Why summer fish kills?

Summer fish kills are as perplexing as they are disturbing to witness. A typical reservoir may have uniform temperatures throughout the lake, from top to bottom, for only a short time in the spring and again in the fall. In the summer, most lakes with sufficient depth are stratified into distinct, non-mixing layers of different temperatures.

The top, warmer layer is referred to as the “epilimnion,” and the colder, bottom layer is known as the “hypolimnion.” These two layers are separated by the “metalimnion,” better known as the “thermocline” — a zone of rapidly changing temperature. Because little sunlight reaches the hypolimnion, photosynthetic oxygen production is negligible and decomposition of dead plant and animal matter leads to depleted oxygen levels as the summer progresses.

The greatest conservation challenge in inland reservoirs in relation to stripers’ thermal habitat is water quality. Dissolved oxygen and temperature are two fundamental measurements of water quality. Water quality is essential day-in and day-out for the survival of adult fish. This is especially true in the summer, when surface temperatures may soar into the 90s and the system lacks sufficient water flow, depth or incoming supply to provide stripers a thermal refuge.

Many reservoirs struggle during the summer to provide enough cool, oxygenated water to support large stripers. This may limit stripers’ overall growth potential, as fish rarely survive long enough to exceed the 15-pound range. Since freshwater stripers seek out thermoclines as refuges, they must rely on incoming water supplies to provide them a safe haven from the lower-oxygen, higher-temperature conditions found during summer. What happens if this already-thin band of living space is diminished, such as in times of extreme heat or drought? More fish in less oxygen results in fish kills.