Prey Supply and Demand in Offshore Waters of Lake Superior: Does Diel Vertical Migration Stabilize Predator-Prey Fluctuations?

Recent studies indicate that prey demand by top predators in Lake Superior is approaching carrying capacity.  While the offshore waters (>80m)of Lake Superior are historically poorly understood, recent surveys provide data sets that now allow examination of trophic supply-demand in this region.  We examined predatory demand of siscowet, the dominant offshore predator, relative to the production of prey in offshore areas in nine locations distributed lakewide in 2005.  Comparisons of annual consumption by siscowet to the production of primary fish prey indicate a surplus production of coregonines (~12,500 MT), a deficit production of deepwater sculpin (4,100 MT) and a balance between production and consumption of rainbow smelt.  Given that deepwater sculpin are the predominant diet item by biomass found in siscowet, our results indicate that deepwater sculpin populations may decline in the coming years.  Furthermore, diel vertical migration (DVM) by coregonines may represent a mechanism that reduces interaction intensity with siscowet.  Individual based model scenarios indicate a substantial reduction in siscowet growth under conditions with coregonines that exhibit DVM when compared to growth in environments with no coregonine movement.  These simulations suggest that DVM behavior by coregonines may dampen interaction intensity.