Otolith shape differences between ecotypes of Icelandic cod (Gadus morhua) with known migratory behavior inferred from Data Storage Tags

Abstract

Otolith shape can be used to identify ecotypes of the Icelandic cod stock. The use of data storage tags (DSTs) has increased our knowledge of the stock structure of the Icelandic cod (Gadus morhua). The profiles of tagged cod reveal different migratory strategies. This has led to definition of two ecotypes within the cod stock. Frontal ecotypes reside in deep waters during feeding season and express a highly variable temperature profile associated with thermal fronts, while coastal ecotypes stay in shallow waters all year round. In this study the DSTs profiles were analysed with Cluster Analysis which revealed existence of an intermediate behaviour that expresses a variable depth profile and a feeding migration which is both shorter in time and not as deep. The main objective was to develop a morphological key based on otoliths to distinguish the ecotypes. The shape of the otoliths was extracted with shape measurements and Fast Fourier Transforms. The results of a discriminant function analysis indicated a difference in morphology between the ecotypes resulting in a successful classification.