A new method based on modelling of seasonal growth increments (GSI) in total length was found useful for assessing the date of onset of annual growth for 16 fish species in a temperate fluvial lake. Model comparisons indicated that polynomial (linear or quadratic) functions provided better fits to seasonal growth and were more likely to avoid convergence problems than alternative non-linear models. There was little evidence for differences in the date of onset of growth between years, nor among age classes within individual species. The onset of growth also was to some extent synchronized among species and was concentrated within a narrow temporal window of c. 2 weeks, between 18 May and 2 June, which corresponded to mean water temperatures between 16·1 and 17·3° C. There was no apparent relationship between date of onset and species’ thermal preferenda or thermal preferences. By producing a point estimate along with appropriate 95% CI, the GSI method provides useful information on the onset of growth and the uncertainty about that estimate. The GSI analyses can contribute to a better understanding of environmental influences on the onset of growth and the length of the growing season, and of thermal thresholds for growth, including their use in calculation of degree-day metrics.