LAUSR

Abstract Growth of sea trout (Salmo trutta L.) is traditionally estimated by ‘reading’ visible increments on sampled scales. This subjective technique relies strongly on expert judgement, and estimates for a given fish can vary widely among readers. We present a novel and objective technique that estimates growth from scale chemistry profiles. The new method is described using a sample of PIT tagged 0 sea-winter sea trout (N = 10), for which actual marine growth was known, because individual fish length was measured before and after marine migration. Scale chemistry elemental profiles for Ba:Ca, Mn:Ca, Zn:Ca, Mg:Ca, Na:Ca and Sr:Ca were collected using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Scale Ba:Ca profiles accurately delineated the freshwater-marine transition of migrating trout, and the marine growth phase. Profiles of other tested elements were unreliable due to post-depositional change following marine migration. A simple equation using scale Ba:Ca profiles and measured lengths at capture of returning tagged fish was developed to estimate marine growth (mm) without recourse to subjective scale readings. The chemistry-based estimates were highly correlated (r2 = 0.97) with measured marine growth. The new method is more accurate and reproducible than visual scale readings, and could potentially support processing of large samples.