IMAGE: Gretchen Hansen “The rate of increase in air temperature has accelerated in recent decades, and this increase in air temperature will affect the thermal habitat for fishes across the region,” Wagner said. “Temperatures are projected to continue increasing across the Midwest – with the greatest increases in average temperature expected in northern areas – so we wanted to know what was happening with walleye populations in the upper Midwest.” Using data provided by the Minnesota and Wisconsin departments of Natural Resources, researchers quantified annual walleye early-life growth rates from 1983 to 2015 in 61 lakes in the upper Midwest. Then they estimated the relationship between early-life growth rates and water growing degree days – an indicator of the temperature the fish are exposed to – over those 32 years. Importantly, they also examined how water turbidity influenced growth rates across the 61 lakes, correlated to an increased number of growing degree days.