The Mediterranean is a subtropical, semi-enclosed and oligotrophic sea, separated from the Atlantic by a narrow and shallow sill in the Strait of Gibraltar (20 km wide, 300 m deep). This sea occupies an area of about 25·106 km2, being 3800 km wide from east to west and having a maximum north-south distance of 900 km. This sea has a high degree of environmental stability for both temperature and salinity below a depth of 200 m. In the western Mediterranean, their values are 12.8-13ºC and 38-38.6‰, respectively.
Except for the industrial fleets fishing large pelagics in open seas, most Mediterranean fisheries are artisanal, with low-capital ventures, where the fisherman is often the owner of the vessel, in contrast to industrial fisheries involving major investments by companies or financial groups
Mediterranean fisheries are enormously diversified, with many fleets based all along the coast in a great many ports. Trawlers clearly predominate, however, purse seiners and small-scale fleets also constitute important elements. Concerning catch composition, and despite the inherent complexity of multi-species landings in Mediterranean ports, there is an identifiable series of target species which, with other species of local interest, constitute the basis of production: (i) sardine (Sardina pilchardus) and anchovy (Engraulis encrasicholus), among the small pelagics; (ii) hake (Merluccius merluccius), red mullets (Mullus surmuletus, Mullus barbatus), blue whiting (Micromesistius poutasou), anglerfishes (Lophius piscatorius, Lophius budegassa), seabreams (Pagellus acarne, Pagellus bogaraveo, Pagellus erythrinus), octopuses (Octopus vulgaris and Eledone cirrhosa), squids (Loligo spp.), cuttlefish (Sepia spp.), pink shrimp (Parapenaeus longirostris), blu and red shrimp (Aristeus antennatus) and giant red shrimp (Aristaeomorpha foliacea), among the demersals; (iii) bluefin tuna (Thunnus thynnus) and swordfish (Xiphias gladius), among the large pelagics. These species represent 70-80 % of all landings.
The Balearic Islands separate the two basins of the western Mediterranean: the Balearic (north of the Balearic Islands) and the Algerian (south of the Balearic Islands) basins, characterized by different oceanographic and geo-morphological conditions. These two basins are connected by a series of sills between depths of 100 to 800 m, which play an important role in the general circulation and the transport of water masses between these areas. Atlantic water flows into the Mediterranean through the Strait of Gibraltar and, into the Alboran Sea, forms one or two anticyclonic gyres. These eddies reach the Balearic sills, creating the Balearic front, and contribute to an intense mesoscale variability. The Algerian basin acts as a reservoir for water of Atlantic origin. By contrast, the Balearic basin has a large cyclonic circulation, with two permanent fronts linked to the Northern Mediterranean Current, which flows southwards from the north along the Iberian slope, and the Balearic Current, which flows from the south following the Balearic slope. These frontal boundary regions, reinforced by the formation of a winter water mass with minimum temperature in the water column, which can be compared to the mode waters of the North Atlantic, are particularly relevant in the general oligotrophic context of the Mediterranean Sea as biologically active locations.