Combined methods of otolith shape analysis improve identification of spawning areas of Atlantic cod

The effectiveness of otolith shape for identifying Atlantic cod (Gadus morhua) spawning populations in the northern North Sea and west coast of Scotland was investigated. Otolith shape was described using Cartesian Fourier analysis of the otolith outline and morphometric variable rectangularity. Pairwise randomization analysis of Fourier descriptors, and discriminant analysis of rectangularity and Fourier descriptors were used to identify differences between spawning stocks. Significant differences and a high classification success (>70%) in otolith shape indicated that spawning groups from Viking Bank and Moray Firth had different shapes from those of cod in other spawning areas. Significant differences in otolith shape of age 2 cod suggested that otoliths from the South Minch and Irish Sea were different from those from the Moray Firth and the Clyde. These results appear consistent with genetic evidence for a low gene flow, and tag-recapture evidence of resident populations. The results of pairwise randomization analysis and discriminant analysis differed slightly, indicating that use of a combination of methods increases the potential to identify spawning groups on the basis of otolith shape.