Validation of otolith d18O values as effective natural tags for shelf-scale geolocation of migrating fish

Abstract

The oxygen isotopic ratio of fish otoliths is increasingly used as a ‘natural tag’ to assess provenance in migratory species, with the assumption that variations in d18O values closely reflect individual ambient experience of temperature and/or salinity. We employed archival tag data and otoliths collected from a shelf-scale study of the spatial dynamics of North Sea plaice Pleuronectes platessa L., to examine the limits of otolith d18O-based geolocation of fish during their annual migrations. Detailed intra-annual otolith d18O measurements for 1997-1999 from individuals of 3 distinct sub-stocks with different spawning locations were compared with d18O values predicted at the monthly, seasonal and annual scales, using predicted sub-stock specific temperatures and salinities over the same years. Spatio-temporal variation in expected d18O values (-0.23 to 2.94) mainly reflected variation in temperature, and among-zone discrimination potential using otolith d18O varied greatly by temporal scale and by time of year. Measured otolith d18O values (-0.71 to 3.09) largely mirrored seasonally predicted values, but occasionally fell outside expected d18O ranges. Where mismatches were observed, differences among sub-stocks were consistently greater than predicted, suggesting that in plaice, differential sub-stock growth rates and physiological effects during oxygen fractionation enhance geolocation potential using otolith d18O. Comparing intra-annual d18O values over several consecutive years for individuals with contrasted migratory patterns corroborated a high degree of feeding and spawning site fidelity irrespective of the sub-stock. Informed interpretation of otolith d18O values can therefore provide relatively detailed fisheries-relevant data not readily obtained by conventional means.