It is linked to the Atlantic Ocean (called the Iroise Sea at this point) by the Goulet de Brest, a strait about 1.8 km wide.

Work is being done on gun emplacements on either side of the passage; the canons will crossfire, but if there were a battery on the rock in the middle, it would be impossible for enemy vessels to enter the roadstead, where more than 10,000 ships would be safe".

These areas have been greatly affected by the human activities in the north-west, but have justified the classification of around half of the roadstead as a zone in Natura 2000.

[3] The rich ecology of the roadstead has been diminished by the past exploitation of certain resources, and by the presence of a number of pollutants including heavy metals and tributyltin which were used as biocides in anti-fouling bottom paints.

The products which have replaced them for small boats (e.g. Diuron and Irgarol) pose similar problems and have been measured in non-negligible quantities in the roadstead by Ifremer in 2003-2004.