Abstract
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.
Highlights
The way in which animal populations are structured in space and time affects how they respond to environmental and humanThis article is published with the permission of the Controller of HMSO and the Queen’s Printer for Scotland.pressures (Schindler et al, 2010)
Through a combination of clustering of near-core ablation chemistry to consider natal sources and assignment of the juvenile component of adult otoliths to baseline nursery sites, the present study provides a framework for examining how population structure is maintained in species with more than a single dispersive life stage
The approach is dependent on spatial variation in otolith elemental composition (Elsdon et al, 2008) and the assumption that differences in elemental values near the core reflect multiple larval origins (Calò et al, 2016)
Summary
This article is published with the permission of the Controller of HMSO and the Queen’s Printer for Scotland. Adult fish of the same year-class sampled on known spawning grounds during the spawning season were assigned to these nursery grounds based on the chemical signature of the equivalent juvenile region of their otolith, determined by WS-ICP-MS The combination of these two approaches carried out on the same individuals gave insights on the roles played by the adopted migrants’ hypothesis or natal homing in shaping cod population structure in the North Sea. otolith evidence was reviewed in the context of reported genetic structuring to consider philopatry
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