Diel hypoxia in marsh creeks impairs the reproductive capacity of estuarine fish populations

Seasonal hypoxia in estuaries and continuous hypoxia in the laboratory significantly reduce the reproductive capacity of estuarine fish, but the impacts of diel hypoxia are unknown. This field study quantifies reproductive capacity in populations of gulf killifish (Fundulus grandis) sam- pled from sites with different dissolved oxygen cycles. Condition factor, gonadosomatic index (GSI), steroid sex hormones, and vitellogenin were measured in F. grandis captured in Weeks Bay (WB), Alabama (July 2004 and 2005) and Pensacola Bay (PB), Florida (July 2005). Compared to a control site with little or no diel hypoxia, testes and ovaries were significantly smaller under both moderate (WB: 2.61 mg l -1 , 0.6 h d -1 ; PB: 2.41 mg l -1 , 1.5 h d -1 ) and severe (0.93 mg l -1 , 3.4 h d -1 ) diel hypoxia. Male 11-ketotestosterone (11KT) concentrations were significantly lower under moderate hypoxia, while both testosterone (T) and 11KT concentrations were significantly reduced under severe diel hypoxia. In females, T concentrations were similar regardless of the occurrence or severity of hypoxia, but estradiol-17β (E2) concentrations were lower under severe diel hypoxia. Since T is unchanged, but its products 11KT and E2 are significantly reduced, hypoxia may affect reproduction by inhibiting spe- cific steroidogenic enzymes in the gonad. The association between diel hypoxia and lower reproduc- tive capacity was consistent across estuaries. Hypoxia-related changes in gulf killifish populations could affect trophic structure in marshes and bays since F. grandis move carbon from the upper marsh surface to the lower marsh, as increased biomass and as prey for juvenile piscivores.