Hypoxia tolerance in two juvenile estuary-dependent fishes

Abstract

Hypoxia events, or low dissolved oxygen (DO) conditions, occur frequently in North Carolina estuaries during the summer. These events may have harmful effects on important fish stocks, including spot ( Leiostomus xanthurus) and Atlantic menhaden ( Brevoortia tyrannus), but their consequences are not well understood. We investigated direct mortality due to hypoxia in juvenile spot and Atlantic menhaden to determine how the extent of mortality varies with the severity of hypoxia and the duration of exposure, and to explore how vulnerability to hypoxia changes across species, fish size, and temperature. Atlantic menhaden and spot were tested at two temperatures, 25 and 30 °C, and three dissolved oxygen concentrations, 0.6, 0.9, and 1.2 ppm. Survival analyses were performed on the data relating survival rate of each species to dissolved oxygen concentration, duration of exposure, fish size, and temperature. The data were analyzed using an LC 50 approach for comparative purposes, and 12-h LC 50 estimates ranged from 0.9 to 1.1 ppm O 2. Spot and menhaden exposed to 1.2 ppm O 2 showed no mortality in 24 h at 25 °C, and only 30–40% mortality at 30 °C. In contrast, both species experienced 100% mortality in 2–6 h at 0.6 ppm O 2. There was an effect of size on hypoxia tolerance, with small spot being less tolerant than large spot, while the converse size effect was observed for menhaden. Spot were consistently less tolerant to hypoxia than menhaden and both species were less tolerant to hypoxia at 30 °C than at 25 °C. Preliminary experiments showed a 24-h acclimation to sublethal levels of hypoxia significantly reduced mortality upon subsequent exposure to lethal hypoxia concentrations. Our results indicate that direct mortality due to hypoxia will vary with species, size, and temperature, but will likely only be substantial when these species are exposed to oxygen concentrations less than about 1 ppm O 2. Given the severity of hypoxia necessary to cause mortality and the ability of fish to behaviorally avoid hypoxia, direct mortality due to hypoxia may have limited impacts on fish population dynamics. Therefore, the greatest effects due to hypoxia may be caused by the stress imposed by sublethal hypoxic conditions alone or in concert with other stressors, or by indirect effects incurred by avoiding hypoxic areas.