Age and Growth Dynamics Suggest Biological Plasticity in Invasive Channel Catfish Ictalurus Punctatus Across Italian Rivers Systems

The North American channel catfish Ictalurus punctatus has successfully established populations across Europe; nevertheless, empirical data on its life‐history traits in this introduced area remain scarce. This study analyses age and growth of I. punctatus in the Po and Arno, two of Italy's principal river systems. Specimens of I. punctatus were sampled from populations of the River Po (Northern Italy) and River Arno (Central Italy). Age estimates were derived from lapilli otoliths for the River Po ( n = 104) and pectoral spines for the River Arno ( n = 769). Initial efforts to apply the standard non‐linear von Bertalanffy growth function (VBGF) produced biologically implausible asymptotic lengths, primarily due to the absence of older individuals (> 6 years) within the sampled populations. Consequently, growth trajectories were evaluated using linear regression and hierarchical linear mixed‐effects models (LMM). Of these, the growth models used 94 Po fish and 768 Arno fish. Growth is reported separately for each river because the rivers were aged with different structures. In the Arno, length‐at‐age slopes were lower than the Po estimates and varied by site; that between‐river difference is descriptive only and is confounded with method. In the River Po, females exhibited steady linear length increases (91.52 ± 5.09 mm year −1 ), whereas males followed a quadratic trajectory (linear coefficient 159.15 ± 14.09 mm year −1 ; quadratic coefficient −11.71 mm year −2 ), with declining increments over the sampled ages. Po male curvature and Arno near‐linear growth are consistent with a short age range and, in the Arno, with spine underestimation in larger fish. These results highlight the invasive potential of this species and growth parameters can provide baseline for predicting population trends and informing management strategies in Europe's freshwater ecosystems.