Migratory fish

Two broad strategies divide this group. Some are born in fresh water, grow at sea and run back up to breed; others do the reverse, like the eel, which is born far out in the Atlantic, spends its adult life in our rivers and returns to spawn where it hatched. In both cases the crossing between the two worlds is the critical point.

That crossing carries a considerable physiological cost. The body must adapt to a complete change of salinity, often within days, in the estuaries. That is where these species concentrate, and it is also where they are most vulnerable — to fishing as much as to pollution.

Their presence in a catchment says something about its condition. A river that sees its migratory fish return is a river whose barriers have been passed and whose water quality holds. Many of these species are among the most closely monitored in Europe.