Fish growth and degree-days II: Selecting a base temperature for an among-population study

The degree-day (DD) is a method of describing the thermal opportunity for growth and development and is becoming increasingly popular when comparing fish growth over large spatial scales (e.g., counter-gradient growth). Temperatures too cold to permit growth are excluded in the DD equation by incorporating a lower temperature threshold (T_o). However, there is no convention for choosing T_o, and unknown is the effect of an incorrect T_o on how growth is perceived. We simulate data to demonstrate how an incorrect T_o may lead to differences in temperature-corrected growth rates among populations. These differences increase with the error in T_o and the thermal range among simulated populations. We then show the same relationships in an analysis of length-at-age data from 81 walleye (Sander vitreus) populations in North America. Together, our results demonstrate that differences in temperature-corrected growth rates among populations can be a statistical artifact rather than a biological phenomenon, especially when populations are distributed over a large thermal gradient.