Historical cisco Coregonus artedi population collapses in Green Bay, Lake Michigan, and Saginaw Bay, Lake Huron, during the 1950s

Coregonus populations across their Holarctic range have often undergone unexplained collapses. Here, we document causes of collapse for two of the largest Coregonus populations in the world, cisco (C. artedi) in Green Bay, Lake Michigan, and Saginaw Bay, Lake Huron, during the 1950s. We used simulation models, rank‐order correlations, and historical literature to evaluate effects of overfishing, interactions with introduced rainbow smelt (Osmerus mordax), and overwinter bottom hypoxia on both cisco populations. Fully selected exploitation rates for cisco were alarmingly high (0.82–0.84), correlations and anecdotal evidence suggested negative interactions with rainbow smelt, and extensive areas of hypoxia coincident with cisco declines suggested incubation of cisco embryos was impaired regionally. Our findings suggested that negative effects of rainbow smelt were mainly responsible for the cisco population collapse in Green Bay and that overfishing and hypoxia had suppressed the Saginaw Bay cisco population before rainbow smelt caused its final collapse.