The Artibai river basin, with some 103 km2 of surface, is the most oriental in Biscay it extends in a south-to-northeast direction. Practically the whole of the length of the river (some 25 km) has been integrated in the space, from its source in Mount (1026 m) to its mouth in the fishing village of Ondarroa.
The only appreciable remains of marshes is situated between Arraveta and Goitiz, formed by a small island artificially closed up by earthen walls, with extensions of vegetated quagmires and subhallophillic communities (wild reads, rushes). In the rest of the stretch with marine influence there are only small patches of inter-tidal and sandy bogs, surrounded by cultivated farmland or urbanised zones.
The main naturalist value corresponds to the presence of the European mink (Mustela lutreola), a semi-aquatic carnivore that is globally threatened. Its population in the central and eastern basins in Biscay (Oka, Artibai) could be amongst the most interesting in its area of distribution.