Population characteristics of Bythotrephes in Lake Michigan

The population characteristics of Bythotrephes were evaluated at seven nearshore (45 m) and eight offshore (80 to 150 m) stations in Lake Michigan during July and September 2000. Bythotrephes was generally most abundant at offshore stations, but mean density was patchy (4 to 1,326/m(2)) among locations. During the year, there was a shift from reproduction by mainly instar III females to reproduction by instar II females. The shift generally reflected a change in the population structure of Bythotrephes at most sites during the same period. Bythotrephes populations in July were generally characterized by small body size at reproduction (instar II and III), large clutch size, and small neonates. Later in the season, body size at reproduction and neonates were larger and clutch size was smaller. Most growth (body length) of Bythotrephes occurred between instar I and II whereas little growth occurred between instar It and III. Spine length of Bythotrephes increased between July and September. The population characteristics and reproductive strategies of Bythotrephes appear to be adaptations to fish predation and food limitations.