Research Document - 2010/101

Herring Ageing: Re-ageing of Atlantic Herring Otoliths and the Development of a Revised Catch at Age

By G.D. Melvin, M.J. Power, and D. Knox

Abstract

Age based analytical assessment models rely on the accurate and consistent estimation of ages to track year-classes or cohorts through the fishery. Biased or inconsistent ageing between readers/institutes can affect the model outputs. Several otolith exchanges conducted between 2002 and 2006 identified significant inconsistencies in assigned ages among readers from regional and international research institutes undertaking production herring ageing. To discern these differences a workshop was held in 2006 to examine individual reader practices and procedures, and, to establish standard protocols to improve the precision between readers. A internal DFO review recommended that two studies be initiated, involving multiple readers from several institutes,  to validate the herring otolith ages: Bomb radiocarbon assay of otoliths (n=96) from the 1962 year-class over several years, and tracking of the 1983 year-class (n=1787) as it progressed through the fishery from 1985 to 1994. The results of these studies validated the ageing method and confirmed the inconsistencies in ages among readers and institutes observed in previous exchanges.

Simulation studies showed that the observed differences between readers could have a serious impact on the VPA output. This led to the suspension in 2006 of the 4WX herring assessment until the ageing issue could be resolved. Furthermore, for the 4WX herring stock it was recommendation that all otoliths from 1999 to 2005 be re-aged. Over the next couple of years new quality control measures, aging protocols, mounting media, and acceptance criteria for inter-reader comparisons were established. Subsets of the 1999 to 2005 otoliths were re-aged, as well as the 2006 to 2009 otoliths which had never been read, following the above procedures. For all years the acceptance criteria of 80% agreement, a CV <5% and no bias was met. The ages were then used to generate an age-length for each year and a revised catch at age created for the 4WX herring stock. This report provides a chronological overview of the exchanges, workshops, analyses, and results that led to the revised catch-at age.

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