Au Sable River assessment: Troy G. Zorn and Steven P. Sendek.

Au Sable River Assessment

reaches of all these tributaries and the mainstem down to the mouth of the South Branch Au Sable
River. Fish species richness at these sites was lower than at warmer upstream reaches (which would
be expected given the cooler temperatures), typically ranging between 7-9 species (Richards 1976).
In 1972, the most coummon fishes in these reaches included: brook trout, brown trout, mottled
sculpin, slimy sculpin, common shiner, blacknose dace, creek chub, white sucker, and johnny darter.
Increased brown trout, slimy sculpin, and round whitefish (all coldwater species) suggested that these
reaches had gotten colder between the 1920s and 1972. Decreased temperatures may have resulted
from removal of old logging dams or increased shading due to forest regrowth (Richards 1976). Less
frequent presence of blacknose shiners, common shiners, pearl dace, and river chubs in the 1972
collections might also reflect decreased temperatures, or an increase in sand and silt in these reaches
since the 1920s. Comprehensive surveys would aid in describing how these reaches have changed
since 1972.
Self-sustaining populations of brook trout and brown trout have existed in upper mainstem, North
Branch, and South Branch Au Sable rivers since these waters were stocked in the late 1800s.
Population estimates of brown and brook trout in several reaches of these waters have been made by
the MDNR, Fisheries Division nearly every year since 1957 (Figures 23 and 24). These data
demonstrate the variability of trout populations over time, with variations being attributable to many
possible causes including habitat and water quality changes, hydrologic and climatic changes, and
changes in angling pressure, regulations, and methods. Explanation of these trends has been
complicated by simultaneous changes in several factors, and a lack of past data for testing the
influence of suspected causes. However, analyses of these data have contributed greatly to our
knowledge of trout biology and population ecology, and provided much of the scientific basis for
trout research and management in Michigan and the United States (Table 18).
Richards (1976) grouped downstream collection sites into "large river" and "below impoundment"
habitat categories. We will briefly discuss changes that occurred among these habitats between the
1920s and 1972 before continuing our description of their present fish communities. The "large
river" habitat category included 6 stations on the mainstem between the mouth of the South Branch
Au Sable River and Lake Huron. Richard (1976) noted substantial species shifts in this habitat type.
Declines of rainbow trout and slimy sculpin, both coldwater species, and of river chub and longnose
dace, fishes intolerant of the silt and turbidity, suggested a change in water quality. Greatest species
shifts were observed among the 4 "below impoundment" stations on the mainstem from Mio
downstream. Longnose dace, blackchin shiner, river chub, rosyface shiner, and redfin shiner, fishes
intolerant of silty and turbid conditions (Trautman 1981), were absent from the 1972 collections. The
mimic shiner, a species relatively tolerant of silt and turbidity, was absent from these four stations in
the 1920s but abundant at all of them in 1972. Richards (1976) attributed the general decline in fish
populations here to hydroelectric peaking operations and the influence of these aging impoundments
on downstream water quality.
From Wakeley Bridge to the upper end of Mio Pond, the river grows considerably in size as it
receives waters from the North Branch and South Branch Au Sable rivers, and Big Creek-South.
Though summer stream temperatures in rivers typically warm downstream, temperatures decline
along this river mainstem valley segment due to groundwater accrual from adjacent ice contact
deposits and tributaries draining them (Farrand and Bell 1982). Lowest stream temperatures were at
the downstream end of this segment at Parmalee Bridge, just above Mio Pond (Coopes et al. 1974).
Three collections were made in this mainstem valley segment in 1972 (Coopes et al. 1974). The
number of fish species per site ranged from 7-14, and averaged 11. Most commonly collected fishes
were brook trout, brown trout, mottled sculpin, slimy sculpin, common shiner, blacknose dace, creek
chub, and white sucker. Sampling by SWQD in 1992 at Parmalee Bridge (Morse 1994), revealed a
"good" fish community having 14 species, the most common ones being brook trout, creek chub,

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