This Sole is most commonly found in large chain retailers because it is the most affordable and accessible. Another fish that needs more research to determine if it is sustainable.
For your convenience we have included the following information from Wikipedia, the free encyclopedia:
The common sole, Dover sole, or black sole, Solea solea, is a species of flatfish in the Soleidae family. It has a preference for relatively shallow water with sand or mud covering the bottom. It is found in the Eastern Atlantic ocean, from the south of Norway to Senegal, and in almost all of the Mediterranean Sea. In the winter it withdraws to the somewhat warmer waters of the Southern North Sea.
In the UK Commercially a small sole is called a "slip".
The small eyes are close to each other on the right-hand side of the body. This gives the fish the possibility of lurking half-buried in the sand for passing prey. The common sole, just like all other flatfish, hatches as an "ordinary" fish with one eye on each side of the body. The young metamorphose to flatfish when they are about one cm long. The common sole approaches a maximum length of approximately 70 cm.
Chefs prize Dover sole for its mild, buttery sweet flavour and versatility and for its ease of filleting. The fish yields fillets that hold together well in a variety of recipes.
The name "Dover" comes from Dover, the English fishing port landing the most sole in the 19th century.
Sustainable consumption 
In 2010, Greenpeace International has added the common sole to its seafood red list. "The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries."
Other species named "Dover sole" 
Because of its prestige, the name "Dover sole" was borrowed to name the eastern Pacific species Microstomus pacificus, a quite distinct species with different culinary properties: the Pacific sole has thinner, less firm fillets and sells for a lower price.