Corroborating age with oxygen isotope profiles in otoliths: consequences for estimation of growth, productivity and management reference points in northern pike (Esox lucius) in the southern Baltic Sea

Abstract

Accurate and precise age estimates are crucial for assessing the life-history of fish and providing management advice for fisheries, but age validation studies remain rare or absent in many species. Aging from scales is common, as it is non-lethal, but potential for underaging old fish exists. Using 85 northern pike (Esox lucius) collected from the southern Baltic Sea in Germany as a model, we corroborated age readings based on annual cycles of oxygen isotopes ({delta}18O) in otoliths to infer the timing and validity of growth, so as to compare results with visual age estimations from scales and otoliths. Otoliths were accurate and precise, while age readings from scales systematically underestimated the age of old pike. Fitting population-level von Bertalanffy growth models to the size-at-age data estimated via{delta} 18O-profiling, otoliths or scales revealed a larger terminal length (L{infty}) and a lower body growth coefficient k in scale-aged fish compared to otolith and corroborated age data. Populating an age-structured model with structure-specific growth model parameters demonstrated that the maximum sustainable yield (MSY) was estimated to be about 37% lower using scale-informed growth models relative to growth models fitted to corroborated and otolith-based size-at-age data. Thus, pike populations assessed and modeled based on scale age readings might appear less productive than they really are. Using scale-based ages to inform management regulations may therefore result in too conservative management and lost biomass yield, while instilling unrealistic angler expectations as to the trophy potential of the fishery.