We examined microhabitat use among an assemblage of fish during summer low-flow in a 242 m section of the Nazko River, British Columbia. Most fish species and size classes within species selected distinct microhabitats, although use of microhabitat often overlapped. Principal component analysis indicates that microhabitat was generally partitioned among fish during daytime along gradients of depth, current velocity and substrate. Size-related variation in microhabitat use was primarily ontogenetic, with most juvenile fish being found in shallow water. Most adult fish used higher current velocity areas with larger substrate, but use of stream depth differed between species. A comparison of holding positions of fish in deeper habitats indicated broad separation of species into benthic and mid-water column groups during the day, but was unable to distinguish interspecific differences within the groups. This broad segregation was less apparent at night, when mid-water column fish species descended towards the stream bottom.
Preferences of most juvenile fish for shallow backwaters and channel margins indicates that these areas should be protected as summer nursery sites. Selection by most adult fish for areas with coarse substrate may be related to the presence of large interstitial spaces providing shelter, particularly at night. Stream management programs must prevent excessive siltation or sedimentation that could reduce availability of these refugia. Increased seasonal sampling over a greater range of streams is recommended to determine the consistency of patterns in microhabitat use within the Nazko drainage and throughout the region.
Porter, Marc, Rosenfeld, Jordan. 1999. Microhabitat Selection and Partitioning by an Assemblage of Fish in the Nazko River. Ministry of Fisheries. Fisheries Project Report. RD 77