Factors Affecting Nesting Success of Smallmouth Bass in a Regulated Virginia Stream

We examined the influences of habitat, temperature, stream discharge, and the timing of spawning on the nesting success of smallmouth bass Micropterus dolomieu . Smallmouth bass began spawning when temperatures reached 15°C, and high flow disrupted spawning five times. Larger males spawned earlier than smaller males; a log‐linear relation between male length and degree‐days accumulated was significant ( r = –0.63, P < 0.01). In all, 45 of 105 nests (43%) and 42 of 81 males (52%) produced free‐swimming larvae. Production of free‐swimming larvae was variable and lower than previously reported, ranging from 98 to 1,802 (mean, 608). Large males (>305 mm total length) accounted for the highest production of free‐swimming larvae and also made the most renesting attempts, which suggests that large males can have a strong influence on year‐class strength. Stepwise discriminant analysis distinguished successful nests from unsuccessful nests by higher flow at time of nest construction, higher mean temperatures, lower mean stream discharge during nest incubation, and shorter distance to shore. High flow (>10 m 3 /s) was responsible for most nest failures (85%). Increased water velocity at nest sites with increased stream discharge was the most likely cause of nest failures. The increase of mean velocity with increased stream discharge was significantly lower for successful nests than for unsuccessful nests, which showed that nest location determines the degree of exposure to high flows. The temporal pattern of streamflow fluctuation appears to be the most important abiotic factor determining nesting success or failure for smallmouth bass in this perennial stream.