A multi-scale approach for understanding factors limiting invasive carp distribution in the Ohio River Basin

The spread of aquatic invasive species (AIS) poses a major threat to freshwater biodiversity. Understanding the mechanisms regulating occurrence of invasive species in invaded systems requires a multi-scale approach, as regional dispersal can facilitate arrival while local processes ultimately determine their persistence across a landscape. We used a dynamic multi-scale occupancy model to test hypothesized factors limiting the occurrence of invasive bigheaded carps (bighead carp Hypophthalmichthys nobilis and silver carp H. molitrix) in the Ohio River, USA. Across the basin, invasion has slowed, with limited evidence of upstream advancement of their invaded range in this system. However, once present, silver carp and bighead carp populations tended to persist even in apparently low numbers. Bighead carp occurrence was not correlated with any environmental variables. On the local scale, silver carp exhibit clear habitat and flow preferences, where populations persisted in low velocity off-channel locations especially when flows are too high in the main channel. We found evidence for decreasing detection of both species in mid-reach and upstream pools, potentially because of lower abundance in these reaches. Our results indicated that while silver carp occurred throughout a large range of the Ohio River, fewer sites were occupied in upstream reaches. However, there was indirect evidence that site-level occurrence was increasing in the invasion front. Understanding these multi-scale dynamics can be helpful for developing effective strategies to contain invasive silver carp and bighead carp in the Ohio River and other systems.