ICES provides advice on two sea bass (Dicentrarchus labrax) stocks; a ‘northern’ stock in divisions 4.b-c, 7.a, and 7.d-h (central and southern North Sea, Irish Sea, English Channel, Bristol Channel and Celtic Sea), and a southern stock in divisions 8.a-b (Bay of Biscay). This Workshop reviewed evidence and proposed plausible stock structure scenarios for the ICES sea bass benchmark in 2023 (WKSEABASS). In summary, evidence suggests that the current ICES stock units do not reflect the biological boundaries and connectivity of sea bass within these areas. Evidence from genetic (section 2.1), tagging (section 2.3) and larval/juvenile pelagic drift (section 2.4) studies, revealed a high degree of connectivity between the northern and southern sea bass stock units. Furthermore, genetic (section 2.3) and tagging (section 2.3) evidence revealed potential bounda-ries within the northern sea bass stock, particularly between the North Sea and the Celtic/Irish Sea. The group agreed that evidence suggested a single meta-population structure, meaning a group of spatially separated populations interacting at some level (i.e. migrating, reproducing, feeding). Based on evidence provided, three plausible population scenarios were identified:
• Hypothesis 1: Three subpopulations, Irish Sea (7.a), North Sea (4b-c), southern Bay of Biscay (8.b); with mixing among the Celtic Sea (7.f-g), Bristol Channel (7.h), English Channel (7.d-e) and Northern Bay of Biscay. Mixing is seasonally dependant with more mixing occurring during summer compared to winter.
• Hypothesis 2: Three subpopulations, Irish Sea (7.a), North Sea (4.b-c) and Bay Biscay (8.a-b); with mixing among the Celtic Sea (7.f-g), Bristol Channel (7.h) and English Chan-nel (7.d-e). Mixing is seasonally dependant with more mixing occurring during summer compared to winter.
• Hypothesis 3: Three subpopulations, Irish Sea (7.a), North Sea/eastern English Channel (4.b-c, 7.d) and Bay Biscay (8a-b); with mixing among the Celtic Sea (7.f-g), Bristol Chan-nel (7.h) and western English Channel (7.e). Mixing is seasonally dependant with more mixing occurring during summer compared to winter.
It was not possible to choose between these scenarios or rank them in order of likelihood. Na-tional data should be requested by ICES Division within the data call as this will allow assess-ment approaches to be developed at relevant scales. ICES stock assessors will need to agree on what data are necessary to consider these three scenarios before the WKSEABASS data call and benchmark in 2023. However, if the available data are insufficient to accurately assess stocks based on these hypotheses, then the group agreed on two other scenarios that could be consid-ered:
• Scenario A: Continue to assess the ‘southern’ (ICES divisions 8a-b) and ‘northern’ (ICES divisions 4.b-c, 7.a and 7.d-h) stocks separately but incorporate mixing between stocks.
• Scenario B: Single meta-population between northern and southern stocks.
While research has provided additional information regarding biologically relevant stock boundaries for sea bass, more research is required to narrow down these scenarios and identify robust biologically relevant stock units. Additionally, similar levels of research are required to elucidate the connectivity of the sea bass populations delineated within this current workshop to West of Scotland and Ireland (ICES divisions 6.a, 7.b and 7.j) and Iberian (ICES divisions 8.c and 9.a) stocks that are not included in this benchmark process.