Predation risk and prey fish vertical migration in Lake Superior: Insights from an individual based model of siscowet (Salvelinus namaycush)

Drivers of movement patterns in prey fishes are often multifactorial in nature, but difficult to discern in complex lake ecosystems. Our objectives were to examine interactions between predatory siscowet lake trout and kiyi and deepwater sculpin, under different modeling scenarios to address the ramifications of prey distribution strategies on foraging by siscowet. We built an individual based model (IBM) of free moving siscowet to examine changes in predation rate on prey species given four scenarios of prey distribution. The scenarios included: 1) the nominal scenario where kiyi perform diel vertical movements (DVM) as observed in western Lake Superior; 2) a random distribution scenario where siscowet move randomly; 3) a no DVM scenario where kiyi maintain position near the bottom 24 h a day; and 4) a no DVM scenario where kiyi maintain position near 35-m 24 h a day. In the nominal scenario, there was strong agreement between simulated distribution, growth and diet of siscowets relative to observations in Lake Superior. In scenarios 3 and 4, when kiyi maintained their distribution at 35-m or near the bottom, predation rates by siscowet were approximately 14 times and 2 times higher, respectively, compared to the normal strategy exhibited by kiyi (scenario 1). These findings indicate that the present kiyi DVM strategy significantly reduces predation compared to other evaluated strategies and helps stabilize predator-prey interactions over relatively long timescales. Future analyses examining the supply and demand in the Lake Superior food web may quantify the specific importance of DVM in stabilizing predator-prey interactions.