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Abstract
Large numbers of steelhead (Salmo gairdneri) annually migrate into many of the rivers located in Michigan's northwestern lower peninsula. Knowledge of the magnitude and dynamics of wild steelhead populations in these rivers is limited. During 1981-1986 I examined the characteristics and dynamics of one of these populations by quantitatively assessing three life stages: parr, emigrant, and returning adult.
Annual densities of age-1+ parr were relatively high and extremely consistent--this consistency was apparently related to an abundance of spawners and stable river discharge. Emigrant populations consisted mostly of age-2 smolts. Smolts were relatively abundant and large. Smolt migrations peaked in mid-May. Overwinter survival to age-2 smolts varied from 13 to 90%, resulting in smolt yields ranging from 10,000 to 72,000. Minimum estimates of annual adult runs during 1979-1986 ranged from 5,800 to 19,800 fish. Adult abundance was high relative to other Great Lakes populations. Returning adult populations showed consistent age structures, sex ratios, and size at lake age. Most returning fish were maiden spawners. Upstream migrations took place during fall-winter and spring periods, while spawning occurred during late winter and spring. Returns of a marked cohort showed that 71% of the returning adults were produced in the river. Minimum estimates of lake survival for two smolt cohorts were 48 and 17%, suggesting that survival in Lake Michigan is extremely high. Fluctuations in adult abundance were apparently influenced both by smolt abundance and by factors which affected the population during lake life. The latter may have included smolt density-dependent survival and random environmental effects.
The biology of the study population likely represents that of populations in rivers located in Michigan's northwestern lower peninsula. These rivers provide spawning and rearing conditions similar to the study river and empty into Lake Michigan. Management efforts need to focus on the preservation and enhancement of these valuable populations.
