A high degree of variation in hydrographic conditions is found in the so-called Iroise Sea, within less than 100 km of the west coast of Brittany. Tidal current maximal velocity, especially, ranges there from about 0·5 knot to more than 8 knots (locally, near the island of Ushant), i.e. practically as wide a range as found over the whole of north-west European shelf seas. Pelagic ecosystems accordingly exhibit a high degree of variety, related not only to classical inshore-offshore gradients, but also to the extent of vertical mixing or stratification. Areas where different physical and biological conditions prevail are generally separated by rather clearcut boundaries. The better-known of these is the Ushant thermal front, which runs in summer across the whole entrance to the English Channel, but also extends into the Iroise. In addition, freshwater runoff results in thermohaline stratification, or at least in the existence of thermohaline vertical gradients, in the two major bays of the west coast of Brittany. The relevant area is limited seawards by a thermohaline front, the Iroise inner front (Grail & Le Fèvre, 1967; Le Fèvre & Grall, 1970), beyond which are found the well-mixed waters inshore of the Ushant front. Fig. 1 sums up these hydrographic patterns in the area taken here into consideration.