Ecology of Shallow and Deep Water Populations of Pontoporeia hoyi (Smith) (Amphipoda) in Lake Michigan

Abstract

Aspects of the ecology of Pontoporeia hoyi (Smith) were studied at a 15-m and a 42-m deep site along the southeastern shoreline of Lake Michigan. At these shallow and deep sites, respectively, average densities were 3234 and 8292/m2, average annual biomasses were 0.90 and 5.58 g/m2, annual productions were 2.82 and 7.07 g/m2/year, and P/B ratios were 3.13 and 1.27. Mean number of eggs per female at 42 m was 17 with a mean weight per egg of 0.0340 mg, and there was a significant correlation between number of eggs and body length. The life cycle of the 15-m portion of the populations was one year while the 42-m portion was two years. At 42 m the major winter reproductive peak contributes an apparently low density to annual recruitment compared with the minor summer reproductive peak. In part, differences may reflect active onshors migration by winter reproductive adults as nearshore waters cool in early winter. Adults remain onshore and release young in early spring, either dying or moving offshore with rising water temperatures. Young remain onshore as evidenced by very predictable onshore population changes.