Science: Effects of dam removal on fish habitat and communities

             Dams impact fisheries communities through habitats alteration and fragmentation.  Habitat alteration can lead to displacement of native species and colonization by non-natives.  Habitat fragmentation can impede access to habitats necessary for reproduction, feeding and survival.  Dam removal is becoming a standard method for rehabilitating river ecosystems.  Burroughs et al. (2001) observed the gradual removal of Stronach dam on the Pine River in Manistee County, Michigan and documented changes in fish habitat and communities.

            Stronach dam was gradually removed between 1997 and 2003.  From 1996 through 2006, the Pine River was assessed to document the physical characteristics and the effects of the dam.  Fish were sampled from 1997 through 2007 using multi-pass removal sampling with an electrofishing boat to estimate abundance and distribution of species. 

            Above the dam’s impoundment there were no significant changes to the habitat.  Just above the dam, progressive erosion of large amounts of sediment increased the gradient and water velocities, which formed a new channel and resulted in steeper banks, increased riffle and pool frequencies, and median substrate particle size.  Downstream of the dam, stream bed aggradation increased width, decreased depth, increased the gradient and increased the water velocity of the river. 

            Prior to the removal of the dam, 31 species were captured: 11 only downstream of the dam, 1 only upstream and 19 both downstream and upstream of the dam.  After the removal, 37 species were captured: 3 only downstream of the former dam, 10 only upstream and 24 both.  All but two species that were caught prior to the dam being removed were caught after, and there were 8 species caught after the dam being removed that were not caught before.  Of 25 taxa with enough specimens to evaluate abundance, 18 exhibited increased mean abundance, six had decreased but not significantly and smallmouth bass Micropterus dolomieu showed no change.  The abundance of both brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss increased by more than two-fold during the study, with the largest increases in the impoundment and upstream reach. 

            Fish habitat was improved due to the dam being removed, but not restored to the condition of the reference reach of the river.  Habitat actually degraded downstream of the dam but should improve as sediment erosion upstream stops.  Dam removal also restored connectivity of all three zones, leading to changes in species composition and increased diversity.  Increases in abundance and minimal changes in abundance indicate that dam removal reduced habitat limitations and increased productivity of fish communities because they could access optimal habitats.  Because historic fish community composition is unknown, the authors were unable to conclude that dam removal resulted in restoration of the community, but can argue that it restored functionality and production potential of the community.  Overall, dam removal appears to be a useful tool for improving fish communities

Implications for anglers:

            The issue of dam removal has surfaced in areas of southeast Wisconsin.  From the perspective of an angler, dam removal is a good thing because, overtime, it improves fish habitat and ultimately fish communities.  However, the changes to the river may not be welcomed by property owners and other user groups.  Anglers should support dam removal projects when they are proposed.  A number of groups support the removal of Estabrook Dam from the Milwaukee River, including Milwaukee Riverkeeper, Trout Unlimited, Milwaukee Steelheaders and Milwaukee Lake and Stream Fly Fishers.  I found an older study on smallmouth bass and common carp Cyprinus carpio response to the Woolen Mills Dam from the Milwaukee River, and hope to have it posted soon.

Definitions:

Stream bed aggradation:  An increase in stream bed elevation due to the deposition of sediment

Community:  All the organisms that inhabit a particular area, living close enough for potential interaction

Taxa:  A taxonomic group of any rank, such as a species, family, or class

Impoundment:  A body of water formed by confining a source of water

Abundance:  the relative representation of a species in a particular ecosystem and is usually measured as the number of individuals per sample

Burroughs, B.A., D.B. Hayes, K.D. Klomp, J.F. Hansen and J. Kiskak. 2010. The effects of the Stronach Dam removal on fish in the Pine River, Manistee County Michigan. Transactions of the American Fisheries Society. 139: 1595-1613.