[1] This work presents two case studies where stock assessment models were used to test hypotheses of spatial heterogeneity and spatial overlap. These hypotheses were raised by previous studies that compiled several sources of information. To test spatial overlapping the stock of plaice in the Skagerrak was used where a migration from the North Sea stock is hypothesised. The assessment model compared two fits of the Skagerrak stock, with and without a year trend in the abundance index. The assumption was that the extra level of abundance caused by the migration would be balanced out in the stock assessment by a trend in the survey’s catchability. To test spatial heterogeneity, stock assessments were run on the total stock and on putative stock sub-units of the North Sea cod stock, which is believed to have 3 approximately closed sub-populations. If the sub-units were closed sub-populations, with little movement across sub-units, the aggregated indicators (SSB and recruitment) should be similar to the indicators estimated for the total stock. In addition to the case studies two simulation studies were carried out to check/test that the stock assessment model was responding as expected. Results show that under the conditions of existing spatial structure, the model was able to detect both stock overlap and the difference between closed sub-units and an incorrectly partitioned unit.
Keywords: spatial structure, stock sub-units, stock assessment, spatial heterogeneity, spatial overlap, stock structure