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

Abstract

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 not appear to rely on memory or recognition of the nearest coast. We postulate that individuals specifically performed barokinesis, whereby individuals assessed the gradient in absolute hydrostatic pressure on the bottom and to choose a heading towards shallower water. Repeated excursions to the bottom may confirm progress, while time spent at the surface is likely associated with surface-linked olfactory cues that indicate proximity to river water entrained along the coast. This is the first evidence that suggests the shoreward gradient in hydrostatic pressure may be used during shoreward orientation, and may represent a class of sensory information not previously considered in aquatic animal navigation.