Demersal groundfish assemblages and depth-related trends on Flemish Cap (NAFO division 3M): 2004–2013

• Definition of the assemblage structure is the first step toward an ecosystem-based management.
• Understanding how assemblages are impacted by fishing and recovery is essential to successful management.
• Flemish Cap is an important fishing ground of the NAFO Regulated Area.
• Three main assemblages are indentified in Flemish Cap correlated by depth.
• Boundaries and composition of the assemblages seem to be similar to the period before the collapse.

We examine groundfish assemblage structure and diversity in relation to depth on the Flemish Cap using data from a multispecies bottom trawl survey performed by the EU. The data include 1699 hauls prosecuted between 129 and 1460 m and collected from 2004 to 2013. We focused on the 29 most abundant species, which made up 99.2% and 99.1% of the fish catch in terms of biomass and abundance respectively. Assemblage structure was strongly correlated with depth. We identified three main assemblages: assemblage I (Shallow, shelf, <250 m), assemblage II (Intermediate, upper-slope, 251–600 m) and assemblage III (Deep, medium-lower, >601 m). Despite dramatic changes in biomass and abundance of the species in the area, the boundaries and composition of the assemblages seem to be similar to the period before the collapse of Atlantic cod in 1998. The main differences between periods were replacements of the dominant species in several assemblages and bathymetric range extension of some species.