Coastal water moving north up the coast of Europe mixes with oceanic water from the Atlantic off the north of Scotland, which leads to a high marine productivity. In summer the mixing tends to be concentrated along fronts which form between the stratified water out at sea and that mixed by coastal currents, and in the sounds between the land masses. Small fish, and the birds which feed on them, frequent these areas, and the birds breed nearby, so that the area is a major nursery for temperate North Atlantic seabirds. Most of the large breeding colonies of pelagic species are not found along the Outer Hebridean island chain, but along the mainland coast and on the outlying islands. There is also a substantial population of birds which feed along the coast scattered throughout the area. When the stratification of the water out at sea breaks down at the end of the summer, many moulting guillemots, razorbills and kittiwakes move inshore with their young for a time. Some birds also tend to move north at this time, presumably following some change in the distribution of the available food. As the weather deteriorates in the winter, many of the seabirds disperse. Some auks move into the more sheltered North and Irish Seas, the shearwaters, small petrels, skuas and terns migrate to the southern hemisphere, and the puffins, kittiwakes and young fulmars scatter throughout the temperate North Atlantic. Variable proportions of many species, including cormorants, gulls, auks and the adult fulmars remain in the area, retiring to more sheltered waters during bad weather. There may have been some long-term changes in populations; thus the southern Manx shearwater declined and the northern puffin increased in the last century, and the process was reversed in this one, possibly as a result of changes in climate. Some species, notably the gulls and fulmar, are now increasing following their exploitation of human wastes for food, and all species are benefiting from the cessation of human exploitation and its replacement by protection over the last century.