Validation of otolith δ18O values as effective natural tags for shelf-scale geolocation of migrating fish

Am Darnaude,E Hunter

doi:10.3354/meps12302

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 delta O-18 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 delta O-18-based geolocation of fish during their annual migrations. Detailed intra-annual otolith delta O-18 measurements for 1997-1999 from individuals of 3 distinct sub-stocks with different spawning locations were compared with delta O-18 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 delta O-18 values (-0.23 to 2.94%) mainly reflected variation in temperature, and among-zone discrimination potential using otolith delta O-18 varied greatly by temporal scale and by time of year. Measured otolith delta O-18 values (-0.71 to 3.09%) largely mirrored seasonally predicted values, but occasionally fell outside expected delta O-18 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 delta O-18. Comparing intra-annual delta O-18 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 delta O-18 values can therefore provide relatively detailed fisheries-relevant data not readily obtained by conventional means.

Highlights

The distributions of marine species are often discontinuous in time and space, usually as a consequence of spatial and temporal shifts in the physical and biological characteristics of oceanic habitats (Hixon et al 2002)
Fine-scale analysis of the right-hand sagittae of 24 of the tagged fish returned with intact otoliths generated 117 measures of intra-annual δ18O to be compared with the δ18O values calculated from expected temperatures and salinities
The comparison was performed at various temporal scales, for each of the 3 sub-stocks, and for the areas seasonally occupied by the fish within the North Sea and the English Channel

Summary

Introduction

The distributions of marine species are often discontinuous in time and space, usually as a consequence of spatial and temporal shifts in the physical and biological characteristics of oceanic habitats (Hixon et al 2002). Distribution shifts further reflect ontogenetic and/or seasonal migrations driven primarily by the physiological requirements of maturation and subsequent annual reproductive cycles (Kimirei et al 2013). Because otoliths continuously log environmental data throughout life, and because otolith material is rarely resorbed or physiologically altered after deposition (Mugiya & Uchimura 1989, Campana 1999), otoliths provide a precisely dated and seasonally resolved record of fish lifetime environmental history (Thorrold et al 1997). The interrogation of otolith structure and chemistry can provide coarse estimates of past geographical location (‘geolocation’), at least in those species that migrate between water masses with sufficiently different characteristics (Campana 1999)

Methods

Results

Discussion

Conclusion