The Fight for Fish Food: Invasive Armored Catfish vs. Native Fishes

Source Article: Quintana, Yasmín, Keppeler, Friedrich Wolfgang, and Winemiller, Kirk O.. 2023. “ Does Invasion by Armored Catfish Shift Trophic Ecology of Native Fishes? Evidence from Stable Isotope Analysis.” Ecology 104(5): e4024. https://doi.org/10.1002/ecy.4024

Featured Image Caption: The armored catfish is a popular aquarium fish, now invasive in many places around the world.Image Source: Paweł Cieśla Staszek_Szybki_Jest, CC BY-SA 4.0, via Wikimedia Commons

Aquarium Fish Gone Rogue

Aquariums are beloved by many. Enthusiasts enjoy nurturing and viewing aquariums full of beautiful and unique fish from around the world. However, the aquarium trade contributes to lots of introductions of fish to ecosystems in which they are non-native. Armored catfish (Pterygoplichthys spp.) are one example of a fish species that has been moved around the world by the aquarium trade and has become invasive in water bodies on several continents.

Armored catfish are a successful invasive species for a few reasons. First, they can survive in a wide range of water conditions. Second, their body is covered in hard scales and spines; they’re literally tough. Third, they can occasionally leave the water and breathe air. And fourth, they’re bottom feeders that mostly eat biofilms on surfaces and detritus, or decomposed organisms (think algae and “goo” in aquariums), which can be found in a wide range of water bodies.

Now that these catfish are invasive in so many places, it is extremely important to figure out what resources they use to understand their impacts on other species. If invasive species eat lots of certain foods, the competition for these foods increases as the invaders increase in numbers. So, how are armored catfish affecting native fish species that they compete with for food?

Isotopes: You Are What You Eat

Scientists answered this question using isotope analysis. Isotopes are different versions of the same chemical element, but with slightly different weights. Plants tend to use a light isotope of Carbon (C) as they live and grow, which means that plants end up having more of the light C isotopes than the air that they took the C from. Because plants can be either more or less selective of light C isotopes, different types of plants end up with slightly different ratios of light versus heavy C isotopes. Nitrogen (N) isotopes in plants differ depending on the source of N. N isotopes also become increasingly heavy moving up the food chain.

For example, when other organisms, like macroinvertebrates or fish, eat plants, they end up being made of C and N isotopes that can be linked to the types of plants they ate. If fish eat macroinvertebrates or fish, they end up being made of C and N isotopes that can be linked to the other organisms that they ate. You are what you eat! This means that scientists can look at isotopes in fish to figure out roughly what they’ve been eating.

Researchers went to two rivers in Guatemala, one where the invasive armored catfish was common and one where it was rare. They collected pieces of plants, plankton, and macroinvertebrates in both rivers, then took pieces of all fish species in the rivers. Scientists figured out the different isotopes present in their samples using a machine called a mass spectrometer. With this method, scientists investigated whether the mix of things armored catfish were eating overlapped with other native fish, causing harmful competition.

Delicious Goo in High Demand

In both rivers, isotope analysis revealed that invasive fish and many native fishes were eating similar foods. This similarity was stronger in the river where the invader was rare. In this river, native fish ate a wide variety of foods. Contrastingly, in the river where the invader was common, native fish ate fewer different foods and particularly ate foods that the invader did not eat.

Isotopes also revealed that the armored catfish mostly ate benthic periphyton, or algae and bacteria on the bottom of the rivers, but that they ate a bit less of this microbial “goo” in the river where they were common. This means that the biofilm was in such high demand that it was all eaten up and the invasive fish had to branch out to eating other foods.

Taken together, these findings mean that when the armored catfish has reached a high population size, it outcompetes many native fish for some foods. These other fish may be able to survive and coexist with the invader by eating different foods. However, native fish were less commonly found in the river where the invader was common, suggesting that there is a negative effect of the invader on native fish.

Research Feeds Conservation Efforts

Invasive armored catfish can adjust their diets in different competition scenarios, which is key information for predicting the invasions by these fish in other water bodies. The adaptability of invasive species to a wide range of conditions and foods plays a large role in allowing many of these species to be successful. Not all native fish can adapt their diets as easily because they may have evolved to eat specific foods in their local habitat. If some native fish are less flexible in their diets than the invaders are, they may be particularly vulnerable. Scientists may be able to use this knowledge to prioritize locations to try to prevent invasions or to fight early invasions before the catfish populations become large. Scientists may also prioritize the conservation of species that are known to have specific diets.

How can citizen scientists help prevent ecological invasions like this? We can clean, drain, and dry boats and other watercraft between uses, and we can use native bait fish when fishing. We can also stop moving around live fish and keep aquarium fish out of natural water bodies. Friends don’t let friends dump aquarium fish in the wild!

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