The Role of Metalimnetic Hypoxia in Striped Bass Summer Kills: Consequences and Management Implications
James A. Rice, Jessica S. Thompson, Jamie A. Sykes, and Christian T. Waters
Abstract.—Historically, striped bass Morone saxatilis summer kills have been attributed to two mechanisms: stressors associated with crowding when striped bass are confined in isolated, cool refuges; and thermal stress or energetic deficit when hypolimnetic hypoxia (dissolved oxygen less than 2 mg/L) forces them into high-temperature surface waters. Here, we present observations suggesting that a third mechanism may account for some of these striped bass mortality events. During summer stratification, many relatively deep southeastern reservoirs develop hypoxia in the metalimnion, as well as near the bottom, isolating a layer of oxygenated hypolimnetic water between them. As these hypoxic zones expand in thickness and severity of oxygen depletion, the oxygenated layer between them shrinks both horizontally and vertically, and its oxygen content declines. Evidence suggests that striped bass summer kills can occur when fish are trapped in this isolated layer and its oxygen concentration declines below 2 mg/L or disappears altogether. The presence of coolwater forage fish such as alewife Alosa pseudoharengus or blueback herring A. aestivalis may increase the risk of striped bass kills by attracting them into the hypolimnetic oxygenated layer where they may become trapped. We draw upon examples from two southeastern reservoirs to illustrate this phenomenon, and discuss its implications for reservoir fisheries management, as well as possible approaches to minimize or avoid impacts.