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www.natureserve.org | Range map. Pink color indicating exotic species. |
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Habitat and Range
Wild brook trout populations are typically associated with low to moderate gradient, rocky mountain stream habitats that have permanent cool or coldwater spring sources. Brook trout populations are generally most successful in perennial streams with water temperatures less than 20°C. Hatchlings suffer high mortality rates in waters with sustained temperatures of 20°C and above and adults can tolerate temperatures up to about 25°C. Closed canopy forest cover is a key common denominator for the persistent long-term success of most brook trout populations within stream habitats.
Brook trout are considered native from the Hudson Bay basin and northeastern Canada, much of the Great Lakes basin, a small portion of the upper Mississippi drainage, Atlantic coastal areas from Maine to New Jersey and interiorly along the Appalachian chain as far south as northern Georgia. Though not as extensive world wide as brown or rainbow trout, brook trout have become widely established via introduction throughout North America and elsewhere, with the exception of Australia and Antarctica.
Behavior
Brook trout are primarily crepuscular being most active near dawn and dusk. During mid day hours, brook trout are more likely to retreat to deeper waters or shaded areas if available as they seem to prefer more overhead security during daylight hours than do brown and rainbow trout. Behaviorally, brook trout are aggressive predators but are cautious and easily spooked.
Diet
The only apparent limitation in the diet of brook trout is likely the size of their intended prey. Their diet includes nearly any aquatic or terrestrial invertebrate or vertebrate of suitable size that either occurs naturally in the stream or happens to fall in. One of the most noteworthy prey items extracted from a brook trout captured within the park was a sub-adult five-lined skink. Small mammals and a wide variety of terrestrial insects are also opportunistically consumed as are other fish, aquatic insects and microcrustaceans.
Ecosystem Role
In the absence of exotic, predatory fishes such as brown trout and the sunfishes, brook trout are the largest, most dominant predatory fish in the mountain stream ecosystem. As noted above, they readily prey on nearly any living organism of suitable size and in turn are preyed upon by northern water snakes, mink, kingfishers, herons and the occasional otter within ShenandoahNational Park. The northern water snake very likely accounts for more brook trout predation within the park than any other natural predator. Water snakes are more than occasionally encountered by the fisheries monitoring crew in the process of ingesting or having recently ingested brook trout of moderate to large size.
Threats
While not generally considered threatening to established, wild brook trout populations, flood and drought events are likely the leading causes of sudden and dynamic change within brook trout populations in park streams. Additional external stressors such as over-harvesting, the presence of an exotic predatory fish population, disease outbreaks and changes in water chemistry due to acidification could result in the loss of a brook trout population from one or more park streams over time.
Currently, brook trout are the most widely distributed fish species within the park occurring in over 66 park streams and many associated tributaries. The few remaining streams that they are not found in are largely a function of small or intermittent stream character in relation to the point along those streams intersected by the park boundary. A number of those streams contain brook trout populations below or downstream of the park boundary.
Acidification is an ongoing concern, particularly if aquatic systems within the park increasingly acidify over time. Currently, stable brook trout populations persist within the park’s most acidified streams that have stable flow and suitable habitat. To their advantage in persisting in acidified waters, brook trout are the most acid tolerant fish of the park’s current suite of fish species, capable of successfully reproducing in water with a pH as low as 4.5. Most park streams currently have pH ranges considerably higher than 4.5.
The presence and proliferation of exotic predatory fish, particularly brown trout has the potential to impact brook trout populations within several of the park’s premier large streams. Currently, the brown trout issue is confined to the lower reaches of three park streams. Brown trout have demonstrated the potential to displace brook trout from entire sections of those three streams that currently contain established brown trout populations.
References
Atkinson, J.B. 2005. Shenandoah National Park Fisheries Monitoring Program Annual Report for 2004. Division of Natural and Cultural Resources, Luray, Virginia 22835. 46 pp.
Jenkins, R.E., and N.M. Burkhead. 1993. Freshwater fishes of Virginia. American Fisheries Society, Bethesda, Maryland. 1079 pp.
Lennon, R.E. 1961. The Trout Fishery in Shenandoah National Park. Special Scientific Report-Fisheries; No. 395. USFWS. 16 pp.
Mohn, Larry and Paul E. Bugas. 1979. Virginia Trout Stream and Environmental Inventory. Dingell-Johnson Project F-32.
Webb, J.R., Cosby, B.J., Galloway, J.N. and Hornberger, G.M 1989. Acidification of native brook trout streams in Virginia. Water Resources Research, 25:1367-1377.
Virginia Department of Game and Inland Fish Species Description
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