Relationship of density to growth and metamorphosis of caged larval sea lampreys, Petromyzon marinus Linnaeus, in Michigan streams

Larval sea lampreys, Petromyzon marinus, of the 1976 year class were studied between May 1977 and October 1981 in five Michigan streams to determine the relationship of density to growth and metamorphosis. Two cages were placed in each stream; one cage contained 25 larvae (low density group) and the other 75 larvae (high density group). Changes in body length, as an indicator of growth, were monitored in all streams from age I to V. Mean lengths ranged from 25 to 55 mm at the beginning of the study and from 102 to 158 mm at the end. Some ammocoetes attained lengths greater than 160 mm. Growth was related mainly to density; crowding affected growth from age I to metamorphosis. Mean increases of total length ranged from 90 to 128 mm among low-density groups and from 54 to 97 mm among high-density groups. Growth was typically greater in the cage with fewer lampreys in each stream. Most growth occurred from May to October when stream temperatures were highest. In two streams in which cages were not vandalized, 96% and 92% of the lampreys in the low-density groups and 68% and 52% in the high-density groups survived to age V. Metamorphosis occurred at age V in low-density groups. Five lampreys (2.4%; range, 133–145 mm long) metamorphosed among the 207 that survived until completion of the study.