POPULATION BIOLOGY OF STEELHEAD IN A STABLE-FLOW, LOW-GRADIENT TRIBUTARY OF LAKE-MICHIGAN

I examined the characteristics and dynamics of a population of wild st eelhead Oncorhynchus mykiss in the Little Manistee River, an extremely stable-flow, low-gradient (sandy) tributary of Lake Michigan. I monit ored autumn parr populations by electrofishing (1981-1983), and I moni tored spring emigrant populations at a weir near the river mouth (1982 -1984) and both autumn- and spring-returning adults at this weir (1979 -1987). Relative to other Great Lakes populations, steelhead in the Li ttle Manistee River had several notable characteristics that are proba bly linked to abundant groundwater and stable flows: (1) high abundanc es of age-1 parr, smolts, and adults, (2) constant autumn age-1 parr d ensities, (3) few emigrant parr, (4) fast growth when in the stream en vironment, and (5) relatively late age at maturity (after 3 years in t he lake). An excessive sand bed load occurs in the low-gradient sectio ns of the river, limiting gravel spawning areas to higher-gradient sec tions. This limited spawning area likely caused the relatively low abu ndance of age-0 parr, and may also be a factor in late age at maturity . Abundance and age structure of returning adults varied because of (1 ) variable survival during the presmolt winter (13-90%), (2) variable survival from smolt to maiden adult (6-29% for spring-run maiden spawn ers), and (3) variable survival to repeat spawn (3-58%). Most adults r eturning from a marked smolt cohort originated in the Little Manistee River (mean +/- 95% confidence limit, 71 +/- 28%). Management of such populations needs to focus on protecting and enhancing the stream envi ronment, and on managing sport harvest. Modeling population dynamics w ill require development of long-term data sets.