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RESPONSE OF ADULI WILD BROWN TROUT(Salmo trutta)
TO LIGHT AND VELOCITY UNDER.SIMULATED
BANK COVER IN STREAM CHANNELS

BY

James Curtis Gruber II

A THESIS
Suhmitted to
Michigan State University
'in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE

Department of Fisheries and Wildlife

1978

ABSTRACT
RESPONSE OF ADULT WILD BROWN TROUT(Salmo trutta)

TO LIGHT AND VELOCITY UNDER.SIMULATED
BANK COVER IN STREAM CHANNELS

By

James Curtis Gruber II

Experiments were conducted in controlled-flow stream channels
to measure daytime response of adult wild brown trout(Salmo trutta) to
light and current velocity stimuli associated with overhead coverts
attached to vertical channel walls. Groups of 25-30cm trout were
offered sets of cover types. Occupation of a specific cover type
represented a choice of known stimuli. Regardless of the light in-
cident to the channel, the trout most often(p 0.025) occupied the
coverts offering greatest darkness within the range of 0.0100-5.0000ft-c°
Water velocity at coverts did not significantly affect cover use from
0-149mm/sec. Cover use was greatest(p 0.005) at lowest relative
velocity within the range of 150-199mm/sec. This suggested that at
0-149mm/sec, the photic stimulus and response governed cover choice

and that at lSO-l99mm/sec, the rheostatic stimulus predominated.

ACKNOWLEDGEMENTS

I would like to extend my appreciation to Dr. Ray J. White,
committee chairman, for the Opportunity to undertake this study and
for his thorough review of the manuscript. Thanks, too, are extended
to committee members, Drs. Howard E. Johnson, Eugene W. Roelofs, and
John A. King for their review of the text.

I am also indebted to the Michigan Department of Natural
Resources, particularly the Grayling staff, for the use of their
labor, materials, and facilities, and to Dr. Jack R. Hoffert of the
Michigan State University Physiology Department and Earl Gillen of
the Physics Department, who loaned me light recording equipment.

The research and my assistantship were financed by funds
from the Michigan State University Agricultural Experiment Station
Project 1169, assigned to Dr. White.

Finally, a special note of appreciation to my family(

Ella & Jim, Scott, Eric, and Dixie) for their moral support throughout

the study and for their physical assistance in the field.

11

TABLE

LIST OF TABLES . . . . . . . .
LIST OF FIGURES . . . . . . .
INTRODUCTION . . . . . . . . .
METHODS AND.FACILITIES . . . .
Experiment 1 . . . .
Experiment 2 . . . .
Data Organization . .
Statistics . . . . .
RESULTS . . . . . . . . . . .

Light Measurements .

OF CONTENTS

Cover-preference, Light . . . . .

Cover-use o o o o o o

Cover-preference, Velocity . . .

Cover-preference, Light-Velocity Interaction

DISCUSSION AND CONCLUSIONS . .

A Suggestion for Further Research

LITERATURE CITED . . . . . . .

APPENDIX . . . . . . . . . . 0

iii

Page

iv

11
11
11
13
13
15
15
15
20
22
25
27

29

LIST OF TABLES

Table Page
1 Numbers of trout occupying coverts, the coverts ranked by
relative light intensity within the prevailing range of
SUb‘CovertintenSitYQQQQoooooooocooocoo 16
2 Numbers of trout occupying coverts, the coverts ranked by
relative light intensity within the prevailing range of
sub-covert illumination difference . . . . . . . . . . . . 17
3 Cover use and non-use by trout at each light range . . . . 18
4 Numbers of trout occupying coverts, the coverts ranked by
relative current velocity within the prevailing range of
V€10C1ties O O O O O O I O O O 0 O O O O O O O O I O O O O 19

5 Numbers of trout occupying the lowest, intermediate, and
highest light intensity coverts within each current-

veIOCity range 0 O O O O O O O O O O O O O O I O O O O O O 21
A1 Daily maximum and minimum.water temperatures for the East

Branch of the Au Sable River at the DNR Grayling Field

Office during the smer Of 1977 o o o o o o o o o o o o o 29

iv

LIST OF FIGURES

Figure Page
1 Raceway showing experimental setup. Not drawn to scale . . 6
2 Top view of cover device, showing locations (A,B,C) of

photocell placement during light measurements beneath it.
Center of cell surface was 3.6cm away from wall and Sam
above 3 treambed O O O O 0 O O O 0 O O O C O O O O O O I O O 8

3 Covert, showing point V where velocity was measured . . . . 10
4 T0p view of channel showing the shaded area at each

observation period. Not drawn to scale. Channel widths
and covert Sizes greater than aCtual o o o o o o o o o o o o 14

INTRODUCTION

A better understanding of the daytime response of wild brown
trout(Salmo trutta) to light intensities and current velocities as-
sociated with instream bank cover should enable closer adaptation of
stream management to the needs of trout. Cover for stream-dwelling
salmonids is defined as a position affording protection or security
by concealment from enemies. Position choice is a reactive1 response
to phototactic, rheotactic, and thigmotactic stimuli which enhances
survival. The behavior initiating a cover position choice is regulated
and modified by the specific requirements of the fish and conditions in
the stream environment (Lindroth 1955, Hartman 1963, Chapman 1966).
Preference increases as more environmental stimuli are associated with
a position, suggesting the response to cover is governed by a summation
of stimuli (Hartman 1963, Haines and Butler 1969). The social environ-
ment and food supply also affect position occupation.

A review of the literature on cover use reveals that negative
phototaxis and response to optimal velocities, relatively low vel-
ocities, are universal characteristics of cover choice for adult
stream-dwelling salmonids, regardless of other conditions(environment
type, seasonal differences, and causative factors of reaponse, hiding,

resting, and feeding). The response to overhead cover is primarily a

 

1
Lindroth (1955) distinguishes between reactive and spontaneous responses.

visual (Bassett 1978) response to the relative light intensity beneath
the cover (Stewart 1970). Stewart (1970) presented evidence that
response by fish to experimental fright-cover devices was strongly
related to light intensity under the structures. Use of overhead cover
by wild rainbow trout increased with increasing structure size, decreas-
ing structure height, and decreasing percentage of holes in the over-
head covert. DeVore (1975) demonstrated that wild adult brown trout
preferred opaque overhead cover placed 10cm above the stream bed

rather than 15 or 20cm above the bed. Butler and Hawthorne (1969)
showed that dominant brown, brook, and rainbow trout preferred the
shaded areas of a 91.5 x 91.5cm overhead covert versus the shaded

area provided by a 61 x 61cm cover. There was very little use of

shade provided by a 30.5 x 30.5cm overhead covert. Kwain and MacCrimmon
(1969) reported that hatchery rainbow trout at ages 10 and 24 months
exhibited a significant preference for the darkest chamber when given

a choice of five light intensities(10.000, 0.500, 0.010, 0.002, and
0.000ft-c). Haines and Butler (1969) showed that juvenile smallmouth
bass preferred black coverts over clear coverts.

Further evidence linking occupation of cover to the relative
illumination under the cover is provided by the reaponse of fish to
reflected light. Bassett (1978) demonstrated that wild brown trout
preferred overhead cover with dark rather than light bed beneath it.
Immature brown and rainbow trout showed an attraction to dark back-
grounds, selecting black rather than white (MacCrimmon and Kwain 1966,
Kwain and MacCrimmon 1967, Ritter and MacCrimmon 1973). However, the

selection preference for dark background gradually decreased with

habituation.

Trout appear to reapond tactually to features of submerged
objects. DeVore (1975) demonstrated that brown trout preferred overhead
coverts with clear plastic streamers fastened under them versus cover
without such a tactile feature. In comparison with overhead cover
possessing the primary feature of incident light diminution, overhead
cover attached or adjacent to the channel bank also provides a lateral
surface which could serve as a tactile stimulus.

Brown trout are the most overhead-cover-oriented among the
trouts (Butler and Hawthorne 1968). Ritter and MacCrimmon (1973)
reported that preference for dark background was higher for brown
trout than for rainbow trout. Overhead bank cover has also been
shown as the most important factor for determining density of brown
trout in stream pools (Lewis 1969).

Preference for current velocity has been studied with regard
to the resting microhabitat and focal point concepts, thus separating
the response to current velocity from the response to cover. Despite
this dissociation, all resting microhabitat and focal points showed
a high spatial correlation with cover (Wickham.1967, Baldes and Vincent
1969).

The objective of this study was to examine daytime responses

of adult wild brown trout(Salmo trutta) to light and to current velocity

 

stimuli associated with overhead bank cover in controlled-flow stream
channels during July and August.
Specific hypotheses concerning cover-use behavior were:

(1) Trout prefer the covert offering the lowest light

intensity relative to that beneath other simultaneously available coverts.
(2) Trout prefer the covert with the lowest water velocity

relative to that beneath other simultaneously available coverts.

METHODS AND FACILITIES

The experimental procedure was to place 2 trout of approx-
imately equal length and weight in a stream channel which contained
bankside coverts of different widths. The positioning of the trout
was then recorded 4 times daily for 5 days following a 5-day accli-
mation.

Raceways of the abandoned fish hatchery at the Grayling
Field Office of the Michigan Department of Natural Resources in
Grayling, Michigan served as controlled-flow stream channels. The
water supply for the channels came from the East Branch of the Au
Sable River. Daily maximum water temperatures, measured by a Taylor
disk-chart continuous-recording thermometer, ranged from 16.1 to
24.40 during the study period, July-August, 1977. On the same days,
the minimum water temperatures were 15.0 and 20.00, reapectively.
The minima for the period ranged from 11.7 to 16.1C (Appendix I).

The brown trout used for the study ranged from 25 to 39cm
in body length(TL). All fish were collected from a lOO-m section
of the East Branch of the Au Sable River within the hatchery grounds
by electrofishing with a 12-volt D.C., variable amp, backpack shocker.

Three raceways, 3.7m wide and 45.7m long, with gravel sub-
strate were used. Each raceway was partitioned with screens into 3
dimensionally identica1 12.9 x 3.7m sections (Figure 1), designated

A, B, and C, which served as the experimental enclosures. Each

.mfimom on 55.3 uoz .msuom Hmucoawummxm M5395 mmaoomm .H shaman

 

section contained 6 coverts, 3 per channel side(a set of coverts).
Coverts were made from l/8-inch masonite sheeting and attached to the
raceway wall at 3.225m intervals 10cm above and parallel to the
channel bed. Each covert was 50cm long, but covers were of various
selected widths. Data on light intensity and current velocity were
recorded under each covert.

Incident light intensity was measured in the atmoSphere
about 2m above the water surface of the channels with a selenium
photovoltaic cell(Weston, Model 856RB) held horizontally and housed
in a waterproof plexiglas casing. The illumination-current output
of the cell was recorded on a microvolt ammeter(Keithley, Model 150B)
and converted to foot candles.

Light intensities under the coverts were measured with the
same photocell also held horizontally 5cm above the substrate and with
the center of the cell 3.6cm from the channel wall. Three measurements
were recorded at intervals of 12.5cm along the length of each cover
(Figure 2). The average of the 3 measurements was used as the light
intensity datum under that covert.

Prior to each experiment, light intensity was recorded beneath
each covert at 0900-1000, 1100-1200, 1500-1600, and l700-1800h (EDT).
The correlation of sub-covert light intensity with incident light
intensity above the channels was recorded daily at l300h (EDT). A
correction factor, determined from comparison of various sub-covert
light intensities with their atmospheric equivalents, was used to com-
pensate for varing atmospheric light conditions encountered throughout

the day. The appearance of cloud cover on an otherwise sunny day

 

3.6cm
T (—12.5—®—12.5—®—125 @—lzmu—>
A B 0

20cm

 

 

 

AL

 

15 50cm . :1.

Figure 2. Top view of cover device, showing locations (A,B,C) of
photocell placement during light measurements beneath it. Center
~ of cell surface was 3.6cm away from wall and 5cm above streambed.

reduced illumination under coverts by an average of 30%, whereas sunlit
periods on cloudy days increased sub-covert light by an average of 333%.
In deriving the sub-covert light values for each observation period,

the 1300-h incident light recording was adjusted according to atmospheric
light conditions(overcast, haze, clear) which prevailed for the half
hour before the observation period.

A current velocity under each covert was recorded prior to
each experiment and was assumed to be constant throughout the experiment.
The current velocity was measured with a pygmy current meter at a point
5cm upstream from the covert, 5cm from the channel wall, and 5cm above
the bed (Figure 3). Constant stream discharge through the channel was
maintained by regulating the depth of the head pool above the raceways.

Water depth in each channel was 38.1cm.

Experiment 1

 

Three coverts which were 50cm long and of 3 widths(l4,l7,
and 20cm) were randomly assigned to positions 3.225m apart on each
channel side in 9 experimental channel sections. Two fish of ap-
proximately equal size and weight were placed in each section. After
a 5-day acclimation, the position of each fish was recorded at 0900-
1000, 1100-1200, 1500-1600, and 1700-l800h (EDT) for 5 days. During
each observation hour, each covert was searched once.

Observation of fish under cover was aided by shining the
light from a 12-volt spotlight into a mirror affixed to the end of
a 2-m.pole. From a channel bank position, the observer, by placing
the mirror underwater beside the covert, could see any fish that might

be under the covert. Positioned initially at the downstream end of the

lO

 

a“
as“

Figure 3. Covert, showing point V where velocity was measured.

11

covert, the mirror was slowly moved forward until a fish was detected.
Trout seldom showed that the light and mirror disturbed them, unless
the beam of light was directed at the head region. Use of cover was
defined as occupation of a position where any part of the trout's

body was under cover (Bassett 1978). Fish which moved into a covert
when inadvertently frightened by the observer were excluded from the
data, along with subsequent observations regarding that covert through-
out the same day. Fish frightened out from a covert were recorded as

having the predisturbed position.

Experiment 2
Three 50cm-long coverts of another range of widths(20,22,
and 24cm) were randomly attached as before, and the previous experi-

mental proceedure repeated.

Data Organization

Light intensity under cover was arbitrarily divided into
3 ranges, 0.0100-0.0999, 0.1000-0.9999, and l.0000-5.0000ft-c. Four
ranges of differences in sub-covert illumination between the lowest
and highest intensity within a set of coverts were established: 0.0010-
0.0499, 0.0500-0.0999, 0.1000-0.4999, and O.5000-3.0000ft-c. Current
velocity data were also partitioned into 3 ranges, 0-99, 100-149,
and 150-199mm/sec. Sets of coverts were placed within the appropriate

light and velocity ranges.

Statistics

 

Analysis to determine cover preference was performed using

relative light intensities and current velocities within the various

12

ranges. Cover use was tested for relationship to light intensity and
current velocity separately. Frequency data were initially tested

for randomness, then analysed using contingency tables employing the
Chi-square test statistic(p$ 0.05). A 2-sample t-test was used to test
the mean difference in daily cover use between experiments(p.<. 0.05).
Cover preference was tested by the Scheffe proceedure for analysing
multiple comparisons among means with designed contrasts. Mean cover-
use per day for each light intensity, low, medium, and high, was sub-
stituted into the apprOpriate contrasts to detect significant preference

at the level of p50.025 (Gill, in press).

RESULTS

‘Light'Measurements

Light intensity under the coverts was a function of the
width of the covert and the magnitude and angle of incident light.
During any given observation period, the small, medium, and large
coverts of any channel side, had the highest, medium, and lowest
light intensities, respectively, regardless of absolute light intensity
above the channel. Thus, the relative intensities of sub-covert il-
luminations within a set of covers was constant.' However, the dif-
ference in absolute light intensity between the smallest and largest
‘cover increased as incident light intensity increased.

Light intensities ranged from 0.0125ft-c under the large
covert(24cm) at the east side of the channel during the first obser-
vation time(O900-lOOOh) on a heavily overcast day to 4.8147ft-c under
the small covert(l4cm) at the east side of the channel during the last
observation time(1700-l800h) on a sunny day. Coverts on the east side
of the channel were shaded by the channel wall throughout the first
2 observation periods of each day, while covers on the west side were
shaded by the wall throughout the final 2 observation periods of each
day (Figure 4). During any given observation period, no overlap
between ranges of light intensities under sets of coverts on Opposite
sides of the channel occured, although overlap of light intensities
under different sized coverts between observation periods, days, and

13

l4

 

   

 

 

 

Observation 1 Observation 2
0900 - 1000 h 1100 - 1200 h
3
II E
I]
I]
N
III I:
II I:
Observation 3 Observation 4
1700-1800 h
S

 

   

 

Figure 4. Top view of channel showing the shaded area at each
observation period. Not drawn to scale. Channel widths and
covert sizes greater than actual.

15
experiments occured frequently.

Cover:preference,7Light

Regardless of the light incident to the channel, the trout
most often(p$0.025) occupied the coverts offering greatest darkness
within the set (Table 1). Variation in magnitude of difference between
least and greatest sub-covert illumination did not significantly
affect positioning by fish (Table 2). There was no significant dif-
ference in the cover-preference response between ranges of light in-

tensities, ranges of illumination differences, and experiments.

Cover-use

Cover use was significantly greater at the lowest light.
range, 0.0lOO-0.0999ft-c, and least at the highest light range, 1.0000-
5.0000ft-c (Table 3). There was no significant difference in cover-
use in the various light ranges between experiments. However, cover-
use was significantly greater(p$0.05) in experiment 2(847.) as Opposed
to experiment 1(7SZ) (Table 3). ‘Mean cover use for both experiments

was 80%.

Cover:preferenceL Velocity

Water velocity at coverts did not significantly affect cover
use within the 2 lowest velocity ranges, 0-99 and lOO-l49mm/sec,
although cover use was greatest(p $0.005) at lowest relative velocity

within the highest velocity range, lSO-l99mm/sec (Table 4).

l6

 

 

 

 

 

 

 

Table 1. Numbers of trout occupying coverts, the coverts ranked by
relative light intensity within the prevailing range of sub-covert
intensity.

Range of sub-covert light intensity (ft-g)

Covert

width(cm) Relative Total

within light 0.0100 0.1000 1.0000 0.0100

set *** intensity -0.0999 -0.9999 -5.0000 -5.0000

Experiment 1
l4 lightest 10 (24%) 25 (19%) 26 (27%) 61 (23%)
17 medium 16 (39%) 53 (39%) 32 (43%) 101 (37%)
20 darkest 15 (37%) 57 (42%) 37 (39%) 109 (40%)

Experiment 2
20 lightest 19 (20%) 42 (25%) 10 (23%) 71 (23%)
22 medium 32 (34%) 49 (29%) 9 (21%) 90 (30%)
24 darkest 42 (45%) 77 (46%) 24 (56%) 143 (47%)

Combined Results
** ** * **
lightest 29 (22%) 67 (22%) 36 (26%) 132 (23%)
medium. 48 (36%) 102 (34%) 41 (30%) 191 (33%)
darkest 57 (42%) 134 (44%) 61 (44%) 252 (44%)
***

A set of coverts consists of 3 coverts attached to one side of a
channel section.

*1:

Within these columns occupancy ratios were significantly different

from each other at p30.025.

*

Occupancy ratio differs at p$0.0S.

17

Table 2. Numbers of troug occupying coverts, the coverts ranked by
relative light intensity within the prevailing range of sub-covert

illumination difference.

 

Within-set difference in light intensity between

darkest and lightest sub-covert shadowg(ft-c)

 

 

 

 

 

 

Covert
width(cm) Relative Total
within light 0.0010 0.0500 0.1000 0.5000 0.0010
set ** intensity -0.0499 -0.0999 -0.4999 -3.0000 -3.0000
Experiment 1
l4 lightest 9 (24%) 5 (24%) 25 (19%) 22 (26%) 61 (23%)
17 medium 14 (37%) 5 (24%) 54 (42%) 28 (34%) 101 (37%)
20 darkest 15 (39%) 11 (52%) 50 (39%) 33 (40%) 109 (40%)
Experiment 2
20 lightest 20 (18%) 19 (24%) 23 (30%) 9 (25%) 71 (23%)
22 medium 41 (37%) 23 (30%) 18 (23%) 8 (22%) 90 (30%)
24 darkest 51 (45%) 36 (46%) 37 (47%) 19 (53%) 143 (47%)
Combined Results
* s * *
lightest 29 (19%) 24 (24%) 48 (23%) 31 (26%) 132 (23%)
medium 55 (37%) 28 (28%) 72 (35%) 36 (30%) 191 (33%)
darkest 66 (44%) 47 (48%) 87 (42%) 52 (44%) 252 (44%)
**

ch

*

A set of coverts consists of 3 coverts attached to one side of a

annel section.

There were no significant differences in occupancy ratios within this
column between sub-covert light-difference ranges.

18

Table 3. Cover use and non-use by trout at each light range.

 

Sub-covert light intensity (ft-c)

 

 

 

 

 

 

 

 

 

 

Total
0.0100 0.1000 1.0000 0.0100
Sets of -0.0999 -0.9999 -5.0000 -5.0000
covert
widths(cm) Use Non-use Use Non-use Use Non-use Use Non-use
Experiment 1
*
(14,17,20). 43 13 136 30 92 46 271 89
(75%). (25%)
Experiment 2
*
(20,22,24) 91 5 167 41 46 10 304 56
(84%) (16%)
Combined Results
** ** **
134 18 303 71 138 56 575 145
(88%) (12%) (81%) (19%) (71%) (29%) (80%) (20%)
**

Differences in occupancy ratios between light-ranges were significant
at p50.001.

*

Differences in occupancy ratios between experiments were significant
at p£00050

19

Table 4. Numbers of trout occupying coverts, the coverts ranked by
relative current velocity within the prevailing range of velocities.

 

Sub-covert water velocity (mm/sec)

 

 

 

 

 

 

Sets of Relative
covert water Total
widths(cm) velocity 0 - 99 100 - 149 150 - 199 0 - 199
Experiment 1
lowest 34 (47%) 55 (31%) 7 (33%) 96 (35%)
(14,17,20) medium 20 (28%) 54 (30%) 8 (38%) 82 (31%)
highest 18 (25%) 69 (39%) 6 (29%) 93 (34%)
Experiment 2
lowest 60 (30%) 20 (34%) 28 (65%) 108 (36%)
(20,22,24) medium 77 (38%) 14 (24%) 5 (12%) 96 (31%)
highest 66 (32%) 24 (42%) 10 (23%) 100 (33%)
Combined Results
** ** * **
lowest 94 (34%) 75 (32%) 35 (55%) 204 (35%)
medium 97 (35%) 68 (29%) 13 (20%) 178 (31%)
highest 84 (31%) 93 (39%) 16 (25%) 193 (34%)
**

Within these columns occupancy ratios were not significantly different.

*
Difference in occupancy ratio was significant at p50.005.

20

Cover-preferenceLVLight-Velocity Interaction

Preference for the position offering lowest relative current
velocity occured at the upper velocity range, 150-199mm/sec. However,
preference between the coverts having different light intensity within

this velocity range was random (Table 5).

Table 5.

21

Numbers of trout occupying the lowest, intermediate, and

highest light intensity coverts within each current-velocity range.

 

Water velocity (mm/sec)

 

 

 

 

 

 

Covert
width(cm) Relative
within light Total
set *** intensity 0 - 99 100 - 149 150 - 199 0 - 199
Experiment 1
l4 lightest 19 (27%) 34 (19%) 7 (33%) 6O (22%)
17 ‘medium 24 (33%) 64 (36%) 6 (29%) 94 (35%)
20 darkest 29 (40%) 8O (45%) 8 (38%) 117 (43%)
Experiment 2
20 lightest 49 (24%) 12 (21%) 13 (30%) 74 (24%)
22 medium 60 (30%) 21 (36%) 14 (33%) 95 (31%)
24 darkest 94 (46%) 25 (43%) 16 (37%) 135 (45%)
Combined Results
* * ** *
lightest 68 (25%) 46 (20%) 18 (28%) 132 (23%)
medium 84 (30%) 85 (36%) 22 (34%) 191 (33%)
darkest 123 (45%) 105 (44%) 24 (38%) 252 (44%)
***

A set of coverts consists

channel section.

*‘k

of 3 coverts

attached to one side of a

Occupancy ratio was not significantly different.

'k

Water velocity range occupancy ratios within these columns for
relative cover light intensities were significantly different from each

other at p.<_ 0.025.

DISCUSSION AND CONCLUSIONS

This study demonstrates that adult brown trout in a "semi-
natural" environment respond preferentially to overhead bank cover
offering the lowest light intensity within the range of 0.0100-
5.0000ft-c. Because this response was consistent in degree at all
light intensities examined, I conclude that light levels never oc-
curred which were lower than the trout's light threshold for cover
seeking behavior. Under laboratory conditions, a light preference
threshold between 0.0005 and 0.0001ft-c has been established for 2-
year-old rainbow trout, based on their choice of 2 chambers with
different illuminations (Kwain and MacCrimmon 1969). However, the
preference threshold for response to sub-covert light may be dif-
ferent than the illumination response threshold in absence of cover.

That the response to light was similar for coverts of both
experiments suggests that the response to the overhead cover was
primarily a visual response to darkness beneath the cover, not a
lateral line reSponse to the mass of the overhead cover.

That cover use was greatest at the lowest sub-covert light
range and least at the highest sub-covert light range is evidence
that position choice was a response to photic stimuli (Table 3).
Light intensities within the lowest range occured only on cloudy days.
Cover-use on overcast days was greater(84.5%) than on sunny days

(75.8%). This may be indicative of a characteristic of cover-seeking

‘22

23

behavior by trout. On clear days, slanting shadows from stream banks
may, by virtue of their great contrast with adjacent sunlit stream
bed, afford protective concealment from viewing by predators above the
channel. On overcast(but still bright) days when lighting is diffuse
and reaches more equally into most parts of the stream, viewing by
predators may not be hampered or distracted by contrasting intensities,
and the trout may have to seek smaller, darker niches, if they are to
have concealment. The greater cover-use in experiment 2 was an arti-
fact of the disprOportionate number of observations in the lowest
light range.

The interaction of light and velocity occurred only within
the highest velocity range, 150-199mm/sec. Thus, I conclude that
the rheostatic stimulus and the response to it afforded the trout a
behavior for minimum energy expenditure and predominated as the major
factor in cover choice at current velocities of 150-199mm/sec(0.49-
0.65ft/sec).

Haines and Butler (1969) reported that areas of low current
velocity are important in shelter use by juvenile smallmouth bass
only as long as an area of darkness and visual reference are present.
The fish positioned themselves beneath black overhead plates which
were enclosed on 2 sides in a mean channel water velocity of 28cm/sec
in strong preference to clear overhead plates enclosed on 3 sides
and in mean velocity of lZcm/sec. This suggested that at these vel-
ocities, the reaponse to photic stimuli predominated over the response
to rheostatic stimuli. Use of higher water velocities might have

elicited a different reSponse. Furthermore, at 12cm/sec the fish

24

significantly preferred a black overhead plate with 3 sides rather

than a slanted black overhead plate with relative higher light inten-
sity and lower current velocity beneath. When the current was increased
to 28cm/sec, the difference in occupancy became nonsignificant.

These findings are in agreement with the present study and indicate

that at low water velocity, the photic stimulus and response govern
overhead cover choice and that at high water velocity the rheostatic
stimulus predominates.

One might suspect that the velocity interaction with light
stimuli may have confounded the results. However, since only 11%
of the observations occurred within the light-velocity interaction
range(150-199mm/sec), the results of this study represent a true
response to light level beneath cover.

Cover use, although random in the velocity range of 100-149
mm/sec, was greatest for the cover possessing the highest current
velocity (Table 4). Thus, there appears to be a preferred range of
sub-covert velocities somewhere between 125 and l75mm/sec. This
study supports the findings of Baldes and Vincent (1969) who reported
that wild brown trout of about 20cm preferred resting microhabitat
water velocities within a range of 122 to 213mm/sec where no overhead
cover was involved.

The 2 trout in a section usually occupied opposite sides of
the channel. When both fish were using cover, those covers were on
Opposite sides of the channel 75% of the time, and only thrice in 224
observations were the 2 fish under the same cover. These observations

support the findings of Bassett (1978), who demonstrated that

25

interactive behavior(social interaction) influenced cover choice by
subordinate avoidance of dominant trout. Despite this avoidance re-
action, if one considers the observations in which both trout occupied
cover on the same side of the channel, they did so on the darker side
of the channel 68% of the time. This preference for the darker side
was significant(p$ 0.01). There was no significant deviation from
this preference on cloudy days. Thus, the disadvantage of having

2 trout per section was that the variability and degree of social
interaction with its associated influence on cover reaponse were
unknown.

To examine the possible influence of body size on reaponse
to cover(light and velocity preference and cover-use), cover occupancy
was analysed for 25-29cm trout and 30-39cm trout. Cover occupancy
for 25-29cm fish accounted for 65% of the observations. There were
no differences between the 2 size ranges, thus I conclude that body
size had no influence on response to cover within the range of 25-
39cm. I also examined the possible influence of time of day on response
to cover, light and velocity preference and cover-use. There were no
differences between observation times, thus I concluded that time of
day had no influence on response to cover between 0900-1800h EDT.

Due to the constantly changing light under the covers, the
problem of Operant reinforcement or conditioning for a particular

site was not a factor determining response to cover.

A Suggestion for Further Research
There is a definite management need for cover-habitat

quantification for stream-dwelling salmonids. This could be achieved

26

by the develOpment of a trout cover rating system based on light and
velocity stimuli and their associated reSponse to cover.

To develOpe such a rating system, further knowledge about
the interaction of light and velocity stimuli as they relate to cover
occupation, must be.acquired. One might, by playing light and velocity
stimuli against one another, determine a cover index function. The
application of this cover index could be used to predict trout pOpula-

tions(biological variables), and thus to manage accordingly.

LITERATURE CITED

LITERATURE CITED

Baldes, R.J. and R.E. Vincent 1969. Physical parameters of macro-
habitats occupied by brown trout in an experimental flume.
Trans. Am. Fish. Soc. 98:230-238.

Bassett, C.E. 1978. Effect of cover and social structure on position
choice by brown trout(Salmo trutta) in a stream. M.S.
Thesis. Michigan State Univ., E. Lansing. 181 pp.

 

Butler, R.L. and V.M. Hawthorne 1968. The reactions of dominant
trout to changes in overhead artificial cover. Trans. Am.
Fish. Soc. 97:37-41.

Chapman, D.W. 1966. Food and Space as regulators of salmonid pOp-
ulations in streams. Am. Naturalist 100:345-357.

DeVore, P.W. 1975. Daytime behavioral responses of adult brown
trout(Salmo trutta) in stream channels. 'M.S. Thesis.
Michigan State Univ., E. Lansing. 38 pp.

 

Gill, J.L. Design and analysis of experiments in the animal sciences.
Volume 1. Iowa State Univ. Press, Ames, Iowa. (In press).

Haines, T.A. and R.L. Butler 1969. Responses of yearling small-
mouth bass(Micropterus dolomieui) to artificial shelter in
a stream aquarium. J. Fish. Res. Bd. Can. 26(1):21-31.

Hartman, G.F. 1963. Observations on behavior of juvenile brown trout
in a stream aquarium during winter and spring. J. Fish.
Res. Bd. Can. 20:769-787.

Kwain, W. and H.Rw MacCrimmon 1967. The behavior and bottom color
selection of the rainbow trout, Salmo ggirdneri, Richardson,
exposed to different light intensities. Anim. Behav. 15:
75-78 0

. 1969. Further Observations on the response of rainbow
trout, Salmo ggirdneri, to overhead light. J. Fish. Res.
Bd. Can. 26(12):3233-3236.

Lewis, S.L. 1969. Physical factors influencing fish pOpulations in
pools of a trout stream. Trans. Am. Fish. Soc. 98:14-19.

27

28

Lindroth, A. 1955. Distribution, territorial behavior, and movements
of sea trout fry in the river Indalsalven. Rept. Inst.
Freshwater Res. Drottingholm, 36:104-119.

MacCrimmon, H.R. and W. Kwain 1966. Use of overhead cover by rain-
bow trout exposed to series of light intensities. J. Fish.

Ritter, J.A. and H.R. MacCrimmon 1973. Effects of illumination on
behavior of wild brown trout(Salmo trutta) and rainbow trout
(Salmo ggirdneri) exposed to black and white backgrounds.
J. Fish. Res. Bd. Can. 30:1875-1880.

Stewart, P.A. 1970. Physical factors influencing trout density in
a small stream. Ph.D. Dissertation. Colorado State Univ.,
Fort Collins. 88 pp.

Wickham, M.G. 1967. Physical microhabitat of trout. M.S. Thesis.
Colorado State Univ., Fort Collins. 42 pp.

APPENDIX

29

Table A1. Daily maximum and minimum water temperatures for the East
Branch of the Au Sable River at the DNR Grayling Field Office during
the summer of 1977.

 

Temperature C Temperature C

 

 

Date High Low Date High Low
7/11 20.6 14.4 7/28 16.7 13.9
12 20.6 16.7 ' 29 17.2 15.0
13 21.2 16.1 30 19.4 15.0
14 19.4 14.4 31 20.0 15.6
15 22.2 17.2 8/ 1 19.4 15.0
16 21.7 16.7 2 17.2 15.0
17 23.3 18.3 3 16.7 12.2
18 20.6 18.3 4 18.3 14.4
19 23.3 16.7 5 18.9 15.6
20 24.4 20.0 6 19.4 16.1
21 22.8 19.4 7 17.8 16.1
22 21.1 15.6 8 18.9 15.0
23 20.6 16.1 9 16.7 13.3
24 18.3 15.6 10 17.8 14.4
25 18.9 15.0 11 16.1 15.0
26 18.3 12.8 12 17.8 11.7
27 17.8 12.2 13 16.1 13.9
14 16.7 12.8

 

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