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Thermal Niche Preference and Mortality of Striped Bass In Reservoirs
By Dan Ashe, Texas Parks and Wildlife Department


Striped bass (Morone saxatilis) by nature is an anadromous species, meaning they spawn in freshwater and spend the majority of their lives in saltwater. When the Santee River in South Carolina was impounded in the 1940's to create Santee - Cooper Reservoir a landlocked reproducing population of striped bass came into existence. Offspring from these fish have subsequently been produced and stocked throughout U.S. reservoirs to provide additional angler opportunities.

Striped bass have been stocked annually into Toledo Bend Reservoir since 1971 by the Louisiana Department of Wildlife and Fisheries and have also since been stocked occasionally by the Texas Parks and Wildlife Department beginning in 1974. Although it is not an annual occurrence, there are occasional die-offs of striped bass in southern reservoirs during the summer months as was evidenced in Toledo Bend Reservoir this summer, particularly this past August. Striped bass are susceptible to die-offs during summer months due to their strict preferences for temperature.

There have been numerous studies conducted documenting temperature (thermal) preference by striped bass. Striped bass are often targeted by fishermen, based on knowledge of the characteristic nature of these fish to orient themselves to a preferred range of temperature (about 45o - 68o F).

Southern reservoirs often stratify during the hot summer months and form thermoclines, which act as barriers preventing the mixing of water of differing temperatures. Surface water is warmer and less dense than the heavier cool water deeper down in the reservoir, with the point of most drastic temperature change being the thermocline. Without water mixing due to thermoclines that have formed during the summer there is also usually an anoxic zone (water without oxygen) in the water column at the edge of the thermocline and below. This is more often the case in productive lakes such as Toledo Bend and Sam Rayburn with a high nutrient load and good plankton bloom.

Most fish that occupy southern reservoirs typically do not have the narrow preferred temperature preferences that striped bass have and can suspend above thermoclines where oxygen levels and water temperatures are higher. Striped bass given their temperature requirements are often forced to stay near (just above or below) the thermocline where the water is cooler, however with little oxygen or forage. If hot conditions persist long enough keeping the lake stratified, striped bass can find themselves in peril of having to exist in an environment that provides little to no forage and one that is without oxygen leading to a die-off.

Although the die-off of striped bass in Toledo Bend Reservoir was noted and observed by department personnel, an estimate of the number of individuals that perished is not available. Accurately enumerating a fish kill on a large reservoir such as Toledo Bend is difficult if not impossible, due to the size of the reservoir (181,000 acres), expanses of emerged timber and vegetation, and the prolonged duration of the die-off (about two months). Those that visited Toledo Bend during late July through August undoubtedly observed dead striped bass floating on the lake's surface. Seeing dead fish is disappointing and most certainly detracts from anyone's fishing experience, however not all should have been lost this summer in Toledo Bend. Telemetry studies have documented striped bass natural mortality rates of 10-15% along with fishing mortality rates as high as 30-65% in reservoirs of similar latitude that support excellent striped bass fisheries. The die-off in Toledo Bend Reservoir may have been higher than this natural mortality estimate, but it is unlikely that the kill was to the extent as to cause a dramatic decrease in abundance that the lake is naturally accustomed to. Annually an average of 715,000 striped bass fingerlings or fry (1995-2004 average) are stocked into Toledo Bend Reservoir with no plans in the future to eliminate this stocking program.

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