Factors Affecting Native Cutthroat Trout Population Dynamics, Abundance, and Distribution in the Logan and Bear River Drainages
2004 Research Initiation Award Report
Investigators
Phaedra Budy—Department of Watershed Sciences, Utah State University
Mark Vinson—Department of Watershed Sciences, Utah State University
Gary Thiede—Department of Watershed Sciences, Utah State University
Summary
Water Initiative funds were combined with monies from the Utah Division of
Wildlife Resources and a variety of other sources to allow this project to go
forward. The overall goals of this study were to monitor and evaluate the
population dynamics, abundance, and distribution of trout in the Logan River,
and to determine the present and potential impacts of disease, habitat, and
interspecific interactions on the present and future status of native
Bonneville cutthroat trout (Oncorhynchus clarki utah). Toward that goal, we
have been monitoring the fish community of the Logan River at eight long-term
index sites since 2001. We collect information on trout distribution,
population abundance, condition, health and disease, as well as measuring key
abiotic variables at each site. We consider the effects of interactions between
native cutthroat trout and exotic brown trout (Salmo trutta) with both field
observations of feeding relations and in large-scale species interaction
experiments. In 2003, we completed enclosure (20 m2) experiments at six sites
arranged along the longitudinal gradient of the Logan River, and reared
cutthroat trout in treatments with (sympatric) and without (allopatric) brown
trout. In 2004, we scaled up and repeated similar species interaction
experiments in 500-m reaches in three nearby tributary streams. We summarize
all this information in an evaluation of the potentially synergistic effects of
abiotic variables (e.g., temperature), competition with brown trout, and
disease (e.g., Myxobolus cerebralis) on the status and trend of native
cutthroat trout using some simple, population modeling.
Depletion-based abundance estimates indicated that in 2004, cutthroat trout
abundance decreased from the previous year at four of the six sites where they
occur with small increases at the other two sites. The Logan River has been
experiencing drought conditions since 2000; mean annual run-off has been less
than the long-term mean. Consequently average summer temperatures are warm and
discharge is low. We suspect that the drought is having some negative influence
on trout abundance; however, 2004 was the wettest year since our monitoring
began in 2001.
In the Logan River we have observed several different patterns which suggest
that native cutthroat trout are competing with exotic brown trout. If, as we
observed, adult cutthroat trout are forced to expand the breadth of their diet
when in the presence of brown trout, and if these other, additional diet items
are less energetically profitable, this shift could result in a reduction of
growth and ultimately fitness of cutthroat trout. The fact that juvenile
cutthroat trout do not demonstrate this increase in diet breadth when in the
presence of brown trout could indicate they do not yet have the plasticity to
expand their diet; if this were true, it could mean that juvenile cutthroat
trout are at an even greater disadvantage, as compared to adults, when in the
presence of brown trout.
Finally, we synthesized our monitoring and evaluation and experimentation within
a population modeling framework. We first estimated trends in population growth
rates at each site, and we then used these observed trends to calibrate a
simple matrix model to three of the sites (Franklin Basin, Twin Bridges, and
Third Dam). We also discussed the different sources of evidence which point to
the importance of the tributaries as refuges. It appears that juvenile or
subadult fish may rear in these less-infected tributary areas until they have
grown to a body size where the risk of mortality due to M. cerebralis infection
(or infection in combination with another factor) is low, before moving out
into more dangerous mainstem areas. In combination these results thus emphasize
the importance of protecting and restoring habitat in tributary spawning and
rearing areas.
Papers resulting from this work:
- McHugh, P., P. Budy., G. Thiede, and E. VanDyke. In Press. Trophic relationships of nonnative brown trout, Salmo trutta, and native Bonneville cutthroat trout, Oncorhynchus clarkii utah, in a northern Utah, USA river. Environmental Biology of Fishes XX:XXX-XXX. (in press)
- McHugh, P., and P. Budy. 2006. Experimental effects of nonnative brown trout on the individual- and population-level performance of Bonneville cutthroat trout. Transactions of the American Fisheries Society 135:1441-1455.
- McHugh, P. and P. Budy. 2005. An experimental evaluation of competitive and thermal effects on brown trout (Salmo trutta) and Bonneville cutthroat trout (Oncorhynchus clarkii utah) performance along an altitudinal gradient. Canadian Journal of Fisheries and Aquatic Sciences 62:2784-2795.
Contact Information
Phaedra Budy
phaedra.budy‹at›usu.edu