Variability in Diel Vertical Migrations by Siscowet Lake Trout in Lake Superior: When Does It Pay to Migrate Every Day?

Recent studies show that Lake Superior’s top predator, the siscowet lake trout (Salvelinus namaycush siscowet), perform diel vertical migration (DVM) that exceeds 150 m in magnitude and is consistent in pattern across seasons.  The DVM behavior takes a significant portion of the population from daytime bottom areas to upper portions of the water column at night.  This migration behavior is linked specifically to following vertically migrating prey species including kiyi (Coregonus kiyi).  However, a small portion (10 to 30%) of the population fails to move vertically on any given day. We conducted individual based model scenarios to explore the possible mechanisms underlying foregoing migration by some individuals in the population. In model scenarios, growth was maximized by following prey only when siscowet were unable to obtain high rations from consuming demersal prey (e.g. sculpins).  When demersal prey availability is high, the cost of migration exceeds the benefit of DVM behaviors.  Our results suggest that individuals make the decision to undergo DVM based on foraging profitability, which can exhibit threshold-like behavior from subtle changes in prey abundance.