A novel length back-calculation approach accounting for ontogenetic changes in the fish length--otolith size relationship during the early life of sprat (Sprattus sprattus)

An individual-based length back-calculation method was developed for juvenile Baltic sprat (Sprattus sprattus), accounting for ontogenetic changes in the relationship between fish length and otolith length. In sprat, metamorphosis from larvae to juveniles is characterized by the coincidence of low length growth, strong growth in body height, and maximal otolith growth. Consequently, the method identifies a point of metamorphosis for an individual as the otolith radius at maximum increment widths. By incorporating this information in our back-calculation method, estimated length growth for the early larval stage was more than 60% higher compared with the result of the biological intercept model. After minimal length growth during metamorphosis, we found the highest increase in length during the early juvenile stage. We thus located the strongest growth potential in the early juvenile stage, which is supposed to be critical in determining recruitment strength in Baltic sprat. Une methode de retrocalcul de la longueur basee sur les individus a ete mise au point pour le sprat (Sprattus sprattus) juvenile, qui explique les modifications ontogeniques de la relation entre la longueur du poisson et la longueur des otolithes. Chez les sprats, la metamorphose des larves en juveniles est caracterisee par la coincidence d'un faible taux d'augmentation de la longueur, d'un taux eleve d'augmentation de la hauteur du corps et du taux de croissance maximum des otolithes. Par consequent, la methode definit le point de metamorphose pour un individu comme etant le rayon de l'otolithe correspondant a la largeur maximum des cernes de croissance. L'integration de cette information a notre methode de retrocalcul a donne un taux d' augmentation de la longueur estime pour le debut du stade larvaire de plus de 60 % superieur au taux predit par le modele d'interception biologique. Apres un taux minimum d'augmentation de la longueur durant la metamorphose, la plus forte augmentation de la longueur est notee au debut du stade juvenile. Ainsi, nous situons le potentiel de croissance le plus eleve au debut du stade juvenile, qui est cense etre un moment determinant de l'intensite du recrutement des sprats. [Traduit par la Redaction]
Introduction An important research focus in fisheries ecology is the relationship between year class strength and variable mortality rates of early life stages. Mortality during early life stages is often [...]