Inter-benchmark process on Sandeel (Ammodytes spp.) in Area 2r (central and southern North Sea, Dogger Bank), and Area 3r (Skagerrak, northern and central North Sea) (IBPSandeel)

The inter-benchmark IBPSandeel evaluated the use of density-dependent survey catchability to reduce retrospective patterns in the assessment of sandeel in area 2r and 3r. IBPSandeel con-cluded that the method proposed is appropriate and can be applied to provide advice on these two stocks.
The sandeel advice on fishing opportunities is highly influenced by good incoming year classes. High uncertainty is usually associated with estimation of high recruitments, which justified the implementation of an F_cap strategy. However, sandeel assessments in 2r and 3r have also strong retrospective patterns in recruitment (i.e. the model has a tendency to overestimate recruitment in the terminal year) which are not properly accounted by the current F_cap.
Sandeel recruitment in 2019 is estimated to be high throughout the entire North Sea, including areas 2r and 3r. For this reason, an adjustment to the assessment model was proposed to provide more reliable estimates of recruitment in the terminal year. A power function was implemented in the assessment of sandeel 2r and 3r to capture density-dependency catchability of age 0 fish in the survey. The adjusted models provided downward correction of the terminal year recruit-ment, which is considered more reliable as suggested by the reduced Mohn's rho statistic, while estimates of stock dynamics (SSB and R) remained highly consistent with the previous assess-ment.