This study investigated the development of a quantitative method for distinguishing stock components of Icelandic cod Gadus morhua based on visual examination of morphology. The stock is known to be structured into genetically distinct geographic components (north and south of
Iceland) and behavioural types that spawn sympatrically. Differences in morphology were tested between locations, genotypes (a proxy for behaviour) and sexes. Results show morphological markers on the head, fins and body of G. morhua that are correlated with the sex, genotype of the
fish at the pantophysin (pan‐I) locus and the location at which the fish were caught. Females were found to have relatively deep bodies, and the pan‐IBB genotype (associated with deep‐water feeding behaviour) have greater gaps between their fins.
Overall, morphology is more useful for distinguishing sympatric genotypes but less powerful at identifying genetically distinct geographic sub‐populations, perhaps because counter‐gradient evolution reduces phenotypic differences even with an underlying genetic cause.