The first step towards a standardized approach for using degree-days in fisheries science

Background/Question/Methods

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 (T_o) 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 T_o 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 T_o 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 T_o 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 20^oC 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 T_o 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 T_o value. Taken together, these findings argue for a set of standardized T_o values for describing fish growth (e.g., 0, 5, 10, and 15^oC). 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.