Hungry flathead catfish are changing everything in the Susquehanna

 



"Flatheads grow fast in this river system, attain large body sizes and can eat a variety of prey," said study first author Olivia Hodgson, a master's degree student in Penn State's Intercollege Graduate Degree Program in Ecology. "Because adult flatheads have few natural predators, flathead catfish can exert strong control over the ecosystem."

Hodgson is working 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. He is senior author on the study. In findings published Sept. 4 in Ecology, the researchers reported that flathead catfish are apex predators.

Flatheads had the highest trophic position -- the level an organism occupies in a food web, based on its feeding relationships -- even higher than resident top predators such as smallmouth bass and channel catfish. Channel catfish had a lower trophic position in areas with flathead catfish. This means they now eat lower on the food chain, likely because they are being outcompeted by flatheads or avoiding them, the researchers explained. In areas with flathead catfish, they found, all species showed broader and overlapping diets.

"This suggests that resident species are changing what they eat to avoid competing with or being eaten by the invader," Hodgson said. "These findings support the 'trophic disruption hypothesis,' that says when a new predator enters an ecosystem, it forces existing species to alter their behavior, diets and roles in the food web. This can destabilize ecosystems over time. Our study highlights how an invasive species can do more than just reduce native populations -- it can reshape entire foodwebs and change how energy moves through ecosystems."

Although the predatory effects of invasive catfishes on native fish communities have been documented -- such as in a recent study on the Susquehanna River led by researchers at Penn State -- the impacts of invasion on riverine food webs are poorly understood, Hodgson noted. This study quantified the effects of invasive flathead catfish on the food web in the Susquehanna by comparing uninvaded river sections to invaded sections, focusing on several key species: flathead catfish -- invader, channel catfish and smallmouth bass -- resident predators, and crayfish and minnows -- prey.

In addition to evaluating trophic position, the researchers analyzed the isotopic niche occupied by the fish species -- the range of carbon and nitrogen markers found within the tissues of an organism, reflecting its diet and habitat, providing insights into its ecological role.

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