FooD INTERRELATIONSHIPS OF THEſom en scULPIN.CŒTUS BĀ
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RAINBOW TROUT, SALĀTO GARDNERI, ÎN A TRIBUTARY OF LAKE SUPERIOR - PAUL R. HANNUKSELA
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ABSTRACT
FOOD INTERRELATIONSHIPS OF THE MOTTLED SCULPIN,
COTTUS BAIRDI, AND JUVENILES OF THE RAINBOW
TROUT, SALMO GAIRDNERI, IN A
TRIBUTARY OF LAKE SUPERIOR
By Paul R. Hannuksela
Young rainbow trout and sculpins occupy similar habitat in
many Michigan streams and thus, may compete for available food.
To examine this possibility, I investigated the production and food
habits of coexisting populations of the mottled sculpin and juveniles
of the rainbow trout in relation to the standing crop of bottom fauna
in a small tributary of Lake Superior. Age 0 and I rainbow trout
production from June to August 1970 was 2.8 g/ m”. Mottled sculpins,
age I and II, produced 1.5 g/m” during the same period. The bottom
fauna was composed mostly of Tendipedidae, Baetidae, Hydropsychidae,
Rhyacophilidae, Limnephilidae, and Gastropoda. The mean standing
crop of bottom fauna was 9.4 g/ m*. The rainbow trout and the mottled
sculpin had similar diets; bottom fauna was eaten in approximate
proportion to their abundance in the stream. - No mutual predation
occurred between the two fishes. The low total food consumption of
1.9 times the mean standing crop of benthos indicated that detrimental
food competition probably did not occur between rainbow trout and
mottled sculpins.
FOOD INTERRELATIONSHIPS OF THE MOTTLED SCULPIN,
COTTUS BAIRDI, AND JUVENILES OF THE RAINBOW
TROUT, SALMO GAIRDNERL IN A
TRIBUTARY OF LAKE SUPERIOR
By
Paul R. Hannuksela
A thesis submitted in partial fulfillment
of the requirements for the degree
Of Master of Science
1973
School of Natural Resources
Department of Wildlife and Fisheries
The University of Michigan
Committee members: -
Dr. Karl F. Lagler, Chairman
Dr. William C. Latta
ACKNOWLEDGMENTS
Drs. Karl F. Lagler and William C. Latta made up my
graduate studies committee and offered suggestions. Personnel
from the Marquette Fisheries Research Station, Michigan Depart-
ment of Natural Resources, assisted with the field work. James R.
Ryckman advised on statistical procedures. Thomas M. Stauffer
reviewed the manuscript. - -
The study was supported in part by funds from the Federal
Aid in Wildlife Restoration Act under Dingell–Johnson Project
F-31-R, Michigan.
ii
TABLE OF CONTENTS
ACKNOWLEDGMENTS . . . . . . . . . .
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . .
INTRODUCTION . . . . . . . . . . . . .
STUDY SITE . . . . . . . . . . . . . . .
METHODS . . . . . . . . . . . . . . . .
Fish Populations and Production
Bottom Fauna. . . . . . . . . . .
Fish Food Habits . . . . . . . .
Fish Food Consumption . . . .
Stream Measurements . . . . .
RESULTS . . . . . . . . . . . . • • * ~ *
Fish Populations and Production
Bottom Fauna. . . . . . . . . . .
Comparison of Food Habits . . .
Food Consumption . . . . . . . .
DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . .
CONCLUSIONS . . . . . . . . . . . . .
LITERATURE CITED . . . . . . . . . .
iii
iv.
Table
LIST OF TABLES
- Page
Estimated numbers of the rainbow trout and the
mottled sculpin and their mean length and weight
in the Little Garlic River, June–August 1970 . . . 9
Mean standing crop, production and food consump-
tion of the rainbow trout and the mottled sculpin in
the Little Garlic River, June–August 1970 . . . . 10
Taxa and number of Organisms in 10 bottom
samples each month, June and August 1970,
Little Garlic River . . . . . . . . . . . . . . . . 12
Percentages of total number and weight Of
organisms found in bottom samples and in the
stormachs of the rainbow trout and the mottled
sculpin, Little Garlic River, June and August
1970 . . . . . . . . . . . . . • • * * * * * * * * * * * 14
Analysis of variance of the total diet of the
rainbow trout and the mottled sculpin in the
Little Garlic River, June and August 1970 . . . . 15
Production of trout, salmon, and sculpins in
various streams, 15 June– 15 August 18
iv.
INTRODUCTION
Many Michigan tributaries of Lake Superior are nursery
streams that support substantial populations of juveniles of the
rainbow trout (Salmo gairdneri). Most of these streams also contain
large populations of sculpins (Cottus bairdi and/or C. cognatus).
During the first 1 to 3 years of life, young rainbow trout are in close
association with sculpins. Thus, trout and sculpins may compete for
available food and prey upon each other.
Prior investigations of food interrelationships have been done
on various species of salmonids and cottids. However, to my knowl-
edge, none has been done on the rainbow trout and mottled sculpin
(C. . bairdi) combination. Dineen (1951) found that the brook trout
(Salvelinus fontinalis), brown trout (Salmo trutta) and mottled sculpin
eat the same food. But he, as did Koster (1939), contended that food
competition was lessened by the ability of trout to feed in all planes
of the water from top to bottom, whereas sculpins are essentially
bottom feeders. Brocksen, Davis, and Warren (1968) concluded that
the reticulate sculpin (C. perplexus) could influence the food consump-
tion and production of the cutthroat trout (Salmo clarki) by cropping the
benthic food supply. They concluded that this reduced the supply of
drift organisms (which are important to trout [Hunt, 1965]), as well
as the benthos of the substrate.
The evidence regarding sculpin predation on juvenile trout
and salmon is conflicting. Koster (1939) and Patten (1962, 1971)
concluded that trout and salmon fry make up a negligible portion of
the diet of several species of sculpins. Conversely, significant
predation by the reticulate sculpin on rainbow trout sac fry (Phillips
and Claire, 1966) and by slimy sculpins (C. cognatus) on brown trout
sac fry (Clary, 1972) has been demonstrated in aquaria. Juvenile
rainbow trout in streams reportedly do not prey upon fish (McAfee,
1966).
To provide information on food interrelationships of popula-
tions of the rainbow trout and mottled sculpin coexisting in one stream
system, I investigated their food habits and production in relation to
the standing crop of bottom fauna.
STUDY SITE
This study was conducted on the Little Garlic River, a rain-
bow trout nursery stream tributary to Lake Superior in Marquette
County, Michigan. Field data were collected during 10–13 June,
8-9 July, and 5-7 August 1970, from a 300-m long stream section
some 2 km upstream from the river mouth.
The Little Garlic River flows for most of its length through
rugged terrain characterized by steep hills forested mainly with
mature hardwoods and some conifers. However, the study Site was
in that portion of the stream which flowed through an area of relatively
low relief. The stream banks were lined with alder (Alnus), aspen
(Populus), and willow (Salix). Emergent vegetation was lacking in the
study section. Submerged vegetation was primarily green and blue-
green filamentous algae sparsely covering the stream bottom.
- The stream bottom in the study section was estimated to be
47% rubble, 35% gravel, and 18% sand. Seventy per cent was riffle
area with pools making up the remaining 30%. Between June and
August, the stream became narrow and shallow because of receding
water levels. Mean width decreased from 6.5 to 4.8 m, mean depth
from 26 to 17 crin, mean volume of flow from 0.4 to 0. 1 m*/sec, and
the area of submersed bottom from 1,950 m” in June, to 1, 770 m?
in July, and 1,440 m” in August.
Conductivity was 153 umho/cm3 in August at 18 C. Mean
water temperature increased from 13 C in June to 18 C in August.
Age 0 and I rainbow trout and mottled sculpins were abundant
and adults of the longnose dace (Rhinichthys cataractae) Welſ’ e COIY) IO, OI).
in the study section. The brook trout, young (age 0) of the coho
salmon (Oncorhynchus kisutch), age II and older of the rainbow trout,
and the mottled sculpin were rare. Young-of-the-year of the rainbow
trout and of the mottled sculpin began to emerge in early June. The
other age groups of both species were present during the entire
study. -
Aquatic insect nymphs and 1arvae, snails, and Oligochaetes
were abundant in the stream substrate.
METHODS
Fish Populations and Production
Population estimates of age 0 (August only) and age I
rainbow trout and age I and II mottled sculpins were made in June,
July, and August using the methods described by Shetter (1957).
The fish were captured, marked by excising part of a fin, and
released one day. The following day the ratio of marked to
unmarked fish was determined. Calculations followed those of
Bailey (1951). All fish captured for the population estimate were
measured to the nearest millimeter, total length, and 10 from each
10-mm length group were weighed to the nearest 0.1 g to obtain
average weights of the age groups for calculating production.
TWO random samples of unmarked rainbow trout and
mottled sculpins were preserved in 10% formalin for determining
age and growth and food habits--35 rainbow trout and 53 mottled
sculpins in June, and 78 rainbow trout and 54 mottled sculpins in
August. These preserved fish were measured to the nearest
millimeter, total length, and weighed to 0.1 g. Scales were
removed from the rainbow trout at an area between the origin of
the dorsal fin and the lateral line and otoliths were excised from
the labyrinths of the mottled sculpin for age assessment. Otoliths,
cleared in hot xylene (Larsen and Skud, 1960), and scales, impressed
on plastic slides, were enlarged with a microprojector to a magnifica-
tion of 107X to determine the age of the fishes (Lagler, 1956).
Production of fish flesh was determined graphically by
plotting standing crops of rainbow trout and mottled sculpins in
numbers on the Ordinate and corresponding mean weights on the
abscissa (Allen, 1951; Chapman, 1965). Areas under the resulting
6
production curves were measured for the period 11 June to 6 August
1970. Estimated production of age 0 rainbow trout was low because
the August population estimate was used as the estimate (minimal)
of their numbers in June and July.
BOttom Fauna
Ten randomly selected samples of benthos (Hildebrand, 1971)
were collected on 12 and 13 June and 10 on 6 and 7 August with a
0.1 m” modified Hess sampler having size-30 mesh netting (Waters
and Knapp, 1961) and preserved in 40% isopropyl alcohol. In
collecting, the sampler was set on the sample site and forced into the
substrate as deeply as possible. Large rocks within the sample site
were individually cleaned of fauna. Fine substrate was sifted by hand,
causing the fauna to drift into the mesh bag of the sampler.
- The sugar flotation method of separating fauna from
inadvertently collected substrate and debris (Anderson, 1959) was
attempted, but filamentous algae hampered the technique. Manual
sorting of the bottom samples proved most feasible. Identification
of the Organisms followed Pennak (1953), with individuals being sorted
into the appropriate taxonomic group and counted.
To get standing crop, average weights of individuals in each
taxon were calculated for both sampling periods in the following
manner. Four bottom fauna samples were randomly selected from
each of the two sets of 10 bottom samples collected in June and
August. The known number of individuals in each taxon were
centrifuged for 2 minutes at 2500 rpm and weighed to 0.001 g.
Trichoptera were weighed without cases. An average weight for
individuals in each taxon was calculated. This average was used
to calculate weight of each taxon present in the stream and the
weight of food at the time of ingestion.
Fish FOOd Habits
Stomach contents of the random sample of preserved rainbow
trout and mottled sculpins were usually identified to family (Pennak,
1953; Ross, 1965), sorted into appropriate taxonomic groups, and
counted. A three-way analysis of variance (Freund, 1952) was used
to detect any differences between food habits of juvenile rainbow trout
and mottled sculpins. I tested the hypothesis that there was no
difference between the diets of the two fishes. Sources of variation
were the sampling dates, six food categories (classes and Orders) with
families as replicates, and the two fish species.
Fish Food Consumption
Total food consumption during my study was calculated by
summing the daily maintenance and the growth ration. The maintenance
and growth factors were adapted from other investigations. Allen (1951)
and Hopkins (1970) used a daily maintenance ration for brown trout of
1.2% of the standing crop of fish and a growth ration of 4.2 times brown
trout production. Bonham (1949) found the growth ration for rainbow
trout to be three times rainbow trout production. For purposes of
this study, I used 1.2% of the average standing crop of the rainbow
trout and of the mottled sculpin as the daily maintenance ration for
each, and three times the production of these fishes as the growth
ration.
Stream Measurements
Stream width in meters, depth in centimeters, rate of
current flow in meters per second (No. 622 Gurley current meter)
were measured at each randomly selected bottom sampling site.
Also, the substrate was classified as rubble, gravel or sand and
the area of each type was estimated as per cent of the total area.
RESULTS
Fish Populations and Production
Several changes occurred in the populations of the rainbow
trout and the mottled sculpin during the course of this study (Table 1).
Young-of-the-year rainbow trout were just beginning to emerge from
their redds in June; by August all had emerged and their population
amounted to 2,845 individuals (2.0/m”). Age I rainbow trout, which
numbered 724 (0.4/m”) in June and 464 (0.3/m”) in July, decreased
significantly to 263 (0. 2/m2) in August. This was probably due to
downstream, 1akeward migration (Stauffer, 1972). In June, age I
rainbow trout averaged 91 mm in length and weighed 6.9 g. They
increased to 99 mm and 9.7 g in July, and to 108 mm long and 11. 7 g
in August. Age 0 rainbow trout averaged 44 mm long and weighed
1.0 g in August. Biomass production of rainbow trout from June to
August was 2, 292 g (1.3 g/m”) for age 0 fish and 2, 480 g (1.5 g/m”)
for age I fish (Table 2).
Although only a few very small young-of-the-year mottled
sculpins were observed in June, many were present in July and
August, but they were too small for me to estimate their numbers.
Age I mottled sculpins numbered 1,930 (0. 9/m”) in June; 2, 879
(1.6/m”) in July; and 1,770 (1.2/m2) in August. Although the point
estimates differed among the three population estimates, the 95%
confidence limits overlapped widely (Table 1), which indicated that
there was no significant change in age I mottled sculpin numbers.
Age II mottled sculpins numbered 70 in June, 26 in July, and 24 in
August. Again, 95% confidence limits overlapped widely. Age I
mottled sculpins grew from means of 49 mm and 1.6 g in June, to
55 mm and 2.0 g in July, and to 61 mm and 2.6 g in August. Age II
8
Table 1. --Estimated numbers of the rainbow trout and the mottled
sculpin and their mean length and weight in the Little Garlic River,
June–August 1970
Population
e © Den- HiO – Mean Mean
Species, age estimate and & ge
Of e Sit mass length weight
and month 95% confidence (m.4) ( /m2) (mm) (g)
limits g g
Rainbow trout
Age 0
June 2, 845& 1.5 0.3 23 0.2
July 2, 845a. 1. 6 0. 7 35 0.4
August 2, 845 + 320 2. 0 2. 0 44 1. 0
Age I
June 724 + 206 0.4 2. 6 91 6. 9
July 464 + 112 0.3 2. 6 99 9. 7
August 263 + 45 0. 2 2. 1 108 11. 7
Mottled sculpin
Age I
June 1, 930 + 624 0. 9 1. 6 ºf $3 1. 6
July 2, 879 + 955 1. 6 3. 3 55 2. 0
August 1, 770 + 440 1.2 3. 2 61 2. 6
Age II
June 70 + 74 <0. 1 0.4 92 10. 9
July 26 + 16 <0. 1 0.2 93 12. 3
August 24 + 29 <0. 1 0.2 95 13. 1
* These values were derived from the August population estimate
and applied to measurements of a sample of young-of-the-year
in June and July to derive the matching values in each of the four
columns to the right.
10
Table 2. --Mean standing crop, production, and food consumption of
the rainbow trout and the mottled sculpin in the Little Garlic River,
June–August 1970
Species Mean e FOOd e
and standing crop Production consumption
age g g/mº g g/mº/ g g/mº/
- day day
Rainbow trout
Age 0 1, 517 0.9 2, 292 0.02 7, 914 0.1
Age I 4, 191 2.5 2,480 0.03 10,307 0.1
Total 5, 708 3.4 4, 772 0.05 18, 221 0. 2
Mottled sculpins
Age I 4, 483 2.6 2, 460 0.03 10, 446 0.1
Age II 466 0.3 - 80 <0.01 559 30. 1
Total 4, 948 2.9 2, 540 0.03 11, 005 0. 1
11
mottled sculpins attained means of 92 mm and 10.9 g in June,
93 mm and 12.3 g in July, and 95 mm and 13. 1 g in August. Change
in ichthyomass of the mottled sculpin production from June to August
was 2,460 g (1.4 g/m2) for age I fish, plus 80 g (0.1 g/m2) for age II
fish (Table 2). -
BOttom Fauna
Thirty-five taxa were found in the bottom samples (Table 3).
The most abundant groups were Tendipedidae, Baetidae, Hydro-
psychidae, Rhyacophilidae, Limnephilidae, and Gastropoda. These
taxa made up 85% of the total number of organisms and 82% of the
total weight. Biomass of bottom fauna was 16.1 kg (8.2 g/m2) in
June and 15.3 kg (10.7 g/m2) in August, with a mean of 15.7 kg
(9.4 g/m2). There was little change in biomass and species composi-
tion of the benthos between June and August.
Comparison of Food Habits
The 35 taxonomic groups were apportioned into 11 major food
categories to test relationships between numbers and weight of various
groups in the bottom fauna and the food habits of the rainbow trout and
the mottled sculpin (Table 4). Those taxa with an abundance of
individuals formed their own food item categories, usually families,
while taxa with few members were combined within appropriate higher
taxonomic groupings. The frequency of Occurrence and weight of
bottom organisms in the stomachs of both fishes was in approximate
proportion to bottom fauna abundance in the stream, with few excep-
tions. The rainbow trout fed slightly more frequently on baetid nymphs
and the mottled sculpin fed a little more often on Trichoptera naiads.
Adult stages of insects made up a small portion (9.5%) of the rainbow
trout diet, whereas, mottled sculpins ate none.
No significant difference existed between the diets of young
rainbow trout and mottled sculpins (Table 5). All observed values of
F for sources of variation and their interactions were not significant.
12
Table 3. --Taxa and number of organisms in 10 bottom samples each
month, June and August 1970, Little Garlic River
June August TOtal
Taxa Num-- Per Nunn - Per Num- Per
ber cent ber cent ber cent
Diptera
Tendipedidae 3, 443 59 2, 069 39 5, 512 49
Ceratopogonidae 145 2 258 5 403 4
Tipulidae 101 2 30 tr 131 1
Rhagionidae 16 tr 15 tr 31 tr
Simulidae 19 tr 3 tr . 22 tr
Tabanidae 2 tr 5 tr 7 tr
Empidae 5 tr 1 tr 6 tr
Cecidomyidae 2 tr 0 0 2 tr
Drosophilidae 1 tr 0 O 1 tr
Unidentified 4 tr 3 tr 7 tr
Ephemeroptera
Baetidae 1, 000 17 523 10 1, 523 14
Heptageniidae 57 1 168 3 225 2
Ephemeridae 16 tr 1 tr 17 tr
Unidentified 2 tr 0 0 2 tr
Trichoptera
Hydropsychidae 197 3 55.2 10 749 7
Rhyacophilidae 43 tr 398 8 441 4
Limnephilidae 265 4 129 2 394 4
Leptoceridae 4 tr 128 2 132 1
Hydroptilidae 100 2 O 0 100 1
Psychomyiidae 13 tr 50 1 63 1
Phryganeidae 2 tr 5 tr 7 tr
Unidentified 17 tr 13 tr 30 t]."
Coleoptera -
Finnidae 63 1 126 2 189 2
Unidentified 17 tr 7 tr 24 t;"
Plecoptera 86 1 90 2 176 2
Odonata 8 tr 4 tr 12 tr
Hemiptera 1 tr 6 tr 7 tr
Collermbola 3 trº 1 tr 4 tr
(continued, next page)
13
Table 3. --concluded
June August Total
Taxa Num- Per Nurm — Per Num — Per
ber cent ber Cent ber cent
Megaloptera 3 tr O 0 3 tr
Lepidoptera 1 tr 0 0 1 tr
Hymenoptera 1 tr 0 O 1 tr
Hydrac arina - 97 2 149 3 246 2
Araneae - 2 tr O 0 2 tr
Gastropoda
Physidae - 2 tr 498 9 500 4
Pelecypoda - 2 tr 7 tr 9 tr
Oligochaeta 90 1 61 1 151 1
Hirudinea 3 tr 11 tr 14 tr
Nematoda 11 tr 7 tr 18 tr
Fish" 6 tr 2 tr 8 tr
Unidentified 0 0 1 tr 1 tr
Totals 5, 850 5, 321 11, 171
Includes sac fry of Catostomidae and Cyprinidae.
14
Table 4. --Percentages of total number and weight of organisms found in
bottom samples and in the stomachs of the rainbow trout and the mottled
sculpin, Little Garlic River, June and August 1970
Percentage of total Percentage of total
Taxa number weight
Bottom Rainbow Mottled Bottom Rainbow Mottled
samples trout sculpins samples trout sculpins
Diptera
Tendipedidae 49. 2 37. 2 47. 1 4.4 5. 8 4. 2
Other 5. 6 10. 0 7.4 1. 8 6.4 3. 8
Trichoptera -
Hydropsychidae 6.7 4. 7 14. 7 62. 5 32. 1 55. 0
Limnephilidae 3. 5 1. 8 2. 2 2. 6 1.0 1. 7
Rhyacophilidae 3. 8 0. 5 0. 0 6. 0 3. 8 0. 0
Other 3. 1 2.4 5.4 1. 8 2.8 5. 5
Ephemeroptera
Baetidae 13. 6 28. 3 8.0 5. 8 24. 5 3. 2
Other - 2. 3 1.2 4. 2 1.9 1.4 6, 7
Other insecta 6. 1 5. 6 2.9 9. 5 10. 0 8. 0
Gastropoda 4. 5 7. O 6.4 1. 5 7.2 5. 9
Oligochaeta 1. 6 1. 3 1. 7 2. 2 5. 0 6. 0
15
Table 5. --Analysis of variance of the total diet of the rainbow trout
and the mottled sculpin in the Little Garlic River, June and August
1970 .
Degrees Sums - F
Source of
variation Of Of Mean
freedom squares square Observed Expected"
Months 1 2. 14 2. 14 0. 72 4. 35
Food groups * 5 19. 38 3. 88 1. 32 2. 71
Fish species 1 5. 28 5, 28 1. 79 4. 35
Months X Orders 5 9. 76 1. 95 0.66 2. 71
Months X species 1 5.36 5.36 1. 82 4.35
Orders X species 5 10. 81 2. 16 0. 73 2. 71
Months X Orders
X species 5 29. 56 5. 91 2. 00 2. 71
Error 20 59. 06 2. 95
Total 43 141. 55
* p = 0.05.
See Table 4 for food groups with families as replicates.
16
Even though rainbow trout fry were present in ever-
increasing numbers in the study section of the stream during June
to August, no fry were found in stomachs of either rainbow trout Or
mottled sculpins. The only fish encountered as a food item of another
fish was a young-of-the-year mottled sculpin in an age II mottled
sculpin.
Food Consumption
Total food consumption during the 57 days of the study was
29.2 kg, of which rainbow trout ate 18.2 kg, while mottled sculpins
ate 11.0 kg (Table 2). Rainbow trout consumed 319.7 g of food per
day (0.2 g/m2) which was 2% by weight of the mean standing crop of
benthos. Mottled sculpins ate 193. 1 g of benthos per day (0. 1 g/m2)
or 1.2% of the mean benthic biomass.
DISCUSSION
Production of the rainbow trout in the Little Garlic River
was within the range of values found elsewhere by others, particularly
in salmonid nursery streams (Table 6). Only one report (Allen, 1951)
had substantially greater salmonid production, but he was able to
compute values for more age groups than I did. Production values
for salmonids of corresponding age groups (Goodnight and Bjornn,
1971; Chapman, 1965) are similar to my findings. Information on
sculpin production is scant, but Goodnight and Bjornn (1971) reported
much lower production values for Cottus sp. than I found for the
mottled sculpin (Table 6).
A comparison of food habits and bottom fauna abundance
revealed that young rainbow trout and mottled sculpins fed on the
various bottom fauna in approximate proportion to their abundance
in the stream. An analysis of variance test confirmed that the
diets of these two fishes were similar.
No occurrences of mutual predation between rainbow trout
and mottled sculpins were found during this study. Even though
large numbers of the fry of both fishes were available for predation,
only one young mottled sculpin was eaten by a larger sculpin. This
supports the contentions by Koster (1939) and Patten (1962, 1971)
that sculpin predation on trout and salmon fry in streams can be
negligible. My finding of an absence of predation by the rainbow
trout on young of the mottled sculpin is augmented by McAfee (1966)
who contended that in general, juveniles of the rainbow trout in
freshwater eat mainly aquatic and terrestrial insects.
I could not accurately measure the extent of food competition.
My experimental design provided me only with data on standing crop of
bottom fauna. Total food consumption by the rainbow trout and
17
18
Table 6. --Production of trout, salmon, and sculpins in various Streams,
15 June – 15 August
Fish e d Produc –
Investigators Stream lS sº º tion
- - ge g D (g/m2)
Present study Little Garlic R., Rainbow trout 0–I 2. 8
Michigan Mottled sculpin I-II 1. 5
Goodnight and Big Springs Cr. , Rainbow trout 0-I 3. 8
Bjornn (1971) Idaho Cottus sp. I-II 0. 5
Lemhi R. , Rainbow trout 0–I 0. 6
Idaho Cottus sp. I-II 0.3
Chinook salmon 0-I 1. 7
Chapman (1965) Deer Cr., Coho salmon 0-I 1.5
Oregon
Flyn Cr. , Coho salmon O-I 1. 3
Oregon
Needle Branch, Coho salmon 0 — I 1. 2
Oregon
Hunt (1966) Lawrence Cr. , Brook trout 0-IV 1.6
Wisconsin
Section A
Lawrence Cr. , Brook trout 0-IV 2. 9
Wisconsin .
Section B
Allen (1951) Horokiwi Stream, Brown trout A11 9. 0
New Zealand
19
mottled sculpin for 57 days was 1.9 times the mean standing crop
of bottom fauna. Bottom fauna production values reported by other
investigators indicate that this level of consumption probably was
much less than the amount of benthos produced in the Little Garlic
River. Allen (1951) estimated that the total annual production of
bottom fauna in the Horokiwi Stream must have been 40–150 times
the average standing crop, based on fish production. Allen's
production estimates are regarded as being too high by Mann (1967).
Mann reported that in the littoral zone of lakes and in streams where
the fish population is at the maximum level permitted by food
resources, annual food production may be 10 or more times the
average standing crop of food. In support of this view, annual
productivity of bottom fauna living on a rock-outcrop in a southern
Piedmont stream was reported by Nelson and Scott (1962) to be
11-12 times the mean standing crop.
Daily food consumption by the rainbow trout was 2.0% of
the mean biomass of bottom fauna; the mottled sculpin consumed
1.2%. This rate of cropping was substantially lower than that which
Hopkins (1970) found for fish in a brown trout nursery stream. He
concluded that a 5 to 20% daily consumption of the standing benthic
population represented a high rate of cropping.
Based on the low total consumption and low daily cropping
rate, food competition between rainbow trout and mottled sculpins
in the Little Garlic River probably was not detrimental to either
because more food was produced than the fishes could consume.
CONCLUSIONS
Populations of the rainbow trout and the mottled sculpin
in the Little Garlic River exhibited production comparable to
salmonids in other waters, particularly salmonid nursery streams.
With minor exceptions, the food habits of the trout and sculpins in
the Little Garlic River were similar. Mutual predation did not
occur between or among the age and size groups studied. Detrimental
food competition probably did not occur because of the low rate of food
consumption compared to the abundance of food organisms.
20
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