Walsh, R.Dornan, T.N.Upadhyay Stæhr, S.Brookes, H.J.Hipsey, M.R.Laws, M.Cassey, P.Brookes, J.D.Closs, G.2025-01-222025-01-222025Marine and Freshwater Research, 2025; 76(2):MF24183-1-MF24183-130067-19400067-1940https://hdl.handle.net/2440/143603Context. In Australia’s Murray–Darling Basin, the invasive common carp (Cyprinus carpio) poses a significant ecological threat. Efforts to manage their population include the potential use of cyprinid herpesvirus (CyHV-3) as a biological control agent. The environmental benefits of carp elimination could be substantial in the long term; our knowledge of the immediate ecological consequences resulting from large-scale fish die-offs remains limited. Aim. This study sought to investigate the effect of a large-scale fish-mortality event on water quality. Methods. Experiments of an increasing scale (bucket, mesocosm and whole wetland) were conducted to assess how decomposing carp carcasses alter dissolved oxygen and nutrient concentrations in water. Key results. In mesocosms, dead carp decayed more rapidly at 18 than 12°C, yielding oxygen demands of 1.022 ± 0.029 and 0.496 ± 0.239 mg kg–1 min–1 respectively. This carp decay released phosphorus, yielding 2121.1 ± 140.4 mg kg–1 . In the wetland experiment, carp addition of 2400 kg ha–1 resulted in anoxic conditions over the following 2 weeks. The release of dissolved organic carbon and lipids led to a peak biological oxygen demand of 95.3 mg L–1 . Conclusions. Carp decomposition considerably contributes to biological oxygen demand and algal growth through nutrient enrichment and is strongly influenced by carp density. Implications. The finding highlights key factorsto consider before using biocontrol agents causing mass carp mortality, including hypoxia, anoxia and increased risk of harmful algal blooms.en© 2025 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons AttributionNonCommercial 4.0 International License (CC BY-NC).biogeochemistry; CyHV-3; Cyprinus carpio; ecology; eutrophication; fish; freshwater; introduced species; limnology; Murray–Darling BasinJournal article10.1071/mf24183726295Walsh, R. [0009-0000-9215-4848]Dornan, T.N. [0000-0002-4998-7577]Cassey, P. [0000-0002-2626-0172]Brookes, J.D. [0000-0001-8408-9142]