The First Step towards a Standardized Approach for Using Degree-Days in Fisheries Science

Although the thermal integral degree-days (DD; °C^.day) is gaining popularity as a way to describe and predict fish growth and development, we have yet to develop a standardized approach to DD or an understanding of the nuances among species and lake characteristics. In this study, we use length, age, and temperature data from both Minnesota and Ontario to estimate lower threshold temperatures for calculating DDs for different freshwater fishes. These thresholds were species-specific and closely approximated by published relationships between specific growth rate and temperature. We also found that DDs based on air temperature describe much of the variation in growth (even for cold-water species such as lake trout (Salvelinus namaycush) and cisco (Coregonus artedi), and that the strength of any length-DD relationship was negatively related to lake characteristics such as depth and surface area. This information improves our ability to describe fish growth and development and the effects of climate change, thermal discharge, and other stressors.