Within‐Ecosystem Comparison of Bigmouth Buffalo <i>Ictiobus cyprinellus</i> and Common Carp <i>Cyprinus carpio</i> Reveals Diverging Population Trajectories, Declining Recruitment, and a Lifespan of 148 Years

The bigmouth buffalo <i>Ictiobus cyprinellus</i> is a long-lived, migratory freshwater fish native to North America whose numbers are declining amidst increasing conservation concerns. Recent studies have uncovered long lifespans, delayed maturation, and episodic recruitment of bigmouth buffalo. Building from previous work in the Qu'Appelle watershed of Saskatchewan, here we quantify otolith-derived population demographics of bigmouth buffalo and invasive common carp <i>Cyprinus carpio</i> across multiple sites in the drainage. The common carp (<i>n</i> = 125) and bigmouth buffalo (<i>n</i> = 173) collected from 2018 to 2024 reveal that common carp reach asymptotic size two times faster, live three times shorter lives, and invest significantly more into reproduction while also exhibiting recruitment stability during the water control era (post-1958). Indeed, invasive common carp now outnumber native bigmouth buffalo in this watershed by at least an order of magnitude. In contrast, only a single year class (1997) was evident for bigmouth buffalo after 1949. Therefore, only one recruitment year was evident for this species since common carp were first detected in the system in 1955. Remarkably, we find that as of 2024 more than 90% of bigmouth buffalo in this system are greater than 75 years old with a known maximum age of 148 years. We now know that the bigmouth buffalo is the 11th longest-lived vertebrate out of more than 66,000 species, and across diverse systems can have recruitment gaps longer than any other animal. Bigmouth buffalo require immediate conservation reassessment amidst ongoing population declines.