AN ABSTRACT OF THE THESIS OF
Craig A. Tinus for the degree of Master of Science in Fisheries Science presented on
March 2, 1999. Title: Territorial Behavior in Juvenile Steelhead Trout (Oncorhynchus
mykiss): How Redside Shiner (Richardsonius balteatus) Influence Intraspecific
Interactions.
Redacted for Privacy
Abstract approved:
Gordon H. Reeves
Juvenile steelhead are known to associate with shiner groups, though they also
compete for food. Steelhead form dominance hierarchies within cohorts and aggressively
defend feeding territories against all other fish. This study focused on the differential
effect of shiner competition on steelhead of different social standing.
Survival of subordinate juvenile steelhead was significantly enhanced by the
presence of redside shiner under laboratory conditions. A factorial experiment in 80 L
tanks examined the relative effects of 0, 3, and 9 shiner at 15° and 20° C on the growth
and survival of 3 juvenile steelhead per tank. No temperature effect was detected and
there was no significant difference in steelhead growth though statistical power was low
(n=5). The largest steelhead did not die in any treatment and no steelhead died in the
presence of 9 shiner. In treatments where no shiner were present mortality in the smallest
steelhead was 80% (p-value < 0.01). Aggressive interactions between steelhead allowed
pathogens to colonize breaks in the skin of stressed fish resulting in death.
In 6800 L recirculating stream channels with natural substrate, 10 steelhead were
held either alone or with 20 shiner at 15° C. No steelhead died and their growth was not
significantly different between treatments, but, in the absence of shiner fin damage was 16
times greater (p-value < 0.01) in the smallest three steelhead. If a shiner group was
present the smallest steelhead appeared to shoal with shiner to avoid attack by dominant
steelhead.
Territorial Behavior in Juvenile Steelhead Trout (Oncorhynchus mykiss): How Redside
Shiner (Richardsonius bahewus) Influence Intraspecific Interactions.
by
Craig A. Tinus
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Completed March 2, 1999
Commencement June 1999
Master of Science thesis of Craig A. Tinus presented on March 2, 1999
APPROVED:
Redacted for Privacy
Y
M or Professor, representing Fisheries Science
Redacted for Privacy
ead of Department of Fish
s and Wildlife
Redacted for Privacy
Dean of Graduat
chool
I understand that my thesis will become part of the permanent collection of Oregon State
University libraries. My signature below authorizes release of my thesis to any reader
upon request.
Redacted for Privacy
Craig A. Tinus, Author
ACKNOWLEDGMENTS
I thank the members of my committee: Gordon Reeves, Mark Hixon, Hiram Li, and
Antonio Amandi, each of whom contributed significantly to the successful completion of
this project. In addition I thank Craig Banner, Richard Holt, Robert Chitwood, Donald
Stevens, Paul Murtaugh, Christian Zimmerman, Kitty Grizwold and Carol Seals. I also
thank Mark Carr who is, for better or worse, the mentor who got me into this business.
Finally, I thank Gay Lynn Timken, and Linda Carroll for helping to keep my head screwed
on.
TABLE OF CONTENTS
Page
INTRODUCTION
1
METHODS
6
Laboratory Tank Experiment
Laboratory Stream-Channel Experiment
RESULTS
8
11
15
Laboratory Tank Experiment
15
Steelhead Survival
Steelhead Growth
15
Stream-Channel Experiments
Growth
Fin Damage
19
19
19
19
DISCUSSION
25
LITERATURE CITED
34
APPENDIX
39
LIST OF FIGURES
Figure
1
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Page
Location of collection sites for both the juvenile steelhead and redside
shiner
7
Set-up of individual 80L tanks used in laboratory
experiments
9
Set-up of artificial stream channels
12
Proportional survival of the largest steelhead
in each treatment
16
Proportional survival of the intermediate sized
steelhead in each treatment
17
Proportional survival of the smallest
steelhead in each treatment
18
Proportional change in wet weight of the largest individual steelhead
in each treatment
20
Proportional change in wet weight of the intermediate size
steelhead
21
Proportional change in wet weight of the smallest individual steelhead
22
Box plots representing steelhead growth in stream-channel
experiments
23
Mean score and SE of fin tears in the smallest three steelhead in each
of two treatments
24
Model illustrating that dominant juvenile steelhead have a strong
negative effect on subordinate steelhead in the absence of other
competitors for food
30
Model illustrating how redside shiner modify the strength and direction
of the interaction between dominant and subordinate steelhead
30
LIST OF APPENDIX TABLES
TABLE
Page
Data indicating the time to death and the general identification of
the most common pathogens cultured on cytophaga agar from those
steelhed which died in the laboratory tank experiments
40
B.
Steelhead survival data for all laboratory tank trials
41
C.
Growth data measured as the proportional change in wet weight of the
smallest three steelhead in laboratory stream channel experiments
42
Accumulated fin damage data of the smallest three steelhead in the
stream channel experiments
43
A.
D.
Territorial Behavior in Juvenile Steelhead Trout (Oncorhynchus mykiss): How
Redside Shiner (Richardsonius balteatus) Affect Intraspecific Interactions
INTRODUCTION
Aggressive interactions between individuals may be considered a competitive
interaction (Krebs and Davies, 1993). In general, competition occurs when a number
of organisms of the same or different species utilize common resources which are in
short supply. However, competition models are not always dependent on resource
supply. If the lack of a certain resource prevents an increase in size or number of a
given organism it is considered to be limiting. When one group or individual removes
a common resource from the available supply it is termed exploitation competition.
For competition to exist in this case the resource must be in short supply. When one
group or individual harms another in the process of attaining resources it is termed
interference competition. When interference competition is modeled there is no
assumption that resources are limiting (Krebs, 1994). In this paper the important
distinction is that shiner do not physically harm competitors when feeding (exploitation
competition). Steelhead (Oncorhynchus mykiss) can and do cause physical harm when
defending territories and competing for food (interference competition) Abbott and
Dill, 1989. Because exploitation competition is difficult to observe directly in nature,
the effects of competitive interactions are often inferred from indirect evidence such as
a change in distribution of one group of fish when another group is present, or from
direct observation of interference competition such as territorial exclusion. The real
effects of competition are difficult to determine because, presumably, the patterns
2
observed in nature over short time scales are the result of competitive interactions
which have achieved stability (Connell 1980)
Competition in freshwater stream fish may result in habitat displacement of a
poor competitor and has thus been shown to influence the structure of some fish
assemblages (Nilsson and Northcote 1981, Magnan and FitzGerald 1982). To an
individual fish competition may come in two forms: interspecifc, meaning competition
from other species, and intraspecific, competition from members of the same species.
Intraspecific competition can be further divided into competition from different age
groups, e.g. older, larger individuals from an earlier cohort, or from individuals from
the same cohort, which may or may not be siblings. Competition within a cohort may
be especially intense when physically similar individuals require similar limited
resources (Keast, 1977).
It is often assumed that competition significantly influences the distribution and
density of fish such as juvenile salmonids (Crisp, 1993). Many researchers have
examined interspecific competition for food and habitat by comparing growth and
survival in single and mixed species groups (see Matthews 1998 for a review). Few
studies have examined simultaneously the effects of intra- and interspecific competition
(Fausch 1988), or have included environmental factors. Stream dwelling juvenile
steelhead provide a good subject for such a study. Juvenile steelhead trout are
aggressively territorial (Hutchison and Iwata 1997, Abbott and Dill 1985). As density
of juvenile steelhead increases, subordinates in the dominance hierarchy are increasingly
unable to defend feeding territories (Fausch and White 1986, McFadden 1969,
Chapman 1962). The fate of these subordinate individuals is poorly understood.
Several researchers examining juvenile salmonids have determined that subordinates
unable to establish feeding territories are more likely to die from starvation, disease, or
predation than territory holders (Crisp 1993, Li and Brocksen 1977). Alternatively,
Metcalfe (1988) suggests some subordinate individuals have higher than expected
survival because they do not expend energy in territorial defense.
Individuals may emigrate or modify behavior to improve their ability to attain
resources. Fish from many taxa have been shown to reduce the negative effects of
competition by modifying behavioral responses (Ryer and 011a 1991, Endler 1988,
Robertson et al. 1976). Ayu (Plecoglossus attivelis), a stream dwelling salmonid-like
osmerid, switch from territorial defense to shoaling depending on several environmental
conditions including food abundance (Kawanabe 1969).
Typically, within-group competition (single species) has been thought of in
terms of its negative effects on subordinate individuals. There are also well
documented advantages to individuals living in groups and they may be equated to
advantages enjoyed by individuals in mixed species groups (Shaw 1978, Pitcher 1986).
Potential benefits to individuals in mixed species groups include enhanced feeding
success and predator avoidance (Pitcher 1986). Matthews (1998) points out that (1) in
streams, fish are not evenly distributed in the available area and (2) mixed-species
groups are common. A patchy distribution pattern suggests a more complex array of
costs and benefits to behavioral interactions than those suggested by simple
competition models. Interactions between two organisms, whether of the same or
4
different species, have been generally considered in isolation. This approach may yield
spurious results by exaggerating the effects of some interactions which would in nature
be balanced or mitigated by other interactions, e.g. associating with food finders or
omnivory.
When three or more organisms are interacting directly the potential exists for a
class of interactions termed "indirect effects" (see Menge 1995 for a review of indirect
effects). For example, when two species compete for a common food resource and a
third species preys preferentially on one of the competitors there is an indirect positive
effect on the competitor not preyed upon. The adage "the enemy of my enemy is my
friend" applies in such cases. In an examination of field data on birds, ants, and
zooplankton, Stone and Roberts (1991) suggest many potentially "competitive"
interactions which have been assumed to be negative may in fact produce many positive
indirect effects. When they modeled community interaction webs, 20-40% of
interspecific interactions needed to be positive for the system to be stable. Their
models of community interaction webs are reportedly in good agreement with empirical
data.
Juvenile steelhead trout are sometimes found in close association with redside
shiner (Richardsonius balteatus) shoals (Reeves 1984, Li personal communication,
personal observation). Reeves et al. (1987) found that shiner and juvenile steelhead
compete directly for food and space but that the outcome of this competition was
moderated by water temperature. Steelhead were competitively dominant (displaced
shiner from a focal point) at 12-15° C and shiner were dominant at 19-22° C.
5
Steelhead which were associated with shiner shoals were not defending territories while
other steelhead in the same habitat were. The implications of this observed distribution
pattern are that there may be multiple strategies for gaining food resources and that the
presence of a shoaling species might encourage subordinates to switch from territory
acquisition to shoaling behavior. The objective of this study was to test the effects of
shiner density and temperature on the survival and growth of subordinate juvenile
steelhead and to determine the mechanisms if significant differences were found.
6
METHODS
Redside shiner and juvenile steelhead were captured by electroshocking in
Steamboat Creek in the Umpqua River basin (Fig. 1). Fish were collected in
September and October of 1995 and 1996 in stream sections where they were
sympatric. Shiner and steelhead were sorted in the field and placed in dark 150-L tubs
lined with black plastic bags. Transport water was taken directly from the stream and
NaCI was added to produce a 1-3% salt solution. This helped to reduce osmotic stress
during transport. Ice placed on top of the bags kept transport water below 15°C.
Battery-operated aerators kept the transport water well oxygenated. Transport time
was always less than five hours and no fish died during transport.
Fish were initially placed in 600-L 0.91-m diameter x 0.91-m deep circular
tanks in the Oregon Cooperative Fisheries Research Unit hatchery, Oregon State
University, Corvallis, OR. Well water at 12.8° C flowed through each tank at 600-L
hour-1 Each tank had six 0.3-m sections of 0.8-m sewer pipe cut into half circles and
placed on the tank bottom to provide both overhead cover and visual isolation to
reduce stress. Steelhead densities ranged from 25-50 fish per tank depending on fish
size. Shiner were held at densities of up to 250 fish per tank. Fish were treated against
external parasites with malachite green and against bacterial infection with
Spectrogram, a furan-based antibiotic. Both species were initially fed live three week
old crickets, about 30 per tank, and frozen krill estimated to equal 2.5% of the biomass
in each tank twice daily until all fish were feeding, generally 3-5 days.
7
Figure 1. Location of collection sites for both the juvenile steelhead and redside
shiner. The trout and shiner were collected in habitats where they occur together.
Steelhead Trout and Redside
Shiner Collection Sites
Steamboat Creek, Oregon
A
Collection Sites
12
Kilometers
8
They were then fed only frozen krill (Euphausia pacifica) in the same amount as before
twice daily. A photoperiod of 10 hours light and 14 hours dark was maintained with
full spectrum lights under timer control. Fish were held under these conditions for at
least two weeks before the experimental trials.
Laboratory Tank Experiment
The experiments were conducted in tanks at the Oregon State Salmon Disease
Laboratory in Corvallis, Oregon. The ten 80-L oval shaped tanks were 0.7-m x 0.35-m
x 0.35-m deep. A single 25-L min.-' submersible pump created water flow within each
tank. One air stone in each tank maintained adequate dissolved oxygen. Clean water
was added to each tank at 0.5 L min:', resulting in a complete replacement of tank
water every 160 minutes. Desired water temperatures were maintained with immersion
heaters. The primary light source was natural light from overhead windows.
One 21 x 9 x 6 cm terra cotta brick and one 15 x 15 cm tile together were used
to create overhead cover and an eddy. These features were designed to mimic the
"pocket water" areas trout defend as feeding territories in natural streams. Two brick
and tile structures were in each tank (Fig. 2).
Frozen krill equaling 2.5% of the total wet weight of fish was placed
midmorning and mid afternoon in a 0.019 m PVC pipe that had its outlet just forward
of the water pump in each tank. Uneaten food was removed from each tank by a filter
in the intake of the water pump
9
The six experimental treatments of this 3 x 2 factorial design were 3 juvenile
steelhead alone, 3 steelhead with 3 shiner, 3 steelhead with 9 shiner at either 15 or
20°C. A ratio of one juvenile steelhead to three shiner was observed in the stream at
the point and time of capture. The experimental density ratios of 1:1 and 1:3 were
chosen to test for a shiner density effect.
Figure 2. Set-up of individual 80L tanks used in laboratory experiments. A
submersible pump (A) maintained a clockwise flow. A slurry of food mixed with water
was introduced through a pipe (B) just in front of the pump. Two structures made of
two bricks and one tile (C) provided cover with a good opportunity to capture food
particles and a submersible heater (D) maintained the temperature in 20° trials.
10
Each tank was randomly assigned a treatment allocation with the constraint that
a single tank did not have the same treatment twice in a row. The trial period was 21
days and there were 5 replicates.
The order in which the tanks had fish placed in them was randomized with the
constraint that steelhead placed in a tank were of similar size. Fish were anesthetized in
buffered methanotricansulfate (MS222) before being weighed and measured. Fish were
measured to the nearest 1.0 mm, weighed to the nearest 0.1 g, and inspected for signs
of disease. Fish were assigned an identification number so individuals could be tracked
throughout the experiment. Individuals were identified by a unique combination of
body marks, size, weight, and tank allocation. Shiner ranged from 65-100 mm fork
length and 3-20 g wet weight. Steelhead ranged from 90-153 mm and 8.6-41.4 g;
ranges similar to those found in the field. Larger steelhead that began to smolt were
not used in the analysis because smolts behave differently from juveniles.
Fish that died or became moribund during the course of the experiment were
removed and identified. Dead and dying fish were not weighed to estimate growth
because of tissue wasting from disease, physical damage, and osmotic imbalance.
These fish were identified by length, body marks, and reference to daily notes that
described the health of each fish. All fish were removed within 8 hours of death and
necropsied. Suspected pathogens were isolated, identified, and roughly quantified.
When a known pathogen was found on a fish in nearly pure culture, it was considered
the cause of death.
11
At the end of the 21-day test period, all surviving fish were removed from the
tanks and killed in MS222. Each fish was weighed, measured, identified, and
necropsied. Growth, general condition, and pathogen load were recorded for each fish.
Mortality and growth were compared in steelhead for all treatments using a Fisher's
Exact Test. Because there was no variation in the largest size category a logistic
regression could not be used to analyze the binary data. A Fisher's Exact Test is a two
by two matrix which compares the actual distribution of data to a random distribution.
Laboratory Stream-Channel Experiment
A second experiment was conducted in two laboratory stream channels at the
USDA Forest Service, Pacific Northwest Research Station, Corvallis, OR. Each oval
channel was 4.3 m x 4.9 m on the sides, 0.8 m wide, and 0 61 m high. Channels were
divided into two equal size sections creating four experimental cells by attaching 4 mm
Vexar mesh in a Plexiglas frame to the walls of the channel (Fig. 3). The paddle
wheels, pump intake, and exhaust were similarly isolated in another section of each
channel (see Reeves et al. 1983). Inner walls were clear Plexiglas. Opaque curtains
surround the channels on both sides, limiting outside light and disturbance, and
allowing an observer to be unnoticed by the fish. Substrate was pea gravel (6-12 mm)
in each pool and gravel (20-50 mm) mixed with cobble (100-500 mm) in the riffles.
Each section had identical structure. Two shallow areas 35 cm deep, and two areas, 50
cm deep were in each section.
Channels were filled with deionized water and allowed to circulate for 24-48
hours before fish were added. Water was continuously pumped through sand filters.
12
Figure 3. Set-up of artificial stream channels. Shown is one of two identical stream
channels.
Paddle wheel unit
I
iffle
Downstream
screen and trap
.W....--­
Riffle
--"'"
Cooling - heating
Flow
4.9 m
Pool
6.4-mm plexiglass side
Pool
Downstream
screen and trap
Riffle
Riffle
/I
6.4-mm
plywood side
4.3 m
13
A plexiglas paddle wheel in each channel maintained water flow comparable with the
areas in the stream where the fish were collected. Ultraviolet sterilizers provided
additional breakdown of metabolic waste as well as limiting pathogens in the water
column. Water temperature was maintained at 15° C. The photoperiod was
maintained as a 15-hour daylight period, including a two hour brightening and two hour
dimming phase that simulated a twilight period.
Krill was delivered through a 25-mm PVC pipe that zigzagged from a head box
down the length of each channel section. The pipe was perforated variously so as to
deliver an equal portion of food to each riffle-pool section (Reeves et al. 1983). Daily
rations (5% of the total wet biomass in each section) were fed in two daily feeding
periods, one mid morning, the other mid afternoon. The krill was frozen in blocks of
ice so particles would be delivered to the system slowly as the ice melted. Each feeding
period lasted about 1 hour to reduce scramble competition which may have been an
important experimental artifact.
Fish were taken from holding tanks and randomly assigned to a channel section.
The four sections were randomly assigned a treatment with the constraint that no
section received the same treatment twice in a row. In this experiment only the
presence of shiner with steelhead varied. This arrangement provided two replicates per
trial. All fish were weighed and measured as in the first experiment, and their unique
marks identified individual fish. Any fin damage was noted for each fish. Ten
steelhead and twenty shiner were added to two sections. Ten steelhead alone were
14
added to the remaining two sections. The experiment was repeated to provide
additional replication.
Shiner were introduced first and 12 hours later the steelhead were introduced.
Trials ran for ten days and no fish died during the trials. After ten days, all fish were
removed from the channels by bubbling CO2 through the water and killed in MS222.
All fish were weighed, measured and identified. Fin tears, which indicate aggressive
interaction between steelhead, were assessed (Moutou, McCarthy, and Houlhan 1998).
Minor fin tears tended to be short while larger tears tended to run the length of the fin.
Fin tears less than 30% of the fin length were given a score of 0.5. Tears greater than
30% of the total fin length were scored as 1.0. The choice of a 30% cutoff was
arbitrary. The total fin damage score for each fish was then recorded. Although ten
steelhead were in each section, several of the largest individuals began to smolt and
could not be used in the analysis. To adjust for this problem, a decision was made to
focus the analysis on the three smallest steelhead in each section, which was reasonable
because the results from the first experiment indicated the smallest individuals suffered
the most from aggressive interactions with other steelhead. Differences in growth and
fin damage between the three smallest steelhead with shiner were compared with those
held without shiner by ANOVA.
15
RESULTS
Laboratory Tank Experiment
Steelhead Survival
Water temperature did not significantly affect steelhead survival or growth
within or between size categories (Fishers Exact Test, p-value >> 0.05). However,
there was a trend in the average time to death (ANOVA p-value = 0.13). At 20° C the
mean time to death was 10.0 days (SE +/- 2.45, n=9). At 15° mean time to death was
13.6 days (+/-1.87, n=10). These data are shown in Appendix A.
There was a significant effect of shiner presence (Fishers Exact Test p-value
<0.01). The largest trout, considered dominant in a social hierarchy, had 100%
survival regardless of the number of shiner present (Fig. 4). The second largest trout
was affected by the presence of shiner (Fig. 5). When shiner were absent at 15° C, only
20% (SE +/- 0.2) of intermediate sized trout survived, at 20° C, 60% (+/- 0.24). When
three shiner were present at 15° C (shiner and trout at equal density) 40% (+/- 0.24) of
intermediate sized trout survived, at 20° C, 80% (+/- 0.2) survived. When nine shiner
were present survival of intermediate sized trout was 100% at both temperatures. The
smallest trout were most affected by the presence of shiner (Fig. 6). When shiner were
absent at 15° C and 20° C only 20% (+/- 0.2) of the smallest trout survived. In
treatments where three shiner were present survival of the smallest trout was 100% at
15° and 80% (+/- 0.2) at 20° C. When nine shiner were present survival of the smallest
trout was 100%. These data are shown in Appendix B.
16
Figure 4. Proportional survival of the largest individual steelhead in each treatment.
The x-axis represents the number of shiner present in each tank. The y-axis represents
steelhead survival with standard error of the mean. The dark bars are treatments at
15°C and the light bars are treatments at 20°C.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
15°
Li..;zzzl 20°
3
Number of Shiner Present
9
17
Figure 5. Proportional survival of the intermediate sized steelhead in each treatment.
1.2
co
cl)
co
10
io
0.8
E
teu­
c
0
E
0.6
0.4
Tu.
02
0
0.
2
0.0
3
15°
Lad 20°
Number of Shiner Present
9
18
Figure 6. Proportional survival of the smallest steelhead in each treatment.
1.2
1:3
cv
1.0
a)
0.8
To
"5
0.6
._
z
0.4
To
t
0
o
0.2
c,2
0.0
9
EM 15°
lam:3 20°
Number of Shiner Present
19
Steelhead Growth
Growth rates between different size categories were not significantly different
(ANOVA p-value = 0.34) (figs. 7,8,9).
There was no significant within treatment temperature effect (p-value > 0.05).
Water temperature effects may be compared by examining the adjacent light and dark
bars in (Figs. 7,8,9).
Stream Channel Experiments
Growth
There were no significant differences in growth of the smallest steelhead
between treatments (p-value = 0 53, n=4) (fig. 10). The mean proportional change in
wet weight of the smallest three steelhead in the absence of shiner was 3.6% (SE +/­
0.036, n=4). In the presence of shiner the smallest three steelhead increased by 0.67%
(SE +/- 0.04, n=4) The data are shown in Appendix C.
Fin Damage
There was no mortality of steelhead trout during the experiments. However,
the smallest three steelhead accumulated sixteen times more fin damage from attacks by
larger steelhead in treatments with no shiner present (p-value < 0.01). In treatments
where shiner were present the smallest three steelhead accumulated a mean score of
0.17 (SE 0.11) fin tears in the caudal and dorsal fins. When shiner where not present
the smallest three steelhead accumulated a mean score of 2.67 (0.57) tears in the caudal
and dorsal fins (Fig 11). The data are shown in Appendix D.
20
Figure 7. Proportional change in wet weight of the largest individual steelhead in each
treatment. The x-axis represents the number of shiner present in each tank. The y-axis
is the mean proportional change in wet weight with standard error of the mean. The
dark bars are treatments at 15° C and the light bars are those treatments at 20° C.
.c3)
0.25
-cs
cu
-c
0.20
a)
0.15
cn
cu
_c
0
0.10
0
0
0
0
a_
0.05
(1)
0.00
0
MI 15°
7779 20°
3
Number of Shiner Present
9
21
Figure 8. Proportional change in wet weight of the intermediate size steelhead.
0
15°
20°
3
Number of Shiner Present
9
22
Figure 9. Proportional change in wet weight of the smallest individual steelhead.
0.20
0.15
0.10
0.05
0.00
-0.05
0
15°
20°
3
Number of Shiner Present
9
23
Figure 10.. Box plots representing steelhead growth in stream channel experiments.
The boxes indicate the Intraquartile Range (IQR) with the median observation
represented by a line. The whiskers are those observations which fall within 1.5 x IQR,
and the dots are observations (outliers) which fall beyond 1.5 x IQR. The median
response is very close to zero both in treatments with a group of shiner present and in
those treatments where shiner were not present. The mean proportional increase in
weight in treatments without shiner was 0.036 (SE 0.036), and the mean increase in
steelhead weight in those treatments with shiner present was 0.017 (SE 0.041) n=4.
0.3
0.2
0.1
0.0
0.1
-0.2
0.3
-0.4
No Shiner
Present
Shiner Present
24
Figure 11. Mean score and SE of fin tears in the smallest three steelhead in each of
two treatments. Where no shiner were present the mean number of fin tears was 2.67,
SE = 0.57. Where a shiner group was present only 0.17, SE = 0.11, fin tears
accumulated, n=4.
4
3
2
0
no shiner
present
shiner present
25
DISCUSSION
The presence of redside shiner had a mediating effect on interactions among
steelhead. In the tank experiments, where steelhead densities were 0.0375 fish/liter, the
smallest steelhead died when shiner were not present. Steelhead densities in the stream
channels were 0.0024 fish/liter and the length of the trials where half as long as those in
the tanks. In the stream channel experiments no steelhead died, but the smallest
steelhead suffered extensive fin damage. The fin damage indicated these steelhead
received injury in aggressive interactions and would likely have died from the effects of
stress and disease if the experiments ran longer (Schreck 1992, Holt et al. 1975).
In these experiments dominance was inferred from fish size. I considered the
smallest steelhead to be subordinate in a dominance hierarchy though some workers
have suggested that other factors such as "fierceness" and not strictly size determine
social standing (Keeley and McPhail 1998, Titus and Mosegaard 1991, Huntingford et
al. 1990). Even if size were not the correct indicator of social position, the experiments
showed that more steelhead were able to live within a given area in the presence of an
equal or greater number of shiner than were able to live without shiner.
The fates of subordinate individuals in territorial fishes such as salmonids are
varied and poorly understood. Several researchers have suggested that subordinates
unable to establish feeding territories die from starvation, disease, or predation (LeCren
1965, Crisp 1993, Frazer 1968, Li and Brocksen 1977). However, Metcalfe (1986) in
a reexamination of Li and Brocksen (1977) found that subordinates may have higher
26
than expected survival suggesting the consequences of different behavioral strategies
may be more complex than previously thought.
Fish mortality is due both to biotic factors (e.g. predation, and competition for
food), and to abiotic factors (e.g. temperature stress and debris torrents). Biotic
factors tend to be "deterministic" meaning that, for example, good competitors for food
are more likely to survive than poor competitors if food is scarce. In contrast abiotic
factors are stochastic and an individual's survival is governed by a probability
distribution. Which kind of factor is most important to fish survival at a particular
period is not well understood. Elliott (1994) states that in brown trout (Salmo trutta)
only territory holders survive the "critical period" and that downstream migrants have
poor survival. Elliott (1994) operationally defines the "critical period" as a
developmental stage when density-dependent forces, such as competition, are acting to
regulate the population. In summer, though temperature stress can be significant, the
frequency of physical disturbance is low in streams in the Pacific Northwest. In winter
it is much higher. Power (1992) found the importance of biological factors are
increasingly important with decreasing frequency of environmental stress. Titus (1991)
found little evidence for strong density-dependent mortality in brown trout and
suggests abiotic factors are most important. If "critical periods" as defined above are in
fact important for juvenile steelhead they would occur during the summer months in
their first and second years of life before going to sea. If, however, abiotic factors are
more important to juvenile steelhead survival, much of the early mortality would be
expected during the first winter when the frequency of disturbance is high. Food
27
resources are low in winter, but because water temperatures are low metabolic
requirements are greatly reduced in fish.
Agonistic behavior between juvenile steelhead may lead to increased mortality
of subordinate individuals (Abbott and
Dill 1985). If
this assumption is true the
following options remain to a subordinate individual trying to secure food resources. A
subordinate juvenile steelhead must take shelter and starve, leave an area which is
known to contain food and other resources but is occupied by other steelhead, continue
to challenge a more dominant individual for food, or modify its behavior such that it
can remain in the area and not suffer from constant attacks. One way of doing this is to
adopt the feeding behavior of another group of fish which successfully feed in a
desirable area. Redside shiner exhibit a large repertoire of behaviors which allow them
to feed successfully in habitats occupied by steelhead as well other species under a wide
range of environmental conditions (Reeves,
1984).
One example is that shiner have
been shown to increase group density as a response to stress (Lindsey and Northcote
1963)
Ryer and 011a
(1991)
show that subordinate chum salmon (Oncorhynchus keta)
form groups to defeat a despotic dominant. Similarly, shiner
will
feed in tighter groups
as a way of competing with territory holding juvenile steelhead.
In this study the shiner spent most of their time in slower water, but during
periods when food was abundant shiner moved into focal points bringing them into
direct competition with steelhead which had already established territories. When only
steelhead were present subordinates continually tried to move into these focal points
suffering physical damage from attack. When shiner were present the subordinate
28
steelhead moved into the focal point within the shiner group. There was a "swarm
effect" when a dominant's territory was invaded simultaneously by many intruders.
When shiner were present the subordinate steelhead were seen repeatedly taking refuge
in a group of shiner when chased by a larger steelhead. The dominant steelhead broke
off the chase when the smaller steelhead were close to a shiner group, possibly because
it could not track its target within a group of similar sized fish. When shiner were
absent the subordinates were driven far from a focal point area, often after being bitten
repeatedly. The "decision" to associate closely with a group of shiner represents a
balance of costs and benefits. The cost to subordinate steelhead from association with
a group of shiner is additional competition for food. The benefit is a reduction in stress
and physical damage which would result if subordinates continually challenged
dominants.
Juvenile steelhead can become very aggressive with other fish, particularly other
salmonids (Harvey and Nakamoto 1996, Everest and Chapman 1972). Under certain
conditions dominant steelhead can have an important negative effect on subordinate
steelhead (figure 12) In this example the negative effect may be seen in a reduction in
growth and survival of less dominant steelhead. The effects come in two forms:
exploitation competition, the monopolization of resources, and interference
competition, the physical damage incurred by the subordinates from attacks by the
dominant steelhead. This physical damage allows pathogens to colonize breaks in the
skin (Holt et al. 1975). Small steelhead turned darker in color after being repeatedly
attacked indicating stress hormones were elevated (Iger et al. 1995). Elevated stress
29
hormones lead to immune system depression (Mau le et al. 1989). Though redside
shiner are thought to be significant competitors for food with juvenile salmonids in
some systems (Crossman and Larkin 1959), shiner can have positive effects on
subordinate juvenile steelhead. Where a shiner group and dominant steelhead compete
equally, subordinate steelhead, by their association with a shiner group can successfully
compete and remain in areas with high food abundance. A subordinate steelhead may
function as a member of a shiner group and gain the benefits of the shiner's behavioral
response to a dominant's defense of a territory. When a group of shiner approach an
area which is being defended by dominant steelhead, the subordinate steelhead enjoy a
net benefit even though the shiner are using food resources because they can remain in
a profitable feeding territory and do not suffer physical abuse.
Figure 13 illustrates how shiner can have a strong negative effect on dominant
steelhead by defeating their ability to monopolize resources. There is a much weaker
negative effect on shiner from the energetic cost of competing with dominant steelhead.
As shown in figure 12, dominant steelhead have strong negative effect on subordinant
steelhead from both exploitation and interference competition. There is a much weaker
negative effect on dominant steelhead from the cost of territorial defense. When shiner
are present there is a strong positive indirect effect from shiner to subordinant
steelhead. In addition there are weak negative effects direct effects. These effects are
the cost of exploitation competition when steelhead and shiner shoal together.
30
Figure 12. Model illustrating that dominant juvenile steelhead have a strong negative
effect on subordinate steelhead in the absence of other competitors for food. The
relative strength of the interaction effect is indicated by the width of the line in the
model.
Dominant Steelhead
(
)
(
)
Subordinate Steelhead
Figure 13. Model illustrating how redside shiner modify the strength and direction of
the interaction between dominant and subordinate steelhead
(-)
Dominant Steelhead
(-)
Shiner
( -)
Subordinate steelhead
(-)
31
Water temperature was expected to alter both shiner and steelhead behavior.
The tank experiments did not show a temperature effect on either growth or survival,
though 15° C and 20°C are within the range typically encountered by these fish in
nature and may not greatly affect behavioral responses. In this experiment survival was
not a sensitive enough measure to detect a temperature effect. To detect a temperature
effect would probably require a more sensitive measure such as growth. Growth data
were recorded but sample sizes were very small and differences between treatments
may have been obscured by high within treatment variation. To improve precision
growth should be measured in dry weight. In addition, food should be delivered more
evenly and for a longer period of time. This helps to avoid a scramble competition
which may cause fish behavior different from that seen in nature (Chapman 1962).
Scramble competition should tend to break down territory boundaries and periodic
delivery of food should encourage boundary maintenance. That territories were
maintained by steelhead in these experiments encourages the assumption that steelhead
behavior in these experiments was similar to that seen in nature. In streams drifting
food particles are most abundant during crepuscular periods, though food is likely to be
present throughout the day (Waters 1968, Rader 1995) Food abundance has been
shown to reduce territory size in steelhead (Slaney and Northcote 1974, Fausch 1984),
but how food distribution in streams effects dominance hierarchies in juvenile steelhead
is poorly understood. One way of providing a natural food distribution would be to
conduct the trials in a natural stream. However, when fish are confined in a small area
in a natural setting the experimental artifacts become the same as in the laboratory. To
32
address this problem the scale must increase. As the scale of the experiment increases,
tracking individual fish becomes more difficult.
The tank experiments were used to efficiently quantify which of multiple factors
were important in influencing steelhead growth and survival. This was a necessary first
step in determining which factors were important. The laboratory stream channels
allowed direct observation of fish which provided insight into the mechanisms
influencing fish behavior. Natural substrate was used allowing for greater visual
isolation and fish densities were similar to those seen in nature. Laboratory
experiments are generally "hard-edged" where even at densities similar to those in
nature fish do not have the option to move back and forth from marginal habitats. If
individuals either cannot or do not leave a distinct area they are referred to as "hard­
edged" habitats. These laboratory experiments are "hard- edged ". If an individual can
and will move in and out of a habitat it requires, the habitat is "soft- edged ". The
geometry of hard-edged and soft-edged territories may have important consequences
for how and how many animals persist in a given area (Stamps et al. 1987). A territory
surrounded by other territories is qualitatively different than one which borders
marginal, undefended habitats. One reason is the rate of territory invasion by
conspecifics is likely to be different, another is that soft edge habitats may have
different predation pressures. There is evidence for both kinds of habitats in nature but
stream dwelling fish like juvenile steelhead probably live in soft-edged territories with
convoluted geometry. Even though steelhead in nature have more options than in a
laboratory environment, the pattern of steelhead response to shiner in these
33
experiments was so strong that similar behavioral patterns are likely to exist in nature.
An experimental manipulation of shiner and steelhead density in natural streams on a
large scale would be an important next step to further study this aspect of fish behavior.
The long term effects of subordinate steelhead associating with shiner are
unknown. Whether or not subordinate steelhead which change their behavior to better
compete with dominants have greater fitness or even survive to smolt is unknown.
However, juvenile steelhead with lower growth rates than larger individuals within the
same cohort may smolt at different times and diverge in their later life history
strategies. Pre-smolt growth poorly predicts ocean growth rates in steelhead (Johnson
et al. 1997). There is some evidence that higher presmolt growth rates lead to later
onset of smolting (Ward et al. 1989). It is possible that if a subordinate steelhead
survives through its first winter it will be a dominant the following summer and smolt
earlier.
To better understand the long-term effects of juvenile behavior it will be
necessary to track individual fish through all its life history stages. Since survival of
steelhead from emergence to spawning age adults is probably less than one percent
(Unpublished data on Fish Creek OR, USFS-PNW ALI Corvallis) the resources
required to track a sufficient number of fish would be prodigious. Additionally, wild
steelhead stocks in the lower 48 states have declined steadily since the turn of the
century (Cone and Ridlington 1996) so future experiments may require the use of
hatchery steelhead for experimentation.
34
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39
Appendix
40
Table A.
Data indicating the time in days to death and the general identification of the
most common pathogens cultured on cytophaga agar from those steelhead which died
in the laboratory tank experiments. In the treatment column the first number indicates
the temperature and the second number indicates the number of shiner present in that
tank.
°C
Number of Shiner
Present
15
0
15
0
20
20
20
20
20
0
Water Temperature
15
15
20
20
0
0
Most Abundant
Pathogen Isolated From
Culture
Flexibacter
psychrophilus
Flexibacter columnaris
F. columnaris
Saprolegnia spp.
F. psychrophilus
F. columnaris
Unidentified lesions
Unidentified lesions
F. columnaris
F. columnaris
F. columnaris
F. columnaris
F. psychrophilus
F. psychrophilus
Unknown lesions
Unknown lesion
F. columnaris
3
F columnaris
0
0
0
0
0
0
0
0
0
15
15
0
15
3
20
3
Time to Death in Days
21
15
7
14
12
4
2
10
11
4
5
17
7
20
21
6
8
21
203Fmcht:ofhil_
15
15
15
41
Table B.
Steelhead survival data for all laboratory tank trials. The trial number is the
order each set of trials was run, the second number is the identifying mark on each
individual tank. The number of shiner present (0, 3,9) is the number of shiner in each
tank, each with three steelhead. The remaining columns indicate whether or not an
individual fish survived.
Trial
Tank ID
Temperature
°C
15°
Number of Shiner
Present
0
9
20°
1
2
7
8
15°
9
1
13
20°
2
2
2
2
15°
15°
3
20°
2
4
15°
2
2
2
7
14
15
20°
0
0
9
9
9
0
0
9
2
16
3
1
3
2
15°
15°
3
3
20°
9
9
9
3
4
15°
3
3
5
20°
3
6
15°
3
7
20°
0
0
0
3
1
1
1
15°
20°
20°
9
3
8
15°
3
13
20°
3
3
15°
0
3
14
15
16
20°
20°
3
3
3
2
15°
15°
5
20°
3
14
16
15°
3
20°
3
4
4
4
4
4
1
9
3
Largest
Steelhead
Intermediate
Steelhead
Smallest
Steelhead
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Survived
Died
Survived
Survived
Died
Survived
Survived
Survived
Survived
Died
Died
Survived
Survived
Survived
Survived
Survived
Survived
Died
Died
Survived
Died
Died
Died
Survived
Survived
Survived
Survived
Survived
Died
Survived
Died
Survived
Survived
Died
Survived
Survived
Died
Survived
Died
Died
Survived
Survived
Survived
Survived
Survived
Survived
Died
Died
Survived
Survived
Survived
Died
Survived
Survived
Survived
Survived
Survived
Died
Died
42
Table C.
Growth data measured as the proportional change in wet weight of the smallest
three steelhead in laboratoy stream channel experiments. In this table the column
headers indicate the number of shiner present in each trial. There is no indication that
the presence of shiner affected steelhead growth.
No Shiner Present
20 %
20 Shiner Present
19.8 %
1
12.7
16.2
1
20.9
4.8
6.2
23.5
1
10
1
0
Trial
1
1
2
-12.9
2
-4
2
- 3.9
- 2.7
-16.7
2
2
4
3
4.8
-28
- 1.4
-10.8
-15.6
- 4.3
0
43
Table D
Accumulated fin damage data of the smallest three steelhead in the stream
channel experiments. The method for scoring is explained in the method section. The
column headers indicate the number of shiner present in each experimental cell.
Trial
No Shiner Present
20 Shiner Present
1
1
1
5
0
0
1
5
1
1
1
2
2
2
2
2
2
0
6
2
4
3
1
1
1
3
0
0
1
0
0
1
0
0
0