Separation of Norwegian coastal cod and Northeast Arctic cod by otolith morphometry

Abstract

For stock assessment purposes, Atlantic cod (Gadus morhua) from the coastal and offshore regions off northern Norway is usually allocated to Norwegian coastal cod (NCC) and Northeast Arctic cod (NEAC) by internal morphological features of their otoliths. As this classification is subject to individual interpretation, this study investigated an alternative objective approach for the separation of the two cod groups, using otolith shape analysis. Otolith samples from coastal areas along northern Norway and from the Barents Sea were analysed by univariate shape descriptors and Elliptical Fourier Analysis (EFA). When combining those methods, the classification score was 89% for NCC and 90% for NEAC. When genetic typing data (Pan I marker) were used as reference, the classification scores were reduced to 83% for NCC and 76% for NEAC. These results imply that differences in internal otolith morphology are translated to a large extent into the outer shape, but that those cannot directly be linked to genetic structure. Environmental conditions, however, seem to have a considerable influence on how otolith growth increments and consequently otolith shapes are formed. As the various fjord systems in Norway provide local habitats and as differences within the NCC with regard to genetic structure and life-history parameters had been found in earlier studies, variation of NCC otolith shapes between three coastal regions was also examined. The region classification scores for reader-typed NCC varied between 60% and 81%. Apart from the outer shape analyses, experimental work on the detection of internal (annuli) shapes was carried out and will be presented as work in progress. Keywords: Stock identification; Otolith shape analysis; Northeast Atlantic; Norway; Atlantic cod; Gadus morhua