Long-Term Treands in the Relative Abundance and Size Structure of Sport Fishes in the Flathead River, Montana, Following Changes in Kerr Dam Operations

Scientific Disciplines
Biological Sciences - Aquatic
interior salish
confederated salish and kootenai tribes of the flathead nation
Fish management
kerr dam
flathead river
smallmouth bass
northern pike
flathead river montana
Volume 16, No. 4

Long-term trends in the relative abundance and size 
structure of sport fishes in the flathead river, Montana, 
following changes in kerr dam operations

Craig A. Barfoot and Les A. Evarts,  Fisheries Program, Confederated Salish and Kootenai Tribes, 
Box 278, Pablo, Montana 59855 craigb@cskt.org
We studied long-term trends in relative abundance and size structure of four sport fish 
taxa in the Flathead River, Montana, following changes in operations at Kerr Dam. In 1997 
Kerr Dam was changed from a power-peaking and load-following facility to a base-load 
facility. New base-load operations were designed to reduce fluctuations by establishing 
within- and between-day ramping-rate restrictions, i.e., maximum hourly and daily rates 
of change. We monitored spring and autumn trends in the relative abundance of two size 
classes (substock and stock) of northern pike (Esox lucius), Oncorhynchus spp., brown trout 
(Salmo trutta
), and smallmouth bass (Micropterus dolomieu) from 1998-2008 using nighttime 

electrofishing. We documented significant (P < 0.05) increasing trends in the autumn catches/
unit effort (fish/hr) of both substock and stock sizes of all taxa, except stock northern pike. 
Trends in spring relative abundances were similar to those in autumn, except that increases 
in smallmouth bass catch rates were not significant (P > 0.05). We also examined long-term 
patterns in the size composition of fishes following changes in dam operations. All four taxa 
had either an initial strong downward shift in annual median total length or a decrease in 
the minimum sizes of fish captured, or both, a pattern consistent with enhanced survival of 
smaller fishes and highly suggestive of benefits from changes in dam operations. Our results 
imply that modifications in the operation of Kerr Dam led to significant increases in relative 
abundance of four sport fish taxa in the Flathead River.