Larval habitat of American brook lamprey

American brook lamprey, Lampetra appendix, and physical habitat information were collected during the summer from 10 second-order streams throughout much of the species' geographical distribution. Larval density, several descriptors of larval length composition, particle size distribution, hydraulic conductivity, porosity, organic content, current velocity, and water depth were measured for a number of sites within each stream. Multiple regression analyses indicated that larval density was positively related to the percentage of medium-fine sand and organic matter in the substrate. Larval density was typically high when the length range was wide and small lamprey were relatively abundant, reflecting similar habitat requirements among age-classes as well as the dynamics between larval recruitment and mortality. Three patterns were observed among the 10 streams. In three streams, larval density was strongly related to both medium-fine sand and organic matter; in two streams, increases in larval density were less noteable; and in five streams, abundance was more strongly related to medium-fine sand than organic matter. Other variables did not significantly affect larval density. The important habitat variables are discussed in the context of the species' ecology.