weighed using a microbalance (Cahn model 27). Non-prey items in stomachs, primarily ..... with field and lab work from Linda O'Bryan and. Bill Wiseman is much ...
Environmental Biology of Fishes 39: 301-311,1994. ©1994 Kluwer Academic Publishers . Printed in the Netherlands .
Diel and density-related changes in food consumption and prey selection by brook charr in a New Hampshire stream
Graham E. Forrester', Jeffrey G. Chace & William McCarthy Department of Zoology, University of New Hampshire, Durham, NH 03824, USA . 'Present address : Department of Biological Sciences and Marine Sciences Institute, University of California, Santa Barbara, CA 93106, U.S.A . Received 28 .1 .1993
Accepted 2 .10.1993
Key words: Feeding rate, Prey-preference, Size-selection, Population density, Salvelinus fontinalis, Salmonidae Synopsis We report the results of a field study testing influences of both density and changes over the diel cycle on food consumption and prey selection by brook charr, Salvelinus fontinalis. Charr density in replicate 35 m long sections of a New Hampshire stream was adjusted to either medium or high levels (relative to natural densities) . Diets of charr and the availability of drifting prey were then sampled every four hours for 24 hours . There were no significant diel changes in the weight of prey consumed by charr per four hours, though there was some indication of reduced feeding at night . Charr fed selectively on different prey taxa, showing most preference for cased caddis larvae . Several species of mayflies and stoneflies were selected more strongly during the day than at night . Charr fed selectively on larger prey during the daytime but showed no sizeselection at night . The density of charr had no significant effects on either their rate of food consumption or on selection for prey of different taxa or sizes .
Introduction Many studies have sought to determine mechanisms by which stream-dwelling salmonids select
invertebrates, are usually more abundant at night than during the day (Waters 1972) . A few studies suggest that salmonids take advantage of this nocturnal increase in prey availability by increased
prey by comparing the composition of the diet to
feeding at night (Jenkins et al . 1970, Elliott 1973),
the composition of prey available . One important aspect of the feeding behaviour of salmonids which
but the majority of studies suggest that most feeding occurs during daylight (Bisson 1978, Allan 1981,
has received limited attention is changes in feeding and prey selection over the diel cycle . Feeding
Walsh et al . 1988, Angradi & Griffith 1990) . Studies of prey selection by trout done during the day have
might be expected to differ between day and night because salmonids feed visually (Robinson & Tash
demonstrated selection for different prey taxa (Cada et al . 1987) and also for larger prey (Irvine &
1979, Tanaka 1970), and so should feed less efficiently, and perhaps also less selectively, after dark .
Northcote 1983, Newman & Waters 1984, Grant & Noakes 1986) . The few diel studies of prey selection
In addition, there are marked diel changes in the
indicate that selection for larger prey may be re-
availability of their prey. Prey, in the form of drifting
laxed at night (Allan 1978, 1981, Sagar & Glova
3 02 1988) but there are no clear diel patterns in selec-
during the study was bimodal, with one peak
tion of prey by taxa (Angradi & Griffith 1990) .
around 55-60 mm fork length (FL) representing young-of-the-year charr and a broader, shallower,
A second potentially important factor influencing feeding and prey selection by salmonids is their local population density. Both growth and survival of salmonids are often reduced at high densities (Fraser 1969, Mortensen 1977, Elliott 1984, Wentworth & LaBar 1984) . Populations of salmonids also show reduced growth (Mason & Chapman 1965, Mason 1976, Wilzbach et al . 1986) and production (Warren et al . 1964) in areas of low food availability.
peak around 80-110 mm FL representing older charr. The only other fish present at the study site were a few black nosed dace, Rhinichthys atratulus (Hermann, 1804) (mean density = 0 .009 m-2 ) . All dace captured while setting up the study were removed and placed in areas downstream of the study site.
There is no direct evidence that trout at elevated densities experience reduced food availability, but one possible mechanism for reduced growth at high density is a shortage of prey. Optimal foraging theo-
Experimental procedures
ry predicts that animal feed less selectively when
Charr density was manipulated in 35 m long sections of stream, enclosed at the ends with 7 mm
food becomes scarce (Stephens & Krebs 1986) . Despite the possibility for influences of population
mesh fences . The fences were constructed on 13-14 August, and extended at least 0 .1 m into sediment
density on feeding rates and prey selection in stream salmonids, we know of no attempts to test
and 0 .5 m above the water's surface. Sections were
for this . We present a field study of brook charr, Salvelinus fontinalis (Mitchill, 1815) designed to answer
separated by 17-20 m long unmanipulated areas . Charr densities within the sections were adjusted to either medium or high levels (Table 1) . Three repli-
two questions : (1) does the feeding rate of charr,
cate sections were assigned to each treatment . Sections containing differing charr densities did not
and selection of prey of different taxa and size
differ in their physical characteristics . Arrange-
change with time of day, and (2) does the rate of feeding and prey selection by charr depend on their
ment of treatments from upstream to downstream was : high, high, zero, zero, medium, zero, high, me-
local population density?
dium, medium. Densities of charr were adjusted by electrofishing (four passes) from 16-18 August . Numbers of
Methods
charr in the sections were then adjusted to the appropriate levels by replacing charr in their section,
Study site
adding charr captured from other sections, or removing charr, as necessary. We attempted to keep
The study was conducted during July and August 1990 in Stoney Brook, a second order stream in New Hampshire, U.S .A . (43° 23'N, 72'01'W, elevation 300 m) . The study area was 0 .6 km long, with riparian vegetation consisting of hemlock and mixed hardwood trees. Mean summer discharge was 3 .74 m3 min- ' and water temperature was 1618° C . Fish densities in the study area were surveyed by electrofishing (Smith-Root model 12) in August 1989, and in June and August 1990 . We used the removal method (3 passes) following Bohlin et al. (1989) . Mean density at the study site was 1 .2 m2 (range = 0 .6-2 .4 m-') . The size distribution of charr
Table 1. Characteristics of stream sections (mean ± SE) stocked with medium and high densities of brook charr .
Medium density Charr density (no . m-') Charr size (g) Canopy (%) % riffle/pool/run Depth (cm) Width (m)
High density
0.88 (± 0 .02) 1 .56 (± 0 .12) 7.67 (± 1 .30) 6 .90 (± 0 .90) 74 (± 6) 72 (± 8) 55/10/40 (± 4/7/4) 60/10/30 (± 7/5/4) 21 (f 9) 26 (±7) 2.6 (±0 .2) 2 .9 (±0 .5)
303 charr size-distributions similar
among sections
when assigning fish (Table 1) .
samples were measured from the screen image using a digitizing pad and computerized image analysis software (Sigma Scan, Jandel Inc .) .
Sampling charr for prey consumption Patterns of prey consumption Charr for diet analysis were captured by electrofishing on 29 August . Two high density and two medium density sections were electrofished every four hours over 24 hours . Start times of sampling were : 1400, 1800, 2200, 0200, 0600, and 1000 h . Each col-
The dry weight of prey consumed by the charr in each section of stream, during each of the six, four hour sampling intervals was calculated, following Elliott & Persson (1978) as
lection took less than one hour . Electrofishing was started at the downstream end of each section, proceeding upstream until six charr has been captured . Areas within sections were not revisited once they
_ (St - S o C`
eR`)Rt
1- e -Rt
where C is the dry weight of food consumed per
had been fished .
charr in a sample interval oft hours, S o and S t are the
Captured charr were immediately killed and preserved in 10% formalin . Their stomachs were later
mean dry weights of food in charr stomachs at the beginning and end of the interval respectively, e is
dissected and the contents stored in 70% alcohol . Prey items in the stomachs were later enumerated
the exponent of natural logarithms, and R is the ex-
under a dissecting microscope . All mayfly and stonefly nymphs (Ephemeroptera and Plecoptera) encountered were measured (maximum head width) using an eyepiece graticule . Stomach contents were then dried at 60° C for 24-48 h and weighed using a microbalance (Cahn model 27) .
ponential rate of gastric evacuation . We estimated R as 0 .051 in a field pilot study done immediately downstream of the study site in June 1990. Thirty charr were captured by electrofishing at 1200 h and placed in a plastic holding tank, filled with approximately 50 1 of stream water . The
Non-prey items in stomachs, primarily cases of cad-
stream water had previously been filtered through 0 .3 mm mesh to remove any food items . The hold-
dis larvae, were not weighed .
ing tank was placed in the stream to keep it at the same temperature as the stream water (16° C) . Five of the charr were then killed every 2 hours, and the
Sampling prey ayailability
prey in their stomachs were later dried at 60° C for 48 h and weighed (as described above) . The rate of
Drifting prey were sampled at the downstream end
gastric evacuation (R) was estimated as the slope of
of each section on 29 August using drift nets with square mounths (area = 0 .09 m2), 1 m long with
the regression of the logarithm of prey dry weight
0 .3 mm mesh . Nets were in place for six, 1 .5-2 hour periods spaced over 24 hours . Drift sampling was interspersed between the electrofishing also done on this date . The start times of the samples were : 1600, 2000, 0000, 0400, 0800, and 1200 h . Disturbance from wading was minimized, both during electrofishing and prey sampling, by walking on the banks or on rocks in the stream . Samples were preserved in alcohol and later sorted and enumerated using a dissecting microscope . The microscope image was filmed and displayed on a video monitor . Head widths of mayflies and stoneflies in the drift
against time since feeding ceased (following Elliott 1972) . We tested effects of charr density and time of day on food consumption using analysis of variance (ANOVA), where stream sections were replicates and samples over 24 hours were considered repeated measures . Food consumption was expressed as the dry mass of food consumed per fish (mg 4 h - ') and as the amount of food consumed per wet mass of fish (mg g 4 h -1 ) to account, for effects of charr size on food consumption .
3 04 Patterns of prey selection by taxa
this sample (nighttime = 2200 and 0200 h only) . Selection indices provide measures of relative, rather
Prey selection was evaluated by comparing the rela-
than absolute, selection for different prey items and
tive abundances of prey consumed to the relative abundances of prey available. Prey selection was
so effects of time of day and charr density on prey selection cannot be tested statistically . Because,
measured using the rank preference index which
however, stream sections contain replicate groups
provides measures of relative preference for differ-
of charr we present information on variability in
ent prey items, and has the advantage over other commonly used indices of being relatively unbiased
preferences and use this information to make some inferences about diel and density-related changes
by the inclusion or exclusion of rare prey items from the analysis (Johnson 1980) . To calculate this index,
in prey selection by charr.
prey available and prey consumed are ranked from most to least abundant . The rank preference index
Patterns of prey selection by size
for a given prey item is the rank of consumption minus the rank of availability, so higher index values
Selection for prey of different sizes was tested using
indicate lower selection for a given prey item .
mayflies and stoneflies because they were common
Drift samples were used as measures of prey available, because observations of charr of a range
prey, and easy to recognize and measure in stomach contents . Measurements of mayflies and stoneflies
of sizes (roughly 40-120 mm FL) in Stoney Brook
from different sections at different times of day were pooled into day or night (as above), and medi-
indicated that most daytime feeding attempts are directed at drifting prey [mean (± SE) = 84% (± 3 .5 %), n = 30, 5 min observations of different fish] . Other studies report similar observations (McNicol et al . 1985, Grant & Noakes 1986) . We al-
um or high density. To test for diel and density-related changes in prey selection we compared the sizes of mayflies and stoneflies consumed versus available, during both day and night, and at high and me-
so used drift samples as measures of prey available
dium charr densities using 3 factor model I analyses
at night, but note that whether charr continue to feed primarily on drift at night is unknown . Our es-
of variance . Again we repeated the analysis with and without the sample of uncertain day/night sta-
timates of the availability of prey were for each
tus (0600 h) .
stream section, rather than for each charr . We therefore used the mean number of prey per stomach for each section as our measure of prey con-
Results
sumed, and calculated rank preference indices for each stream section, rather than for each charr indi-
Patterns of prey consumption
vidually. To compare diel and density-related changes in prey selection, we pooled the samples collected
The mean size of charr captured did not vary among
during the daytime (1400, 1800, and 1000 h) and
sections and times of day (2-way ANOVA, p always > 0 .23) and did not differ systematically among sec-
nighttime (2200, 0200 and 0600 h) within each section . We pooled the samples into day and night be-
tions with differing charr densities (t-test, p > 0 .4) . Charr had on average 8 .4 prey in their stomachs
cause these represent periods of distinctly different prey abundance (Forrester 1992), and of visual conditions for feeding . Separate rank preference indices were then calculated for daytime and nighttime feeding by the charr in each section . Because there was some doubt as to the day/night status of the 0600 h sample (sunrise occurred during the sampling period) we also repeated the analyses without
(± 22 .4 SD), weighing 72 .8 mg (± 24 .7 SD) . Contents of charr stomachs, in terms of both numbers and weight of prey per stomach, did not vary significantly over the 24 hour period (repeated measures ANOVA on log transformed data, p always > 0 .3) . The lack of significant diel pattern was also true when stomach contents were standardized by charr mass (repeated measures ANOVA, p always
3 05 a
> 0 .25) . There was, however, some tendency for the weight of prey in charr stomachs to be lower during
20 -
the nighttime samples (2200 and 0200 h) than during the daytime (Fig. 1) . Numbers of prey per stom-
8 816-
ach showed a similar diel pattern to weights of prey
E
and so are not presented .
n12-
Rates of food consumption by charr, both in terms of prey consumption per fish (mg 4 h -1 ) and prey consumption per unit mass of fish (mg g 4 h -1),
a
8-
were highly variable and did not change significantly over the diel cycle (repeated measures ANOVA, p always > 0 .5) . The similarity between the two estimates was not surprising because the relationship between the mean size of charr in a section and their h-1) rate of food consumption (mg 4 was very weak
01400 1800 2200 0200 06001000
b
Table 2 . Overall numeric composition of brook charr diets, and of drifting prey available. Values are percentages. Only taxa comprising more than two per cent of drift or stomach contents were listed separately, rarer taxa were pooled into higher taxo-
T
nomic units . Prey taxa
Stomach contents
drift
1
Aquatic prey Ephemeroptera
Baetis Paraleptophlebia Ephemerella Eurylophella Stenonema
2 .23 2.16 1 .24 0.74
8 .26 6 .60 3 .11 1 .85
2.65
8 .34 9 .03
1400 1800 2200 0200 06001000 0.83
Trichoptera Caseless Cased Diptera Chironomidae
2.69 8.06 30.0
4 .82 0 .77
0 1 .24
7 .76 17 .1
Culicidae'
11.07 1.06
0 2.19
0.90 0.83
3.66 0.24
8 .96
2.33
15 .12 9 .10
6 .53 0
Other taxa' Terrestrial prey Unidentified
Fig. 1 . a - The average weight (mg ± SE) of prey contained in brook charr stomachs at six sampling times over 24 hours and b estimates of food consumption during 6, 4 hour intervals over 24 hours . Horizontal bars under x axes indicate nighttime .
12.3
Simuliidae Tipulidae Other Diptera Coleoptera (adults) Oligochaeta
T
5 .64
2.98
Plecoptera
Peltoperla Utaperla
T
This taxon was found in the stomach for only two charrs, one of which had consumed very large numbers of these prey . b Other
(r2 = 0 .115, n = 144) . Despite the lack of significant diel changes in food consumption, there was reduced feeding between 2200 and 0200 h (Fig . 1) . The method we used to calculate food consumption assumes that food is consumed at a constant rate during the time interval (Elliott & Persson 1978) . If this was not the case then the estimates will be biased . One interval where we feel this is possible was between 0200 and 0600 h . Charr captured at 0200 h
a
taxa included Odonata, Megaloptera, Coleoptera larvae, Hemiptera, Mollusca and other rare Ephemeroptera and Plecoptera .
had relatively empty stomachs, compared to those captured at 0600 h (Fig. 1) . Few prey items at 0600 showed signs of digestion, suggesting a commence-
3 06 (a) Day (o) vs . Night (a)
Prey taxa
(b) High (o) vs . medium (.) charr density
Ephemeroptera
t--I F--O HO H
Baets Ephemerella Stenonema Paraleptophlebia Eurylophella
0 I--o
0i
0--I 0-4
0-+I
Plecoptera
H
Utaperla Peltoperla Trichoptera caseless cased Diptera Chironomidae Tipulidae others Oligochaeta Coleoptera (adults) Terrestrial Insecta
I-5
0-4
H
I-1 H I
I
-9
0-1
I-o--~ 0-~
H
-15
1 00
I-O •---+
-9 0
0-4 I-O H 0--I
I
i
-3
I
I
0 3
Mean rank preference
I
I
9
15
-15
I
I
-9
I
o
51
I
I
I
-3
0
3
I
1
9
I
15
Mean rank preference
Fig. 2 . Patterns of selection for common prey taxa by brook charr feeding during the day versus at night (a) and stocked at different densities (b) . Lower values for preference indices indicate stronger selection for a given prey item . Error bars are standard errors.
ment of feeding at dawn just prior to electrofishing at 0600 h, rather than continuous feeding from
Patterns of selection for different prey taxa
0200-0600 h. If most of the prey in the stomachs of
The overall composition of the diet of charr and of
charr sampled at 0600 h were consumed just prior to their capture, our estimate of food consumption
prey available is shown in Table 2 . Common prey
from 0200-0600 h will be biased upwards . If the estimate of food consumption for 0200-0600 h is biased upwards, this would add support to a nighttime decline in feeding (Fig . 1) . Nevertheless, some feeding occurred at all times of night and day . Charr at medium and high densities did not have significantly different numbers and weights of prey in their stomachs (repeated measures ANOVA on
types in the diet'included midges (Chironomidae), terrestrial insects, caddis larvae (Trichoptera) and mayfly nymphs (Ephemeroptera) . Rank preference indices indicate that cased caddis larvae were consistently the highest ranked prey taxon (Fig . 2) . Inspection of Figure 2 suggests greater selection for mayflies and stoneflies during the day, compared to during the night. The difference in prey se-
log transformed data, p always > 0 .45) . Charr at
lection between day and night was particularly striking for Paraleptophlebia, Eurylophella and
high density contained on average 6 .1 prey (± 0 .2 SE), weighing 13 .0 mg (± 0 .9 g SE), whereas mean
Utaperla. The density of charr, however, did not appear to affect prey selection . For all taxa, there was
stomach contents of charr at medium density were 10.8 prey (± 1 .4 SE), weighing 10.9 mg (± 0 .5 mg h-1) SE) . Food consumption rates (mg 4 were similar for charr at high (mean = 6 .1 ± 0.6 SE) and medi-
more variation in rank preference within, rather
um (mean = 4 .9 ± 0 .5 SE) densities (repeated measures ANOVA, p = 0.80) . Food consumption rates
Patterns of selection for different prey sizes
standardized for charr size (mg g 4 h -1 ) also did not
Analyses on individual prey taxa indicated that
vary among charr densities (repeated measures ANOVA, p = 0 .62) .
charr consumed Ephemerella, Stenonema and Peltoperla selectively by size, but the extent of size-se-
than between, densities of charr .
307
Consumed o
Night
Available 0 ∎
pay
Prey taxa o-i
t- -o
Baetis Paraleptophlebia
H S
F-o
I
o-4
Eurylophella
o-a
F-0
ha .l
Ephemerella
a
i--a
HN
Stenonema H a
Peltoperla Utaperla
o-i
F-a
F-0
NN
o-i
All combined I
I
I
0.4 0 .6 0.8
I
I
I
1 .0 1 .2 1 .4
I
I
I
I
I
I
I
I
0.2 0 .4 0 .6 0 .8 1 .0 1 .2 1 .4 1 .6
Head width (mm)
Head width (mm)
Fig. 3. Diel patterns of selection for different sized prey by brook charr feeding on five mayfly and two stonefly taxa . Plotted are mean sizes (with standard errors) of drifting prey and prey consumed by charr . Mean sample size per point for individual taxa = 29 .3 .
lection differed between night and day (ANOVAs, p always < 0 .03),, Multiple comparison tests indicat-
trends was significant individually (ANOVAs, p al-
ed that Peltoperla consumed by charr during the day
combined there was a general tendency for charr to selectively consume larger prey during the day
and night did not differ significantly in size (Tukey's HSD, p > 0.05), whereas the Peltoperla that drifted during the night tended to be larger than those drifting during the day (Tukey's HSD test, p < 0 .05) (Fig . 3) . Ephemerella and Stenonema consumed by charr during the day were significantly larger on average than those available in the drift (Tukey's HSD test, p < 0 .05), whereas at night drifting prey and prey
ways > 0 .05), when all mayflies and stoneflies were
(when prey consumed were significantly larger than those in the drift ; Tukey's test, p < 0.01), but to show no size-selection at night (when drifting and consumed prey were not significantly different in size ; Tukey's test, p > 0 .2) . There were no significant effects of charr density on selection for prey of different size (ANOVAs, p always > 0 .15) .
consumed by charr were of similar sizes (Tukey's HSD test, p > 0 .05) . For the remaining four mayflies and stoneflies, the mean size of prey consumed by
Discussion
charr during the day was larger than the mean size of prey available. In contrast, the prey consumed by
Diel changes in feeding and prey selection by charr
charr at night tended to be either smaller or of similar sizes to prey available. While none of these latter
The density of drifting invertebrates at the study
308 site showed a pronounced increase during the night
in Sagar and Glova's (1988) study of chinook salm-
(Forrester 1992), as it does in most streams (Waters
on . Diminished selection for these taxa during the
1972) . There was, however, no corresponding increase in food consumption after dark . Other stud-
night might perhaps reflect reduced ability of the charr to detect subtle differences between drifting
ies of brook charr found that more feeding occurred
prey at low light levels . Better understanding of se-
during the day than at night (Allan 1981, Johnson &
lective feeding by salmonids would thus be further-
Johnson 1982, Walsh et al . 1988), suggesting that brook charr are predominantly diurnal feeders .
ed by direct observations of charr feeding behaviour in the field at night to determine where and
Our observation of a decline in feeding at night, though non-significant, is consistent with the find-
how they feed, and by laboratory tests of their capabilities to detect and capture different types of prey
ing of these other studies . Studies of other salmonid
at low light levels .
species have uncovered a wider range of diel feeding patterns, including dawn, evening and daytime peaks of feeding activity (Elliott 1970,1973, Sagar &
Cased caddis larvae were the most strongly selected prey during both day and night . Cased caddis larvae are large and conspicuous and field studies
Glova 1988) . There is variation within, as well as
often find that they are selected by salmonids (Grif-
among, species in diel feeding periodicity . Rainbow
fith 1974, McNicol et al . 1985, Hubert & Rhodes 1989) . These findings are perhaps surprising, given
trout, for example, have been observed to feed mostly during the day in some streams (Bisson 1978, Angradi & Griffith 1990), but in other streams they show pronounced evening peaks of feeding (Elliott 1973) . Reasons for this variation in diel feeding chronicity are not clear. Salmonids detect prey visually and so are unable to feed at light levels below which they cannot see (Robinson & Tash 1979, Tanaka 1970) . One reason for variation in the rate of feeding at night may, therefore, be the amount of light available, as determined by the moon phase and weather . Indirect evidence for this is that the usual increase in the abundance of drifting prey at night is suppressed during full moon periods (Anderson 1966) . This alteration of prey behaviour implies that the risk of predation from fish is increased during full moon, presumably because drifting prey are more visible in bright moonlight . More systematic observations of diel feeding rates under varying moonlight conditions might therefore help to explain differences in the extent of nighttime feeding . Brook charr during this study tended to select larger mayflies and stoneflies during the day, but
the purported anti-predator function of caddis cases (e .g . Otto & Svensson 1980) . Why cased caddis larvae were strongly selected at night, while selection for mayflies and stoneflies was reduced after dark, is uncertain . Cased caddis rarely drift, and so their consumption has been inferred to be the result of benthic foraging (Bisson 1978) . If charr feed on mayflies and stoneflies mostly from the drift, but on caddis larvae from the benthos, then continued selection for cased caddis at night may be because benthic foraging is more effective than drift feeding after dark . A caveat to the above is that inferences about prey selectivity from comparison of stomach contents to food availability rely on the assumption that prey are evacuated from the stomach at similar rates (Gannon 1976, Kolok & Rondorf 1987) . Cased caddis larvae in stoney cases may be digested more slowly than other prey . An alternative explanation for the frequent occurrence of cased caddis larvae in charr stomachs is, therefore, that these prey were evacuated more slowly than other prey types . Con-
showed no size-selection at night . A similar diel pat-
firmation of patterns of prey selection identified in the field by this study, and others, should be corrob-
terns of size-selective predation on mayflies has been reported in other streams for brook charr (Al-
orated in the laboratory, where consumption of prey can be observed directly .
lan 1978,1981) and chinook salmon (Sagar & Glova 1988) . Mayflies and stoneflies also seemed to be selected more strongly relative to other prey taxa during the day than during the night, a finding mirrored
309 Density-related changes in feeding and prey selection
A third potential explanation for the absence of measurable effects of charr density on feeding is high variation among individuals in feeding success.
Mean drift rates of prey did not differ significantly between areas containing high and medium densities of charr (the only exceptions being the mayflies Baetis and Paraleptoplebia) (Forrester 1992) . The mean number of prey available per charr should, therefore, have been lower in stream sections containing high densities of charr, than in sections with
The abundance of drifting prey varies substantially at small spatial scales (e.g . Baker & Hawkins 1990), suggesting that individual charr will experience different supplies of drifting prey. In addition, most stream dwelling salmonids, including brook charr, tend to occupy small feeding territories (Dill et al . 1981, McNicol et al. 1985, Puckett & Dill 1985) . Ter-
medium densities . Despite this fact, we detected no effects of charr density on their feeding rate and se-
ritory sites vary markedly in profitability, defined as
lectivity for different prey types .
quired to maintain position (Fausch 1984) further adding to variation in food supply. It may be, then,
One possible reason for the apparent lack of effects of population density on food consumption is
the food delivery rate relative to the energy re-
that over the range of densities tested, variation in
that the interval between manipulation of charr
access to food among individual charr is much
density and assessment of feeding behaviour (11-13 days) was too short to allow the charr to establish
greater than any influence of population density at larger scales (35 m sections of stream) .
normal territorial and feeding behaviour. Our observations showed no differences in rates of feeding and agonistic encounters of charr observed before and after manipulation, but we cannot rule out the
Acknowledgements
possibility that the charr were not behaving normally during the experiment .
Many thanks to Peter Sale, Doug Fraser, Jim Haney, Bobbi Peckarsky, Jim Taylor and the reviewers
Another possible reason for the lack of effects of
for advice and comments on the manuscript. Help
density is that food was available in excess and so no effects of density were likely . Brook charr in this
with field and lab work from Linda O'Bryan and Bill Wiseman is much appreciated . We are grateful
study consumed on average 1 .75 % of their dry body
to Freeport Development Inc . for allowing us to work on their land . J .G .C. thanks Stuart Fisher for
weight per day (calculated assuming charr dry wt = 24% of wet wt, Elliott 1975) . We do not have the information necessary to determine whether this rate of food consumption by the charr was adequate for them to maintain their body weight, or allow
use of his microbalance. Research funds were provided (to G .E .F.) by a Grant-in-aid-of-research from Sigma Xi, a Theodore Roosevelt Memorial Fund Grant from the American Museum of Natural
growth. This rate of food consumption is, however,
History, a Central University Research Fund Grant
at the low end of food consumption values reported by Walsh and coworkers (1988) for young-of-the
from the University of New Hampshire and a Dissertation Improvement Grant from NSF (BSR
year (YOY) brook charr in Quebec (1.27-9 .75 %) . It is also lower than values reported for other salm-
9016445) . G .E .F. was supported in part by Summer Fellowships and a Dissertation Fellowship from the
onids, e.g. 2 .5 % for juvenile sockeye salmon (Doble
University of New Hampshire.
& Eggers 1978), 5 .6% for YOY rainbow trout (Angradi & Griffith 1990), 8 .3% for juvenile chinook salmon (Sagar & Glova 1988) and 6 .6-13 .1% for juvenile pink salmon (Godin 1981) . The fact that rates
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