Experimental Evidence Shows Instream Cover as a Limiting Resource for Stream-Dwelling Trout

Most ecological investigations examining processes that either limit or regulate populations have focused on space, food, or inter- or intraspecific interactions, and less on cover as a key resource. Cover is often studied from an individual perspective (e.g., habitat use or selection), but influences on populations are infrequently demonstrated.  We addressed the importance of variable cover on populations of trout in realistically-scaled, semi-natural outdoor experimental stream channels where food availability and space were held constant.  Emigration was  found to be a driving force for grouping and cover use behavior. Grouping behavior was pronounced when individuals were prevented from emigrating, but it was replaced by emigration and cover use when populations could emigrate.  Cover use by individuals was dependent on body size, with larger individuals showing more frequent use of cover.  Smaller individuals employed a wider range of behavioral tactics (i.e. sharing cover with a larger individual, shifting among cover positions). Overall, our results show that cover alone can strongly influence individual behaviors and numbers of individuals, indicating that cover merits serious consideration among other more commonly studied factors in either limiting or regulating local populations.