The physiological basis for well-known correlations between summer air temperature indices and year-class strength in northern smallmouth bass (Micropterus dolomieui) populations was examined. Field and laboratory studies demonstrated the existence of two critical stages in early life when smallmouth bass are particularly vulnerable to features characteristic of many natural water temperature regimes. The first stage extends from fertilization until the young leave the nest; high mortality results from exposure to extreme temperatures. The second stage extends over the first winter, when the young subsist on accumulated energy reserves. Because the ratio of energy stored to basal metabolic rate increases with size, large fish can withstand winter starvation better than small fish. The results from these and other studies were incorporated into a deterministic model of the relations between temperature and first-year survival of small-mouth bass. Analysis of water temperature time series data from many locations led to the development and parameterization of a stochastic model capable of simulating variations in water temperature characteristic of the littoral zones of typical North American lakes. The stochastic physical and deterministic biological models were used together to assess the effects on first-year survival of changes in climate and of realistic changes in the magnitude and frequency of short-term temperature fluctuations. The model successfully predicted the observed northern limit of the species' range. It also generated approximate environmental criteria for judging when year-to-year variation in survival, over one or both of the critical life history stages identified, is likely to reach a level sufficiently great to determine the pattern of year-to-year variation in recruitment to the adult stock. The combined model was also used to assess the effects of thermal loading from a nuclear power plant on a particular population.