Gulf sturgeon migratory patterns, habitat utilization and aspects of reproductive biology in the Choctawhatchee River system, Alabama and Florida

Although most species of sturgeons worldwide are threatened with extinction, information on specific habitat requirements is lacking in many cases. Both ultrasonic and radio telemetry methods were used to identify critical freshwater and estuarine/marine habitats for adult Gulf sturgeon (Acipenser oxyrinchus desotoi) from the Choctawhatchee River system, Alabama/Florida. Additional insights were also developed on the roles that gender and maturational stage play in both migratory timing and habitat utilization in this threatened species. For both sexes, ripe fish entered the Choctawhatchee River significantly earlier and migrated further upstream than did nonripe individuals. Gulf sturgeon utilized hard bottom substrate in upstream areas for spawning from late March through early May. Gulf sturgeon moved downstream into marine and or estuarine waters in the fall. During winter and spring, telemetered Gulf sturgeon were distributed non-randomly along nearshore shallow (2–4 m deep) sandy areas within Choctawhatchee Bay. Telemetry results indicate that female Gulf sturgeon appear to utilize the waters of the Gulf of Mexico more frequently than males. Declines in habitat quality in Choctawhatchee Bay and the Gulf of Mexico may hinder recovery of this species, since adult Gulf sturgeon rely on these areas for nourishment during periods of gonadal growth. The lack of sexual dimorphism in most sturgeon species makes it difficult to ascertain gender or maturational stage. The first information on blood levels of the gonadal steroids testosterone (T), 11-ketotestosterone (11-KT), estradiol 17β (E₂), 17, 20β-dihydroxy-4-pregnen-3-one (17,20β-P), and the egg-yolk precursor protein, vitellogenin (VTG), in Gulf sturgeon is presented. Plasma levels of E₂ and VTG were greater in females than in males. A trend towards decreasing levels of E₂ at the beginning of final maturation in females corresponds in time with a trend of increasing levels of VTG. Gulf sturgeon levels of the androgens T and 11-KT did not differ with sex or reproductive stage prior to their spring spawning migration. Discriminant function analysis utilizing 11-KT, E₂, and VTG correctly classified 95% of females correctly while males were classified correctly 85% of the time.