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

The abundance and life history characteristics of Mysis relicta were evalua ted 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 th e 1970s and 1980s. Mean densities of M. relicta during May through Septembe r each year were higher offshore (210/m(2) to 373/m(2)) compared to nearsho re (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 si ze distribution of M. relicta suggested that reproduction occurred year-rou nd with the most consistent influxes of juveniles in the spring (April thro ugh June) each year; winter (February and March) and summer (July through S eptember) influxes of juveniles did nor occur consistently each year. Mean length of females with broods differed between nearshore (14 mm) and offsho re (16 mm). Brood size and the proportion of females with broods did not di ffer between stations. Abundance estimates were equal or higher and life hi story characteristics were similar to reported data from the 1970s and 1980 s. However ongoing declines in the benthic macroinvertebrate Diporeia may r esult in higher fish predation pressure on M. relicta in the near future.