How Delays in Fall Spawning May Affect Spring Emergence Times for Brook Trout in Mountain Lakes

The timing of life-history events – phenology – is highly sensitive to changing climate conditions in many species.  In this study we explored how changes in spawning phenology could translate to changes in emergence timing of juvenile salmonids as mediated by degree day accumulation in redds. Prior to the 2013 spawning season, we buried temperature loggers in each of three redds across three Adirondack Mountain lakes.  Loggers were buried to a depth of 5-10 cm - comparable to estimated egg pocket depth.  We recorded mean daily temperature in these redds through winter 2013-2014, and we determined degree day accumulation rates in each redd starting on the median spawning date of brook trout in each lake.  Degree day accumulation is key in determining egg development, and ultimately potential emergence times.  Because temperatures in the redds were warmer in the fall, degree days accumulated faster during this period. A delay in spawning in fall could therefore have a disproportionate impact on the date at which a given degree day threshold is met.  Using a 450 degree day threshold for emergence, we found delaying spawning by a week in fall could result in a potential delay in emergence of three weeks or more.