|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.
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.
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.
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.
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..
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
Boulder > 256
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.
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.
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.