Ecology of Larval and Metamorphosing Lampreys

Abstract

Ammocoetes are relatively sedentary burrowing animals. Movement is related to water discharge, temperature, and season, and occurs predominantly downstream and at night. Growth is asymptotic and seasonal. At the end of larval life, the ammocoete ceases to increase markedly in length and starts to accumulate lipid. Length–frequency curves and data on kidney growth indicate that, in relatively stable and productive sites, ammocoetes of long established populations of the landlocked and anadromous sea lamprey take ~ 5 yr to reach metamorphosing length. Many animals probably enter transformation within a further 3 yr. information from an isolated population in the Big Garlic River and from other tributaries of lakes Superior and Michigan, some of which had been treated with larvicide, shows that the onset of metamorphosis can be highly variable and is apparently related to the growth rates and size of larvae. A short larval life is usually associated with a fast growth rate of ammocoetes, as is sometimes found in rivers where the use of larvicide has reduced population density. The landlocked sea lamprey tends to metamorphose at a longer length and at a greater age than other parasitic lampreys. During metamorphosis, which usually begins in the summer, lampreys maintain length but lose weight as a result of mobilization of lipid. The time between initiation of transformation and onset of feeding is generally 4–10 mo. The downstream migration of metamorphosed animals is nocturnal and is influenced by freshwater discharge. Comparisons are drawn between the sex ratios of sea lampreys in the upper Great Lakes and those of other populations.Key words: ammocoete, habitats, growth, mortality, larvicide, lipid, metamorphosis, migration, sex ratio, Great Lakes