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January 2018 - Electrofishing in the South Platte River

05 Jan 2018 8:29 AM | Natalie Love (Administrator)

As part of ongoing biomonitoring, scientists from Metro Wastewater participate in an annual electrofishing program.  This monitoring program began in 1986 and there are currently thirteen sites spread over a forty-mile stretch of the South Platte River.  The same sites are sampled annually in the fall to compare historical data and change over time.  The main purpose of the program is to gather information on the species, size, quantity, weight, and health of the fish in Segment 15 of the South Platte River.  Various entities, including the Colorado Department of Public Health and Environment (CDPHE), use the data for river assessment studies, resegmentation, and habitat and aquatic life preservation projects.


CDPHE’s Water Quality Control Commission sets regulations to control surface water quality.  Regulation 38 establishes classifications and numeric standards for four rivers including the South Platte River and its tributaries.  Each river segment is assigned a stream classification, which may contain up to four designated uses including Aquatic Life, Recreation, Water Supply, and Agriculture.  Each combination of designations comes with ranges and standards for temperature, nutrients, and other parameters designed to protect these uses and meet the goals of the Clean Water Act ensuring every river segment is fishable and swimmable.


Colorado Parks and Wildlife (CPW) does regular fish surveys in the upper South Platte River drainage around the same time.  CPW and Metro sometimes survey at different times of the year for a specific study or construction project.   The National Park Service in conjunction with the US Fish and Wildlife Service surveys National Parks (like RMNP) rivers and creeks every year as well. Consultants may be hired to conduct fish surveys when projects take place in or near a body of water, such a building a bridge over a river, or destroying a pond for a parking lot.  For example, GEI Consultants performs surveys for clients all over the Western US using bank shockers (like MWRD) and backpack shockers. Some dischargers in South Dakota and Idaho are required by their states to monitor the instream fish populations, so they monitor upstream and downstream for fish, bugs, and habitat. Some participating entities have long-term data sets for some rivers going back 20 years, which is valuable information when observing the transitions of fish populations over time through weather events and flood years. Graduate students and researchers may also perform fish surveys as needed.


An electrofishing day begins by taking flow measurements.  The river flow must be below 300 cfs (cubic feet/second) on sampling days for safety concerns and accordingly, the flow is slow enough to successfully catch the fish once they are stunned.   Flags are placed along a 100-meter reach of the river to mark the sampling zone.  Two or three electrodes are connected together by power cords and held by members of the crew in the river.  These electrodes are on poles with a circular ring that is dipped into the water the entire time.  The electrodes are connected to a generator sitting in the back of a truck parked on the bank that is also attached to a cathode in the water to complete the circuit.  About 3-4 amps of alternating current hits the fish within about a 6-foot diameter area in the water.  The current temporarily paralyzes the fish much like a stun gun, thus making them easy to catch with nets.  Alternating current is used because it causes the least amount of harm to the fish and actually draws them towards the electrode so they do not float away too fast.  The amount of current will vary depending on the size of the river, flow, and conductivity levels of the water.


Crewmembers wear waders to keep them dry and to separate themselves from the current in the water.  They should still use caution.  If the electrodes are in the water and an analyst touches the water directly, they get a shock.  Depending on their proximity to the electrode itself, they could feel as little as a slight tingle or as much as a good wake up jolt.  Serious damage could occur if the electrode was touched directly, but alternating current does offer some protection.  Analysts also watch for frogs, turtles, crawfish or other critters and try to remove them from the electrode path to save them the trauma of being unnecessarily shocked.


The crew zigzags back and forth across the sample reach about 30 times to cover the entire area from bank to bank.  As fish are netted, they are taken to coolers of water on the banks while they recover and wait until the process is complete.  The cooler keeps the water temperature cool and closer to the temperature of their natural habitat.


Once the shocking and collection process is complete, the counting begins.  The fish caught typically range in size from about an inch or two to a foot long, with an occasional larger fish.  Some sites have been known to house the occasional 2-foot long carp.  A total of 29 different fish species have been counted by Metro in the South Platte over the years including the typical yield of Fathead Minnow, White Sucker, Largemouth Bass, Johnny Darter, Green Sunfish, Longnose Dace, and Sand Shiner. Each fish is measured in millimeters on a fish board, weighed in grams, and identified per species.  Any unusual wellness indicators may also be noted during the counting process.  Then the fish go back into the river unharmed, although a little confused. (Similar to an alien abduction).  All the data is logged and compiled into a database for a variety of future uses.



Michelle Neilson, Water Quality Technician, has been with Metro Wastewater for 8.5 years.  She has a B.S. in Chemistry, and has 19 years of experience in the Environmental field.  Michelle has worked for USGS, contract laboratories, and several municipal wastewater and drinking water labs prior to Metro Wastewater.


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