Substrate Influence on Presence of Mottled Sculpin (Cottus bairdi) and
Brown Trout (Salmo trutta) Densities in a Central Wisconsin Stream
R. A. Krause
Abstract. Substrate serves many purposes within the ecology of many organisms, as well
influences species composition in a given stream section. This study conducted at Black
Earth Creek, west of Madison, WI, examined relationships between dominant substrate
type and mottled sculpin(Cottus bairdi) and brown trout (Salmo trutta)densities. Fish
were collected along transects marked every 5m, using a tow-boat electrofisher and
dominate substrate was determined by dividing each transect into thirds and looking at
longest dimension present. Mottled sculpin density was highest among gravel substrate
but decreased as substrate increased. Brown trout densities remained constant although
lower than mottled sculpin densities for all three substrates. Substrate preference may be
a factor in the variations of species densities during different spawning seasons.
Introduction
Substrate is essential in determination of stream habitat suitability for fish species.
Many species will need large, boulder, or rocky type substrate in order to hide from
predators while others seek out sand or fine gravel during the spawning season. Juvenile
fish may sometimes need to grow where the quick flow of velocity will not cause pieces
of large substrate to fall on top of them(when they are smaller), and so influence of
substrate can also be harmful if the right type is not available. Species densities can also
vary since one population will not stick around for long if competition oh habitat or
predation proves too stressful. Additionally, prey abundance can lead to different
densities of species if the prey is hard to find in the predator’s normal substrate.
Mottled sculpin (Cottus bairdi) exhibit this latter behavior, as seen in a study
conducted by Petty and Grossman (1996), who found that habitat preference was strongly
influenced by prey abundance. Based on preference for macro invertebrate prey
(Rashleigh and Grossman 2005) it would make sense that presence of mottled sculpin
would be found among substrate where the prey resided. Streams, especially the
shallower areas are typically where mottled scupin can be found. With a dorsal ventrally
compressed body form and a bottom dweller lifestyle, mottled sculpin tend to never be
more than a few inches above the ground; regardless of whatever substrate the preferred
prey lives in. Sculpin are also good indicators of trout, and one study carried out at a
rocky mountain watershed found mottle sculpin among high densities of brown trout
(Salmo trutta) and fine substrate. (Quist et al.2004a) During the spring, sculpin spawn
and males defend their burrows which lie underneath rocks on the streambed. Brown
trout spawn during late summer and autumn, with juveniles displaying a large preference
for gravel substrate. (McRae and Diana 2005) Brown trout also prefer shallow waters, as
well as rocky, and boulder present areas. Large brown trout populations can be found at
Black Earth Creek, a spring fed, mineral rich, ample groundwater with a diverse insect
population and sufficient substrate suitable for spawning. (Born et al 1997)
Based on the large abundance of brown trout at Black Earth Creek, it would be
interesting to determine if large populations of mottled sculpin were present as well and
explore whether brown trout were good indicators of mottled sculpin. After observation
in the field and thinking about the various substrate/species interactions, this study aims
to examine whether mottled sculpin spawning in the spring will be more present among
gravel substrate than brown trout or if there will be preference for a different substrate.
Additionally, brown trout and mottled sculpin densities will be examined to see whether
there is similar presence of both species among any of the substrates.
Methods
Fish and substrate data was collected on March 21, 2007 from Black Earth Creek,
a tributary stream west of Madison, WI containing constant, mineral-rich water and deep
undercut banks. Agriculture characterizes the primary land use but sufficient plant growth
and abundance of aquatic organisms contained here create an excellent habitat for trout
populations to grow. Many farms and small town developments are scattered along the
length of the creek utilizing this abundant land, and the section of the creek where this
study was conducted was near the town of Cross Plains.
Fish were collected by 3-4 students using a towboat electrofishing process
involving electric current, and beginning at the downstream end of the reach and moving
upstream. For each section of the reach, one pass was made with the electrofishing boat
for five minutes, and stunned fish were collected with nets. Captured fish were identified,
weighed, measured and then released when analysis was complete.
The stream reach was measured and upstream and downstream boundaries were
determined. Transects were marked using flags every 5 m perpendicular to water flow of
the sampling section. Dividing the transects into thirds from one side of the creek to the
other allowed a student to observe a range of substrate present by looking down into the
water. Substrate was classified by its longest dimension given in millimeters. (Table 1)
The computer program Microsoft Excel was used as the database for this study.
Dominant substrate for the entire stream was determined by dividing the number of
transects per reach of a substrate type (n) by total number of transects (t): (n/t) x 100.
Figures were created to examine densities of mottled sculpin and brown trout species
across the various substrates, and simple linear regressions were run to identify any
variance between those densities based on substrate type. P values from this analysis
were determined to indicate significance if p< 0.05.
Results
Cobble, silt, and gravel were found to be the most dominant substrates within this
section of the stream, yielding percentages of (26%, 44.5% and 47.5%) respectively.
High P value’s (> .05) resulted in no significant relationship between densities of either
species for all three dominant substrates.(Fig. 1, 2, and 3) Out of the three dominant
substrates, only one substrate indicated close to 50% variance in explaining mottled
sculpin density while brown trout was somewhat lower. (Fig. 2) The remaining substrates
for both mottled sculpin and brown trout contained such low variance that there were
probably other factors to explain presence of species density. (Fig. 2, Fig. 3) Among
increasing amounts of cobble and silt substrates, density of mottled sculpin increased,
whereas for gravel substrate, substantial mottled sculpin decrease was evident.(Fig. 2)
There did not appear to be substantial brown trout density changes as substrate amount
increased in all three figures.
Discussion
There are many factors that contribute to presence of species in certain habitats.
Substrate is no different from factors such as velocity, riparian cover, or depth of water. It
is however, important in determining where certain species will choose to spawn and
deposit eggs/raise larva. One example of this was found in a study conducted on juvenile
brown trout that occupied sites with high gravel percentages because gravel was the
preferred substrate for spawning. (McRae and Diana 2005) The fact that brown trout
spawn later in the season than when this study was conducted could account for the
smaller population of brown trout among gravel substrate.
The higher density of mottled sculpin among the gravel however, supports the
idea that sculpin also spawn in the same environment as brown trout. Reasons for this
include observations of decrease in mottled sculpin density among only gravel substrate,
which could result from male guarding and defending of their nests, potentially driving
away others among increased area of gravel substrate. Additionally, in a study conducted
on sexual behavior of mottled sculpin, it was suggested that males possessed the ability to
“ingest any other sculpin 40 mm smaller in total length than itself.” (Downhower et al
1983) This would account for the decrease in density and also the higher populations
among other substrates, since those sculpin entering nests and guarding areas (gravel
substrate) might inevitably be eaten.
The lack of change in brown trout densities among the three substrates suggests
that substrate did not seem to have particular effects on population structure, although
examination of other environmental factors might suggest differently. After review of
other literature, similarities were found supporting what was observed in this study,
namely that mottled sculpin tend to prefer gravel substrate during spawning just as brown
trout do, only there is a difference in when the two species spawn..
Literature Cited
Born, S. M., J. Mayers, A. Morton, and B. Sonzogni 1997. Exploring Wisconsin Trout
Streams: the Angler’s Guide. University of Wisconsin Press
Downhower, J. F., L. Brown, R. Pederson, and G. Staples 1983. Sexual Selection and
Sexual Dimorphism in Mottled Sculpins. Evolution 37: 96-103
McRae, B. J. and J. S. Diana 2005. Factors Influencing Density of Age-0 Brown Trout
and Brook Trout in the Au Sable River, Michigan. Transactions of the American
Fisheries Society 134: 132-140
Petty, J. T. and G. D. Grossman 1996. Patch selection by mottled sculpin (Pisces:
Cottidae) in a southern Appalachian stream. Freshwater Biology 35:261-276
(a) Quist, M. C., W. A. Hubert, and D. J. Isaak 2004. Factors Affecting Allopatric and
Sympatric Occurrence of Two Sculpin Species across a Rocky Mountain Watershed.
Copeia 3: 617-623
(b) Quist, M. C., W. A. Hubert, and D. J. Isaak 2004. Fish assemblage structure and
relations with environmental conditions in a Rocky Mountain watershed. Canadian
Journal of Zoology 82: 1554-1565
Rashleigh, B. and G. D. Grossman 2005. An individual-based simulation model for
mottled sculpin (Cottus bairdi) in a southern Appalachian stream. Ecological Modelling
187:247-258
Table 1. Classification of Substrate by longest dimension
Substrate
Dimension (mm)
Silt
< 0.1
Sand
0.1-2
Gravel
2-16
Pebble
16-64
Cobble
64-256
Boulder
> 256
Mottled Sculpin
y = 0.5097x + 81.026
R2 = 0.0549
140
Species/M^2
120
Brow n Trout
Linear (Mottled
Sculpin)
Linear (Brow n
Trout)
100
80
60
40
y = 0.0426x + 24.786
R2 = 0.0025
20
0
0
10
20
30
40
50
% Cobble
Fig. 1. Very little variance is explained by this type of substrate for either species. There
must be some other factors explaining species density for cobble substrate. Simple linear
regression analysis resulted in p=0.655 for mottled sculpin and p=0.925 for brown trout.
Mottled Sculpin
y = -0.913x + 121.42
R2 = 0.4678
140
Brow n Trout
Species/M^2
120
Linear (Mottled
Sculpin)
Linear (Brow n
Trout)
100
80
60
40
y = -0.299x + 36.422
R2 = 0.3244
20
0
0
20
40
60
% Gravel
Fig. 2. Almost half the variance for mottled sculpin density in this stream section can be
explained by gravel substrate, while 20% less variance is explained for brown trout.
Analysis resulted in close P value’s of the two species, with p=0.134 for mottled sculpin
and p=.238 for brown trout.
140
Species/M^2
120
y = 0.3649x + 78.9
R2 = 0.1143
Mottled Sculpin
100
Brow n Trout
80
Linear (Mottled
Sculpin)
Linear (Brow n
Trout)+ 20.652
y = 0.1971x
R2 = 0.2156
60
40
20
0
-5
15
35
55
75
95
% Silt
Fig. 3. Very little variance is shown here (based on R² values) so prevalence of either
species density cannot be explained completely by silt substrate, there must some other
factor(s) involved. Simple linear regression analysis yielded p values of 0.51 for mottled
sculpin and 0.35 for brown trout. lending no relationship between silt substrate affecting
density of either species.