Ecology of the burrowing amphipod, Pontoporeia affinis, in Lake Michigan / by Wayne Paul Alley.

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The only winter benthic samples were obtained at stations C-l, C-2, and
C-3 on 27 January 1967, and no brooding or spent females were observed
in those samples.
It appeared that at depths less than 35 m, in Lake Michigan, reproduction is completed by late May-early June. Beyond 35 m it seemed
that Pontoporeia bred intermittently throughout the sampling season. An
examination of the bottom temperature records indicated that this amphipod
did not breed at any depth when the bottom temperature exceeded 7 C.
The fact that adults die after reproduction simplifies the recognition of year classes. The size frequency distributions for station B-8,
average depth 11 m, described a population of amphipods that matured
in one year (Fig. 10). The two major size groups of 2 May indicated
that the population was in the middle of the breeding season. The large
2 mm size group and small proportion of larger individuals on 2 June
showed that reproduction was in its terminal phase. The large 5-6 mm
size group of 15 October indicated that the cohort of the young of the
year has grown approximately 4 mm in six months.
At station E-6 where the average depth was 33 m, amphipods required two years to mature. Both brooding females and newly released
young as well as juveniles from the previous year were present in the
21 April sample. Recently released young, juveniles from the previous
yea; and a few adults were present in the 2 June sample. The two size
groups of the 5 October sample represented the young of the year and the