Flathead catfish, renowned as opportunistic predators, are native to the Mississippi River basin. However, their introduction or invasion into other river systems poses significant threats to both native and recreational fisheries, disrupting existing ecosystems. A collaborative research effort involving experts from Penn State, the U.S. Geological Survey (USGS), and the Pennsylvania Fish and Boat Commission has shed light on how these invasive species are affecting the food web and energy flow within the Susquehanna River in Pennsylvania, where they were first detected in 1991. The population of flathead catfish in this region has surged dramatically over the past few decades.
According to Olivia Hodgson, the study's first author and a master’s degree student in Penn State's Intercollege Graduate Degree Program in Ecology, flathead catfish thrive in the Susquehanna River, growing rapidly and attaining impressive body sizes. Their diverse diet allows them to consume various prey, making them formidable predators in the ecosystem. With few natural predators of their own, adult flatheads can significantly influence the dynamics of the river ecosystem. Hodgson collaborated with Tyler Wagner, a scientist with the USGS Pennsylvania Cooperative Fish and Wildlife Research Unit Program and a Penn State affiliate professor of fisheries ecology, who serves as the senior author of the study.
Published on September 4 in the journal Ecology, the researchers discovered that flathead catfish occupy the highest trophic position in the Susquehanna River food web, surpassing resident top predators such as smallmouth bass and channel catfish. The presence of flathead catfish correlates with a decline in the trophic position of channel catfish, suggesting they are either being outcompeted or are avoiding flatheads altogether. This shift indicates a significant alteration in feeding relationships among species in the area.
The researchers observed that in habitats populated by flathead catfish, the diets of all species exhibited broader overlaps. Hodgson explained that this dietary change likely results from resident species adapting to avoid competition or predation from the invasive flatheads. These observations support the trophic disruption hypothesis, which posits that the introduction of a new predator can force existing species to modify their behavior, diets, and ecological roles, potentially destabilizing the ecosystem over time.
This study emphasizes that invasive species have the potential to do more than just diminish native populations; they can fundamentally reshape entire food webs and alter energy transfer within ecosystems. While the predatory impacts of invasive catfishes on native fish communities have been documented, such as in previous studies on the Susquehanna River, the broader implications of these invasions on riverine food webs remain poorly understood.
To quantify the effects of invasive flathead catfish on the food web, the research team compared sections of the Susquehanna River that were invaded by flathead catfish to those that were not. They focused on several key species, including flathead catfish (the invader), channel catfish and smallmouth bass (the resident predators), as well as crayfish and minnows (the prey). In addition to assessing trophic positions, the researchers analyzed the isotopic niches occupied by these fish species, which reflect their diets and habitats.
Using stable isotope analysis, a well-established technique for understanding food web dynamics, the researchers measured carbon and nitrogen isotopes within the tissues of the fish and crayfish. This analysis allowed them to deduce dietary habits, habitat preferences, and potential shifts in fish migration. A total of 279 fish and 64 crayfish samples were collected for this purpose, including 79 flathead catfish, 45 smallmouth bass, 113 channel catfish, and 42 minnows from nine different species.
The results of the stable isotope analysis provided valuable insights into the food web dynamics of the Susquehanna River, revealing how the invasion of flathead catfish affects interactions among species. Hodgson noted that this research helps elucidate the links between different species and how invasive species can lead to changes in competition, dietary habits, and habitat displacement among native species.
In summary, this comprehensive study highlights the critical need for ongoing research into the ecological effects of invasive species like flathead catfish, particularly in the context of river ecosystems. Understanding these dynamics is essential for developing effective management strategies to preserve native biodiversity and maintain the health of aquatic ecosystems.
