Otolith morphology differences between ecotypes of the Icelandic cod stock
The use of DST tags has increased our knowledge of the stock structure of the Icelandic cod (Gadus morhua). The depth profiles of tagged cod reveal different strategies during the feeding season. This has led to the definition of two ecotypes within the cod stock, one which migrates to deeper waters during feeding, and another which stays in shallow waters year round. These ecotypes are termed Frontal and Coastal, respectively. In an ongoing project (CodMorph) the morphological differences between these two types are being studied, both in otoliths and body morphology. The aim is to develop a morphological key to distinguish the ecotypes. Here we report on the first results from this project with respect to the otolith morphology. The definition of the ecotypes has been such that cod spending most of their time at depths less than 200m and having a temperature profile with a normal annual cycle with maxima in the summer are characterized as Coastal. Frontal cod show deep water migrations below 200m and have a variable temperature profile. Here the DST profiles were analyzed by Cluster Analysis, which indicates that there exists an intermediate group that has a variable depth profile and a feeding migration which is both shorter in time and not as deep as the Frontal cod. The morphology of the otoliths was analyzed with shape measurements and Fast Fourier analysis. The results indicate a difference in the morphology between the Coastal and Frontal cod if the intermediate group is filtered out.