Abstract Introduced Suckermouth Armored Catfish (SAC; family Loricariidae) have invaded freshwater ecosystems globally. In the San Marcos River, Texas, control of invasive SAC includes spearfishing through public tournaments and contracted spearfishing, yet the effectiveness of these control efforts is unquantified. We used a recently developed length-based Bayesian biomass estimation method to assess spearfishing mortality relative to natural mortality and existing biomass relative to an unexploited population. During 2014–2018, 6,046 SAC were removed and measured (total length, cm) from the San Marcos River through spearfishing. Using the length-based Bayesian biomass, we found fishing pressure increased mortality 1.50- to 1.75-fold relative to natural mortality, and that relative biomass during 2016–2018 was significantly below the threshold at which stock depletion occurs. Our application of fishery stock assessment provides quantitative benchmarks for invasive species control and can be applied to other invaded systems where control methods are unassessed but length data from removed individuals are available.