Coupling Otolith Microstructure Analysis and Hydrographic Backtracking Suggests a Mechanism for the 2000s North Sea Herring Recruitment Failure

No abstracts are to be cited without prior reference to the author.

The North Sea autumn spawning herring (Clupea harengus L.) has, since the 2002 year class, shown an unprecedented sequence of ten years of sharply reduced recruitment, in spite of a high spawning biomass and low fishing mortality. Recent work has identified this reduction in recruitment level (or stock productivity) as taking place during the larval over-wintering phase: however, the underlying mechanism remains elusive. In this study we analysed archived larval samples captured both before and after the onset of the reduced survival to test the hypothesis of a reduction in the larval growth rate. Individual larval growth rates, averaged over the 30 days prior to capture, were estimated for two hundred larval otoliths from four different years using a model-based analysis of the ring widths. The otolith measurements were complemented with additional information derived from hydrographic backtracking models (e.g. average temperature experienced, time available for feeding and spawning origin) to reconstruct the recent history of the larvae. A mixed-modelling approach was then employed to analyse the combined data: after correcting for the effect of the other variables, a significant reduction in larval growth rate, associated with the onset of the reduced recruitment, was identified. These results suggest that the reduced recruitment is associated with a reduction in the growth rate of the larval survivors, most probably through changes in either the amount or quality of the available food. Furthermore, this study demonstrates how coupling two different techniques (otolith microstructure analysis and hydrographic modelling) can yield unique insights into fish ecology.