In European cookery, there are several species which may be considered 'true' soles, but the common or Dover sole Solea solea, often simply called the 'sole', is the most esteemed and most widely available. (Davidson)
The name 'sole' outside Europe presents a confusing picture. Davidson says:
Indeed, in North American restaurants, the name 'sole' is often used to name any small flatfish, especially when filleted.
The True Sole solea solea is sufficiently broadly distributed that it is not considered a threatened species; however, overfishing in Europe has produced severely diminished populations, with declining catches in many regions. For example, the western English Channel and Irish Sea sole fisheries face potential collapse according to data in the UK Biodiversity Action Plan.
Sole, along with the other major bottom-feeding fish in the North Sea such as cod, monkfish, and plaice, is listed by the ICES as "outside safe biological limits." Moreover, they are growing less quickly now and are rarely older than six years, although they can reach forty. World stocks of large predatory fish and large ground fish such as sole and flounder were estimated in 2003 to be only about 10% of pre-industrial levels. According to the World Wildlife Fund in 2006, "of the nine sole stocks, seven are overfished with the status of the remaining two unknown."