A loon surfaced intermittently a few feet off the southern shore of Santuit Pond as several ospreysârecently returned from an intercontinental migrationâcircled overhead or perched in still-leafless trees. To a voice memo app on his phone, Jim Carroll noted the date (March 31, 2021), the time (a little past 5:30 PM), the weather (80 percent clouds) and air temperature (11 degrees Celsius, or 51 degrees Fahrenheit). Mr. Carroll dipped a thermometer over the edge of the Santuit fish ladder, where a few dozen alewife herring gathered in pools. After a moment, he retrieved the thermomet...