Do Boats Kill (Or Scare) Fish? Basic Facts [Explained]

While this sounds like a straightforward question, the answer is complex and the subject of much study and debate.

Boats come in many shapes and sizes, from one-person kayaks to cruise ships that carry thousands of passengers. However, when we talk about the largest impact on fish, we are talking mostly about motorboats.

While there is no doubt that boats can kill fish through direct contact, it’s the indirect impact that isn’t so obvious.

Boats can have large and unintended consequences for the local environment and ecosystem.

Here is How Boats can Harm Fish:

Wind and paddle-powered craft may scare a few fish, but motor-powered boats are the main killers and disrupters of fish. They produce water and air pollution from the exhaust and the petroleum motors, and noise pollution and turbulence from boat motors can kill or disturb fish populations.

Here is a list of ways boats cause problems for fish:

Direct Impacts:

Boats generally scare fish away before they run into them, but occasionally boats will strike fish, often killing them.

Some species like carp seem to be particularly affected by this and are sometimes killed by boat impact.

It is unclear why this happens, but the phenomenon is well documented. 

Disrupting the Spawn:

Boats can disrupt spawning fish in several ways, but the primary problem is spooking them off their nests.

Because there are precious few studies on the subject, it is unclear how much this impacts fish populations. Still, boats cause fish to abandon their nest for longer periods, leaving the eggs and fry vulnerable to predation.

Oddly, slower-moving boats have a greater impact than fast-moving boats, probably due to the duration of time they are in the area.

Another issue is the effect sound might have on the development of fish embryos, which brings us to the next problem boats cause for fish.

Noise Pollution:

Motor-driven boats have no shortage of noise.

In the water, this noise can be problematic for fish. From potential disruption of embryonic development to masking natural noises, it is clear that human-produced noises, including boats, are a likely problem for some fish.

The noise can also increase when the boat cavitates.

Some species make noises to detect each other, particularly in mating, while others need to key in on noises like a reef’s activity to be successful.

When there is a lot of ambient noise from boats, fish lose the ability to hear these important biological sounds. Noise pollution also increases fleeing behaviors in fish, causing increased swim speed, altering schooling behavior, and increasing avoidance of noisy areas.

Not all noises repel fish. Although it may sound unlikely, some motors produce frequencies that attract fish. These are usually big diesel motors; however, some fishing guides use a certain outboard motor style to attract catfish on some freshwater lakes.

Fish can also be habituated to some noise.

Regular boat traffic in the same area every day is not as disruptive to fish as they become accustomed to the noise.

Water Pollution:

Boats have many materials onboard that are potentially toxic to fish.

From hull paint and battery acid to garbage and fuel, many things you carry on your boat are potentially harmful.

Boat exhaust can also increase carbon monoxide levels in the water, potentially causing harm, although you need a large amount of boat traffic to cause significant issues.

The larger concern is things that harm the plankton and small insects at the food chain base.

Hull paint and cleansers used to wash boats are problematic and can leach toxic materials in sediment. Bulk garbage, especially plastic, is unsightly and can negatively impact the ecosystem as well.

An often overlooked toxin is nicotine. Nicotine is a natural insecticide and can harm the small insects and larvae crucial to many fish.

If you use tobacco, take extra precautions to ensure that you do not allow any of your waste, like a cigar and cigarette butts, to end up in the water.

Wakes and Turbulence:

Motorboats produce energetic wakes that stir up sediment, increase turbidity, and disrupt crucial habitat.

When a boat stirs up sediment by traveling in shallow water or through the effect of the wake, it can destroy plant life and disrupt insect life cycles, which negatively impacts fish.

It also clouds the water, making it difficult for species that depend on sight to feed or flee from predators.

Wakes and turbulence may also displace structures like submerged branches, plants, or even small rocks that fish use for cover.  

Tips to Minimize Your Boat’s Impact on Fish:

So far, we’ve painted a picture that may sound bleak, but there are steps you can take to decrease the impact your boat has on fish.

Here are a few ideas to get you started:

1. Know Your Local Water

Most boaters stick to one area or body of water. Make sure you know about your local habitat, paying particular attention to spawning times.

If you venture out in saltwater, know the local species and how boats may impact their environment.  

2. Obey the Rules

Many no wake and vessel exclusion zones are set up for the benefit of wildlife, including fish.

Be sure to follow all posted signs and stick to the main boating channels.

Always obey no-wake zones and stay at least 100ft(30m) from shore(500ft or 150m if you can).

3. Watch Your Wake

Wakes can be especially damaging, causing everything from erosion to habitat disruption.

It is better to travel at speed if you are out of a no-wake zone.

This may seem counterintuitive, but studies have shown that boats at speed are less disruptive to fish, probably because you are there and gone quickly.

Also, planing hulls cast less of a wake at speed, decreasing the local habitat’s impact.  

4. Use Marine Friendly Products

You may not control what paint was used on your hull, but you can control what you use to clean your boat.

Always choose cleansers that are approved for marine use and are environmentally friendly.

5. Contain Caustic Materials

Ensure you have secondary containment for batteries and that anything that might leak from the engine has no route to the water.

This is difficult for outboards, but you can minimize environmental impact by keeping your engine clean and well-maintained.

6. Minimize Garbage

Bulk pollution, such as garbage, can be a real problem. Minimize garbage by removing any unnecessary packaging before going aboard.

Many products, like canned drinks, have plastic packaging that can trap fish.

Remove this packaging and dispose of it before getting underway to decrease its risk of ending up in the water.

Ensure that you have a container aboard for garbage with a lid, so garbage doesn’t inadvertently fly out of the boat.

You can also organize or participate in local cleanup efforts to remove bulk garbage.

7. Get Involved

Find a local conservation group and learn what you can do locally to preserve fish habitat.

Speak with politicians and policymakers to establish no wake and vessel exclusion zones in the shallows and spawning areas.

Encourage your local universities and wildlife authorities to commission studies into human impact on local waters.

It may seem like a daunting task, but the more boaters that participate, the easier conservation efforts will become.

Conclusion:

Human activity often has unintended consequences. Boating does affect fish, but the impact boats have on fish populations is unclear.

More study is needed to fully understand how much long term effect boats have on local wildlife. Watch your wakes, control what goes into the water, and know the local wildlife patterns to minimize your negative impact.  

As always, I wish you pleasant travels, calm winds, and fair seas. Be safe and watch out for your fellow boaters!