Fish diversity, microhabitat use, and behavior
in secondary channels of the
Bitterroot River, Montana
Troy Brandt
Environmental Studies
University of Montana, Missoula, MT 59802
The central Bitterroot River floodplain is
characterized by a complex secondary channel network that provides a range of
aquatic environments for native and nonnative fish. To better understand fish communities using these aquatic
habitats, I used snorkeling and electrofishing to evaluate habitat variation,
fish community diversity and microhabitat use in six secondary channels between
August 1998 and September 1999. Secondary
channels provide a range of habitats and microhabitats influenced by secondary
channel proximity to the Bitterroot River, upwelling groundwater, and inchannel
habitat complexity created by woody debris.
Secondary channel morphologies remained stable over the year, while
secondary channel water chemistry varied seasonally and in relation to the
mainstem Bitterroot River. Fish community diversity was greatest in complex channel
reaches and tended to decrease with distance from the mainstem.
Of the eight fish species commonly encountered, microhabitat use and
day-night fish behavior patterns were apparent.
Among young-of-year and juvenile age classes of different species,
microhabitat use overlapped and day-night behaviors were similar.
During the day, young fish generally selected microhabitats associated
with dense cover, while at night these fish moved into less protected, low water
velocity microhabitats. Adult fish
of larger species used microhabitats that differed from those used by
young-of-year and juveniles of the same species. Adult fish occupied deeper microhabitats or those
associated with large woody debris or overhead bank cover. However, these results were species-specific in many cases
and were influenced by site-to-site differences in microhabitat availability.
Bitterroot River secondary channels provide a variety of habitats
occupied by numerous fish species and age classes. Protection of the Bitterroot
River floodplain from inappropriate development is critical to maintaining the
region’s habitat and fish diversity.