Interactions Between Native And Nonnative Species: Consequences Of A Brown Trout Introduction On A Coldwater Stream Community
Investigator: Bruce Vondracek
Student: Julie Zimmerman (Fisheries- University of Minnesota)
One of the most widely introduced sportfish is the brown trout (Salmo
trutta) that was introduced from Europe and has established reproducing
populations in 39 states of the continental U.S. Introductions of brown
trout have often resulted in increased biodiversity in a stream because
native fish fauna have not been extirpated. However, nonnative brown trout
have been shown to compete with native fish species and may cause species
replacement or changes in community structure or function. Although several
studies have examined interactions between introduced brown trout and
native brook trout, little information is available on effects of brown
trout introductions on stream communities (including non-game fish and
macroinvertebrates). Brown trout have been found to reduce growth of slimy
sculpin (Cottus cognatus) in Valley Creek, Minnesota and may also reduce
abundance of invertebrate prey (primarily Gammarus pseudolimnaeus).
The goal of this research was to examine how stream communities change
in a stream where native brook trout have largely been replaced by nonnative
brown trout. We designed a field experiment using enclosures/exclosures
(1m2) that was replicated 6 times during 2002-2004. Specifically, we investigated
the effect of interactions between brown trout, brook trout, and slimy
sculpin on benthic invertebrate assemblages, fish diet composition, and
fish growth. Our objectives were to: (1) assess whether small enclosure
experiments conducted in natural stream systems were an appropriate method
to investigate interactions between fish, or if using enclosures biased
experimental results; (2) investigate whether interactions between brown
trout and slimy sculpin were similar to interactions between brook trout
and sculpin, by examining growth; and (3) compare indirect interactions
between each trout species and sculpin, by examining effects of interspecific
interactions on benthic invertebrate assemblages and prey selection of
each fish species.
The 1m2 enclosures did not reduce total invertebrate drift density or
drift rate. Thus, enclosures did not likely bias our experimental findings.
We found that introduced brown trout have effects on the stream community
in Valley Creek that are different than native brook trout. Interactions
between brown trout and slimy sculpin led to reduced growth of both large
slimy sculpin and large brown trout in enclosures, whereas presence of
brook trout was not associated with changes to sculpin growth. Presence
of brown trout also led to indirect effects on food web dynamics that
may have resulted in these observed changes in growth. Brown trout was
the only species associated with a decrease in abundance of a benthic
invertebrate taxon, suggesting that brown trout may feed in the benthos
to a greater extent than brook trout. Diets of brook trout, brown trout,
and slimy sculpin had high overlap (measured by biomass), and were dominated
by Gammaridae. Brown trout were also associated with shifts in sculpin
diets, indicating a change in sculpin feeding behavior when sculpin were
with brown trout that was not evident when sculpin were alone or with
brook trout. Feeding behavior and intraspecific interactions among brown
trout, along with shifts in feeding behavior of sculpin when brown trout
were present, likely was a mechanism underlying reduced growth of both
species.
Acknowledgments: Funding for this project was provided by a fellowship
from Austen Cargill II, a doctoral dissertation fellowship by the Graduate
School of the University of Minnesota, and grants from the American Fisheries
Society, the Kalamazoo Valley Chapter of Trout Unlimited, the U. S. Geological
Survey, Cooperative Research Units Program, and the Dayton-Wilkie Natural
History Fund of the Bell Museum of Natural History. We thank Janice Odegaard,
the Afton Land Limited Partnership and the Meier family for allowing us
access to Valley Creek.
Publications and Dissertation:
Zimmerman, J. K. H. and B. Vondracek. 2006. Stream enclosure effects on drifting invertebrates and fish growth: do enclosure experiments produce biased results? (Accepted by the Journal of the North American Benthological Society scheduled for scheduled for volume 25, issue 2).
Zimmerman, J. K. H. 2005. Interactions between native and nonnative species: consequences of a brown trout introduction on a coldwater stream community. Ph.D. Dissertation, University of Minnesota, St. Paul, Minnesota.
Minnesota Cooperative Fish and Wildlife Research Unit
University of Minnesota
Department of Fisheries, Wildlife, and Conservation Biology
1980 Folwell Avenue, 138 Hodson Hall
St. Paul, MN 55108
phone: 612-624-3421
fax: 612-625-5299
coopunit@tc.umn.edu