F-80-R-3, Michigan
Study 665
665. Name of Study:
Investigation of causes of declines in Au Sable River brown trout populations.
A.
Problem: Present abundance of larger brown trout in the North Branch Au Sable River (NBAR) and
Mainstream Au Sable River (MSAR) is substantially lower than it was during the 1960's- to early
1980's. Fishing satisfaction has declined along with the fish populations. Significant declines in
brown trout populations in the MSAR were first observed during the 1970’s. The Michigan
Department of Natural Resources (MDNR) has attempted to restore the fishery in the MSAR to its
former level by applying various regulations including no-kill. Although these regulations had effects
on population dynamics they have not been successful in restoring the large brown trout fishery in the
MSAR. Large brown trout were also abundant in the NBAR during the 1970’s to about the mid1980’s. Since that time their abundance has declined precipitously. Declines in large trout
abundance have also been observed in some sections of the South Branch Au Sable River (SBAR).
Thus, it appears that there may be some watershed-scale changes that are being reflected in lower
abundance and production of trout. There is also evidence that abundance of larger trout has declined
in other northern Michigan trout streams since the 1970’s. The long-term data set on trout
populations (up to 35 years of population estimates) at index stations on the Au Sable River system
makes this an ideal location to look for linkages between changes in watershed characteristics and
factors such as changes in thermal regimes, discharge characteristics, or prey availability that may be
causing the observed declines in large trout abundance.
B.
Objectives: To continue to conduct annual trout population surveys at index stations on the Au Sable
River. These data will be analyzed in relation to environmental variables to explore possible
mechanisms causing the declines in larger trout. They will also be used to evaluate the effectiveness
of large woody debris enhancement in the NBAR and MSAR (management action that will be funded
by the private sector).
C.
Justification: The Au Sable River is one of Michigan’s premier Blue Ribbon Trout Streams. The
long term data on trout populations collected here since the 1950’s has been used to test the
effectiveness of a variety of fishing regulations and to develop population dynamics models that are
used to model expected effects of management actions proposed for other Michigan rivers. Because
the Au Sable River is a very popular and high profile system a variety of data on water chemistry,
primary productivity, and secondary productivity have been collected over the years. We have a
greater pool of historical data for this watershed than for any other system in Michigan except Hunt
Creek. We need to continue collecting, summarizing, and analyzing the survival, growth, standing
crop, and production of trout at the Au Sable River population index station sites so that we can
measure the effects of LWD enhancement efforts and regulation changes that will occur in 2000.
Following is a general description of what is proposed in the comprehensive study: The
“comprehensive study” will assemble and summarize data collected in the past on parameters such as
water chemistry, particularly nutrient levels, and water temperatures. We will address river
productivity in several ways. Existing data on nutrients, primary production, and macroinvertebrate
abundance will be compiled and compared to contemporary production at these trophic levels. Trout
shelters are being repaired in the NBAR in an attempt to increase trout survival. The long-term data
set on trout populations in the NBAR makes this a good location to quantitatively test the
effectiveness of this treatment. If these treatments are successful in increasing trout production they
may provide guidance for management of riparian corridors and watersheds to enhance trout
production. In part, this study is intended to provide more information on how the Au Sable River
ecosystem functions so that human resources are not wasted on ineffective restoration efforts.
F-80-R-3, Study 665 - 2
Hopefully, the study will provide more understanding of management options for restoring both the
Au Sable River fisheries and other depleted trout resources in Michigan. Hypothesis for observed
declines in trout production that may be examined in the comprehensive study (some may not be
immediately addressed if sufficient private funds can not be obtained) include: Increased use of the
resource, disease, increased predation, increased sedimentation, toxins, declines in natural and manmade fish cover, and losses of nutrients, plants, and invertebrates.
D.
Status: The abundance of larger brown trout (those 8 inches or longer) in portions of the NBAR and
MSAR has declined precipitously since the 1970's. Fall mark-and-recapture estimates of abundance
of brook and brown trout in the NBAR have been made at Twin Bridges and Dam 4 during 35 years
between 1957 and 1995. At the Eamons station on the NBAR trout population data are available for
28 years between 1962 and 1993. Annual fall population estimates of trout abundance have been
made at two stations on the MSAR during 28 years between 1959 and 1995.
Abundance--The abundance of larger brown trout in the MSAR peaked in 1971 when 8-11.9-inch
fish were over 10 times as abundant as in 1994, and 12-inch-or-longer brown trout were over 14
times more abundant in 1971 than in 1989. Similar dramatic decreases in the abundance of larger
brown trout since the 1970's have also been observed at NBAR sampling stations. In the MSAR
temporal declines in abundance at the Wa Wa Sum and Stephan Bridge stations are most evident for
brown trout 1 or more years old whereas the trend in abundance of age-0 brown trout is relatively
level. Age-1 brown trout abundance appears to have declined more than abundance of older age
classes. At NBAR sampling stations abundance of brown trout 1 or more years old was generally low
during the late 1950's and early 1960's. Abundance increased during the 1960's, peaked during the
1970's and early 1980's and then declined precipitously after about 1986.
Growth--Reduced growth rates account for some of the reduction in numbers of larger brown trout in
the MSAR. From 1960-63 the mean length of all age groups of brown trout was larger than during
subsequent sampling periods. Fall age-3 brown trout were up to 2.7 inches longer during the early
1960's. During this time many age-3 fish were 12 inches long by the month of June. By contrast,
brown trout now do not grow to 12 inches until sometime at age 4 and because approximately 90
percent of age 3 brown trout die (or emigrate) by age 4 there are presently far fewer brown trout 12
inch or longer in the MSAR than in the past. Higher productivity in the MSAR during the 1960's is
the most probable reason for higher brown trout growth rates during that time. Effluent from the City
of Grayling’s sewage treatment plant was diverted from the river in November 1971.
There are also indications of slight long-term declines in growth of brown trout in the NBAR. Brown
trout growth in the NBAR may be inversely related to trout density. Brown trout mean lengths at age
were generally higher during the periods of lower abundance observed during the late 1950's and
early 1960's and again after 1985. Clear interpretation of growth data for all branches of the Au
Sable is complicated by differences in sampling dates over the period of record. Year-to-year
variation in growth of brown trout in the NBAR has been found to be related to the quality of the
growing season, which was characterized by an index derived from the seasonal pattern of water
temperature (Clark and Nuhfer 1995). Seasonal variation in water temperatures influences the
productivity of plants, invertebrates, and fish in complex ways. Several studies are presently being
conducted to better define food web linkages in smaller trout stream ecosystems. Objectives of Ph.D.
research presently being conducted by L. Hinz are to 1) evaluate trout population responses to
temperature variation in field populations; 2) evaluate trout population responses to variation in prey
availability in field populations; and 3) evaluate the interaction between the thermal regime and the
prey community (standing stock/composition) on growth and production of trout populations.
Continuous data on water temperatures (hourly measurement, year-round) at Au Sable River trout
index stations has been collected under past MDNR studies, Hinz’s Ph.D. research, and ongoing
F-80-R-3, Study 665 - 3
MDNR study 633 (Clark, Wiley, and Hinz) which ends in 1996. Most prey availability data collected
under MDNR study 633 is from trout streams that are smaller than the NBAR or MSAR, although
some data on large prey availability was collected during 1995 at Au Sable River index stations.
Survival--Survival of most year classes of brown trout in both the MSAR and NBAR have generally
been declining over time. On average, 40% of age-0 brown trout survived to age 1 during 1974-93.
During the same time average annual survival was 61% for 1-year-old brown trout, 48% for 2-yearold brown trout, and 9.5% for 3-year-old fish. The lower abundance of larger fish in the MSAR since
1974 appears to be largely due to poor survival of multiple age classes, particularly during the late
1980's and early 1990's.
Reduced survival of multiple age classes of brown trout in the NBAR also appears to be the primary
cause of the present low abundance of larger fish. Survival of 1- and 2-year-old brown trout was
frequently 2 or more times higher during the 1970's and early 1980's than it is today. Regression
analysis indicates that 53% of the variation in abundance of age-3 brown trout at Twin Bridges, and
35% of the variation in abundance of age-3 brown trout at Dam 4 is accounted for by the abundance
of 1-year-old brown trout two years earlier. Similarly, 29% of the variation in the number of age-3
brown trout at Dam-4 is accounted for by the abundance of age-0 brown trout 3 years earlier. This
suggests that management actions need to address survival of younger fish. Survival of age-3 brown
trout in the NBAR in recent years has usually been at or above average. However, because 3-year-old
brown trout are so scarce larger fish remain rare in the population.
Water quality changes-- An assessment of water quality in the Au Sable River has shown that
nutrient levels throughout the system to levels are generally 2-3 times lower than they were in the
1960s and 1970s when trout standing stocks were at higher levels (Zorn and Sendek in press). There
is some quantitative data available on aquatic invertebrate standing crops and drift, water chemistry,
and trout diets and growth during the period when nutrient and organic inputs to the Au Sable River
were elevated by sewage effluent and after they were removed (Gislasion 1971, Reger 1973, Coopes
1974, Heckathorn 1977, Stauffer 1977, Merron 1982, STORET data).
However, the issue may not be solely limited to the Au Sable River because declines in trout
populations from 1960s and 1970s levels have been reported for other streams in Michigan and those
in other states (e.g. Vermont). Investigation into relations between nutrient levels and trout biomass
for other Michigan streams will help determine the geographic extent of trout population changes i.e.
whether declines in the Au Sable River brown trout are due to habitat changes unique to the river or
result from general declines in nutrient levels that may have occurred since the 1970s. Historic water
quality data are electronically accessible from the Michigan Department of Environmental Quality
Surface Water Quality Division. Data on historic trout levels occur in Fisheries Division records and
research reports (e.g. Gowing and Alexander 1980). Therefore, we are expanding the scope of this
study to include an investigation of trout streams outside the Au Sable River basin (Jobs 1, 2, and 3
were revised). Also, we added another year to the schedule to complete the final report.
Literature Cited:
Clark, R.D. Jr., and A.J. Nuhfer 1995. Trout population characteristics and environmental
interactions in Michigan trout streams. Michigan Department of Natural Resources, Federal Aid
in Fish Restoration Final Report, Project F-35-R-18, Ann Arbor.
Coopes, G.F. 1974. Au Sable River watershed project biological report (1971-73). Michigan
Department of Natural Resources Fisheries Management Report 7, Lansing.
F-80-R-3, Study 665 - 4
Eyman, L.D. 1969. A comparison of invertebrate drift in three Michigan streams. MS thesis,
Michigan State University, East Lansing.
Gislasion, J.C. 1971. Species diversity of benthic macroinvertebrates in three Michigan streams.
MS thesis, Michigan State University, East Lansing.
Gowing, H., and G.R. Alexander. 1980. Population dynamics of trout in some streams of the
northern Lower Peninsula of Michigan. Michigan Department of Natural Resources, Institute for
Fisheries Research, Fisheries Research Report 1877, Ann Arbor.
Heckathorn, C. 1977. Au Sable River study, July 21-22, 1975. Michigan Department of Natural
Resources, Environmental Protection Bureau Publication 4833-5135.
Merron, G.S. 1982. Growth rate of brown trout (Salmo trutta) in areas of the Au Sable River,
Michigan, before and after domestic sewage diversion. Michigan Department of Natural
Resources, Institute for Fisheries Research, Fisheries Research Report 1900, Ann Arbor.
Reger, S.J. 1973. Benthic macroinvertebrate diversity in three differentially perturbed Michigan
streams. MS thesis, Michigan State University, East Lansing.
Weisberg, S. 1993. Using hard-part increment data to estimate age and environmental effects.
Canadian Journal of Fisheries and Aquatic Sciences 50:1229-1237.
Zorn, T.G., and S. Sendek. In press. Au Sable River Assessment. Michigan Department of Natural
Resources, Institute for Fisheries Research, Fisheries Division Special Report 20, Ann Arbor.
E. Procedures:
Job 1. Collect trout population data from stations on the North, South, and Mainstream Au Sable
Rivers. MDNR personnel will conduct electrofishing sampling during the fall at Eamons
and Dam 4 sites on the NBAR. MSAR trout population index stations will be at Wa Wa
Sum and Stephan Bridge. South Branch stations will be at Chase and Smith Bridges.
During 1999-00 we will identify rivers having historic data on nutrient levels and brown
trout abundance. Field surveys to determine present water quality conditions and brown
trout population characteristics will be conducted during the summers of 2000-01.
Scale samples collected from subsamples of trout will be aged to determine growth and
survival rates. We will collect data on the individual weights of trout that are scale sampled
so that contemporary length-weight relationships can be compared with past data.
Job 2. Monitor water temperature at selected index station sites using electronic recording
thermometers. We will obtain hourly temperature readings at Wa Wa Sum and Stephan
Bridge on the MSAR. Twin Bridges and Dam - 4 on the NBAR, and at Chase and Smith
Bridges on the SBAR. Water temperature will also be recorded electronically in the
additional Lower Peninsula rivers sampled in 2000-01.
Job 3. Analyze trout population data. We propose to re-evaluate the age-structured population data
to provide better estimates of mortality, growth, and reproductive rates. New techniques (e.g.
Weisberg 1993) have been developed for growth rate analysis that may provide more insight
into temporal changes than existing analyses. Data on water temperatures in the Au Sable
River system has been collected under past studies, and stream discharge data is available
F-80-R-3, Study 665 - 5
from USGS gauging stations. Substantial modeling of linkages between these factors is
currently being done under study 633. Research funded by the National Science Foundation
is also examining food web linkages (M. Wiley, University of Michigan, personal
communication). Thus, we will be able to verify and possibly further develop models
developed under other studies using data collected under this new study and privately funded
studies. In addition, we will be able to evaluate the effectiveness of privately funded efforts
to repair trout shelters on the MSAR and NBAR. Unaltered trout population index stations
will be used as experimental controls. Possible linkages between nutrient levels and brown
trout abundance will be explored across both temporal and broader geographic scales.
Job 4. Write final report.
Job 5. Publish report through the Fisheries Division's editing and finishing process for Research and
Technical reports.
G. Geographical Location: Au Sable River watershed in Crawford County and Michigan’s Lower
Peninsula.
H. Personnel: MDNR district 7 personnel, Andrew J. Nuhfer and Troy Zorn, Research Biologists, Hunt
Creek Fisheries Research Station; Cooperative investigator is Dr. Daniel Hayes, Michigan State
University; Research Administrative personnel, and contract editor.