I took a fishing road trip out West last summer, and I noticed a few things about where we caught certain species—cutthroat in the upper river, riffle-type water; brown trout in slower, deeper, silted parts of the river; and rainbows in between. Is there something in a trout’s makeup that makes it prefer certain habitat?
Habitat selection for all animals, be they rainbow trout in a small coastal stream, or mink standing on a bank, is a consequence of the species’s evolutionary history and contemporary environmental conditions. A species’s evolutionary history is a template upon which contemporary physical (depth, temperature, velocity, shelter) and biological (inter- and intraspecific competitors, prey availability, predators) components are laid. Frequently, the template comprises a larger “space” than that occupied by members of a population, but sometimes a population occupies all habitats that are physiologically suitable. This may be common for trout populations because they frequently display strong intraspecific competition, suggesting that all favorable habitats are occupied.
Anecdotally, this is why large trout replace other large fish in a particular lie. The physical and biological characteristics of the position occupied by an individual trout are termed microhabitat, and the ecological benefits of a particular microhabitat are a function of the physical and biological characteristics mentioned above.
Trout species possess different evolutionary templates because they typically evolved in habitats with biological and physical characteristics that were dissimilar over evolutionary time. Hence, the habitat template of rainbow trout, which evolved in the Pacific Rim, was shaped by highly productive, cool, temperate physical conditions, plentiful rainfall, and multiple cohabiting salmonid species (arctic grayling, Pacific salmon, coastal cutthroat trout, and Dolly Varden). Whereas, the template of brown trout, which evolved in Europe and Asia, likely was shaped by harsher, lower-productivity conditions, and fewer potential competitors (grayling and juvenile Atlantic salmon in coastal regions).
In conclusion, species do possess evolutionary limitations that influence habitat preferences today. Nonetheless, these preferences are not fixed—natural selection is an ongoing process, and it is likely that brown trout introduced into North America occupy a wider range of habitats than their European progenitors.
Gary D. Grossman, PhD, is Professor of Animal Ecology in the University of Georgia’s Warnell School of Forestry and Natural Resources. He is also the 2014 recipient of the American Fisheries Society’s Carl Sullivan Award for excellence in fishery conservation. If you have a question for Dr. Trout, email it to firstname.lastname@example.org.