By definition, the body temperature and metabolic rate of ectotherms is largely determined by the ambient temperature of the environment and can quantified using degree-days (DD; °C.day) summed over a period of interest. Degree-days are useful when describing concurrent patterns of growth and development and typically incorporate a base temperature (To) below which growth does not occur (or occurs very slowly). Although degree-days have been used for decades to describe growth and development in plants and insects, this approach is rare in fish science. Recently the use of degree-days in fish science has increased in popularity but a review of the literature suggests limited continuity in its use. At times as many as 7 To values are being used to calculate degree-days for any one species and justification for these values is rarely provided. In this study, we use a large database of freshwater fish records and air temperatures to determine i) how variation in To affects the ability of degree-days to explain variation in immature growth rate both within and among waterbodies, and ii) the scope for using standardized To values in future studies of fish growth. We focused on immature individuals so that growth patterns were not confounded by the allocation of surplus energy to reproduction.
Results/Conclusions
Our results indicate that within a water body and regardless of species there is a broad range of threshold values (spanning 20oC in some cases) that are equally effective at explaining variation in growth. This result stems from a high level of correlation between degree-days at different To and depends largely on the range of temperature values experienced at a given latitude. However, our results also suggest that fish growth among waterbodies is best described by a single, species-specific To value. Taken together, these findings argue for a set of standardized To values for describing fish growth (e.g., 0, 5, 10, and 15oC). Standardization would i) eliminate the need to identify threshold values for each waterbody, and study; ii) facilitate coherent and compatible data sets, comparative studies (e.g., multi-species studies, meta-analyses) and studies at broad spatial scales; and iii) promote the use of degree-days in future research.