How fishes find the shore: Evidence for orientation to bathymetry from the non-homing sea lamprey

Orientation to a shoreline is the critical first step for aquatic organisms that navigate to coastal waters, estuaries, and rivers to feed or reproduce. Most studies of animal migration have focused on homing-based navigation while non-homing navigation is poorly understood. We quantified the navigation behavior of sea lamprey during their non-homing return migration to a coastline in the Laurentian Great Lakes. Acoustically tagged sea lamprey were displaced 3.3 km from shore into the center of an acoustic listening array that provided high-resolution (30 s intervals, <5 m accuracy) three-dimensional paths. Eighty-one percent of individuals arrived at the nearest coast by moving towards shallower water. A biphasic sequence of movement was documented for most individuals, a more tortuous movement closer to the bottom associated with orientation, and a faster more linear movement we associate with directed search. Sea lamprey oriented to shallow water even when that was not the shoreward direction, and did ...