Does aquatic invasive species removal benefit native fish? The response of gizzard shad (Dorosoma cepedianum) to commercial harvest of bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix)

Bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix), collectively known as Asian carp, are planktivorous fishes that have invaded aquatic ecosystems throughout the Mississippi River Basin of North America. Consequently, Asian carp management plans (including contractual harvest) have been implemented to prevent range expansion into vulnerable systems such as the Great Lakes. Asian carp harvest also provides an opportunity to assess if control efforts can benefit native fishes. To answer this question, 26 years of standardized electrofishing data were analyzed, focusing on gizzard shad (Dorosoma cepedianum) body condition (W_r) and abundance (catch-per-unit-effort, CPUE) from the Illinois River (Illinois, USA). Gizzard shad data were categorized into pre-Asian carp establishment (1990–1999), after Asian carp establishment (2000–2009), and harvest (2010–2014) periods. Gizzard shad W_r and small gizzard shad CPUE decreased in the entire river after Asian carp invasion, and W_r and large gizzard shad CPUE rebounded where harvest occurred. Contrastingly, small gizzard CPUE has not yet exhibited a positive response to harvest. These mixed results indicate a time-lag may exist between management action and ecosystem response. Ultimately, Asian carp removal may be facilitating improvements in gizzard shad body condition (by increasing forage availability), which may portend a future gizzard shad population rebound.