Otolith shape and temporal stability of spawning groups of Icelandic cod (Gadus morhua L.)

During the past 5e10 years, studies exploring small-scale structure of cod populations have provided accumulating evidence for the existence of local populations. In Iceland, all cod have been thought to belong to a single management unit. Recent studies on genetic and life history variation have, however, indicated the existence of local populations. The main objective of this study is to explore the potential existence of local populations by use of otolith shape to discriminate between spawning groups of Icelandic cod. Otoliths were sampled from mature and spawning cod at spawning locations around Iceland in 2002 and 2003, and otolith shape was described using variables correlated with size (otolith area, length, width, perimeter, and weight) as well as shape (rectangularity, circularity, and 10 Fourier coefficients). Only standardized otolith variables were used so as to remove the effect of otolith size on the shape variables. Cod were on average larger and older south of Iceland, where ambient temperature was higher than northwest, north, and east of Iceland. Otolith shape effectively discriminated between cod north and south of Iceland, and it was also possible to discriminate among cod spawning below and above 125 m at spawning locations south of Iceland. Recent genetic and tagging studies have indicated differences between cod at these two depths. Correct classification of groups of cod at the different spawning locations ranged between 0% and 44%. Incorrectly classified cod were in most cases classified to adjacent spawning locations, and a high percentage of cod south of Iceland was classified to other southern locations and cod north of Iceland to other northern locations. The temporal stability of otolith shape was studied at seven spawning locations in two consecutive years. Otolith shape differences were greater between locations than among years within a location. The spawning groups are therefore likely to have remained separate during much of their lifetime.

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