Diel hypoxia alters fitness in growth-limited estuarine fish ( Fundulus grandis)

Abstract

In shallow estuaries and tidal creeks circadian cycles of water column photosynthesis and respiration can produce daily periods of hypoxia (dissolved oxygen (DO) ≤ 2 mg/L). Sustained exposure to cyclic hypoxia is associated with smaller gonadosomatic index (GSI, gonad mass/body mass without gonad) and lower sex steroid hormone concentrations in wild Gulf killifish ( Fundulus grandis Baird & Girard, 1853; Cheek, Landry, Steele, and Manning. 2009. Mar. Ecol. Prog. Ser. 392:211–221). The aims of this study were to (1) quantify the effect of diel hypoxia on reproductive success, (2) identify any statistically significant associations between egg production and measures of physiological status, (3) estimate fertilized egg production of wild F. grandis using equations derived from (2), and (4) identify any statistically significant associations between environmental conditions and egg production. Experiment 1 measured reproductive success, growth, and physiological status in response to simulated Gulf of Mexico tidal creek conditions: 3 h episodes of hypoxia (DO ≤ 2 mg/L) daily for 30 days. Growth was negative and similar between treatments. Daily and cumulative egg productions were significantly lower in the hypoxia-exposed fish, but fertilization rate was unchanged. Experiment 2 quantified reproductive success, growth, and physiological status under longer hypoxic episodes, 5 h daily for 30 days. Growth was positive and similar between treatments. Neither egg production nor fertilization rate was altered by exposure to diel hypoxia. GSI and steroid hormone concentrations were similar under normoxia or diel hypoxia of 3 or 5 h duration. In fish with negative growth, female GSI was a strong predictor of cumulative fertilized egg number: y = 110.28x − 12.66, where y is √(cumulative fertilized eggs per female) and x = arcsin√GSI (r 2 = 0.67, F = 7.9, p = 0.05). Using this equation, egg production by wild fish captured in Weeks Bay, AL and Pensacola Bay, FL was estimated from measured GSI. At sites with diel hypoxia, egg production estimates were 50–85% lower compared to sites with little or no daily hypoxia in the same estuary. Across estuaries and years, body size and mean DO during the 48 h prior to fish collection were positively associated with female GSI. Identification of statistical associations between egg production and GSI and between GSI, body size, and 48 h mean DO can support mathematical models predicting population responses to hypoxia.