A two-stage biomass model for Irish Sea herring allowing for additional variance in the recruitment index caused by mixing of stocks

Abstract

An assessment of Irish Sea (ICES Division VIIa, North) herring using a two-stage biomass model was undertaken given lack of internal consistency in the survey age-disaggregated index. The model constrains recruitment variability and reduces the number of catchability parameters to address concerns of overparameterization. An extra parameter was incorporated to estimate the component of the variance resulting from the mixing of Celtic Sea and Irish Sea juvenile herring in the survey area. This additional variance, although imprecise, represented a large part of the total variance associated with the recruit survey data. More information on the variability of Irish Sea herring recruitment is required to estimate additional variance in the context of the two-stage biomass model presented. Limitations in the age-composition data and potential interannual variation in the selection pattern of the fishery favour an assessment method, such as the two-stage biomass method, which is based on a simplified age structure and does not require separability assumptions. Crown Copyright © 2009.