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| Dates: 2002-2005 |
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Background:
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| Thermal
testing facility at the Bozeman Fish Technology Center.
Click on the photo for more info. |
Historically, westslope cutthroat trout (Oncorhynchus clarki lewisi) ranged widely over western Montana, Idaho, and portions of eastern Washington and Oregon. Like many other cutthroat and native trout species, westslope cutthroat trout now persist in only a small portion of their native range, and are listed as a “species of special concern” in Montana.
Leading causes for their decline are habitat degradation and displacement by non-native rainbow, brook, and brown trout. Water temperature is considered a key element influencing the abundance and distribution of coldwater species like trout, yet the thermal requirements of westslope cutthroat trout are largely unknown. Proper conservation and management of westslope cutthroat trout requires detailed knowledge of its temperature requirements. |
Project Goals:
The goal of this laboratory study is to characterize the thermal biology of westslope cutthroat trout, specifically with respect to the lethal and optimal temperatures for this subspecies, and to compare its thermal biology with that of rainbow trout, a non-native competitor. We are using a laboratory design that we developed for thermal testing with bull trout (Selong et al. 2001). This design allows simultaneous assessment of fish growth and survival under many different temperatures over long time periods. In addition, the thermal preferences of both species are being determined to better aid in characterizing the thermal requirements of each species. |
| 1. |
To define the upper lethal and optimal temperature ranges of westslope cutthroat trout. |
| 2. |
To define the upper lethal and optimal temperature ranges of rainbow trout. |
| 3. |
To determine the thermal preference of westslope cutthroat trout and rainbow trout. |
| Facility photos: |
Click on a photo for a larger
view and more information. |
Head tanks which supply flowing water to the 36 thermal testing tanks (there are... |
Close-up of a thermal testing tank. Water flows into the tank from the head box... |
Rearing tanks where fry are raised until they reach appropriate test size. Tank... |
Westslope cutthroat trout in the rearing tanks which are supplied by constant fl... |
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Abstract
Westslope cutthroat trout Oncorhynchus clarkii lewisi have declined throughout their native range in the Northern Rockies and were considered for listing under the federal Endangered Species Act. Water temperature is widely regarded as playing a key role in determining their persistence, but specific lethal levels and thermal optima for this cutthroat trout subspecies had not been precisely defined. This laboratory study used the acclimated chronic exposure method to determine tolerances and thermal optima of westslope cutthroat trout and rainbow trout Oncorhynchus mykiss, a potential non-native competitor now occupying much of the former range of westslope cutthroat trout.
Westslope cutthroat trout and rainbow trout show little spatial overlap in the field despite similar physiological optima and behavioral preferences. Remaining populations of westslope cutthroat trout are primarily confined to cool, headwater stream reaches whereas non-native rainbow trout predominate in warmer, lower-elevation stream sections historically occupied by westslope cutthroat trout. The research team assessed whether differing thermal preferences could account for the spatial segregation of these species in the field. Thermal preferences of age-1 westslope cutthroat trout and rainbow trout (125 to 150 mm total length) were assessed in the laboratory using a modified annular preference chamber at acclimation temperatures of 10º, 12º, 14º, and 16ºC. Westslope cutthroat trout and rainbow trout had similar thermal preferences, with final preferred temperatures of 14.8º to 14.9ºC when tested in a thermal gradient of 11º to 17ºC. The similar thermal preferences parallel previous results showing the two species also have very similar optimum growth temperatures (13.1º to 13.6ºC). The high degree of overlap in physiological and behavioral responses to temperature indicates the two species have highly similar thermal niches and a high potential for competition.
The ability of rainbow trout to survive prolonged exposure to temperatures greater than 20°C and to grow over a wider range of temperatures than westslope cutthroat trout, may account for the increased occurrence of rainbow trout at lower elevations and warmer temperatures. In contrast, the lower thermal tolerance and narrower growth range of westslope cutthroat trout may further explain their greater distribution in cooler, headwater streams, where colder water temperatures may allow them to remain more competitive with non-native salmonids more tolerant of warmer temperatures. Although the research team found low selection of temperatures below 12ºC and above 16.5ºC by both species, assessing temperature preferences over a wider range than they examined would identify avoidance temperatures of each species and help elucidate if rainbow trout more strongly avoid colder, and westslope cutthroat trout warmer, temperatures. Because thermal preference can be strongly affected by food availability, and headwater streams where westslope cutthroat trout persist tend to have poor growing conditions because of low temperatures and low productivity, examining thermal preferences under reduced rations would also be informative for gauging differing species distributions in response to elevational gradients in temperature. In particular, experiments comparing species performance in sympatry and allopatry are needed to fully assess the degree of temperature-mediated competition between the two species.
Temperature may play an important role in determining the degree of spatial overlap between the species and in predicting suitable habitat for westslope cutthroat trout reintroduction. Such predictions of habitat suitability will be vital in prioritizing conservation efforts with respect to reintroduction and translocation of westslope cutthroat trout.
Completion Date and Deliverables:
A Master’s thesis was completed on this project by graduate student, Beth Bear, in April 2005, comprising the first two study objectives on upper thermal tolerance and thermal optima for westslope cutthroat trout and rainbow trout. The thesis was provided in the July 2005 WFHI Semi-Annual Report. Study results were presented by Beth Bear at the American Fisheries Society national conference in Anchorage, Alaska, in September 2005.
Thermal preference experiments were successfully completed in early 2005, and analysis of preference videos was completed in June 2005. Video data analysis was completed in summer 2005, and a final report was completed and submitted to the Montana Water Center on January 1, 2006 (see top of page to download). |
Graduate
Research Assistant
Beth A. Bear
Department of Ecology, Montana State University - Bozeman
Bozeman, Montana 59717 |
Principal
Investigators
Thomas E. McMahon
Department of Ecology, Montana State University - Bozeman
Bozeman, Montana 59717 |
Alexander V. Zale
Montana Cooperative Fishery Research Unit
Department of Ecology, Montana State University - Bozeman
Bozeman, Montana 59717 |
Bill Krise
USFWS Bozeman Fish Technology Center
Bozeman, MT 59715 |
| 1Selong, J.H., T.E. McMahon, A.V.Zale,
and F.T. Barrows. 2001. Effect of temperature on growth and
survival of bull trout, with application of an improved method
for determining thermal tolerance in fishes. Transactions of
the American Fisheries Society 130:1026-1037. (Copy of article
can be obtained from: tmcmahon@montana.edu) |
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