Influence of Invasive Bigheaded Carps on Abundance of Gizzard Shad in the Tennessee River

The Tennessee River basin and its cascade of reservoirs are home to some of the most diverse freshwater fish assemblages in the world. This unique system is threatened by the ongoing invasion of Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis, hereafter bigheaded carps. Bigheaded carps may directly compete for food resources with native clupeid species such as Gizzard Shad Dorosoma cepedianum, and this potential interaction could have damaging ecological and economic consequences. High relative abundances of Gizzard Shad are crucial to the Tennessee River food web and associated fisheries because of their role as a forage base for piscivorous species. We analyzed a collection of annual gillnetting and electrofishing data spanning from 1990 to 2017, to test whether Gizzard Shad relative abundances have changed in Tennessee River reservoirs since the arrival of bigheaded carps. Our analyses indicated that Gizzard Shad abundances have been declining but were already declining prior to the arrival of bigheaded carps in the Tennessee River. At this stage in the invasion, we could not attribute a cause‐and‐effect to the inverse relationship between Gizzard Shad and bigheaded carps, but we advise continued monitoring of indicators of harmful interactions.