Hypoxia-induced growth rate reduction in two juvenile estuary-dependent fishes

Abstract

As eutrophication of coastal waters increases, water quality issues such as hypoxia have come to the forefront of environmental concerns for many estuarine systems. Chronic hypoxia during the summer has become a common occurrence in numerous estuaries, degrading nursery habitat and increasing the potential for exposure of juvenile fish to low levels of dissolved oxygen (DO). We conducted a laboratory study to investigate how hypoxic conditions and temperature affect growth rates of two juvenile estuary-dependent fish: the Atlantic menhaden ( Brevoortia tyrannus) and spot ( Leiostomus xanthurus). For a 2-week period, we exposed the fish to one of four constant DO levels (6.0, 4.0, 2.0 or 1.5 mg O 2 l −1), at one of two temperatures (25 or 30 °C). A fifth DO treatment, included for spot at 30 °C, allowed DO to fluctuate from 10.0 mg O 2 l −1 during the day, to 2.0 mg O 2 l −1 at night. This diel fluctuation approximated the natural DO cycle in tidal estuarine creeks. Size measurements were recorded at the beginning, middle and end of experiments. Growth rates were generally unaffected by low DO until concentrations dropped to 1.5 mg O 2 l −1, resulting in 31–89% growth reductions. Our results suggest that DO levels must be severely depressed, and in fact, approaching lethal limits, to negatively impact growth of juvenile spot and Atlantic menhaden.