What's In Your Water?

How many fish are in your favorite stream? Do you know how many trophy-sized fish there are in your lake nearby? How healthy is the population? These are all questions that fisheries management professionals try and answer with field research, sampling, and models. Naturally, we can't count every fish in the river or lake so we rely on methods that take a small, representative sample of the larger population and make inferences from there. Without getting too dry and boring, suffice it to say that annual population surveys are how managers keep their finger on the pulse of our favorite waters.

One of the bread and butter methods of sampling is electrofishing. Used in streams and rivers, electrofishing works by passing electricity from the negative end of a rectified DC current causing muscles spasms (electrotaxis) in the fish which results in the fish swimming in the direction of the positive anode where waiting biologists scoop them up with nets. It sounds simple in theory but it can be rather complex. Balancing voltage, wave lengths, and pulse rates to get an effective but non-lethal current is sometimes more art than science. Contrary to popular belief, this does not kill the fish and is often less lethal than catch and release fishing.

Electrofishing units can be carried on one’s back, or on a barge, raft or jetboat depending on the size of the waterway. Fish that are stunned are netted and dropped into a live-well. After the section is completed, the fish are anesthetized and measurements are taken. Tissue samples from a small part of the dorsal or tail fin are also taken for DNA analysis. These data and more are recorded and added to a database that allow managers to get a consistent and accurate look at how the fish population changes over time.

Electrofishing is pretty ineffective when it comes to lakes and still waters. With so much room, the fish simply swim away before the current can capture them so biologist turn to nets. The two most prolific types are gill nets and trap nets. Gill nets are long, curtain-type nets with square mesh that hangs in the water column. Often made of monofilament, these nets are nearly invisible to the fish who swim into the nets. The mesh expands and to allow their heads through but only past their gills which flare when they swim backwards, effectively snaring them in the mesh. These types of nets are often lethal but can be made non-lethal by frequent checking and removal of fish. Trap nets are essentially funnels that force fish to swim into a net box or hoop that functions in a very similar way to a metal minnow trap. As is the case with electrofishing, these nets only catch a small portion of the fish population and from there managers use statistics and population models to estimate and make inferences on the size and shape of the population.

So how does all this relate to you and your next fishing trip? These surveys are vital to fisheries managers in their monitoring and understanding of the complex ecosystems that occur in our waterways. Results of these surveys dictate harvest and creel limits and whether to increase or decrease stocking rates. If a survey indicates fish are more abundant and bigger than in the past, it could suggest the right management plan is in place. Conversely, an upcoming population crash could be indicated if there are few or no juvenile fish.

Game agencies usually provide the results of these surveys to the public. With a little digging and some research on your own, you can often identify the river section with the most or biggest fish. That lets you better target your efforts. Lastly, many wildlife agencies are short-staffed and can often use volunteer help. Volunteering is a great way to get involved and give back to the watershed while learning about the ecosystems that we use and recreate in.