North Sea sole is commonly distributed along the Southern half of the North Sea. Spawning takes place in shallow waters on the Southern coasts of the North Sea. Episodic large recruitment events take place at irregular intervals (ICES 2015). 

The North Sea sole is defined to be a single stock in ICES Area 4 (ICES 2015)(ICES 2020). The stock assessment is done accordingly, assuming sole in the North Sea is a closed stock. This approach was supported by a thorough population genetic analysis using neutral microsatellite markers and a mitochondrial marker (Cuveliers et al. 2012). This study showed genetic differences at a large scale, along a latitudinal gradient from the Skagerrak/Kattegat to the Bay of Biscay. At a smaller spatial scale within the North Sea, however, the subpopulations seemed genetically homogeneous, probably due to a high level of gene flow and/or the high effective population size preventing strong effects of genetic drift. Concerning the temporal aspect, a remarkably high genetic stability was found from the 1950s up to the present (Cuveliers et al. 2011).

Based on Single Nucleotide Polymorphisms (SNP) markers, (Diopere et al 2018) found that the population structure of sole in the NE Atlantic can be explained by isolation by distance pattern and local adaptation along a latitudinal cline. Genetic differentiation was detected among the Baltic Sea transition 9 zone, the North Sea and a southern group constituted by the Bay of Biscay and Portuguese samples. A separate population covering the Irish Sea and Celtic Sea was detected only by outlier loci.

A pattern of isolation by distance was reported also at a finer scale (Moan et al. 2019), which showed significant population structure for sole in the North Sea, Baltic Sea and the transition zone, suggesting that the divergence between the North and Baltic Sea sole occurred in presence of high levels of gene flow.

No new information was available on stock identity and substock in the last benchmark (ICES 2020).