Distribution, larval stage duration and growth of the sea lamprey ammocoetes, Petromyzon marinus L., in a highly modified river basin

Abstract  – Larval stage duration of the sea lamprey, Petromyzon marinus , in the River Mondego is estimated to last for 4 years. The number of annuli provides reliable age estimates when compared with length–frequency distributions analysis. The growth rate of the sea lamprey ammocoetes displays strong seasonal patterns, and reaches its highest value during the first 2 years of larval stage. About 69% of the length increment between hatching and metamorphosis is attained at the end of the second year. There is a longitudinal gradient associated with ammocoete distribution along the river. Relative abundance of ammocoetes decreases downstream from the Açude-Ponte dam, the first obstruction encountered by the adult sea lampreys in their upstream spawning migration along the River Mondego.