Marinda Grady
Interspecific Competition between Brown Trout (Salmo trutta) and mottled sculpin (Cottus
bairdi): considering the role of predation on relative densities in a Central Wisconsin
Stream
Abstract
The role of predation on mottled sculpin by brown trout was studied at Black Earth Creek
in Black Earth, WI. There has been much focus on stocking and sustaining the brown trout
population in this stream, but it is important to see how this effects other species in the
ecosystem. No significant relationship was observed between the densities of brown trout and
mottled sculpin along 14 sections of the stream that were sampled (p-value = 0.1131). Several
other factors likely play a role in sustaining both species populations within the central
Wisconsin stream.
Introduction
Interspecific competition and predation between species can have profound effects on the
composition and sustainability of fish communities. An example of this type of competition can
be observed with brown trout (Salmo trutta) that prey on mottled sculpin (Cottus bairdi).
Mottled sculpin are strictly benthivorous creatures and gain some protection from the trout by
occupying muddy areas of their habitat (Dahl, 1996). Brown trout are drift feeding predators that
consume both benthic and terrestrial prey, but only consume exposed food organisms and will
not burrow or root out their prey. (Dahl, 1996). Competition between the two species may be
enhanced by the fact that both species also prey on several common bottom organisms such as
mayflies, caddisflies and aquatic dipterans (Dineen, 1951).
Black Earth Creek is located in south central Wisconsin near the town of Black Earth.
This stream spans approximately 12 miles, originating in a marsh, but gains rich quality because
it is spring fed, and eventually empties into the Wisconsin River (Born, 1997). Black Earth
Creek is also known to cover a good temperature range (Born, 1997). Despite varying
temperatures and habitat substrates brown trout and mottled sculpin are found throughout the
entire stream. A great deal of effort has been put into this stream by several agencies and Black
Earth town members to create an ideal living environment for brown trout (Born, 1997). Brown
trout populations in this stream are now self-sustaining, but it is important to consider the effects
they may have on the rest of the stream ecosystem.
Data collected from two sites along Black Earth Creek was used to quantify the densities
of each species in several sections along the stream. Comparing the densities of these two
species will help to determine how significant effects are on mottled sculpin abundance from
predation due to brown trout. It was hypothesized that as the brown trout density increased,
mottled sculpin density would decrease and an inverse relationship would be observed between
abundance of the two species due to predation.
Methods
Two testing sites along Black Earth Creek were selected for data collection
approximately 1 mile apart from each other. The first testing site, referred to as site 1, was
sampled on March 20, 2007 in the afternoon. The second site, site 2, was sampled on March 21,
2007 at approximately the same time of day as site 1. Each site contained several different
habitat substrates and water flow patterns helping to gain a sense of fish abundance throughout
the entire stream system.
At each testing site seven sections were blocked off using 12.2 mm mesh nets to prevent
fish from moving between them. A towboat electrofishing unit with pulsed DC current was used
to collect the fish from each section of the stream. One pass was made through each section with
electrofishing boat. The fish were then identified, counted and released back into the stream.
The temperature, length and width of each stream section were also measured.
Temperature was recorded in ºC. Most of the stream sections were about 30 meters in length,
and width was measured at every five-meter interval. The average width of each section was
used for area calculations.
The densities (fish/m^2) of brown trout and mottled sculpin were determined for each
stream section from the data collected. Brown trout density (fish/m^2), the explanatory variable
was graphed against the response variable, mottled sculpin density (fish/m^2). The results were
analyzed using a simple linear regression, and p-values were calculated to show significance.
Results
Figure 1 models the density of brown trout and mottled sculpin collected from both
testing sites along Black Earth Creek. The result was a positive correlation between the densities
of the two species. The p-value was 0.1131 and the R2 value was 0.1958.
To observe if temperature variations between the two testing locations had any effect on
the results the data from each site was graphed seperately. Figure 2 models the density of brown
trout vs. mottled sculpin in from the first testing site sampled on March 20, 2007. Another
positive correlation is observed with a p-value of 0.2377 and an R2 value of 0.2644.
Temperatures ranged from 3.3 to 3.9ºC along the sections sampled at site 1. Figure 3 is a graph
of the data collected from site 2 on March 21, 2007. At site 2 the temperature range was from
10.5 to 10.9ºC. Once again a positive correlation is shown. The p-value was 0.6106 and the R2
value was 0.0556.
Discussion
The results from the analysis of this data fail to support the hypothesis. The positive
relationship modeled between brown trout density and mottled sculpin density suggest that the
effect of predation on mottled sculpin does not have a significant effect on their overall
abundance. The temperature difference between the two test sites also did not prove to be a huge
contributor to predation effects.
Other studies have found more significant effects, and may give insight into why the
results of this study were not significant. In a study looking at the effect of temperature on
brown trout feeding behavior, it was found that the optimal temperature for brown trout feeding
was at 18ºC (Forseth, 1994). The temperatures of both sites in this study were well below this
optimal feeding temperature, and does not necessarily represent the preferred diet of the brown
trout.
Another study showed that temperature and season has a substansial effect on the feeding
behavior of brown trout (Langeland, 1991). In warmer temperatures brown trout were more
aggressive than arctic charr, and drove them out of the littoral zone, but in months of colder
temperatures they spread out and occupied the same habitats and shared food sources with the
charr (Langeland 1991). The tendency for trout to feed in different zones in relation to
temperature also suggests that the diet composition of trout may vary throughout the year as the
seasons change. . The samples from Black Earth Creek were taken between spring and winter
during times of extreme temperature variability. Water levels had also been changing recently
due to melting snow. Not only did this greatly impact the temperature of the water it may have
had an effect on feeding habits that the data could not take into consideration. If the sampling
had been done during a time of more constant temperature patterns, more constant feeding
patterns would have likely been established in the trout population, possibly leading to more
significant relationships between the species.
The positive correlation in brown trout and mottled sculpin density from Black Earth
Creek suggests that competition and predation is not so extreme that they cannot occupy the
same habitat. It also makes sense that with similar diets they would be found in similar places.
While the results of this study at Black Earth Creek do not define a significant relationship
between predation on mottled sculpin and their abundance, sampling the same areas under
different conditions may yield more significant results.
Literature Cited
A. Langeland, J.H. L’abee-Lund, B. Jonsson and N. Jonsson. 1991. Resource Partitioning and
Niche Shift in Arctic Charr Salvelinus alpinus and Brown Trout Salmo trutta. Journal of Animal
Ecology. 60: 895-912
Born, S.M., J Mayers, A. Morton, and B. Sonzongni. 1997. Exploring Wisconsin Trout
Streams: The Angler’s Guide. University of Wisconsin Press. Madison, WI.
Dahl, Jonas and L. Greenberg. 1996. Impact on Stream Benthic Prey by Benthic vs Drift
Feeding Predators: A Meta-Analysis. Oikos. 77: 177-181.
Dineen, Clarence F. 1951. A Comparative Study of the Food Habits of Cottus bairdii and
Associated Species of Salmonidae. American Midland Naturalist. 46: 640-645.
Forseth, T. and B. Jonsson. 1994. The Growth and Food Ration of Piscivorous Brown Trout
(Salmo trutta). Functional Ecology. 8: 171-177.
Figure 1
Brown Trout and Mottled Sculpin Density in
Black Earth Creek
1
Mottled Sculpin Density
(fish/m^2)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.05
0.1
0.15
0.2
0.25
0.3
Brown Trout Density (fish/m^2)
Figure 1 – This figure shows the combined data from two testing locations on Black Earth Creek
sampled on March 20 and 21, 2007. Seven consecutive sections were sampled each day at
different locations along the stream. 239 brown trout and 939 mottled sculpin were sampled.
(R2 = 0.1958, p-value = 0.1131)
Figure 2
Brown Trout and Mottled Sculpin Density in
Black Earth Creek - Testing Site 1
Mottled Sculpin Density
(fish/m^2)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.02
0.04
0.06
0.08
0.1
Brown Trout Density (fish/m^2)
Figure 2 – This figure models the data collected along Black Earth Creek in Black Earth, WI on
March 20, 2007. Seven consecutive sections were sampled that ranged in temperature from 3.3
to 3.9 ºC. 91 brown trout and 413 mottled sculpin were sampled. (R2 = 0.2644, p-value =
0.2377)
Figure 3
Brown Trout and Mottled Sculpin Density in
Black Earth Creek - Testing Site 2
1
Mottled Sculpin Density
(fish/m^2)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.05
0.1
0.15
0.2
0.25
0.3
Brown Trout Density (fish/m^2)
Figure 3 – This figure models the data collected along Black Earth Creek in Black Earth, WI on
March 21, 2007. Seven consecutive sections were sampled that ranged in temperature from 10.5
to 10.9 ºC. 148 brown trout and 526 mottled sculpin were sampled. (R2 = 0.0556, p-value =
0.6106)