Bioenergetic and limnological foundations for using degree-days derived from air temperatures to describe fish growth

Degree-days (DD) are an effective metric for quantifying the thermal opportunity for ectotherm growth. There is strong empirical evidence to suggest that DD are useful for describing fish growth and that immature growth increases linearly with DD. However, fish ecology lags behind other disciplines in the widespread adoption of DD. We provide (1) a foundation for the observed linear relationship between immature fish growth and DD and (2) justification for using DD derived from air temperatures as a proxy for DD derived from water temperatures in fish science. We use bioenergetics models and both simulated and empirical water temperatures to show that immature annual and interannual fish growth are approximately linear with water DD. We then use simulated and empirical data to show that air and surface water temperatures are often highly correlated and that immature fish growth is also approximately linear with air DD. By connecting the dots among air temperature, water temperature, and fish growth, we lay the foundation for wider adoption of DD in fish science.