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 M icropterus dolomieu. Smallmouth bass began spawning when temperatures reached 15 degrees C, and high how disrupted spawning five times. Larg er males spawned earlier than smaller males; a log-linear relation bet ween male length and degree-days accumulated was significant (r = -0.6 3, P < 0.01). In all, 45 of 105 nests (43%) and 42 of 81 males (52%) p roduced free-swimming larvae. Production of free-swimming larvae was v ariable and lower than previously reported, ranging from 98 to 1,802 ( mean, 608). Large males (>305 mm total length) accounted for the highe st 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 nes t 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 li kely cause of nest failures. The increase of mean velocity with increa sed stream discharge was significantly lower for successful nests than for unsuccessful nests, which showed that nest location determines th e degree of exposure to high Rows. The temporal pattern of streamflow fluctuation appears to be the most important abiotic factor determinin g nesting success or failure for smallmouth bass in this perennial str eam.