Foraging mechanisms of siscowet Lake Trout (Salvelinus namaycush siscowet) on pelagic prey

Abstract

Little is known about the foraging mechanisms of siscowet (Salvelinus namaycush siscowet), the most abundant piscivore within Lake Superior because they occupy light-limited environments as a result of diel vertical migration. The reaction distance, angle of attack, and foraging success were determined for siscowet during laboratory trials under lighting conditions that approximated downwelling spectral irradiance and intensity (0-10 lx) at daytime depths. Siscowet reaction distance in response to golden shiners (Notemigonus crysoleucas) was directly correlated with increasing light intensity until saturation at 0.01 lx, and afterwards the relationship was asymptotic within our range of tested light intensities. In total darkness, lateral line sensory detection was sufficient to locate prey at 24.9 ± 1.7 cm, while increasing light intensities increased reaction distance up to 58.6 ± 2.3 cm at 10 lx. Larger prey elicited higher reaction distances than smaller prey at all light intensities while moving prey elicited higher reaction distances than stationary prey at the higher light intensities (0.001-10 lx). The capture and consumption of prey similarly increased with increasing light intensity while time to capture decreased with increasing light intensity. The majority of orientations toward prey occurred within 120° of the longitudinal axis of the siscowet's eyes, although reaction distances among 30° increments along the axis were not significantly different. Our predictive model will help determine reaction distances for siscowet in various photic environments and will help identify the mechanisms and behavior that allow for low light intensity foraging within freshwater systems.t>