July, rainbow numbers averaged
120/150 ft, and brown trout averaged 40/150 ft. The YOY rainbow
trout showed a 85% decline in numbers, while brown trout declined
only 50%. Histological examination of both species throughout the
summer showed rainbow trout exhibited more severe infections of the
Myxobolus cerebralis trophozoite than was found in young brown
trout, which showed only light infections. Clinical signs of the
disease such as cranial deformities, caudal deformities, blacktail
and whirling behavior were also noted in up to 50% of YOY rainbow
trout examined. Clinical signs were not observed in YOY brown
trout. Wild trout population estimates conducted in 1995 on the
four study sections showed further declines in the adult rainbow
trout numbers in all study sections except Norris, which as of
spring 1995 had not been infected with Myxobolus cerebralis spores.
Brown trout populations continued to be stable with no significant
change from numbers observed in the 1970s and 1980s.
As a result of the discovery of whirling disease in the Madison
River, the documented impacts the disease has on wild rainbow trout,
and the discovery of the presence of the disease in additional
Montana waters, the MFWP has adopted the following:
(1) Inform Montana anglers of the infection and warn them of its
possible harm to Montana's wild and native trout fisheries.
(2) Initiate a plan to prevent or impede the spread of the infection
to additional wild trout streams.
(3) Increase the amount of whirling-disease-related research,
especially as it relates to wild trout populations.
(4) Initiate new angling regulations that could benefit whirling
disease populations.
(5) Determine the distribution of whirling disease infections in
other Montana waters, including all state, federal, and private fish
hatcheries.
(6) Determine which native Montana salmonids may be susceptible to
whirling disease.
(7) Carefully research management strategies so as not to compromise
Montana's policy of wild trout management for the state's salmonid
waters.
References
Vincent, E. R. 1977. Madison River temperature study. Montana
Department of Fish, Wildlife, and Parks, Federal Aid in Fish
Restoration, Project Report F-9-R-25, Job Iia, Helena.
---. 1984. Fishing regulation evaluation on major trout waters.
Montana Department of Fish, Wildlife, and Parks, Federal Aid in Fish
Restoration, Project Report F-9-R-32, Job Iic, Helena.
---. 1987. Effects of stocking catchable-size hatchery rainbow
trout on two wild trout species in the Madison River and O'Dell
Creek, Montana. N. Am. J. Fish. Manage. 7:91-105.
Copyright c 1996 by the American Fisheries Society. All rights
reserved.
....
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