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The first elements of the French–German cooperation arose in 1993 at a sturgeon meeting in Moscow, and the actual initiation represented the transfer of 40 juvenile Acipenser sturio from France to Germany in 1996. This resulted in an extensive exchange of ideas, material, and data concerning the biology and restoration of A. sturio between the two institutions Cemagref and Leibniz-Institute of Freshwater Ecology and Inland Fisheries, and in addition stimulated many studies on sturgeon biology in general. This will be outlined in this chapter, describing in short the main results and adding the appropriate references. We believe that the French–German cooperation was the key point for the success of the restoration efforts both in France and Germany.

This chapter provides data on age at puberty, duration of sexual activity, frequency of spawning, period of spawning, and zones of spawning grounds. Previous data as well as recent information are reported, mainly for the Atlantic population in France. When available, other data have also been mentioned, particularly from the River Guadalquivir (Sp) and the Elbe/Eider Rivers (D). Puberty was reached between 13 and 15 years for males and 19 and 22 years for females. Corresponding length (TL) was 140–145 cm for males and 185 cm for females. Duration of sexual activity was probably in the range of 6–10 years for males and 17–22 years for females. Frequency of spawning is mostly under-documented. The main spawning season is May–June in the Garonne basin. Some characteristics varied with latitude, e.g., age at puberty, onset of reproduction migration. Characteristics of puberty and sexual cycles in farmed sturgeon are given, including the few available data on A. sturio.

The neurohypophysis (Nh) of adult male and female A. ruthenus L. has been studied in chronic (15‰) and acute (22‰ and 32‰) experiments. The animals were maintained in water with the ionic composition of the Caspian Sea. The amount of neurosecretory material (NSM) in the Nh was estimated visually. The Nh of the fish is characterized by considerable hyperemia during all stages of the acute experiments with a lethal end, after 80 min in 32‰ and 5.5 h in 22‰ salinity. In group 1 (32‰) the Nh of the fish contained 3.8–4.1 units NSM (fluctuation 0.3 units). The average content of NSM in controls was 4 units. Blood osmolarity (BO) in group 1 increased from 315 to 395 mosM. In group 2 (22‰) 3.4–4.8 units NSM was found in the Nh (fluctuation 1.4), and 4 units in control. A tendency towards decreasing NSM content for 1–3 h was observed in group 2. An increase of BO from 265 to 395 mosM was visible in group 2, as was the case in group 1, though a tendency to decrease was noted for 1–3 h. Three phases of changes in the functional state of the Nh and BO were shown in group 3 (15‰). In phase 1 (up to 3 h) a considerable rise of both NSM content (up to 4.1 units versus 3.6 units in control) and BO (up to 320 versus 190 mosM in control) was seen. In phase 2 (6–12 h) a sharp decrease in the NSM content of the Nh (up to 2.5 units) was found. This provides circumstantial evidence that a large amount of neurohormone is discharged from the Nh into the blood circulation. In phase 3 (2–4 days) an increase in NSM content (up to 4.5 units) occurs; there is also a return to a normal level of blood osmolarity (185 mosM) on the second day and, later, stabilization. Different degrees of depressed release of neurohormones from the Nh under an extreme (32‰) and a strong (22‰) osmotic load is demonstrated. Release of the neurohormones providing water-salt homeostasis takes place under a bearable load (15‰).