Population dynamics of Mysis relicta in southeastern Lake Michigan, 1995-1998

The abundance and life history characteristics of Mysis relicta were evaluated at an offshore (110-m) and a nearshore (40 to 45-m) station during 1995, 1996, and 1998. Data were collected ro monitor mysids relative to ongoing ecological changes in Lake Michigan and as a comparison to studies from the 1970s and 1980s. Mean densities of M. relicta during May through September each year were higher offshore (210/m(2) to 373/m(2)) compared to nearshore (41/m(2) to 168/m(2)). Growth rates ranged between 0.026 to 0.041 mm/day and did not differ between stations or with age. Juvenile mysids (< 10 mm) dominated the population, and accounted for 30 to 90% of the catch. The size distribution of M. relicta suggested that reproduction occurred year-round with the most consistent influxes of juveniles in the spring (April through June) each year; winter (February and March) and summer (July through September) influxes of juveniles did nor occur consistently each year. Mean length of females with broods differed between nearshore (14 mm) and offshore (16 mm). Brood size and the proportion of females with broods did not differ between stations. Abundance estimates were equal or higher and life history characteristics were similar to reported data from the 1970s and 1980s. However ongoing declines in the benthic macroinvertebrate Diporeia may result in higher fish predation pressure on M. relicta in the near future.