We studied life-history traits focusing on growth and condition of the pikeperch Sander lucioperca to evaluate its phenotypic plasticity when introduced to new environments. Pikeperch is a non-native fish introduced to Iberian freshwater fauna in 1998 that quickly spread to other river basins through human-mediated activities, occupying now a wide variety of habitats along mainland Portugal.  Condition (K and SMI), fork length at age, and length-weight relationships were studied for Portuguese populations. Pikeperch fork length for ages 1, 2, 3 and 4 was different between several populations. We applied generalised linear models (GLM) to study the influence of habitat type, latitude, altitude, time after first detection and fish prey richness on pikeperch populations size at age 4 and condition. We observed higher condition values on populations from lower altitudes at lentic systems more recently introduced. But higher fork length at age 4 was found in populations from higher altitudes, on older populations with higher prey richness. Habitat type, time since first detection and fish fauna composition are discussed as the main environmental factors explaining the observed phenotypic plasticity with concerns on predatory impact on native fauna.

In the laboratory, specimens were measured (Fork Length – FL, to nearest 1 mm) and weighed (Eviscerated Weight – EW, to the nearest 0.01 g). Since not all populations were sampled at the same time, we used the eviscerate weight to avoid the influence of the gonad size and stomach fullness, giving more reliable values for the condition of the fish. When possible, sex was determined by gonad macroscopic examination.

As a simple, expedite and common methodology for pikeperch age estimation (e.g. Argillier et al. 2021; Pérez-Bote & Roso 2012, Nolan & Britton 2018) we removed around10 scales above the lateral line and below the anterior part of the dorsal fin that were posteriorly cleaned and mounted on microscopic slides. Selected scales were photographed under a binocular lens and examined using freeware Fiji image analysis program