Aggregation and spawning by lampreys (genus Ichthyomyzon) beneath cover

Abstract

Lampreys are generally reported to spawn in shallow water on open, gravel bottoms. During surveys in Wisconsin and Minnesota, we regularly observed aggregations of adult Ichthyomyzon c.f. gagei, I. castaneus, and I. fossor beneath such cover objects as boulders, woody debris, and, at one site, vegetation. In some cases, observations of eggs or rapid quivering by individual lampreys indicated that spawning was occurring. The literature includes scattered anecdotal reports of similar behavior in other populations of Ichthyomyzon. Our data for I. c.f. gagei suggest that aggregations beneath cover objects occur at a greater range of depths than those in the open, but that aggregations in the open can contain greater numbers of individuals. Facultative spawning beneath cover objects may permit lampreys to spawn in deep waters with swift current where spawning could not otherwise occur. Moreover, this behavior may reduce the vulnerability of spawning lampreys to some types of predators.