Large-scale serial replacement of invasive tench (Tinca tinca) by invasive carp (Cyprinus carpio) in the Murray-Darling River system, Australia

Abstract Ecosystems worldwide are increasingly being invaded by multiple species, and the rate of biological invasion is accelerating, leading to more interactions among invasive species. One such interaction that has received little attention is the phenomenon of ‘serial replacement’ or ‘over-invasion’, where an established invasive species is supplanted by a second invasive species. Understanding this interaction is important as controlling the second species could inadvertently lead to an increase in the abundance of the first species. We used a hierarchical state-space model to analyse changes in annual abundances (commercial catch-per-unit-effort) of two invasive fish species, the tench (Tinca tinca) and the common carp (Cyprinus carpio), in the Murray-Darling River system (MDRS), Australia between 1954–2002. Tench were present at low abundances until the mid-1970s, after which they declined to the point of no commercial catch post-1989. The rapid decline in tench coincided with a significant increase in carp abundance, suggesting that carp may have driven the tench decline, consistent with findings from European studies that show substantial dietary overlap between these species and the disappearance of tench from ponds with intensive carp farming. While carp were present in the MDRS for the duration of the study, the rapid increase in carp abundance in the early 1970s coincided with the introduction of a specific genetic lineage – the “Boolarra strain” of carp. Our analysis of long-term changes in invasive fish species abundances provides compelling evidence of serial replacement of long-established tench by invading common carp triggered by the introduction of a novel carp strain.