Is silence golden?

The effects of boat noise on fish

Is there is an avid angler who hasn’t wondered about the effects of outboard motors, trolling motors or even the sounds of fishing on fish?

In the 1960s, Al and Ron Lindner, the founders of In-Fisherman, developed an incredibly affective method for catching walleye. Most Mississippi Sportsman readers couldn’t care less about catching walleye (although the method worked for bass, too), but the point of this story is about outboard motor noise and catching fish.

The Lindner’s method was a highly refined method of structure fishing called “backtrolling”— fishing directly under the boat and moving the boat in reverse with the outboard for precise speed and position control.

Why a gasoline engine? Two reasons: First, trolling motors had just been invented and weren’t very powerful and, second, this method worked in areas like fly-in camps where there was gasoline for outboards but no electricity to charge trolling motor batteries.

One of the repeated questions to the Lindners was whether the engine noise spooked the fish. Millions of walleyes caught by backtrolling are clear testimony that outboard noise didn’t affect all walleye.

But here’s another fish story. Some years ago, a fishing buddy and I found a hydrilla-filled cove where bass would crush buzzbaits.

Two days later, on the day of the Starkville Bass Club tournament, we returned to the cove in separate boats. After 30 fishless minutes of churning through the hydrilla, I left. Bill stayed and patiently, quietly and successfully fished the hydrilla.

He won that club tournament.

I’m sure every reader can add many stories to this start. But what say the scientists?

Science hasn’t been much help on the affects of boat noise on fish behavior, but a recent study on fish you might have never heard of gives some insight. (As you will see, the fact that you might not be familiar with these fish might not be important.)

The study occurred in a small lake in Denmark. The Danish scientists measured changes in the movement of radio-tagged fish when exposed to boating, and to boating and fishing.

The boating treatment consisted of moving a boat with the outboard motor for one to two minutes, mooring the boat at a pole for 15 minutes, and then moving the boat again. This process was continued for several hours until the entire lake had been traversed.

Boating with fishing was the same, except anglers cast lures for 15 minutes at each mooring.

The movement of fish was monitored by radio receivers in the lake. This allowed fish movement to be measured when no boat was on the lake and compared to movement when a boat was on the water.

One of the three fishes monitored is called a roach. Roach are in the minnow family, and the fish monitored were 7 to 10 inches long. The roach became more active when exposed to boating or boating and fishing, and their swimming speed increased five fold compared to undisturbed fish.

The second fish was the European perch, a cousin of the yellow perch that swim in the Tennessee River and are abundant in northern waters. Like the roach, the tracked perch were 7 to 10 inches long.

Perch became more active during the first hour of boating or boating and fishing, but then their swimming speed declined to speeds more similar to undisturbed fish.

The third fish was northern pike, the same fish that swims in northern U.S. waters. These pike were 18 to 39 inches long. Their movement behavior did not change when disturbed by boating or boating and angling.

Conclusion: Boating affects some fish but not all.

The differences in behavior of the three fishes can be explained by their position in the food web.

The roach, which greatly increased activity when disturbed by boat noise, are forage fish that occupy open water. Their survival depends on being vigilant and responsive to disturbances. When disturbed by outboard noise, they moved away from the disturbance, grouped up and swam actively.

Perch were active only during the first hour of boat noise. Perch, too, are a forage fish, and their survival depends on responding to disturbances. But unlike the roach that grouped up and swam fast throughout the disturbance period, the perch were initially active and then sought refuge on the bottom.

The pike are predators that often live along shorelines. Their lack of response to boating is consistent with the role of a large predator that does not have to be responsive to threats.

Applying these results to our native fishes, boating or boating plus angling would have little effect on bass or large catfish movement, fishes that are large predators like the pike. Sunfish and crappie might be expected to behave like the perch — initially become more active until they find refuge on the bottom or in cover. Shad, like the roach, are open-water forage fish, and would be expected to become more active and group into schools.

The results of this study might give you a clue about the movement and shelter-seeking behavior or your favorite sportfish, but the study leaves unanswered how disturbance affects the fish’s likelihood of biting a lure or live bait. That would be a very difficult experiment to conduct. How do you measure the biting or striking behavior of undisturbed fish?

Whether you believe sound affects the behavior of your favorite finned quarry is up to you. If you do think sound adversely affects fish, forget about sneaking up on them; sound travels through water at about 3,200 mph and travels farther through water than air.

Let me put this in more understandable units. Sound travels about one mile per second in water, and fish can hear your boat at least four times farther away than you can hear a boat coming.

Since you can’t sneak up on fish, maybe look at the effect of boat noise differently. If the noise increases shad movement and schooling, might that not be a good thing for anglers fishing for fish that eat shad?