The life-cycle of the European sturgeon, the largest amphihaline migratory fish on the continent, is spent successively in fresh water, brackish water and then salt water.  The sturgeon is essentially a bottom-feeder and moves around on the river bed or sea floor searching for food that is mainly composed of benthic invertebrates.  The most recent and detailed information on the species’ river and estuary habitats was obtained from the Gironde catchment basin where the last remaining potential reproduction areas and natural population of the European sturgeon can be found.

 

The species’ fresh water habitats are situated on the lower stretches of rivers and are limited to the spawning grounds of the reproductive adults present at the time of reproduction and the still little-known rearing areas of larvae, alevins and juveniles, which are progressively attracted to the estuary zone.  Jego et al. (2002) determined the optimal physical characteristics of the reproductive zones of the species in the Garonne and Dordogne.  They are situated in areas that are rather deep (over 5 metres) with strong currents running over a bed of pebbles and blocks.

 

The brackish waters of the estuaries are mainly used by the juveniles (areas of low salinity) and the immature sturgeon (areas with higher salinity) during their growth phase.  These estuaries are also an obligatory point of passage for the adults on their way to the spawning grounds.  Scientific work carried out on the Gironde estuary has shown the existence of several rearing grounds (Rochard et al., 2001 ; Brosse, 2003) in which the young sturgeons gather to feed (Brosse et al., 2000 a & b).  These muddy or sandy-mud zones have differing salinity (from 5% to 25%) and are situated at an average depth of between 4 and 8 metres (Brosse, 2003).

Studies made of the species’ range in maritime zones have shown, based on by-catches made at sea, that the European sturgeon (adults and juveniles of over 8 to 10 years), having left the banks of the estuaries, confine themselves to the coastal waters of the Continental Shelf at a depth of less than 100metres and mainly at depths of less than 40 meters (Rochard et al., 1997).  The presence of large size individuals has however been noted in the past at depths of between 100 and 200 metres in the Adriatic Sea.  The nature of the depths that they appreciate during this marine phase still remains largely unknown.  Between 3 and 8 years, the juveniles make seasonal return trips between the estuary and the nearby coastal areas.  Scientists suspect that, here too, there is a “concentration area” that is rich in food.