How Hungry Flathead Catfish Transform the Susquehanna River

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In the intricate web of our ecosystems, every species plays a vital role, contributing to the balance of life. However, when a non-native species is introduced, the delicate equilibrium can be disrupted, leading to substantial ecological consequences. A recent study involving a collaborative effort from Penn State, the U.S. Geological Survey (USGS), and the Pennsylvania Fish and Boat Commission has shed light on the impact of the flathead catfish, an opportunistic predator native to the Mississippi River basin, on the food web of the Susquehanna River in Pennsylvania.

First detected in 1991, the flathead catfish population has burgeoned, raising concerns about its potential to decimate native fish populations and alter the dynamics of local ecosystems. Olivia Hodgson, a master’s degree student in Penn State’s Intercollege Graduate Degree Program in Ecology, leads the research and highlights the fast growth and large body sizes of flathead catfish. “Flatheads grow fast in this river system, attain large body sizes and can eat a variety of prey,” she explains. With few natural predators of their own, these apex predators exert significant control over their environment.

The findings of this study, published in the journal Ecology on September 4, reveal that flathead catfish occupy the highest trophic position in the Susquehanna River ecosystem, even surpassing resident top predators like smallmouth bass and channel catfish. This shift means that channel catfish now find themselves competing for food lower on the food chain, likely due to the competitive pressure exerted by the flatheads. The study uncovered that in areas populated by flathead catfish, all fish species exhibited broader and overlapping diets, suggesting a forced adaptation in resident species to avoid competition and predation.

Hodgson elaborates on these findings, stating, “This suggests that resident species are changing what they eat to avoid competing with or being eaten by the invader. These findings support the ‘trophic disruption hypothesis,’ which posits that the introduction of a new predator can compel existing species to alter their behaviour, diets, and roles within the food web. Over time, this can destabilise ecosystems.” The research underscores the profound effects invasive species can have, reshaping food webs and shifting energy flow throughout ecosystems.

While the predatory impacts of invasive catfish on native fish communities have been documented previously, the broader implications on riverine food webs remain largely unexplored. Hodgson and her team sought to quantify the specific effects of the flathead catfish on the Susquehanna River’s food web by comparing sections of the river where flatheads are established with those that remain uninvaded. Their focus centred on key species: flathead catfish, channel catfish, and smallmouth bass as resident predators, alongside crayfish and minnows as their primary prey.

To gain deeper insights into the interactions within the food web, the researchers employed stable isotope analysis. This powerful tool enables scientists to discern patterns within a food web, highlighting connections between trophic positions and the breadth of dietary niches. In the context of invasion ecology, stable isotope analysis is invaluable for understanding how introduced species reorganise trophic structures and affect the feeding behaviours of resident species.

When fish consume food, they incorporate the isotopic signatures of their diet into their bodies. By analysing these signatures through tissue sampling, researchers can derive important insights into dietary habits and habitat use. The team collected a total of 279 fish and 64 crayfish for stable isotope analysis, including 79 flathead catfish, 45 smallmouth bass, 113 channel catfish, and 42 minnows spanning nine species. Each sample underwent meticulous preparation—oven-dried and ground to a fine powder—before being sent to Penn State’s Core Facilities and Michigan State University for isotope determination.

“Stable isotope analysis explained patterns within the Susquehanna food web in habitats invaded and not invaded by the flathead catfish,” Hodgson notes. “It allowed us to understand links between different species in the river food web and how invasive species might lead to changes in how native species interact and compete, what they eat, and how their diets overlap.” This comprehensive analysis provides a clearer picture of resource utilisation, offering insights into potential competition dynamics and how they shift with the establishment of flathead catfish in the river system.

Contributors to this pivotal research include Sydney Stark, a recent Penn State graduate with a master’s degree in wildlife and fisheries science; Megan Schall, an associate professor of biology at Penn State Hazleton; Geoffrey Smith, a biologist specialising in the Susquehanna River for the Pennsylvania Fish and Boat Commission; and Kelly Smalling, a research hydrologist with the USGS New Jersey Water Science Center. The study was made possible through funding from Pennsylvania Sea Grant and the U.S. Geological Survey.

As we reflect on these findings, it becomes increasingly clear that the introduction of invasive species like the flathead catfish can have cascading effects within our ecosystems. The ability of these predators to outcompete native species not only threatens biodiversity but also disrupts the intricate relationships that have developed over millennia. Understanding these dynamics is critical as we strive to protect our waterways and preserve the ecological integrity of our natural habitats.

The implications of this research extend beyond the Susquehanna River, resonating with broader discussions around biodiversity conservation and ecosystem management. As we grapple with the challenges posed by invasive species, it is vital to foster awareness and engage in proactive measures to safeguard our rivers and the myriad species that inhabit them. Collaborative research efforts, such as this study, are essential for informing strategies that mitigate the impacts of invasive species and promote the resilience of our ecosystems.

In conclusion, the story of the flathead catfish serves as a stark reminder of the complexities of ecological balance. As champions of net-zero and environmental sustainability, it is our collective responsibility to advocate for informed management practices that protect biodiversity and enhance the health of our ecosystems. By embracing research and fostering a deeper understanding of the interconnectedness of species, we can work towards a future where our rivers thrive, supporting both wildlife and human communities alike.