Working Group on Methods of Fish Stock Assessments (WGMG)

Shrinkage  (either  by  year  or  by  age)  is  a  relatively  ad  hoc  device  that  was  imple‐mented in the XSA model to try to reduce unwanted assessment fluctuations driven by noise rather than signal. We summarize the history of shrinkage in XSA and con‐sider how shrinkage is being used in current ICES assessment working groups. We conclude  that  a)  shrinkage  should  where  possible  be  “light”,  and  b)  what  “light”  means needs to be determined by reference to estimation weights (rather than poten‐tially dubious metrics such as retrospective bias). More generally, we should turn to models that use data (rather than ad hoc assumptions) to generate inferences.