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Identifying the mechanisms maintaining population structure in marine fish species with more than a single dispersing life stage is challenging because of the difficulty in tracking all life stages. Here, a two‐stage otolith microchemistry approach to examining life‐stage movement was adopted, tracking a year‐class from the juvenile to adult stage and inferring larval sources from clustering, in order to consider the mechanisms maintaining population structuring in North Sea cod. Clustering of near‐core chemistry identified four clusters, two of which had either a southern or northern affinity and were similar to juvenile edge chemistry. The other two clusters, common to the central North Sea, had intermediate chemical composition and may have reflected either larval mixing in this region or a lack of geographic heterogeneity in the elemental signature. From the comparison of whole juvenile and the corresponding component of adult otoliths, adults from the southern North Sea mostly recruited from adjacent nursery grounds. In contrast, many adults in the northern North Sea had a juvenile chemistry consistent with the Skagerrak and juveniles from the northern Skagerrak site had a near‐core chemistry consistent with the northern North Sea. Similarities in otolith chemistry were consistent with retention of early life stages at a regional level and also juvenile and adult fidelity. The links between the northern North Sea and Skagerrak indicate natal homing, which when considered in the context of genetic evidence is suggestive of philopatry. The approach used here should be useful in exploring the mechanisms underlying population structuring in other species with multiple dispersive life stages and calcified hard parts.

The ratios of stable isotopes 18 O: 16 O and 13 C: 12 C were measured in otolith carbon taken from nine species of fishes caught within mangroves and on the reef at Gazi Bay, Kenya. Before analysis, otoliths were divided into 'larval' 'post-larval' and 'adult' sections using a drill. Fishes were putatively classified as 'mangrove residents' 'offshore residents' or 'migrants' on the basis of information from the literature, and depending on where they were caught (mangroves only, offshore only or both mangroves and offshore) in the present study. Eight of the species exhibited an increase in otolith 13 C: 12 C with age, but this was significant only in the two migrant species Lethrinus harak and Lutjanus fulviflammus. There were no consistent patterns in 18 O: 16 O with age, or between migrants and non-migrants. These results suggest that comparing absolute values of otolith oxygen and carbon isotope signatures between fish species is not a useful way of determining migration patterns at this site, because of species-specific differences in carbon metabolism and insufficiently steep gradients in temperature and salinity. Changes in carbon isotope signatures between life stages within a species, however, do hold promise as migration tracers.

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