Is analysis of otolith microstructure a valid method for investigating early life history of Western Baltic cod?

Rene Plonus,Uwe Krumme,Steffen Funk,Axel Temming,Kate Mcqueen,Claudia Günther

doi:10.1007/s00227-021-03834-x

Abstract

Daily formation of fish otolith micro-increments is frequently assumed, however applying inferences about timing of life history events and formation of otolith macro-structures requires further validation of the periodicity of micro-increment formation. We analysed micro-increments from Western Baltic cod (WBC, Gadus morhua) otoliths marked with tetracycline-hydrochloride as part of an age validation study to test the assumption of daily formation of micro-increments. We found that the number of counted micro-increments consistently underestimated the age of cod aged 1 and older. Time at liberty was also underestimated, especially for fish at liberty during winter. In contrast, micro-increment counts of otoliths from wild-caught young-of-the-year (YOY) cod could be used to realistically estimate timing of hatch and translucent zone formation. Under ambient conditions, settlement did not correspond to any visible pattern within the otoliths, but could be inferred from the prey switch observed from stomach content analyses. We therefore conclude that micro-increments can be assumed to form on a daily basis until the first winter, and can therefore be used to investigate early life history of YOY WBC. However, the periodicity of micro-increment formation appears to vary seasonally in older individuals, with the number of micro-increments formed during the winter period being particularly low.

Highlights

Micro-increments are visible structures within fish otoliths that often form on a daily basis (Panella 1971), and have been widely used to study the early life history of juvenile fish
Water temperatures measured in the Fehmarn Belt and at the pound nets in 2015 were slightly lower compared to water temperatures in the same region in 2016 but followed the same pattern (Supplementary Fig. S7)
Micro-increments, when formed on a daily basis, may provide valuable insights into the timing of life-history events, the formation of otolith macro-structures used for routine aging, and growth rates of juveniles

Summary

Introduction

Micro-increments are visible structures within fish otoliths that often form on a daily basis (Panella 1971), and have been widely used to study the early life history of juvenile fish. Micro-increment counts have proven useful for estimating age (e.g.,Geffen 1995; Hüssy et al 2003a; Neat et al 2008) and fitting growth curves of juvenile fish (e.g., Bolz and Lough 1988). Interpretation of changes in the microstructure of fish otoliths has been used to estimate timing of life history events, including settlement or metamorphosis

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