The Influence Of Stream Size On Sculpin Population Dynamics.

The roles of predation and competition in regulating sculpin population dynamics were examined in streams of different sizes. Sculpins (Cottus bairdi and C. cognatus) are small, sedentary, benthic fishes inhabiting many northern streams. Sculpins in large streams have faster individual growth rates and higher fecundities than those in small streams, but occur at lower density. This suggests that growth is density-dependent and that predation lowers density in large streams. In laboratory experiments, young sculpins are at greatest risk to predation by rock bass when they are on simple gravel substrates or in deep water. Young will shift to shallow areas when rock bass are present. Risk therefore depends upon predator composition and microhabitat characteristics which differ in different sized streams. When densities of young sculpins are manipulated in field enclosures, both competition and the range of available microhabitats influence sculpin growth rates. The growth of young is density-dependent, even where natural growth is rapid and adults are excluded. However, growth rates are slower at all stocking densities in the smaller stream, so the importance of this regulatory mechanism changes with stream size. Controlled field experiments reduced densities of adult sculpins in a stream of intermediate size (8 m wide) and in a small tributary. In the larger of these streams, the relative quality of microhabitats remained stable from year to year, and young shifted habitats and grew faster when adults were removed. In the small stream, the quality of microhabitats fluctuated between years, and young did not shift habitat use when adults were removed. The young grew slightly faster, however. This study shows that sculpin populations are influenced by environmental fluctuations and low resource productivity in very small streams. In streams of intermediate size, they are more strongly regulated by intraspecific competition for both food and space. In larger streams, young are apparently at greater risk of predation because changes in depth and substrate size influence the effectiveness of their predators.Ph.D.Aquatic sciencesBiological SciencesEcologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127642/2/8324134.pd