Larval timeseries in stock assessment (KB-14-012-054)

Different methodologies have been developed to process the data collected during the herring larvae surveys, the MLAI and the SCAI. Both approaches applied statistical methods to combine the larval abundances into one time-series as input to the North Sea herring stock assessment. In both approaches however the LAI data is smoothed twice, once in the statistical method and thereafter in the assessment, thereby losing its potential to be informative on changes in herring biology and the conditions they live in. The current assessment model for North Sea herring was modified to allow direct incorporation of the LAI data.

The results show that the proportional contribution of each spawning area to total SSB changes markedly over time. In addition, estimating proportional contributions inside the assessment model opens new possibilities in the treatment of other data sources in the assessment, such as acoustic and catch data.

A quality check of the time-series of herring larvae surveys was carried out. A new datasheet was compiled to allow for survey background information, such as weather conditions, flowmeters used, to be saved as well. This information is important to understand the larvae data and how to use them in the assessment.