Manipulation of Fish Populations Through Reservoir Drawdown

A summer drawdown was implemented on Little Dixie Lake, an 83 ha (205-acre) Central Missouri impoundment, to improve growth rate of largemouth bass (Micropterus salmoides) and to re-establish populations of desirable size. Water level was lowered 2.4 m (8 ft) between 19–29 July, 1964. Surface area was reduced by 42% and the volume by approximately 58%. As a result of draining water from the bottom of this lake the hypolimnial waters were removed. Temperature gradient in the remaining epilimnial waters was broken and the lake became homothermos at 30 C (86 F). Stomach analysis of largemouth bass indicated an increase in feeding activity following drawdown. Grams of food found in the stomach per 0.454 kg (1 lb) of bass was used as an index to feeding. These values increased from 4.0 g of food per 0.454 kg (1 lb) of fish prior to drawdown to 10.7 following drawdown. Per cent of empty stomachs also decreased during the post-drawdown period. Scales collected from largemouth bass indicated an acceleration of growth in the year of drawdown, 1964. The increase in this increment of growth for 1964 over the previous year for age groups I, II, and III was 4.0, 4.0, and 3.3 cm (1.6, 1.6, 1.3 inches) respectively. Harvest of bluegill increased immediately following the drawdown and declined during the succeeding two-month period. Largemouth bass harvest was reduced during August but there were increases in the numbers and average size of fish caught through September and October. Shoreline seining before and after the drawdown indicated the density of fry and intermediate bluegill (Lepomis macrochirus) was reduced. It was thought the drawdown was responsible for a portion of this reduction through stranding of small sunfish in weed beds and shallow pools, through increased predation by bass on these small sunfish, and through exposure of nests as the water was lowered.