Fish growth and degree-days I: selecting a base temperature for a within-population study

Degree-days (DD) are an increasingly popular method for explaining variation in fish growth and development. By including a base temperature (T o ) the DD formula limits calculations to temperatures that are relevant to growth. However, our review of growth studies shows multiple T o values in use for a given fish species. To determine how T o affects the ability of DD to explain within-population growth variation, we first show that the ability of DD to describe a growing season is robust to low values of T o . We then analyze immature length data from eight species and 85 water bodies in North America to show that there is a broad range of T o values that effectively explain growth variation. Based on these results, we argue that precise T o estimates are unwarranted for most single-population studies and recommend standard T o values (0, 5, 10, 15 °C). Standardization facilitates comparative studies and promotes the use of DD in future research. To this end, we provide equations for converting annual DD at a given T o to annual DD at a standard T o .