Written By - The Fish Geek - 08/7/2007
Link to Original Article here

The Fish Ecology Lab I work in is responsible for long term population monitoring in the Logan River, Utah, which is home to an important metapopulation of Bonneville cutthroat trout.  A large part of this monitoring project involves collecting fish via electrofishing at eight different long term index sites throughout the river and its tributaries.  In the main stem of the Logan, high flows require that we use a canoe electrofishing unit and a large team of students, biologists, technicians, and volunteers.

In order to complete the task, we rig up a canoe with a large generator, an electroshocking unit, multiple probes (anodes: +) operated by individuals, and a large cable cathode (+) hanging from the canoe. 

When the generator and shocking unit are turned on, and all probes and safety switches operating, an electric current is sent through the water between the probes and the cable hanging from the canoe.  This current causes fish nearby to turn sideways and become unable to escape the electrical field. 

A row of netters stand behind the people with probes and capture the fish that are caught in the electric current.  Fish are then transferred to the plastic tubs in the canoe, which contain cool river water oxygenated with bilge pumps.

The individual pushing/pulling the canoe operates the master safety switch (we like to call it the ‘dead-man switch\'), which has to be depressed for the unit to be shocking.  This person is responsible for watching everyone and ensuring that nobody gets zapped.  If about to stumble or reach into the water to get a fish between the rocks, a person calls out ‘off!\', upon which the switch is disabled and shocking postponed.

After finishing shocking the site, workers quickly transfer fish to the station set up for data collection.  Here, all fish are measured and weighed, and adult fish receive individually numbered floy tags for future ID.  Some fish are saved for whirling disease testing, diet sampling and aging, and the majority are returned to the river.

This process is repeated three times within an area enclosed by blocknets.  Fish captured on the first two runs are placed outside of the blocknets so that only new fish captured on the second and third runs.  Using the numbers of fish caught in each run, we can then calculate a capture efficiency, which we use to estimate the total population of fish within the blocked-off area (usually 200 meters).  This population estimate is expanded throughout the areas adjacent to the sampling site to provide general information about the area\'s fish population, and is used to monitor trends in population dynamics over time.

We\'ll be back out the next three days completing our Logan River long-term sampling.  In addition to graduate students and technicians in our lab, we are usually assisted by a number of volunteers, along with people from the Utah Division of Wildlife Resources Dedicated Hunters Program, where hunters complete fish and wildlife related conservation projects and receive additional hunting priveliges in return.  A reporter from the local newspaper will be joining us during the next couple of days as well, which we hope will shed more light on what we do in the river and why we do it.