The Stock Identification Methods Working Group (SIMWG) reviews new methods for the defi-nition and investigation of stock structure and provides guidance to other ICES expert groups on how to interpret patterns of population structure.

Over the past three years, there has been a proliferation of applications of stock identification methods to ICES stocks, as well as several notable advances in stock identification methods with many results relevant to ICES science and advice. SIMWG provided annual updates on recent applications and advances in stock identification methods. Over the past three years, SIMWG received requests for guidance on stock structure of (1) European sardine (Sardina pilchardus), (2) Beaked redfish (Sebastes mentella) stock affiliation on the East Greenland slope, (3) Atlantic cod in ICES area 6a, and (4) red gurnard (Aspitrigla cuculus) stock structure in ICES subareas 3-8. These requests came from a range of ICES working groups including North Western Working Group (NWWG), the Workshop on Atlantic Sardine (WKSAR) and the Working Group on Widely Distributed Stocks (WGWIDE). SIMWG did not find sufficient evidence to support the recent change in stock structure of European sardine adopted in the 2017 benchmark and recom-mended review of new genetics research prior to the next benchmark. SIMWG did not recom-mend a combined assessment of beaked redfish along the east Greenland slope with the Icelandic slope. SIMWG reviewed and provided feedback on a proposed cod genetics study in ICES area 6a which promises to resolve some critical questions of stock identity of cod in ICES area 6a. SIMWG did not find strong evidence for splitting the red gurnard stock into two stock areas (North Sea/west of Scotland and Celtic Seas/Biscay). Over the past three years, SIMWG also made progress on reviewing and reporting on advances in mixed stock analysis. This work will continue through our next three year to and is relevant to resolving mixed stock composition issues in assessment and management. Understanding stock structure is a fundamental require-ment before any assessment or modelling on a stock can be contemplated and SIMWG will con-tinue to work with ICES expert groups to address pressing stock identification issues.