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Some Don't Like it Hot: Trout and Temperature
By Jim Habera - Coldwater Fisheries Biologist

The native range of trout and other salmonids (salmon, charr, grayling, whitefish, ciscoes, etc.) encompasses the higher latitudes of North America, Europe and Asia.  These species are adapted to the relatively low water temperatures that occur throughout that area and are collectively referred to as “coldwater” fish.  Great Basin redband trout, a subspecies of rainbow trout found in CA, NV, and OR streams, may occur at temperatures approaching 80°F; however, 70°F is typically the upper limit for coldwater habitat.  Basically, coldwater species (and coldwater fisheries) can exist on a year-round basis where water temperatures remain below 70°F.  Other waters where summer temperatures exceed 70°F can be managed as seasonal coldwater fisheries (e.g., many of the stocked trout streams at lower elevations in east TN).  The table below provides selected temperature ranges preferred by rainbow and brown trout:    

                                                                      Rainbow trout             Brown trout
Tolerance range*:                                                 32-77°F                   32-81°F

Optimal range for good growth and survival:             54-64°F                   54-66°F

Preferred for spawning:                                          42-55°F                    45-48°F

Optimal range for egg development, hatching
success, and fry emergence:                                   45-54°F                    45-54°F

*Although the upper tolerance limit is relatively high for rainbows (77°F) and browns (81°F), they cannot endure those temperatures for extended periods of time.  Sub-lethal effects begin to occur at temperatures below the upper tolerance limits (e.g., rainbow trout growth stops at 73.4°F).

In the southeastern U.S., temperature is the single most important variable with respect to habitat suitability for trout.  With the exception of streams above an elevation of about 2,000’ in the Appalachian Mountains, there would be little natural trout habitat in the region.  However, because of the cooler waters present at higher elevations in the mountains, wild trout populations are fairly abundant and occur as far south as GA and SC.  This may change in the future depending on the amount of climate change warming that actually occurs.  The lower elevation of trout water in general may shift upstream in the southern Appalachians and brook trout, which prefer colder water than rainbows or browns, will potentially loose habitat as well.  Brook trout, which are the only native trout in the region, have already had their distribution much reduced by other forms of habitat degradation and the introduction of non-native trout (browns and rainbows).         

There are also numerous ‘artificial’ trout habitats in the Southeast as a result of the many Tennessee Valley Authority and U.S. Army Corps of Engineers dams that have been built for flood control, hydroelectric power production, and navigation.  Several of the tailwater rivers below these dams currently support trout (coldwater) fisheries where warm and cool water fish communities (bass, panfish, catfish, walleye, etc.) once existed.  The reservoirs behind these dams thermally stratify during the summer, with warmer water near the surface and cold (denser) winter-chilled water below.  Water intakes that supply the turbines are typically located at depths within the cold water zone, so water released downstream of the dam during power generation creates ideal trout habitat.  Although cold enough for trout, this water can be low in dissolved oxygen, but this problem is addressed with turbine vents, re-regulation weirs, forebay oxygen diffusion, and other measures.  The cold water supply in some reservoirs can be depleted in years with above average rainfall in the spring and early summer.  This occurs because of the increased generation necessary to keep reservoir levels within operation guides.  The result is elevated tailwater temperatures in late summer and early fall, which can impact trout populations.  

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