Assessing the role of ontogenetic movement in maintaining population structure in fish using otolith microchemistry

Abstract

Identifying the mechanisms maintaining population structure in marine fish specieswith more than a single dispersing life stage is challenging because of the difficulty intracking 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 adultstage and inferring larval sources from clustering, in order to consider the mechanisms maintaining population structuring in North Sea cod. Clustering of near-corechemistry identified four clusters, two of which had either a southern or northernaffinity and were similar to juvenile edge chemistry. The other two clusters, commonto 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 theelemental signature. From the comparison of whole juvenile and the correspondingcomponent of adult otoliths, adults from the southern North Sea mostly recruitedfrom adjacent nursery grounds. In contrast, many adults in the northern North Seahad 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 ata regional level and also juvenile and adult fidelity. The links between the northernNorth Sea and Skagerrak indicate natal homing, which when considered in the context of genetic evidence is suggestive of philopatry. The approach used here shouldbe useful in exploring the mechanisms underlying population structuring in otherspecies with multiple dispersive life stages and calcified hard parts.