Reproductive Function in Estuarine Fishes as an Indicator of Ecosystem Health

Until recently, estuarine and coastal hypoxia has been viewed as a problem for bottom-dwelling, low mobility species such as shellfish. Mobile species such as fish were assumed to migrate away from hypoxia. Recent tracking studies of Atlantic croaker in East Coast estuaries show that even mobile species stay in hypoxic waters for extended periods. This new information puts the problem of hypoxia in a different light: if fish are staying in hypoxic areas, what are the sub-lethal consequences for individuals and how are populations affected? The overall goal of this project is to evaluate reproductive function in Gulf killifish (mud minnows, scientific name Fundulus grandis) as an early warning indicator of fish population hazards due to estuarine hypoxia. The objective of this project was to compare reproductive function between normoxic and hypoxic sites. Fish were collected from marsh creeks in Pensacola Bay, FL and Weeks Bay, AL in 2004 and 2005. Collection sites were characterized as normoxic, mildly, moderately, or severely hypoxic based on how low dissolved oxygen (DO) concentration dropped each day, how long DO stayed below a critical level (2 mg/L), and how often low DO occurred. Condition factor (an index of plumpness used to indicate overall health), gonadosomatic index (the relative size of the gonad), sex hormone concentrations, and egg yolk protein concentrations were compared between sites within each estuary. Condition factor was not significantly affected by DO, suggesting that Gulf killifish may be able to maintain body growth under a wide range of DO conditions. Some reproductive indicators were more sensitive to hypoxia than others: relative gonad size and the male-specific hormone 11-ketotestosterone (a testosterone critical for sperm production in fish) were consistently reduced at hypoxic sites, whether hypoxia was mild, moderate, or severe. Testosterone and estrogen were less sensitive to hypoxia, decreasing only at severely hypoxic sites. Female egg yolk protein (vitellogenin) appeared to be reduced by mild hypoxia only.