Hypoxia Tolerance in Two Juvenile Estuary-Dependent Fishes
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Date
2004-01-14
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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. As part of a larger study examining effects of hypoxia on juvenile estuary-dependent fishes, I investigated direct mortality due to hypoxia in juvenile spot and Atlantic menhaden. The objectives of these experiments were 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, temperature, and fish size.
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, temperature, and fish size. The data were also analyzed using an LC₅₀ approach for comparative purposes, and 12-hour LC₅₀ estimates (concentrations causing 50% mortality) ranged from 0.9-1.1 ppm O₂. Spot and menhaden exposed to 1.2 ppm O₂ showed no mortality in 24 hrs at 25°C, and only 30-40% mortality at 30°C. In contrast, both species experienced 100% mortality in 2-6 hrs at 0.6 ppm O₂. There was a modest 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 that a 24-hour acclimation to sublethal levels of hypoxia caused significantly reduced mortality upon subsequent exposure to lethal hypoxia concentrations.
This study is part of a larger effort integrating lab experiments and field observations in a spatially-explicit, individual-based model to quantify changes in fish survival, growth and distribution in response to water quality changes. Results from this study indicate that while direct mortality due to hypoxia will vary with species, size, and temperature, mortality will likely only be substantial when these species are exposed to oxygen concentrations less than about 1 ppm O2. Given the severity of hypoxia necessary to cause mortality and the ability of fish to behaviorally avoid hypoxia, direct mortality due to hypoxia may not occur on a large scale. Therefore, the greatest impacts due to hypoxia may be indirect, due to density-dependent effects on growth and survival as fish avoid hypoxic areas, or via mechanisms caused by stress imposed by sublethal hypoxic conditions alone or in concert with other stressors.
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spot, Atlantic menhaden, hypoxia, mortality
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Degree
MS
Discipline
Zoology
