Persistent high hatchery recruitment despite advanced reoligotrophication and significant natural spawning in a whitefish

Many lakes of the pre-Alpine region suffered from severe eutrophication that affected natural reproduction whitefish (Coregonus spp.) and necessitated large-scale supportive breeding programs. With advanced reoligotrophication, it is now important to evaluate relevance continued artificial for population dynamics. We focused on a lake has reached phase III i.e., biomass production declining since 2012 in response low phosphorus concentrations. show most eggs are naturally spawned, observed oxygen concentrations would again support embryo development at all depths, ready-to-hatch embryos can indeed be found spawning grounds. marked hatchery-produced 2014 cohort with Alizarin red, stocked them as usual (at larval or early juvenile stages), recaptured over period five years. Fish were aged yearly growth rings scales, otoliths checked marks. 90.3 % hatchery-born. This ratio did not decline fish age. also determined cohorts 2012–2020 (based first annual ring scales) stocking intensity predicted (r2 = 0.67). strong density dependence individual suggests largely size nine years reoligotrophication III. conclude large areas grounds allow successful embryogenesis, quantities but recruitment significantly reduced by ecological evolutionary factors, e.g., competition hatchery-born fish, desynchronization trophic interactions, long-term effects fishing- hatchery-induced evolution.