Anomalous Spawning of Smallmouth Bass in Nebish Lake, Wisconsin: Implications for Early Spawning and Over-Winter Survival

Abstract

We observed that the smallmouth bass (Micropterus dolomieu) population in Nebish Lake, Wisconsin spawned twice-once during the traditional spring period and then again in summer, well beyond the typical spawning season for north-temperate lakes. We documented this anomalous spawning behavior and compared the characteristics of smallmouth bass nests built during the two distinct spawning seasons. Smallmouth bass built 463 nests (1 10.2 n e s t s h shoreline) as water temperature was rising toward 20°C. During the summer spawning period, 24 nests (5.7 nestslkm shoreline) were constructed between 1 1 August and 29 August as water temperature was falling. Nests built in summer were significantly farther from shore, in deeper water, farther from cover, larger in diameter, and built by larger males than in the spring. INTRODUCTIONThe smallmouth bass (Micropterus dolomieu) has adapted to life in northtemperate lakes by adopting a spring spawning behavior. Rising spring water temperature triggers nest building by males, usually in shallow, gravel areas, and often associated with physical cover to increase survival on nests (Hubbs and Bailey 1938, Scott and Crossman 1973, Baylis et al. 1993). The largest males spawn first (Ridgway et al. 1991), procuring larger broods, which presumably results in better survival and fitness for offspring (Wiegmann et al. 1992). It is also presumed that producing eggs earlier confers greater fitness to offspring because earlier-spawned individuals can attain greater size during the growing season by acquiring more food and can reduce size-dependent mortality (Oliver et al. 1979, Latto 1992, Ludson and DeVries 1997. Because mortality of young-of-year smallmouth bass can exceed 95% by early fall (Frey et al. 2003), increased growth of earlier-spawned young-of-year is believed to translate into increased survival.Another presumed ecological advantage that the larger, earlier-spawned young-ofyear smallmouth bass have is that by attaining a larger size these fish are more prepared for the rigors of over-winter survival, a period of time when little feeding occurs (Oliver et al. 1979, Shuter et al. 1980. Shuter et al. (1 980) demonstrated through modeling and in laboratory studies that smallmouth bass survival to later year classes is contingent on growing large enough by their first fall to sustain them through the winter. They also used this work to explain the northern limit of smallmouth bass distribution. Lakes north of the 16.6"C mean July air temperature isotherm generally had lower over-winter survival because young-of-year were not able to acquire enough lipid stores during the short growing season. They concluded that smallmouth bass cannot become established in lakes with successive winter-induced year-class failures.It seems logical from a bioenergetics standpoint that spring-spawned fish should survive best and production of these individuals should regulate future year classes and