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Biodiversity Blog


Pets as Invasive Species: Fish Gone Wild

by Nicole Elmer and Adam Cohen, Ichthyology Collection Manager

 Illustration: Nicole Elmer

Pet fish may not purr and curl up in your lap or bark when they see you, but because of their colors, anatomy, and behaviors they can be interesting and beautiful to observe in their aquariums or backyard ponds. But sometimes their owners decide they just can’t care for their fish anymore. They may decide to get rid of them by dumping them into natural waterways. Most of these do not become established populations and die off due to predation or unsuitable environmental conditions like cold winters, but some can and do become invasive.

People dump fish into Texas waterways for reasons other than being unwanted pets. For example, fish are dumped for food, as bait, or even for ritualistic ceremony. But the fish we list below are some that come from the aquarium trade. The examples out there are plenty and the Ichthyology Collection is actively documenting the invasion via specimens. Go to the Fishes of Texas project website to learn more.

Bluefin Killifish (Lucania goodei): These are popular aquarium fish, native to Florida, most likely transported to Texas waterways via aquatic nursery plants, the kinds one can put in their ponds or aquariums. They can now be found in areas near Houston, Powderhorn Lake, and Anderson County. Little is known about how it may effect the native ecosystem, but it seems to be spreading rapidly and needs more study. Learn on FoTX.

Variable Platyfish (Xiphophorus variatus): This species, featured previously in our blog, is native to northern Mexico, but because of their small size and beautiful coloring, they are also kept as freshwater aquarium pets. The Hendrickson Lab first collected this species in Waller Creek in 2004, in Austin, and believe it was established due to an aquarium release. Variable platyfish could be a vector for introduction of the pathogen Mycobacterium mariumLearn on FoTX.

Flag Fish (Jordanella floridae): This species, endemic to Florida, is an increasingly popular aquarium species. It is a recently-detected invader in Texas that gets its name from the unique patterning of the male. It is still rare in Texas and only known from tributaries of Port Bay. Little is known about how it may effect the native ecosystem. Learn on FoTX.

Armored Catfishes (in the genera Hypostomus and Pterygoplichthys): Often called “plecos” or “algae eaters” in the aquarium trade this group is now widely distributed across the state. They start small and unknowing aquarists soon find that they outgrow most aquaria. They have a habit of burrowing into banks, where they lay their eggs, thus facilitating bank erosion. Learn about Hypostomus and Pterygoplichthys disjunctivus on FoTX.

 Illustration: Nicole Elmer

Goldfish (Carassius auratus): Perhaps the most iconic of pet fish, as many of us have had one at some point in our lives. As innocent as they appear, when released into the wild they have the potential to produce large populations and transmit several parasites and bacteria to native species. They also increase in size after a few generations, and thus require more resources that native populations would otherwise use. Learn on FoTX.

Lionfish (Pterois volitans): The lionfish is another beautiful example, with their showy pectoral fins, and bands in red, white, black, and offwhite. They are popular with aquarists. Native to coral reefs in the tropical waters of the South Pacific and Indian Oceans, outside of this range they pose serious threats to other coral reef ecosystems in the Atlantic, Gulf of Mexico, and Caribbean. They are now numerous in the Flower Garden reefs of Texas. They are voracious predators and not much in their non-native range keeps their numbers in check. As they eat fish that otherwise would keep coral reef algae under control, algal growth can become unchecked and harm the health of coral reefs. They also decimate local fish populations, which negatively impacts local subsistence anglers as well as commercial and recreational fisheries. Learn on FoTX.

Rio Grande Cichlid (Herichthys cyanoguttatus): This is a popular pet among Cichlid enthusiasts for its coloring and interesting nesting and parental care. It was featured in one of our blogs as well. It’s native to the Rio Grande drainage (and adjacent coastal basins) in Texas, but has been introduced throughout the state where they are well-established in the Edwards Plateau and coastal drainages eastward to Houston. It has also been introduced to Florida and Louisiana. In Louisiana, they pose such a threat that it is illegal to own one, and if you happen to catch one, you are required to kill it. There are even private groups that run fishing contests to catch them. Raising even more concerns, a second closely related species has been discovered in Louisiana, the Lowland Cichlid (Herichthys carpintis) by biologists at Case Western Reserve University and at our own Biodiversity Collections. Learn on FoTX.



So what can we do about these problems? Some organizations propose the use of certain invasive fish species as food. Lionfish are one example. With particularly difficult fish species, changes in infrastructure, like the introduction of electric barriers on invasive carp, are attempts to stop the spread. Other attempts include botanic pesticides, carbon dioxide as toxin and deterrent, and underwater acoustic deterrents. These methods are not fail proof however. Oftentimes, they do little.

For each of us individually, the most direct solution is to not release non-native fish into natural waterways or flush them down the toilet. “Rehoming” is the process to find a pet fish a new place to live, in an artificial environment. A great place to start to learn about rehoming your fish is this website: https://firstquarterfinance.com/what-to-do-with-unwanted-pet-fish/

If you think it is indeed time to find a new home for your pet, someone else may have the space for them and you can list your pet on websites like Facebook or NextDoor. (Note that Facebook will only allow a “rehoming” of a pet, not a sale involving money.) 

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The Shelter Project (http://theshelterpetproject.org/pet-search/) is a place to input your zipcode and learn about the local adoption and pet shelters that accept pet fish. The internet often has forums for aquarists where you may be able to find a new owner.

Often, schools will take pet fish as educational resources for their classrooms. A local aquarium may also take it. Additionally, some locally-owned pet stores are more likely to take an unwanted pet fish, rather than a chain store. Still, some larger chain stores like PetCo list resources and guidance for finding a home for your pet. (https://www.petco.com/content/petco/PetcoStore/en_US/pet-services/resource-center/caresheets/environment-habitattitude.html)

Last but not least, euthanizing still remains a better option than releasing the fish into the wild. Texas Invasives lists humane ways to do this (https://www.texasinvasives.org/neverdumpyourtank/)

Check out our other blogs on pets as invasive species! Start here with the intro. Then check out birds, cats, dogs, and amphians and reptiles.


Asian Swamp Eel/Rice Eel, 2014. Texas Invasive Species Institute (accessed online: http://www.tsusinvasives.org/home/database/amphipnous-cuchia

“Case Western Reserve biologist leads discovery of new invasive fish.” The Daily. June 14, 2021. (accessed online: https://thedaily.case.edu/case-western-reserve-biologist-leads-discovery-of-new-invasive-fish/)

Ecosystems: Impacts of Invasive Lionfish. NOAA. March 30, 2020 (accessed online: https://www.fisheries.noaa.gov/southeast/ecosystems/impacts-invasive-lionfish)

Variable platyfish (Xiphophorus variatus) Ecological Risk Screening Summary, U.S. Fish & Wildlife Service, October 2019. (accessed online: https://www.fws.gov/fisheries/ANS/erss/uncertainrisk/ERSS-Xiphophorus-variatus-FINAL-Oct2019.pdf)

Wild Goldfish. Texas Invasive Species Institute. (accessed online: http://www.tsusinvasives.org/home/database/carassius-auratus)

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