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GROWTH RATE OF BROWN TROUT (SALMO TRUTTA) IN AREAS
OF THE AU SABLE RIVER, MICHIGAN, BEFORE AND
AFTER DOMESTIC SEWAGE DIVERSION
by
Glenn S. Merron
A thesis submitted in partial fulfillment
of the requirements for the degree of
Master of Science
in Fisheries
School of Natural Resources
The University of Michigan
1981
Committee members:
Dr. Karl F. Lagler, Chairman
Dr. W. Carl Latta
To Dr. Howard L. Huddle
Class of 1967
ii
ACKNOWLEDGMENTS
This research was supported by the Institute for Fisheries
Research, Michigan Department of Natural Resources. I am
especially indebted to Richard D. Clark, Jr., who suggested this
study and gave great assistance during the progress of the study.
I would like to express my sincere gratitude to Dr. Karl F. Lagler,
for advice during this study and for help in the preparation of the
manuscript. Gaylord R. Alexander provided useful information
about the wild brown trout of the Au Sable River.
Thanks also go to Margaret S. McClure for typing the final
draft of the study and to Alan D. Sutton for drafting the figures.
iii
TABLE OF CONTENTS
DEDICATION
ACKNOWLEDGMENTS
LIST OF TABLES .
LIST OF APPENDIX TABLES.
LIST OF FIGURES . .
ABSTRACT .
INTRODUCTION
STUDY AREA .
MATERIAL AND METHODS .
RESULTS .
DISCUSSION
LITERATURE CITED
APPENDICES
iV
Page
viiiviiViV
11
25
31
34
LIST OF TABLES
Table
Page
Regression statistics for the relationship between
body length (y) and scale radius (x) for brown
trout from the mainstream, South Branch, and
North Branch Au Sable River for years 1960-61
and 1973-77 . . . . . . . . . . . . . . . . . . . . . . 12
LIST OF APPENDIX TABLES
Appendix
A.
Average back-calculated length (mm) of trout by
age group (sample size in parentheses) with 95%
confidence limits and attained significance of t test
(cº = 0.05) for brown trout grouped before and
after sewage diversion from the mainstream, South
Branch, and North Branch Au Sable River .
Average back-calculated increment of growth in
length (mm) of various age groups of brown trout
from the mainstream Au Sable River
Average back-calculated increment of growth in
length (mm) of various age groups of brown trout
from the South Branch Au Sable River .
Average back-calculated increment of growth in
length (mm) of various age groups of brown trout
from the North Branch Au Sable River
Average back-calculated increment of growth in
length (mm) of trout by age group (sample size in
parentheses) with 95% confidence limits and attained
significance of the t test (c. = 0.05) for brown trout
grouped before and after sewage diversion from the
mainstream, South Branch, and North Branch
Au Sable River .
Vi
Page
34
35
36
37
38
LIST OF FIGURES
Figure
10.
11.
The Au Sable River system, Michigan, showing the
location of the sampling sites . © C C tº ge
Average total length at age for brown trout from the
mainstream Au Sable River for years 1960–61 and
1973–77
Average total length at age for brown trout from the
South Branch Au Sable River for years 1960-61 plus
1973 and 1974–77 . © & © G & © º e º e º 'º -
Average total length at age for brown trout from the
North Branch Au Sable River for years 1960–61 and
1973-77. © tº º & G & © e º e G tº
Average back-calculated increment of growth in
length of various age groups of brown trout from the
mainstream Au Sable River .
Average back-calculated increment of growth in length
of various age groups of brown trout from the South
Branch Au Sable River .
Average back-calculated increment of growth in length
of various age groups of brown trout from the North
Branch Au Sable River .
Average increment of growth in length per year as
calculated by fish of different age groups from the
mainstream Au Sable River . tº e º e º º º
Average increment of growth in length per year as
calculated by fish of different age groups from the
South Branch Au Sable River. - tº º e º 'º e
Average increment of growth in length per year as
calculated by fish of different age groups from the
North Branch Au Sable River © tº º G → q) →
Total weight of trout from the Wa Wa Sum
station, mainstream Au Sable River, for the years
1959-63 and 1971–80
Page
13
14
16
17
18
19
22
23
24
27
ABSTRACT
Comparisons of growth rates for brown trout (Salmo trutta) were
made for two intervals, one during and the other after termination of
the discharge of primary treated domestic sewage effluent into parts of
the Au Sable River system, Michigan.
The ages of a total of 3,394 brown trout from the mainstream,
South Branch, and North Branch Au Sable River were assessed from
scale samples. Estimations of length at age and the annual growth
increment in length were obtained by conventional back-calculation
methods.
The growth rates of brown trout after termination of discharges
from sewage treatment plants into the mainstream at Grayling and into
the South Branch at Roscommon were found to be significantly slower
than during the discharge period. No change in growth rate occurred
for the same time intervals On the control, the North Branch, into
which no sewage plants have discharged.
The sewage treatment effluents formerly discharged into the
Au Sable River stimulated biological production of aquatic plants and
invertebrates. Increased trout production resulted through better
growth rates.
Following cessation of sewage input, aquatic production declined
in the affected river sections. In terms of growth of brown trout, this
was apparently due most directly to lowered food production,
specifically of the amphipod Gammarus fasciatus and the isopod
Asellus militaris.
viii
Back calculation of trout lengths at various ages, made from
scale measurements, tended to become progressively longer as older
fish were used. This is the reverse of the usual manifestation of
Lee's phenomenon of apparent change in the rate of growth. Size
selective avian predation of the smallest trout of a cohort is suggested
as the principal cause for this reversal.
ix
INTRODUCTION
The section of the mainstream of the Au Sable River from
Burton's Landing to Wakeley Bridge in Crawford County, Michigan,
is one of the premier trout fishing waters in the Midwest. In the
early 1970's many longtime anglers of this river section became
concerned that large brown trout were not as numerous as they had
been in the 1960's, and trout population samples taken by Fisheries
Division personnel of the Michigan Department of Natural Resources
confirmed there were fewer large trout. Fall population estimates at
sampling stations within the Burton's Landing to Wakeley Bridge
section indicated the numbers of brown trout in this area had remained
relatively constant, whereas the average size for each age group was
significantly smaller in the 1970's than in the 1960's (Alexander et al.
1979). Apparently, the growth of brown trout had declined.
Several hypotheses were presented to explain this decline in
growth. First, many anglers blamed the reduction in catch of large
trout on increasing angler use of this section. They believed that the
river was being overfished. This prompted the Fisheries Division in
1973 to alter the fishing regulation for brown trout from a 254-mm
minimum size limit, 5 fish per day, and fly fishing only, to a 304-mm
minimum size limit, 3 fish per day, and fly fishing only (Alexander
et al. 1979). This new regulation was intended to decrease fishing
mortality on 254-mm to 304-mm fish, and allow more fish to survive to
an older age and larger size (Clark et al. 1979). Later, when
2
populations of larger trout did not develop, the 304-mm size limit was
blamed for slowing growth. It seemed to stockpile too many fish in the
203-mm to 304-mm size range (Clark et al. 1979) which could have slowed
growth through increased competition for a limited food source. In
support of this idea, White et al. (1975) found that growth of brown
trout was poor in the special regulation water from Burton's Landing to
Wakeley Bridge when compared with growth both upstream and downstream
where trout density was lower. Also, Alexander and Ryckman (1976) found
that brown trout had higher densities and slower growth in the sections
of the North Branch of the Au Sable being fished under more restrictive
fly fishing regulations (228-mm minimum size limit, 5 trout creel limit,
artificial flies only) than in sections being fished under normal
statewide regulations (177-mm minimum size limit, 10 trout creel limit,
any lure permitted).
Another explanatory hypothesis was proposed by Favro, Kuo, and
McDonald (1979) who attempted to explain the decline in trout growth
through a population genetics model. The model was based on the idea
that fishing mortality, when applied to a population under a minimum
size limit regulation, would cause the larger fish of a cohort to die faster
than the smaller fish. Many of the larger fish would be harvested by
anglers as they grew over the minimum size limit, whereas the Smaller
fish would be protected. Therefore, the smaller fish would have a
better chance to survive and reproduce which, presumably, would cause
genetic selection for slow growth.
Habitat degradation was suggested as yet another reason for the
disappearance of large fish. Biologists of the Fisheries Division pointed
out that many of the large holes and submerged log sweepers which
served as "hides" for large fish were gone. Also, numerous silt beds
3
that produced food for trout had become less abundant. Many of these
changes were attributed to the increasing bedload of sediment, mainly
sand particles, from the construction of Interstate 75 in the early 1960's
(Coopes 1974). Furthermore, a tremendous number of camping and
canoeing related activities could have contributed to the alteration of
trout COver .
Finally, it was suggested that a decline in stream fertility was
responsible for the decline in growth (Alexander et al. 1979; Clark et al.
1980). This decline in fertility came about when the town of Grayling,
Michigan, in 1971, converted from a system that discharged primary
treated domestic sewage into the river to a land disposal system
(Coopes 1974) and also when the Grayling State Hatchery phased out
operations in the mid-sixties and attendant waste discharge ceased
(Alexander et al. 1979). It was opined that the organic effluent,
although low in volume but high in phosphates and nitrates, may have
had a beneficial impact on the trout fishery by increasing the food supply.
available for fish.
The objective of this study was to test the last hypothesis
concerning the impact of sewage discharge. A unique opportunity
for such a test presented itself as a matter of coincidence. On three
branches of the Au Sable River, the mainstream, the South Branch, and
the North Branch, there existed markedly different sewage discharge
situations, while at the same time their trout populations were being
monitored by the Michigan Department of Natural Resources for evaluation
of trout fishing regulations. The South Branch continued to receive
sewage effluent until 1974, which was 3 years longer than the 1971 cutoff
date for the mainstream. The North Branch received no municipal sewage
at all during the period, and therefore, could serve as a control stream.
4
In view of these events, the trout population data were analyzed to
determine if the nature and timing of the decline in growth of brown
trout were correlated with organic sewage diversion from the
mainstream and the South Branch.
STUDY AREA
The mainstream of the Au Sable River system originates from the
confluence of Kolka and Bradford creeks about 24 km north of the town
of Grayling, in northern lower Michigan (Fig. 1). The average width of
the study section of mainstream from Burton's Landing to Wakeley
Bridge, respectively 10.3 and 24.3 km below Grayling, is 28.8 m with a
mean discharge rate of 4.95 cm/s (Gowing and Alexander 1980). The
average depth is 0.76 m (James Failing, United States Geological Survey,
Grayling, Michigan, personal communication).
In November 1971, the town of Grayling, population 2143 in 1970,
diverted primary treated domestic sewage effluent from the mainstream
to a land disposal system (Coopes 1974). Primary treatment is the
physical removal of most of the suspended solids from sewage. When
sewage effluent is discharged into a river, the nutrients are processed
by natural physical, chemical, and biological means. On the mainstream,
increased growth of bacterial, algal, and macrophyte communities
occurred below the discharge pipe, and odors of putrification were
often present (Coopes 1974). The primary treatment plant was built in
1937, and modernized extensively in 1962 with the addition of facilities
for screening, primary sedimentation, and sludge digestion and had an
average daily outfall of 1.15 million liters (Michigan Water Resources
Commission 1966). Beginning in 1971, at a site 1 mile Southeast of
Grayling, effluent was passed through three 1-acre aerated lagoons
and then to a 7-acre leaching pond where it percolates to the
!
O 5 10
KILOMETERS
} North Branch
| Eaman's Landing º
| Dam 4 mainstream
Burton's Landing
S Wa Wa Sum
s? Stephan's Bridge
& - Wakeley Bridge
Gºt ING WSQa
South Branch ſh
.N- Marlabar
ROSCOMMON Chase Bridge
\\º
s^_^ *Sº
Figure 1. --The Au Sable River system, Michigan, showing the location
Of the sampling sites.

7
groundwater (Heckathorn 1977). Until the mid-sixties, the mainstream
received additional nutrient loading from the Grayling State Fish
Hatchery (Alexander et al. 1979). This hatchery on the northeasterly
side of Grayling, used water from the small East Branch of the Au Sable
River, and returned it to the branch which entered the mainstream
almost immediately.
The South Branch, which originates in Lake St. Helen 63 km
above its mouth, passes through Roscommon and joins the mainstream
some 27.5 km below Grayling. It is the narrowest of the study streams
averaging 23 m wide and with a mean discharge rate of 4.53 cm/s
(Gowing and Alexander 1980). The average depth is 0.70 m (James
Failing, personal communication). Like the mainstream the South Branch
is a habitat that favors aquatic macrophytes. Shelter in the river for
trout is abundant and of excellent quality. It consists of many pools,
log jams, and good bank cover (Alexander 1974a).
In late 1973, the town of Roscommon, population 810 in 1970
(Coopes 1974), began diversion of primary treated sewage from the
river to a land disposal system. Diversion was complete by June 1974
(Ray Moore, Sewage Treatment Plant, Roscommon, Michigan, personal
communication). The original primary treatment plant was built in 1957,
and included sedimentation, gravity sludge removal, and sludge digestion
and had an average daily outfall in August 1966 of 0.568 million liter
(Michigan Water Resources Commission 1966). Beginning in 1973-1974,
domestic sewage was diverted to a lagoon about 2 miles east of town
as part of a spray irrigation disposal System. The new system consists
of oxidation lagoons and a large holding pond where the effluent is
diffused by spray irrigation to seepage beds (Ray Moore, personal
communication). Thus, the mainstream and South Branch differ in two
8
major respects: (1) timing of effluent diversion, and (2) quantity of
nutrients received.
The North Branch, which has never received sewage effluent,
served as the control. Its headwaters are in Otsego Lake, 53 km above
the mouth and joins the mainstream some 10.5 km downstream from the
mouth of the South Branch. The average width of the North Branch
study section is 33.8 m with a mean discharge rate of 3.25 cm/s (Gowing
and Alexander 1974a). The average depth is approximately 0.55 m
(James Failing, personal communication).
The total drainage area of the Au Sable watershed, including all
branches, is 4662 km2 (Michigan Water Resources Commission 1966). The
area is largely forested, with only a little agriculture (Gislasion 1971).
The Au Sable system is highly acclaimed for its excellent trout fishing,
canoeing, and other water related activities (Alexander and Shetter 1967).
Sport fishermen and outdoor enthusiasts from this and other states place
an ever increasing recreational demand upon the basin, and the economy
of the area is largely dependent thereon.
The sampling sites for this study were stations used by personnel
of the Fisheries Division to obtain fall trout population estimates (Fig. 1).
All are located in Crawford County and comprise: (1) two on the main-
Stream, Wa Wa Sum and Stephan's Bridge, respectively 11.6 and 14.1 km
downstream from Grayling; (2) two on the South Branch at Chase Bridge
and Marlabar, respectively 8. 2 and 11.7 km downstream from Roscommon;
and (3) two on the control, the North Branch, Eaman's Landing and
Dam 4.
MATERIAL AND METHODS
As this is a comparative growth study, overall conventional and
well established methodologies were employed. Scale samples for the
brown trout were collected by personnel of the Fisheries Division from
fish captured by dc electrofishing gear annually in the period from late
September to the end of October. The total length was measured for
each individual and a scale sample obtained. In the early 1960's the
samples were removed from the left side of the fish above the lateral
line and above the anal opening, but during the 1970's, from the left
side of the fish above the lateral line and below the anterior edge of
the dorsal fin. For age assessment and measurement, a subsample of
the scales from each fish was impressed on cellulose acetate slides and
examined with a microprojector.
For most years, 10 fish per 25-mm length group could be randomly
selected for age and growth analysis; in only a few years were fewer -
fish available for certain of the specified length groups. The total
number of fish analyzed was 3394--1304 from the mainstream, 1111 from
the South Branch, and 979 from the North Branch.
At time of analysis, for each fish the following data were
recorded: sampling station code, year of capture, and month of
capture. Records were made of total scale radius and distance from
focus to each annulus along the same anterior radius. The least
squares regression analysis was used to obtain the body: scale
relationship. The data were then grouped and analyzed according to
10
stream site and time interval for the years 1960–61 and 1973-77. This
grouping by time period was required because of the difference in sites
of scale removal as described previously. Classically, it is assumed
that the body: scale relationship would be different for scales taken from
different areas of the body. To test this assumption for the present
data, analysis of covariance was used to determine whether or not there
were significant differences between the body: scale regressions as
derived from the two scale sample locations used in the two time intervals.
A FORTRAN program based on the traditional back-calculation
formula was used to calculate the average length at age and annual growth
increment in length at age for all fish used in this study. This formula
can be expressed in the following form:
L. = S Po - a + a
t t Se
Where:
Pt = total length at age t;
Pe = total length at capture;
St = Scale measurement to annulus t;
S
=
total scale radius at capture; and
a = intercept of the body: scale regression.
The Student t test was used to detect significant differences
in mean length at age and annual growth increment in length at age of
brown trout grouped before sewage abatement (1960-61 for the mainstream
and 1960-61 plus 1973 for the South Branch) and after sewage discharge
abatement (1973-77 for the mainstream and 1974-77 for the South Branch).
In the control, the North Branch, time intervals were 1960–61 and 1973-77.
RESULTS
For all stream sections studied the classical linear relationship
between body length and scale radius for brown trout was obtained
(Table 1). Covariance analysis showed that the body: scale regressions
were significantly different (a) = 0.05) between the two time periods for
the same stream section. This difference was most likely due to the
change in area of the body from which the scales were removed. As
expected, the size of scales was different from different areas of the
body. Covariance analysis also revealed differences in the body: scale
regressions from one stream section to another within the same time
period. However, these differences between stream sections may be
due to the slightly different environmental conditions in the streams.
In view of these differences, scales from each stream section and time
period were back calculated separately using their respective body: scale
intercept values (Table 1).
In the mainstream, length at the various ages declined
significantly between 1960–61 and 1973-77 (Fig. 2 and Appendix A),
in agreement with Alexander et al. (1979), Stauffer (1977), and White
(1975). Fish from the South Branch also showed a significant decrease
in mean length at various ages between 1960-73 and 1974-77 (Fig. 3
and Appendix A). This is similar to the findings by Stauffer (1977).
The mainstream exhibited a greater decrease following effluent cutoff
than the South Branch, probably because the mainstream received twice
the discharge of domestic sewage as the South Branch. Also, below
11
12
Table 1. --Regression statistics for the relationship between body length
(y) and scale radius ( x) for brown trout from the mainstream, South
Branch, and North Branch Au Sable River for years 1960–61 and 1973-77.
River branch Number y-intercept Slope R value
of fish
Mainstream
1960-61 432 12. 631 2. 218 0. 943
1973–77 872 5. 409 2.364 0.950
South Branch
1960-61 293 18. 162 2. 185 0.958
1973–77 686 16. 683 2. 328 0.951
North Branch
1960-61 324 8. 149 2. 0.91 0.953
1973–77 787 14. 935 2. 379 0. 939
13
3OOH-
|
2
O
O
->
1OOH- — 1960-61
— — — 1973-77
| || |||
Age
Figure 2. -- Average total length at age for brown trout from the
mainstream Au Sable River for years 1960–61 and 1973-77.

14
3OOH-
G
5
-C
g
d5 200H
—l
Tº
º
H.
1OOH- — 1960-61,73
— — — 1974-77
| || |||
Age
Figure 3. --Average total length at age for brown trout from the South
Branch Au Sable River for years 1960-61 plus 1973 and 1974–77.

15
Grayling the mainstream picked up additional nutrient loading from the
operations of the State Fish Hatchery in the early years.
Average lengths for the successive age groups of brown trout
in the North Branch from 1960-61 to 1973-77 remained relatively constant
except that age-II fish were somewhat longer in the latter interval
(Fig. 4 and Appendix A). These results agree with the findings of
Alexander et al. (1979) who suggested that the North Branch may be
experiencing a progressively increasing nutrient load by seepage from
numerous dwellings that have been built along the river since the 1960's.
The average growth increment in length at various ages was
calculated for years before and after effluent diversion for the various
streams (Appendix B, C, and D). The results showed growth declined
abruptly on both the mainstream and South Branch after organic effluent
was terminated (Figs. 5 and 6). The growth increments were significantly
lower for all age groups of trout in the mainstream (Appendix E), and
also significantly lower for all on the South Branch, except for age II
(Appendix E).
On the North Branch the growth increment for brown trout
remained nearly constant over time (Fig. 7 and Appendix E). Growth
of trout in the North Branch was as good or better between the two
time periods.
In the back calculation of growth a reverse "Lee's phenomenon"
(Lee 1912) was discovered (Appendix B, C, and D). The older the
age group of fish used for back calculation the greater was the growth
for the early years of life. Most often, Lee's phenomenon of apparent
change in the rate of growth shows the opposite effect and has been
attributed, among other things, to size selective mortality of larger fish
brought about by angler cropping. For example, selective catching by
16
3OOH-
|
2
O
O
1OOH- — 1960-61
— — — 1973-77
| || |||
Age
Figure 4. --Average total length at age for brown trout from the North
Branch Au Sable River for years 1960–61 and 1973-77.

17
15Or- SEWAGE
D|VERSION
125- \v | --
F
£
5 100| \º |
E N_2^S
QD
Ö
C
-C
s 75
9
CD ~ ||
5OH- TN/~
l | | -N | |, | | | | 1



5 L |
1958 59 6O 6 71 72 73 74 75 76 7
Year
Figure 5. --Average back-calculated increment of growth in length
of various age groups of brown trout from the mainstream Au Sable River.
18
150 T- SEWAGE
DIVERSION
Ny— | TS_>
125H
# | \º
#100). 7
5
SD |||
O N-
C /
# 75H
S _^
CD
5OH-
–



5 | | | | | l |
1958 59 6O 6 N: 71 72 73 74 75 76 7
Year
Figure 6. --Average back-calculated increment of growth in length
of various age groups of brown trout from the South Branch Au Sable
River.
19
15OT-
125H
F \l | T--
£ N-
*** ||
51OOH
8–
QD
É
5 75- ~ ||
s ~~
S
CD
50H
*EEEEEEE - H,


Year
Figure 7. --Average back-calculated increment of growth in length of
various age groups of brown trout from the North Branch Au Sable River.
20
anglers of fast growing fish of a cohort has been documented for the
brook trout (Salvelinus fontinalis) in the Pigeon River, Michigan
(Cooper 1951). However, Cooper's data also show a pattern of reverse
Lee's phenomenon for brown trout from age I-III. Of the fish used in
my study, approximately 90% were smaller than the minimum size limit
on the respective stream section, so fishing had little impact on my
results. Thus, some form of size selective mortality, possibly predation,
bearing most heavily on the slowest growing fish of a cohort, must be
acting on these populations.
Several studies indicate that the Common Merganser (Mergus
merganser) and the Great Blue Heron (Ardea herodeas) are very effective
predators upon several species of fish, especially the brown trout
(e.g., Salyer and Lagler 1940; Alexander 1974, 1976). Food studies of
the merganser in captivity (Latta and Sharkey 1964) suggest that the
birds tend to select those fish which are from 102 to 229 mm in length.
Alexander (Hunt Creek Trout Research Station, Lewiston, Michigan,
personal communication) has suggested that because the slow growing
fish are in the preferred size range for two growing seasons, they might
suffer greatest losses to avian predation. Thus, more of the fast growing
fish in this size range would survive.
The reverse Lee's phenomenon could have introduced a bias when
calculating the average growth increment of an age group based on fish
from different year classes. For example, in Fig. 5 the average age-I
increment in 1958 is based on the back-calculated history of age-III fish,
whereas for 1977, the average age-I increment is based on only age-I
fish. Therefore, a difference in the back-calculated mean length would
exist between the years as a result of reverse Lee's phenomenon which is
independent of any effect of sewage. In an effort to overcome this bias
21
when determining growth changes related to sewage effluent, the data
were plotted by age group for each year class of fish (Figs. 8, 9,
and 10). The results still support the hypothesis that a decrease in
brown trout growth coincided with the termination of domestic sewage
discharge into the river.
22
15Or- SEWAGE
2^ 2^ DIVERSION
125– / |
/
1OOH- — — — .
- - - - ||
||| |
E 75H. AGE O – || |NCREMENTS |
É | | | | -N- | | | | l | |
# 125ſ
QD
5
92 T-
É 1OOH- 2^
+: ,’ Sºº-----,
> |
3 75H AGE | – || |NCREMENTS
CD | | | | N- | | I | | | _l
75r-
_T
50– TN2-s
25– AGE || – || |NCREMENTS
| | N- | | | | l | |
1958 59 6O 61 71 72 73 74 75 76 77
Year
Figure 8. --Average increment of growth in length per year as
Calculated by fish of different age groups from the mainstream Au Sable
River.

23
150ſ SEWAGE
/ DIVERSION
S/ _2^
125- / >
1OOH- — — —||
----||
||| |
E ºf age o- Nchevents |
E. | | | | -N | | 1 | l |-
# 125r-
ă N —- 2 ,
É 1OOH- _2^ \ /
-C \ /
3. : N/
3 75F AGE | – || |NCREMENTS
CD | | | | N. l | | | | | 1
100 r
2^ -
75–
_T-
50– AGE || – || |NCREMENTS
|— | l | -N- | | I | | 1 |
1958 59 6O 61 71 72 73 74 75 76 7
Year
Figure 9. -- Average increment of growth in length per year as
calculated by fish of different age groups from the South Branch
Au Sable River.

24
125

1OO
7
5
1
2
5
1OO
7
5
1OO
75
5O
T-N Y ~ 4T ~~
N —-e-2
N smºmº me <
__” * _^
_* S-- "
AGE O - | INCREMENTS
————N–––––––

N---
AGE | – || |NCREMENTS
————N-1—1–1————

*
AGE || – || |NCREMENTS
l | | | | |
|--N
1958 59 6O 6 71 72 73 74 75 76 7
Year
Figure 10. --Average increment of growth in length per year as
calculated by fish of different age groups from the North Branch
Au Sable River.
DISCUSSION
Each of several different hypotheses, which attempted to explain
the decline in growth of brown trout in the mainstream of the Au Sable
River, can be reassessed in light of the present study. First, the
hypothesis that increasing angler use and /or fishing regulations were
responsible seems unlikely. Actual fishing pressure in the Burton's
Landing to Wakeley Bridge section was down 29% from 1960 to 1976
(Alexander et al. 1979). Moreover, the growth rate of brown trout
seems to have declined before the 305-mm limit was implemented. Also,
the growth rate diminished on the South Branch where fishing regulations
were not changed.
Second, the population genetics model of Favro et al. (1979)
did not appear to fit my results. If angling cropped the fastest growing
individuals, those fish genetically superior with respect to growth, then
presumably, a gradual decline in the average increment should have
been expected, not an abrupt one as occurred. It is now apparent from
the data that growth dropped rather abruptly at the time of sewage
diversion on both the mainstream and South Branch. Also, if the genetic
theory was true it probably should be spread over many streams.
However, the bulk of the empirical data available for trout streams do
not appear to support this theory. In fact, several studies have shown
that the growth rate of trout in streams has remained relatively constant
over long periods of time, even in heavily fished populations (e.g.,
Clark et al. 1980).
25
26
The timing of the growth decline seemed to rule out the hypothesis
of habitat degradation in the form of loss of cover and concomitant decline
in fish food production. Extensive stream improvement efforts to increase
food production and create holes and "hides" for large trout in the
Burton's Landing to Wakeley Bridge section of the mainstream have not
demonstrated any improvement in either trout stocks or fishing (Alexander
et al. 1979).
Finally, however, the sewage diversion hypothesis definitely
coincides with the decline in growth. The nature of the decline in growth,
based on scale reading, was found to correlate with the timing of sewage
diversion. The mainstream exhibited a greater decrease in growth
following effluent cutoff than the South Branch. This may have been
so because the South Branch received less sewage effluent than the
mainstream and thus the brown trout were not benefiting to the same
degree as fish on the mainstream. Gislasion (1971) has demonstrated
for the mainstream that the abundance of pollution-tolerant benthic
invertebrates, especially the amphipod Gammarus fasciatus and the
isopod Asellus militaris increased as the level of organic enrichment
increased.
Other chemical and biological data also support this hypothesis.
Records show a 70% decrease in nitrogen and a 10% decrease in phosphorus
in the mainstream after 1971 (Heckathorn 1977, Coopes 1974). The fall
population estimates of fishes made by Fisheries Division personnel at the
Wa Wa Sum station confirm that the total weight of trout decreased
substantially after 1971 (Fig. 11). On the South Branch similar decrease
in nitrogen and phosphorus levels occurred between 1973–74 (Ray Moore,
personal communication). Benthic macroinvertebrates collected by Reger
(1973) on the mainstream above and below the Grayling sewage treatment
27
13O - SEWAGE
DIVERSION
11O-
90|- “Oe

7
O
º
5
O
Fº
3
O
T5E5 E5 ETEETNTTF571 75757775.75 Eo
Year *
Figure 11. --Total weight of trout from the Wa Wa Sum station,
mainstream Au Sable River, for the years 1959–63 and 1971–80.
28
plant exhibited a greater number of aquatic invertebrates and a larger
biomass downstream from the plant. A significant contribution to the
increased standing crop came from the isopod Asellus militaris.
In Wisconsin, Brynildson and Mason (1975) found that production
of both brown and rainbow trout below a sewage plant effluent was
elevated where numerical density of trout was high. They suggested
that limited amounts of domestic sewage outfall could benefit trout growth.
A study conducted by Ellis and Gowing (1956) on Houghton Creek,
Michigan, demonstrated a significantly higher coefficient of condition (K)
for brown trout below than above a domestic sewage treatment plant.
They collected benthic macroinvertebrates and showed that two important
brown trout food organisms, the isopod Asellus militaris and the
amphipod Gammarus fasciatus, which are both pollution-tolerant species,
were greatest in abundance below the sewage treatment plant. They
concluded that the importance of these crustaceans as brown trout food
organisms was greatest in the latter part of the summer when important
aquatic insects like the Trichoptera and Ephemeroptera were often in
low supply due to seasonal oscillations in their populations. However,
throughout this time of year population levels of both the crustaceans
remained relatively high.
The Bow River of Alberta, Canada, has experienced an increase
in trout growth and numbers through enrichment (Martin Paetz, Fish
and Wildlife Division, Edmonton, Alberta, personal communication). At
Calgary, where the Bow receives a highly concentrated phosphate and
nitrate effluent, per kilometer harvest of trout, mainly the rainbow
(Salmo gairdneri), above the city averages only 90 whereas below
Calgary it is 433.
29
Phosphorus, especially Orthophosphorus, is often a significant
limiting factor in aquatic production. The growth of algae in both
natural and laboratory cultures exhibits a dependency on the amount
of available phosphorus (Wetzel 1975). Southworth (1974) found that
the standing crop of benthos increased following the addition of phosphate
fertilizer into the Pigeon River, Michigan, with algal production being
chiefly augmented. Coopes (1974) and the Michigan Water Resources
Commission (1966) have both shown that enrichment of the Au Sable
system below the communities of Grayling and Roscommon stimulated
growth of algae and aquatic macrophytes.
Increased diurnal dissolved oxygen fluctuations from heightened
levels of primary production possibly could affect the energy requirements
of trout. Brown trout under such a stress would have to shunt more
energy into body maintenance and less into growth processes. However,
there is no evidence of record that fish in the study sites ever
experienced oxygen deficiency as a result of increased primary
production or of the biochemical oxygen demand of the sewage plant
effluent. This is most directly the result of a uniform streamflow
throughout the year due to a remarkably constant groundwater recharge
(Coopes 1974). Also, the numerous riffle areas would also serve to
compensate for oxygen sags, by affecting physical reaeration of oxygen
depleted waters.
Levels of insect diversity and sensitive aquatic invertebrates
have increased on the mainstream, especially in the former zone of
pollution immediately downstream of the sewage treatment plant, and
are believed to be due to the elimination of effluent that formerly
entered the river at Grayling (Michael Quigley, National Oceanic and
Atmospheric Institute, Ann Arbor, Michigan, personal communication).
30
Heckathorn (1977) sampling below Grayling in 1975, found densities of
aquatic weed growth minimal when compared with densities prior to 1971.
In conclusion, the data presented in this study support the view
that the apparent decrease in brown trout growth in the mainstream and
South Branch of the Au Sable River is correlated with sewage diversion
to a land disposal system. The explanation appears to lie in the
reversion of the composition and level of the brown trout food supply to
its natural level after being inflated artificially by nutrients of sewage
Origin. Food has long been recognized as a major limiting factor in the
numbers of large fish.
LITERATURE CITED
Alexander, G. R. 1974a. Results of fishing and angler questionnaire
on the South Branch Au Sable River, Mason Tract, Crawford
County, Michigan, during the burrowing mayfly hatch, 1973.
Michigan Dep. Nat. Resources, Fish. Res. Rep. 1808. 24 p.
Alexander, G. R. 1974b. The consumption of trout by bird and
mammal predators on the North Branch Au Sable River.
Michigan Dep. Nat. Resources, Fish. Res. Rep. 1855. 26 p.
Alexander, G. R. 1976. Diet of vertebrate predators on trout waters
in north central lower Michigan. Michigan Dep. Nat. Resources,
Fish. Res. Rep. 1839. 18 p.
Alexander, G. R. , W. J. Buc, and G. T. Schnicke. 1979. Trends in
angling and trout populations in the main Au Sable and North
Branch Au Sable rivers from 1959-76. Michigan Dep. Nat.
Resources, Fish. Res. Rep. 1865. 59 p.
Alexander, G. R. , and H. Gowing. 1976. Relationships between diet
and growth in rainbow trout (Salmo gairdneri), brook trout
(Salvelinus fontinalis), and brown trout (Salmo trutta).
Michigan Dep. Nat. Resources, Fish. Res. Rep. 1841. 41 p.
Alexander, G. R. , and J. R. Ryckman. 1976. Trout production and
catch under normal and special angling regulations in the North
Branch of the Au Sable River, Michigan. Michigan Dep. Nat.
Resources, Fish. Res. Rep. 1840. 14 p.
Alexander, G. R., and D. S. Shetter. 1967. Fishing and boating on
portions of the Au Sable River in Michigan, 1960–63. Trans.
Am. Fish. Soc. 96(3): 257- 267.
Brynildson, O. M., and J. W. Mason. 1975. Influence of organic
pollution on the density and production of trout in a Wisconsin
stream. Wisconsin Dep. Nat. Resources, Tech. Bull. 81. 15 p.
Clark, R. D., G. R. Alexander, and H. Gowing. 1980. Mathematical
description of trout-stream fisheries. Trans. Am. Fish. Soc.
109 : 587- 602.
Clark, R. D., G. R. Alexander, G. T. Schnicke, and W. J. Buc. 1980.
Fly fishing on the Au Sable River mainstream. Michigan Dep. Nat.
Resources, Fish. Div. Pamphlet. 14 p.
31
32
Cooper, E. L. 1952. Growth of brook trout (Salvelinus fontinalis)
and brown trout (Salmo trutta) in the Pigeon River, Otsego
County, Michigan. Pap. Mich. Acad. Sci., Arts, Lett. 37:
151–162.
Coopes, G. F. 1974. Au Sable River watershed project biological
report (1971-73). Michigan Dep. Nat. Resources, Fish. Manage.
Rep. 7. 296 p.
Ellis, R. J., and H. Gowing. 1957. Relationship between food supply
and condition of wild brown trout in a Michigan stream.
Limnol. Oceanogr. 2(4): 299-308.
Favro, L. D., P. K. Kuo, and J. F. McDonald. 1979. Population-genetic
study of the effects of selective fishing on the growth rate of trout.
J. Fish. Res. Board Can. 36:552-561.
Gislasion, J. C. 1971. Species diversity of benthic macroinvertebrates
in three Michigan streams. MS thesis, Michigan State University,
East Lansing. 53 p.
Gowing, H., and G. R. Alexander. 1980. Population dynamics of trout
in some streams of the northern lower peninsula of Michigan.
Michigan Dep. Nat. Resources, Fish. Res. Rep. 1877. 45 p.
Heckathorn, C. 1977. Au Sable River study, July 21-22, 1975.
Michigan Dep. Nat. Resources, Env. Protection Bur., Publ.
4833–5135. 65 p.
Klein, L. 1966. River pollution and control. Butterworth and Company,
London, England. 484 p.
Latta, W. C., and R. F. Sharkey. 1964. Feeding behavior of the
Common Merganser in captivity. Michigan Dep. Conserv. ,
Res. Develop. Rep. 20. 17 p.
Lee, R. M. 1912. An investigation into the methods of growth
determination in fishes. Conseil Permanent International
pour l'Exploration de la Mer, Publ. de circonstance, No. 63.
Reger, S. J. 1973. Benthic macroinvertebrate diversity in three
differentially perturbed Michigan streams. MS thesis,
Michigan State University, East Lansing. 60 p.
Ricker, W. E. 1975. Computation and interpretation of biological
statistics of fish populations. Fish. Res. Board Can.
Bull. 191. 382 p.
Salyer, J. C. II, and K. F. Lagler. 1940. The food and habits of
the American Merganser during winter in Michigan, considered
in relation to fish management. J. Wildl. Manage. 4(2): 186-219.
33
Southworth, G. R. 1974. The effects of low level enrichment with
inorganic phosphate on the composition and structure of the
benthic invertebrate community of a trout stream. MS thesis,
University of Michigan, Ann Arbor. 67 p.
Wetzel, R. G. 1975. Limnology. W. B. Saunders Company,
Philadelphia, Pennsylvania. 743 p.
34
Appendix A. --Average back-calculated length (mm) of trout by age
group (sample size in parentheses) with 95% confidence limits and
attained significance of t test (cº = 0.05) for brown trout grouped
before and after sewage diversion from the mainstream, South Branch,
and North Branch Au Sable River.
95% COnfi- 95% COnfi- Attained
Location Age Before dence After dence significance
limits limits (c4 = 0.05)
1960-61 1973–77
Mainstream I 127 124, 129 106 103, 108 0.0000
(401) (524)
II 236 231, 240 203 200,206 0.0000
(178) (323)
III 308 301, 315 254 249,260 0.0000
(64) (102)
1960–61+73 1974–77
South Branch I 129 127,131 109 106, 112 0. 0000
(494) (168)
II 237 233,241 224 219, 230 0.0004
(211) (132)
III 328 320, 336 303 294,311 0.0002
(47) (60)
1960-61 1973–77
North Branch I 113 111,116 114 112, 116 0. 5961
(269) (532)
II 221 216, 226 232 228,235 0.0003
(128) (256)
III 305 295, 315 305 297, 313 0.9980
(29) (64)
35
Appendix B. --Average back-calculated increment of growth in length (mm)
of various age groups of brown trout from the mainstream Au Sable River.
Year Age at Number Increment
class capture of fish 0–I I-II II–III
1957 III 46 135 109 61
1958 II 62 132 92
III 18 144 107 65
1959 I 82 112
II 44 139 101
1960 I 95 128
1970 III 8 124 96 54
1971 II 38 117 88
III 8 115 86 56
1972 I 29 102
II 51 122 84
III 17 123 95 45
1973 I 46 104
II 45 118 91
III 42 108 92 52
1974 I 54 109
II 63 108 92
III 27 102 94 49
1975 I 70 89
II 32 108 87
1976 I 48 93
36
Appendix C. --Average back-calculated increment of growth in length (mm)
of various age groups of brown trout from the South Branch Au Sable River.
Year Age at Number Increment
Class capture of fish 0–I I-II II–III
1957 III 9 137 118 77
1958 II 49 123 94
III 24 131 106 87
1959 I 35 124
II 25 144 103
1960 I 59 132
1970 III 7 132 111 89
1971 II 12 127 117
III 7 135 110 86
1972 I 26 118
II 47 128 113
III 31 132 113 65
1973 I 79 129
II 68 134 95
III 16 113 108 69
1974 I 36 114
II 14 128 74
III 13 117 118 66
1975 I 41 100
II 21 108 107
1976 I 43 109
37
Appendix D. -- Average back-calculated increment of growth in length (mm)
of various age groups of brown trout from the North Branch Au Sable River.
Year Age at Number Increment
class capture of fish 0–I I-II II-III
1957 III 9 124 108 79
1958 II 63 122 103
III 20 120 99 82
1959 I 74 107
II 36 109 103
1960 I 67 110
1970 III 2 118 108 71
1971 II 12 125 111
III 4 115 115 72
1972 I 10 113
II 36 119 111
III 12 117 106 71
1973 I 59 113
II 31 113 105
III 24 117 118 70
1974 I 68 104
II 58 124 110
III 22 122 123 68
1975 I 72 106
II 55 121 111
1976 I 67 111
38
Appendix E. --Average back-calculated increment of growth in length (mm)
of trout by age group (sample size in parentheses) with 95% confidence
limits and attained significance of the t test (a^ = 0.05) for brown trout
grouped before and after sewage diversion from the mainstream, South
Branch, and North Branch Au Sable River.
95% COnfi- 95% confi- Attained
Location Age Before dence After dence significance
limits limits (~< = 0.05)
1960-61 1973–77
Mainstream I 127 124, 129 106 103, 108 0.0000
(401) (524)
II 100 97, 103 91 88, 93 0.0000
(178) (277)
III 62 58, 66 51 47, 54 0.0001
(64) (94)
1960–61+73 1974–77
South Branch I 129 127, 131 109 106, 112 0.0000
(494) (168)
II 107 104, 110 105 99, 110 0. 5252
(211) (48)
III 85 79, 92 68 61, 74 0.0003
(47) (29)
1960–61 1973–77
North Branch I 113 111,116 114 112, 116 0. 5961
(269) (532)
II 103 99, 106 112 109, 114 0.0000
(128) (256)
III 82 73, 90 70 66, 74 0.0062
(29) (64)
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