Do Aquatic Herbicides Cause Sores on Fish?

Redear sunfish/shellcracker with an abnormality.
Redear sunfish/shellcracker with an abnormality.

What is that!?! The fish I caught has sores! People around the state have been increasingly reporting fish with abnormalities or sores on social media. These fish sores appear concerning and are often thought to be a sign of poor resource management. In particular, many stakeholders have formed unfounded conclusions that fish sores are caused by aquatic herbicide use. While there is no doubt fish often contain sores or abnormalities, they are likely not related to herbicide use. Fish sores are commonly due to infection by microorganisms.

Additionally, these infections often occur after fish have been stressed or experience some event that interferes with the protective “slime” coat on their scales. In fact, when fish spawn, they often develop sores due to infection caused by biting and rubbing on each other or on the sediment during the construction of nests. Fish sores and abnormalities are also common after confinement stress. This stress is a major issue in aquaculture production; however, catch and release fishing where live wells are used can also stress fish.

Largemouth bass with an abnormality.
Largemouth bass with an abnormality.

Red sore disease is one of the most common fish abnormalities which can be caused by either a protozoan or a bacterium. Fish infections also become more prevalent after warm weather and high nutrient levels. Similar to how some people are more prone to becoming ill, fish populations also contain some individuals that are more sensitive to infection regardless of stressful events. In 2020, the Florida Fish and Wildlife Conservation Commission conducted fish surveys across over 30 lakes in the state to document the frequency of largemouth bass with abnormalities. In most lakes, abnormality rates ranged from 0 to 20%.

Furthermore, lakes with greater historical herbicide use did not result in increased abnormality rate.

Largemouth bass with an abnormality.
Largemouth bass with an abnormality.

In order for herbicides to receive a Federal registration for aquatic use, extensive ecotoxicology or potential for environmental harm testing must be completed and accepted by the United States Environmental Protection Agency. Also, herbicide registration costs are commonly to the tune of tens of millions of dollars. Therefore, registrants would not attempt this process with a product that would cause environmental harm and result in removal from the market as registration costs could not be recouped.

One herbicide in particular blamed for most of the fish abnormalities is diquat. It is unclear why this particular herbicide has received more backlash than others but we decided look into its toxicology closer. Upon a literature search, we actually found published data (Darwish and Mitchell 2009) demonstrating the use of diquat to lower channel catfish mortality rates from bacteria in aquaculture. So, not only does the herbicide suspected of causing fish sores not hurt fish, it actually has been used to protect fish from infection in production settings.

To read more about this topic read: “Red Sore Disease” in Game Fish by Peggy Reed and Ruth Francis-Floyd

References: Darwish AM, Mitchell AJ (2009) Evaluation of diquat against an acute experimental infection of Flavobacterium columnare in channel catfish, Ictalurus punctatus (Rafinesque). Journal of Fish Diseases 32:401-408

All photos were taken by Jason Dotson, Daniel Nelson, and Ted Lange of FWRI section of FWC.

Written by Dr. Benjamin P. Sperry, UF/IFAS CAIP Research Assistant Scientist. For more information about the UF/IFAS Center for Aquatic and Invasive Plants, please visit Be sure to follow us on social @UFIFASCAIP. To read more blogs like this one visit

UF/IFAS CAIP, Turning Science Into Solutions.


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Posted: February 28, 2021

Category: Coasts & Marine, Conservation, Invasive Species, Natural Resources, UF/IFAS, UF/IFAS Extension, UF/IFAS Research, Water
Tags: Agriculture, Herbicide, Invasive Plant Control, Invasive Plants, UF/IFAS Extension, Water

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