Trophic Niches of Native Fishes in Large‐River Tributaries: Associations Among Invasive Silver Carp, Mainstem Gradient and Catchment

The Ohio River of North America drains from the forested catchments of the western Appalachian foothills to the floodplains of the Mississippi River Basin. Invasive planktivorous silver carp ( Hypophthalmichthys molitrix ) are absent upstream but are established and increasingly more abundant downstream. The invasion gradient of silver carp within tributaries along the Ohio River may modify food webs in different ways from low productivity upstream to high productivity downstream.

Based on patterns in other productive, invaded Great Rivers of North America, silver carp, which feed on phytoplankton and zooplankton, were predicted to reduce the trophic position of top predators and alter the trophic niches of native planktivores in established downstream tributaries even though these systems have high, agriculturally driven productivity.

Body condition and stable isotopes (δ ¹⁵ N and δ ¹³ C) of native fish species and zebra mussels ( Dreissena polymorpha ) were quantified in tributaries along the entire Ohio River in 2020 to compare trophic niche and fish condition as a function of the presence and absence of invasive silver carp and river productivity.

The isotopic niches of native planktivorous gizzard shad ( Dorosoma cepedianum ) and piscivorous largemouth bass ( Micropterus nigricans ) were broader and more depleted in the lower Ohio River where tributaries were agriculturally dominated, highly productive and silver carp were abundant and reproducing. Additionally, gizzard shad and largemouth bass were in poorer condition in the lower reaches of the Ohio River despite the high productivity there, suggesting that silver carp disrupt food webs from their middle‐out trophic position.

Silver carp populations are likely subsidized by high sources of production in the lower Ohio River, allowing them to become densely populated and dominate food webs, perhaps explaining why the species has not rapidly invaded and established in the upper Ohio River where productivity is lower. Invasive species in rivers worldwide may be facilitated by catchment‐level processes such as farming, urban use and forestation, allowing them to rapidly establish, persist, reach high densities and ultimately drive native food web interactions. Because silver carp are among the most farmed fish in the world, the risk to rivers worldwide is going to increase as their use as an economical protein source rises to meet food security needs, especially in highly productive river tributaries as this study suggests.