Research Document - 2007/071

Ageing inconsistencies and sensitivity analysis for 4WX herring

By Melvin, G.D. and M.J. Power

Abstract

The assessment of 4WX herring has been hampered in recent years by uncertainty in ageing. Recent exchanges of herring otoliths amongst research institutes indicated inconsistencies in reader assigned ages that are both significantly different and biased. This paper looks at the divergence, when it may have occurred and the potential effects on the assessment model. Several approaches were examined to investigate the sensitivity of VPA estimates of fishing mortality and biomass to ageing errors. Age length keys (ALK) were compared and then used to generate a new Catch at Age (CAA) and age disaggregated indices of abundance from 1999 to 2006 for the VPA input. An iterative process to fit a new ALK based on the length distribution of samples from another year’s data to investigated to examine the sensitivity of the VPA to the input ALK data. The model did not perform very well and the resulting CAA and VPA outputs were unrealistic. Alternatively, the ALK’s from several selected years were applied directly to the length frequency data from 1999-2006 to generate a revised CAA and an index of abundance for these years. Comparison of these VPA results indicated that slight changes in the age structure can have a profound impact on the final output. Estimates of F and biomass from the VPA analyses were found vary by a factor of 2 or 3 based solely on the different growth model inputs. It was concluded that within the range of age differences examined for the sensitivity analysis, accurate ageing is critical in the estimate F and stock biomass. It was also concluded , that regardless of the scenario , the 4WX herring stock remains at a historically low level with little signs of recent improvement. Fishing mortality is still well above F0.1 even with a substantial decrease in catches over the past 2 years and the acoustic index of abundance shows only a slight upward trend in 2006.

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