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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

References cited

of food consumption by brook charr during our

Allan, J .D . 1978 . Trout predation and the size-composition of stream drift . Limnol. Oceanogr . 23 :1231-1237 .

study were low, relative to most other reported values, suggests that food was not superabundant in the study area.

Allan, J.D. 1981. Determinants of diet of brook trout (Salvelinus

310 fontinalis) in a mountain stream . Can . J . Fish . Aquat . Sci . 38:

zooplankton by alewife, Alosa pseudoharengus, on determinations of selective feeding. Trans . Amer. Fish. Soc. 91 : 89-95 .

184-192. Anderson, N .H . 1966 . Depressant effect of moonlight on the activity of aquatic insects . Nature 209 : 319-320 .

Godin, J .J. 1981 . Daily patterns of feeding behaviour, daily ration and diets of juvenile pink salmon (Oncorhynchus gorbusha) in

Angradi, T.R. & J .S. Griffith. 1990. Diel feeding chronology and diet selection of rainbow trout (Oncorhynchus mykiss) in the

two marine bays of British Columbia . Can . J . Fish . Aquat . Sci. 38 :10-15 .

Henry's Fork of the Snake River, Idaho . Can. J . Fish . Aquat . Sci . 47 :199-209 .

Grant, J .W .A . & D .L .G . Noakes . 1986 . A test of a size-selective predation model with juvenile brook chary, Salvelinus fontina-

Baker, A.D. & C.P. Hawkins . 1990 . Patch-specific variation in drift density of Baetis. pp. 269-274 . In: I .C. Campbell (ed .)

lis . J . Fish Biol . 29 (suppl . A) : 15-23.

Mayflies and Stoneflies, Kluwer Academic Publishers, Dor-

Griffith, J .S. 1974 . Utilization of invertebrate drift by brook trout (Salvelinus fontinalis) and cutthroat trout (Salmo clarki) in

drecht . Bisson, PA . 1978 . Diel food selection by two sizes of rainbow

small streams in Idaho. Trans. Amer . Fish . Soc . 103 : 440-447. Hubert, W.A. & H .A . Rhodes. 1989. Food selection by brook

trout (Salmo gairdneri) in an experimental stream . J . Fish . Res . Board Can . 35 : 971-975 .

trout in a subalpine stream. Hydrobiologia 178 : 225-231 . Irvine, J .R . & T.G . Northcote . 1983. Selection by young rainbow trout (Salmo gairdneri) in simulated stream environments for live and dead prey of different sizes. Can. J . Fish . Aquat . Sci .

Bohlin, T., S. Hamrin, TG . Heggberget, G . Rasmussen & S.J . Saltveit.1989 . Electrofishing-theory and practice with special reference to salmonids . Hydrobiologia 173 : 9-43 .

40:1745-1749 .

Cada, G .E, J.M . Loar & D.K. Cox. 1987 . Food and feeding preferences of rainbow and brown trout in southern Appalachian

Jenkins, TM ., C .R. Feldmuth & G .V. Elliott . 1970 . Feeding of rainbow trout (Salmo gairdneri) in relation to abundance of

streams . Amer. Midi . Nat . 117 :374-385 . Dill, L.M ., R.C. Ydenberg & A .H .G . Fraser . 1981 . Food abun-

drifting invertebrates in a mountain stream . J . Fish . Res . Board Can . 27 : 2356-2361 .

dance and territory size in juvenile coho salmon (Oncorhynchus kisutch) . Can . J . Zool . 59 :1801-1809 .

Johnson, D.H . 1980 . The comparison of usage and availability measurements for evaluating resource preference . Ecology 61 :

Dobie, B .D . & D.M . Eggers . 1978 . Diel feeding chronology, rate

65-71 .

of gastric evacuation, daily ration and prey selectivity in Lake Washington juvenile sockeye salmon (Oncorhynchus nerka) .

Johnson, J .H . & E .Z . Johnson . 1982 . Diel foraging in relation to available prey in an Adirondack mountain stream . Hydrobio-

Trans . Amer. Fish . Soc . 107 : 36-45 . Elliott, J .M . 1970. Diel changes in invertebrate drift and the food

logia 96 : 97-104 . Kolok, A .S . & D .W. Rondorf. 1987 . Effects of differential gastric

of trout Salmo trutta L . J. Fish Biol . 2 : 161-165 . Elliott, J .M. 1972 . Rates of gastric evacuation in brown trout

evacuation and multi-species prey items on estimates of daily energy intake in juvenile chinook salmon . Env. Biol . Fish. 19:

(Salmo trutta L.) . Freshw. Biol . 2 : 1-18. Elliott, J .M . 1973 . The food of brown and rainbow trout (Salmo trutta and S. gairdneri) in relation to the abundance of drifting

131-137. Mason, J.C. 1976 . Response of underyearling coho salmon fry to

invertebrates in a mountain stream . Oecologia 12 : 329-347 . Elliott, J .M . 1975. Number of meals in a day, maximum weight of

40 :775-788 . Mason, J .C. & D .W . Chapman . 1965 . Significance of early emergence, environmetal rearing capacity, and behavioral ecology

food consumed in one day and maximum rate of feeding for brown trout, Salmo trutta L . Freshw. Biol. 5 : 287-303 . Elliott, J .M . 1984 . Numerical changes and population regulation in young migratory trout, Salmo trutta in a Lake District stream. J . Anim . Ecol . 53 : 327-350 . Elliott, J.M . & L . Persson . 1978 . The estimation of daily rates of food consumption for fish . J . Anim . Ecol . 47 : 977-991 . Fausch, K.D . 1984. Profitable stream positions for salmonids : re-

supplemental feeding in a natural stream . J . Wildl . Manage.

of juvenile coho salmon in stream channels . J. Fish . Res . Board Can . 22 :173-190. McNicol, R .E ., E. Scherer & E .J. Murkin. 1985 . Quantitative field investigations of feeding and territorial behaviour of young-of-the-year brook charr, Salvelinus fontinalis. Env. Biol. Fish . 12: 219-229. Mortensen, E . 1977. Density dependent mortality of trout fry

lating specific growth rate to net energy gain . Can . J . Zool. 62 :

(Salmo trutta L .) and its relationship to the management of

441-451 . Forrester, G .E. 1992 . Predator-prey interactions between fish and insects in streams . Ph. D. Dissertation, University of New

small streams . J . Fish Biol . 11 : 613-617 . Newman, R .M . & T.F. Waters . 1984 . Size selective predation on Gammarus pseudolimnaeus by trout and sculpins. Ecology 65:

Hampshire, Durham . 102 pp. Fraser, F.J. 1969. Population density effects on survival and

1535-1545 . Otto, C . & B .S . Svensson . 1980 . The significance of case material selection for the survival of caddis larvae . J . Anim . Ecol . 49:

growth of juvenile coho salmon and steelhead trout in experimental stream channels . pp. 253-265 . In: T.G . Knothole (ed .) Symposium on Salmon and Trout in Streams, Institute of Fisheries, University of British Columbia, Vancouver . Gannon, J .E . 1976. The effects of differential digestion rates of

855-865 . Puckett, K .J . & L .M . Dill . 1985 . The energetics of territoriality in juvenile coho salmon (Oncorhynchus kisutch) . Behaviour 92: 97-111. Robinson, R .W. & J .C . Tash . 1979. Feeding by Arizona trout

311

(Salmo apache) and brown trout (Salmo trutta) at different light intensities . Env. Biol . Fish . 4: 363-368 . Sagar, P.M . & G.J . Glova . 1988 . Diel feeding periodicity, daily ration and prey selection of a riverine population of juvenile chinook salmon, Oncorhynchus tshawwytscha (Walbaum) . J . Fish Biol . 33 : 643-653 . Stephens, D .W. & J .R . Krebs . 1986 . Foraging theory. Princeton University Press, Princeton . 272 pp. Tanaka, H . 1970 . On the nocturnal feeding activity of rainbow trout (Salmo gairdneri) in streams . Bulletin of the Freshwater Fisheries Research Laboratory (Tokyo) 20 : 73-82. Walsh, G., R. Morin & R .R. Naiman. 1988. Daily rations, diel feeding activity and distribution of age-0 brook charr, Salvelinus fontinalis, in two subarctic streams . Env. Biol. Fish . 21: 195-205 .

Warren, C.E., J .H. Wales, G .E. Davis & P Doudordorf. 1964 . Trout production in an experimental stream enriched with sucrose . J. Wildl . Manage . 28 :617-660 . Waters, T.F. 1972 . The drift of stream insects . Ann . Rev. Entomol.17: 253-272. Wentworth, R .S . & G .W. LaBar . 1984. First year growth and survival of steelhead stocked as fry in Lewis Creek, Vermont. N. Amer. J . Fish. Manag . 4:103-110 . Wilzbach, M .A., K .W. Cummins & J.D . Hall . 1986 . Influence of habitat manipulations on interactions between cutthroat trout and invertebrate drift . Ecology 67 : 898-911. Winer, B.J ., D.R . Brown & K .M . Michels . 1991 . Statistical principles in experimental design. McGraw-Hill, New York . 1057 pp.