Vol.:(0123456789) 1 3 Aquat Sci DOI 10.1007/s00027-017-0525-4 RESEARCH ARTICLE Are long-term fsh assemblage changes in a large US river related to the Asian Carp invasion? Test of the hostile take-over and opportunistic dispersal hypotheses Mark Pyron 1  · Jesse C. Becker 1  · Kyle J. Broadway 2  · Luke Etchison 1  · Mario Minder 1  · Dawn DeColibus 1  · M. Chezem 1  · Kevin H. Wyatt 1  · Brent A. Murry 3   Received: 16 May 2016 / Accepted: 27 February 2017 © Springer International Publishing 2017 on other, more deplete, carbon sources than indicated by the algae, and a slight increase between the δ 13 C time peri- ods. Although all functional feeding groups of fshes indi- cated some reduction in δ 15 N, the diferences were only signifcant for omnivores, mussels, and planktivore/detri- tivores. Although Asian Carp may have contributed to the collapse of Gizzard Shad populations, other stressors were likely more important drivers of their decline. This is the frst, albeit indirect, evidence of opportunistic “invasion” as opposed to the historically presumed hostile takeover model. Keywords Fish assemblage structure · Stable isotope ratio · Invasive species Introduction Introduction of non-indigenous invasive species to river- ine systems has altered ecosystems worldwide (Sala et al. 2000; Kolar and Lodge 2001; Olden and Pof 2005). Inva- sive species can induce cascade efects throughout riverine food webs, altering competitive interactions and trophic structure (Strayer et al. 2008; Delong 2010) and resource availability (Karatayev et al. 2014). Invasive species also have the capacity to trigger community assembly shifts (Anderson et al. 2009), alter nutrient cycling and process- ing rates (Sousa et al. 2009; Capps and Flecker 2013), and modify the assemblage composition in rivers (Olden and Pof 2005; Delong 2010). Aquatic invasive species with negative consequences in North American rivers include Zebra Mussels (Driessena polymorpha), Round Goby (Neogobius melanostomus), Rusty Crayfsh (Orconectes rusticus), Asian Carp (Silver Carp, Hypophthalmichthys nobilis, and Bighead Carp, H. molitrix; Lodge et al. 1998; Abstract Gizzard shad is a dominant planktivore/detriti- vore in the Wabash River, and populations crashed in the early 1990s. Previous work (1974–2008) identifed a sub- stantial shift in body-size structure and functional trait composition in the Wabash River fsh assemblage between 1989 and 1996. Invasive Asian Carp appeared in the Ohio River basin including the Wabash River in the 1990s. Our goal was to test for temporal changes in assemblage compo- sition and trophic structure relative to the invasion of Asian carp from the early 1990s. We hypothesized that estab- lishment of Asian Carp was a contributor to the assem- blage composition shift, and that their presence altered the trophic pathways and food sources of native fshes includ- ing Gizzard Shad (hostile takeover hypothesis). Alterna- tively, Asian Carp may have found success through capital- izing on an empty niche, likely left vacant by the decline in Gizzard Shad, or abundance changes in other trophic groups (opportunistic hypothesis). We utilized archival fsh and mussel collections to test for trophic changes in the ecosystem using δ 13 C and δ 15 N stable isotope analyses. We examined stomach contents of the dominant plankti- vore/detritivore consumer, Gizzard Shad, from archival and recent collections to test for changes in the phytoplankton community. Stable isotopes indicated a community reliance Aquatic Sciences * Mark Pyron mpyron@bsu.edu 1 Department of Biology, Aquatic Biology and Fisheries Center, Ball State University, Muncie, IN 47306, USA 2 Biology Department, Institute for Great Lakes Research, Central Michigan University, Mount Pleasant, MI 48858, USA 3 Caribbean Landscape Conservation Cooperative, US Fish and Wildlife Service, International Institute for Tropical Forestry, Rio Piedras, PR 00926, USA