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

120

on the bottom in the deepest parts of Lake Michigan in mid-winter where
they seek the warmest water of the lake. In late winter and early spring
they move shoreward in dense schools, spawn near shore, in rivers, and
in bays during the late spring and summer, and begin to migrate back to
open waters of the lake in the fall. He feels that when the alewife is
concentrated inshore in the summer, the deeper water, previously inhabited by fishes that were replaced by the alewife, are essentially unoccupied and the invertebrate fauna is incompletely utilized. He feels
that similar situations occur during other seasons as the alewife moves
from zone to zone.
Morsell and Norden (1968) reported that as the length of alewives
increased beyond 119 mm they progressively relied more and more on
P. affinis as a source of food. They found, by stomach analysis, that
Pontoporeia constituted nearly 80 percent of the dry weight for the total
stomach content of alewives living in the littoral and sublittoral regions
of the lake.
The seasonally discriminant exploitation of Pontoporeia by alewives,
as well as amphipod predation by other bottom feeding fishes, could
possibly contribute the observed seasonal fluctuations.
PATTERNS OF SPATIAL ABUNDANCE
Powers and Robertson (1965) found the standing crop of Pontoporeia
higher in the inshore areas of the northern basin of Lake Michigan than
comparably placed stations located in the southern basin. Their results