Physiological Justification for Using Air-Based Degree-Days to Describe Annual Fish Growth

Degree-days (DD) are an effective metric for quantifying the physiologically relevant thermal scope for growth for fishes and other ectotherms. However, fish science lags behind other disciplines (e.g., agronomy, entomology) in the widespread use of DD, likely because of the popularity of other metrics (e.g., mean air temperature) and a perceived disconnect between air temperatures and fish growth. In this study, we establish the physiological foundations for using DD derived from air temperatures to describe annual fish growth. We begin by using a bioenergetics model to reinforce the relationships between water temperature and fish growth. We then use both simulated and empirical temperature data to show that air and surface water temperatures are often highly correlated in lacustrine systems. Third, we use simulated and empirical data to show that air-based DD are as effective as water-based DD in describing annual fish growth. Finally, we discuss some conditions under which this air temperature-fish growth relationship may not hold, and potential methods for addressing them. By connecting the dots between air temperature, water temperature, and fish growth, we aim to provide sufficient justification for using air-based DD to describe annual fish growth.