Small mountain trout streams can often be sampled with a single battery-powered backpack electrofisher.

How many fish are out there?  That is one of the first questions a fisheries manager would typically want to answer about a stream or river trout population.  Abundance estimates, along with an understanding of growth, survival, and angler exploitation (harvest) rates, provide the basic information necessary for determining management strategies and establishing regulations.  Anglers, for obvious reasons, often want to know how many trout a particular stream or river holds as well.

It is almost never possible to actually count all the trout present in a given water body, so it is necessary to estimate abundance.  Several techniques have been used to do this over the years, including toxicants (e.g., rotenone and creosote), explosives (e.g., primer cord), snorkeling (i.e., direct observation), and electrofishing.  Currently, electrofishing and snorkeling are the two most frequently used techniques.  Snorkeling is generally limited to smaller streams and is considerably affected by habitat complexity.  Electrofishing is much more widely applicable in terms of stream size and flow, is less affected by habitat complexity, and has the advantage of making each fish available for the collection of other data such as lengths, weights (for biomass estimates), otoliths (for ageing), etc.  Even abundance estimates obtained by snorkeling are typically calibrated using electrofishing estimates.

Boat-mounted electrofishing systems are used to sample larger river and tailwater habitats. Lights permit sampling after dark, when larger fish are more vulnerable.

Trout abundance is readily determined in small, wadeable streams where depths rarely exceed three feet or so and widths are generally less than about 75 feet using a technique known as depletion sampling.  Its methodology involves blocking off a representative reach of stream (typically 300 to 600 ft. in length) with nets and making a series (usually three) of passes through the area using backpack electrofishing units (one per every 9-12 ft. of average stream width).   In the southern Appalachian Mountains, water conductivity is often extremely low, so outputs are usually 400-600 volts.  Research has shown that this results in very low levels of mortality (<10%).  Fewer trout are caught with each successive pass (the “depletion”), permitting use of a statistical model to generate population estimates based on the declining catch pattern for each species.  Because this sampling technique routinely captures 70-75% of the trout present, it yields reliable population size estimates.  Using the surface area of the sample site, population estimates can then be converted into density (number/unit area) and biomass (weight/unit area) estimates for direct comparisons over time (within a stream) or among streams.  Wild trout populations in the southeast typically have biomasses that range from 10-50 lbs/acre.

Trout abundance in larger rivers and tailwaters is usually more difficult to assess.  The widths, depths, and flows present in these habitats preclude the use of backpack electrofishing units.  Additionally, sample areas cannot be blocked off with nets to maintain a closed population, so depletion sampling cannot be used to estimate population size.  In these situations, more powerful boat-mounted electrofishing systems are used.  Timed (10 or 15 minute) runs are made along established transects (shoreline or mid-channel) and average catch rates (fish/hour) are determined.  These catch rates can then be compared over time or among rivers/tailwaters.  Where hazardous conditions (e.g., shoals) do not exist, larger rivers and tailwaters are often sampled at night as catch rates (particularly for large fish) tend to be better after dark.  Catch rates for tailwaters with more abundant trout populations can range from 200-300 fish/hour.  It is possible to obtain biomass estimates for large rivers and tailwaters, but it requires marking a large group of fish prior to stocking, then making a subsequent sampling effort to determine the ratio of marked to unmarked fish.

Wild trout abundance in several east Tennessee streams is monitored annually through the cooperative efforts of three natural resource management agencies.

Many fisheries management agencies estimate trout abundance in their more important stream and river fisheries on yearly basis.  In Tennessee, biologists from the Tennessee Wildlife Resources Agency, National Park Service, and U.S. Forest Service work together in the fall to complete this task on several streams.  This monitoring data provides a valuable means of assessing annual variability and for documenting trends that may be related to environmental events (e.g., droughts and floods).  Abundance data are also useful for determining the effectiveness of angling regulation changes and habitat improvement projects.