The European sturgeon is an anadromous migratory species that reaches reproductive maturity late in its life cycle.  The greater part of its existence is spent in salt water (brackish water and then sea water) but the species reproduces in the lower fresh water stretches of rivers.  From recent studies carried out on the ecology of the natural population of the Gironde catchment basin, its life cycle shows the existence of migratory movements that are specific to each stage of its development (Rochard, 1992; Rochard et al, 2001).  The cycle of development of the Acipenser sturio, such as has been monitored in the Gironde population, can be broken down into five phases:

1. The adult sturgeons reach sexual maturity at the age of 8 to 10 years for the males and 12 to 16 for the females.  It seems that the male is able to reproduce every two years whereas the female only undertakes upstream migration in order to spawn every three or four years.  The spawning migrations from the sea to the fresh water spawning grounds of the Gironde catchments takes place in Spring during the high water periods; reproduction takes place during the month of May and the beginning of June.  However, this migration period may vary according to the rivers and temperature (Magnin, 1962).  Up to now, circumstances at the time of spawning and the associated behaviour of the spawners have never been directly observed.

2. The fertilised eggs, which drift before adhering to the substrata, develop over a period of about ten days.  The larvae, rapidly developing into more mobile alevin that feed on their large yolk sac, remain in the river close to the spawning grounds throughout the summer following hatching.  This is one of the most little known phases of the biological lifecycle of the species (Jatteau, 1998).

3. With the autumnal renewal of the river waters, the young sturgeon gradually undertake the journey downstream towards the estuary.  There they reach brackish waters where they spend their first winter.  At this time, they will be about 6 to 8 months old and have already reached a length of between 20 and 25cm. 
Their journey to the sea, which takes place before the end of their third summer, is sometimes interrupted by short migrations towards the upper reaches of the estuary.

4. Between the age of 3 and 10 years, the juveniles make seasonal return trips between the estuary, where they stay during the summer months, and the saline waters of the coastal area at the mouth of the estuary during the winter period.  In the Gironde, this migration is specific to this species and is called the “movement of Saint John” by local fishermen.  The details and determinism of this phenomenon are still unknown but are probably linked to seasonal variations in the conditions of life (temperatures and feeding) in these milieus.

5. From the age of 8 to 10 years, the sturgeon, known as late juveniles or pre-spawners move out of the estuary.  They spread out over the Continental Shelf, most often in areas that do not exceed 40 metres in depth and they only return occasionally to the rivers in which they were born and grew when they have reached sexual maturity.  With a life expectancy of over 70 years, the European sturgeon can attain impressive lengths of 3.5m and over 300kg.  The distribution of the French population in marine areas currently stretches from the south of the Bay of Biscay up to Scandinavia.  The European sturgeon therefore undertakes long sea migrations (reproductive or feeding migrations).  Even though there is little documented knowledge of the migration routes, the sturgeon’s range of distribution crosses national borders, as is shown by capture-recapture operations that have enabled younger fish tagged in the Gironde estuary to be later identified in the North Sea.