Effects of seasonal habitat limitations on the distribution and energetics of stocked salmonids in Lake Moomaw, Virginia

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Virginia Tech


Lake Moomaw, a 1024 ha flood control reservoir in western Virginia, retains sufficient cold, oxygenated water (< 21°C, ≥ 5.0 mg/L) to allow trout survival throughout most of the summer. However, trout habitat declines to an annual minimum in September. vertical gillnets and ultrasonic telemetry were used to determine the horizontal and vertical distribution of two cohorts of brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss, relative to ambient temperature and dissolved oxygen. Capture of trout in gillnets afforded the assessment of condition (K), relative liver weight (HSI), and daily consumption (CD) of age 1+ brown and rainbow trout in relation to habitat dynamics.

Most age 1+ brown and rainbow trout were distributed in the metalimnion (12 - 21°C) from July to October, even when dissolved oxygen declined below 5.0 mg/L. Location in the metalimnion placed young trout near optimum temperatures and maximum prey abundance. The growth, condition, relative liver weight, and daily consumption of age 1+ brown and rainbow trout were apparently unaffected by habitat limitations. Age 1+ brown trout preyed almost exclusively on alewives, Alosa pseudoharenqus, whereas aquatic and terrestrial insects constituted a substantial portion of young rainbow trout diet.

Age 2+ rainbow trout were distributed in the metalimnion from July to September, but were located in the hypolimnion (<12°C) in October. Adult brown trout were located in the metalimnion during July, but were distributed in the hypolimnion from August to October. Adult trout in the hypolimnion were at low temperature (10°C) and diminished dissolved oxygen concentrations (<5.0 mg/L). Distribution in the hypolimnion also resulted in isolation from primary forage alewives. Age 2+ brown trout effectively were not feeding, as only one of 16 adult brown trout collected in two years of sampling contained food. Adult rainbow trout consumed primarily alewives in August and aquatic insects in September, but were apparently not feeding in October. Small sample size precluded the direct measurement of the effects of this isolation on the growth of age 2+ trout in Lake Moomaw. A bioenergetics model predicted that even brief durations of isolation from prey could severely limit the growth of age 2+ brown and rainbow trout.