How fishes find the shore: evidence for orientation to bathymetry from the non-homing sea lamprey Canadian Journal of Fisheries and Aquatic Sciences

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.

对于前往近岸水域、河口与河流觅食或繁殖的水生生物而言，海岸定向是其导航过程中至关重要的第一步。现有动物迁徙研究多聚焦于归巢导航，而非归巢导航的相关机制却鲜有探明。本研究对劳伦琴五大湖沿岸非归巢性回迁的海七鳃鳗（sea lamprey）导航行为进行了定量分析。经声学标记的海七鳃鳗被从岸边转移3.3公里至声学监听阵列中心，该阵列可采集时间分辨率为30秒、定位精度小于5米的高分辨率三维运动轨迹。81%的个体通过向浅水区移动抵达了最近的海岸。多数个体呈现出双阶段运动模式：一是贴近水底、更为蜿蜒的定向运动，二是速度更快、轨迹更线性的定向搜索运动。即便浅水区方向并非朝向岸边，海七鳃鳗仍会朝向浅水区定向，且似乎并未依赖对最近海岸的记忆或识别。我们推测，海七鳃鳗通过特定的压趋运动（barokinesis）感知水底绝对静水压力梯度，以此选择朝向浅水区的行进方向。反复贴近水底的巡游行为或许用于确认行进进度，而在水面停留的时间则可能与水面相关的嗅觉线索有关——这类线索可指示沿岸裹挟的河水所在区域。本研究首次提供证据表明，水生生物可利用朝向海岸的静水压力梯度完成近岸定向，这类感官信息此前并未被纳入水生动物导航机制的研究范畴。