by Matt Miller
I couldn’t see any fish.
Standing calf-deep in a clear stream so narrow I could easily leap across it in a single bound, it seemed like I should see something swimming around. Maybe fish weren’t using this newly restored habitat after all?
I carried the tool to find the answer: a large backpack with various control panels and dials, hooked up to the probe I held in my hand.
Steve Herrington, director of freshwater conservation for The Nature Conservancy in Florida, tapped me on the shoulder. “Give it some juice,” he said.
I pressed lightly on a small trigger, sending electric voltage into the water. Immediately, a different picture of this stream emerged: As fish twitched on the surface in front of me, giving me just enough time to scoop them up with the net attached to the probe.
A closer look revealed an astonishing diversity for such a small stream: Madtoms with their big heads, whiskers and venomous fins. Eel-like lampreys, some without eyes. Colorful minnows. Endemic salamanders.
In minutes, they would all be released back into the stream.
Electrofishing—as this technique is called—is one of the best tools for fisheries biologists and conservationists studying anything fishy—the success of freshwater habitat restoration, migration routes, invasive species and more.
As any angler knows, it’s difficult to know what lies beneath the water’s surface. I’ve stood in crystal-clear trout streams for hours, ultimately convinced the trout had disappeared.
This feeling may be frustrating for anglers, but it’s unacceptable for fisheries biologists. Are fish in a river thriving or declining? Are rare species using a newly restored stream as hoped, or are they avoiding it? Are non-native fish taking over a waterway, or are they actually not a threat?
These and other questions simply cannot be answered through observation.
To gather data, you need a tactic that brings the fish to you: electrofishing.
Electrofishing involves sending electric current underwater at a set frequency. The frequency and voltage depend on a variety of factors—the size of fish, the size of the waterway, the conductivity of the water’s chemistry.
The electric current causes the fish’s muscles to twitch and its air bladder to inflate. It appears stunned near the surface, and momentarily swims towards the electric current.
The fish is not killed: Cut the juice, and it swims away.
Biologists net the fish for their studies. Often, fish are weighed and measured. Sometimes, a few are killed to sample the fish’s health, diet or toxin levels.
Electrofishing doesn’t lie. Sometimes, the results surprise scientists.
On the tiny Florida stream I electrofished with Steve Herrington, an earthen dam and an unpaved road had been removed five years ago. Herrington is doing a study to determine if fish would reestablish populations in the water above the dam.
Prior to these removals, the diversity of fish species was low. Herington found many fish below the dam and in other nearby streams that could not be found above the dam.
Worse, only one fish was found upstream of the road crossing after two years of surveys.
Now, five years later, our shocking revealed dozens of fish from many different species throughout the stream — an indication that the removal and habitat restoration had been successful.
The previous day, we had electrofished on a different scale—the Apalachicola River in the Florida Panhandle. On this large river, fisheries biologists surveyed populations of Alabama shad, a silvery plate-sized fish that was now being moved over dams using shipping locks. Conservancy biologists wanted to know if the assisted migration technique worked.
On this river, we shocked from a boat. Large probes dangled from the front of the boat, and netters turned on the voltage by stepping onto a rubber pad. As I stood with net in hand, it felt like being on the set of River Monsters, as strange and unusual species rolled on the surface: from the longnose gar to giant grass carp to turtles.
But these were only of secondary interest. Shad shimmied to the surface, where they were netted and tagged. Some were fitted with receivers to allow the fish to be radio tracked. From shad records and radio tracking, biologists have determined that thousands of fish (in 2010, almost 100,000) use the shipping lock to migrate.
Prior to this effort, biologists really didn’t know just how many shad lived in the Apalachicola River.
Anglers and conservationists alike talk about the mysteries of rivers and streams. Biologists don’t want mystery. They want data. Those electric currents only temporarily stun fish, but they provide the evidence needed for effective conservation.June 13, 2012
Matt Miller is senior science writer for The Nature Conservancy. Read his regular blogs at Cool Green Science.