446
THE RELATION BETWEEN GROWTH AND FOOD
CONSUMPTION IN THE BROWN TROUT
(SALMO TRUTTA)
BY F. T. K. PENTELOW
Fisheries Research Station, Alresford
(Received 15 February 1939)
(With Six Text-figures)
THE relation between the amount of food consumed and the growth made is one
of considerable ecological importance, though the study of it has hitherto been
confined almost entirely to domesticated animals living on artificial foods. In the
case of fishes there is a good deal of information available in American and German
literature on the food requirements of fish in fish farms and hatcheries but practically
none on the relation between the amount of natural food eaten and the growth rate.
Dawes (1930-1) studied this relation for the plaice, using chopped mussel (Mytilus
edulis) as food, and recently Surber (1935) has described an experiment, similar to
those on which this paper is based, on feeding brook trout (SalveHnus fontinalis)
and rainbow trout (Salmo gairdneri) with Gammarus fasciatus.
The experiments described in this paper were designed to compare the relations
between food and growth in a fresh-water fish with those of the plaice, and also
as a step towards the understanding of the part played by a carnivorous fish in the
ecology of a stream.
The experiments began in October 1933 and were continued, with some intermissions, until November 1935. Subsequently a series of experiments on the loss
of weight of starved fish were carried out in 1936.
GENERAL DESCRIPTION
Brown trout (Salmo trutta L.), obtained from a commercial hatchery and kept
in stock ponds at the Alresford station until required, were used for these experiments and proved to be very suitable. They withstood confinement well (two fish,
for example, lived in the experimental tanks from October 1933 until the summer
of 1935), were easily handled and their principal food in the Itchen, Gammarus
pulex, was easily obtainable in sufficient quantity.
The experiment was begun with twelve fish which were approximately eight
months old, having been hatched in the early spring of 1933. They were small for
their age, their weights varying from I-I to 3-0 g. Each fish was accommodated in
a wooden tank 56 cm. long, 26 cm. wide and 27 cm. deep (internal measurements)
fitted with a gauze lid. For convenience the tanks were arranged in a double row
supplied the tank immediately below. The two top tanks were fed by a bifurcated
pipe from the main laboratory supply, of river Itchen water. Although this water
was untreated it is unlikely that it carried any appreciable food, for animal plankton
(trout appear to be entirely carnivorous) is very scanty in the Itchen, and the fish
in the top tanks, which would have had the best chance of getting any food from
this source, showed no better growth than the others. Any larger organisms, which,
incidentally, were never observed in the water, would be excluded by the gauze
lids which also prevented the access of any air-borne food in the form of insects.
Fig. 1, The experimental tanks.
To prevent any loss of food the outlets from all tanks were covered with bronze
gauze of 1 mm. mesh. The tanks were numbered 1-6 on one side and 7-12 on the
other, and the fish were identified by the number of the tank they occupied. The
maximum and minimum water temperature was recorded each day in tanks 1 and 12.
With very few exceptions the fish were weighed weekly. They were dried gently
with a towel to remove as much surface water as possible and then placed in a tube
of water of known weight and weighed on an air-damped balance. The weights of
small fish were recorded to the nearest milligram, for, although it was originally
assumed that an accuracy of o-i g. would be sufficient, it was found that duplicate
weighings of the same fish gave values which did not generally differ by more than
a centigram. When the fish became so big that they could no longer be weighed
on a sensitive balance, they were dried, placed in a weighed towel and weighed on
a rough balance to the nearest gram.
448
F. T. K. PENTELOW
The fish were fed on live G. pulex (the fresh-water shrimp), obtained freshly
for each meal from the river Itchen or channels connected with it. They were
usually fed six times a week, no food being given on the day they were weighed,
but when it was necessary to miss a day they were given a double ration on the day
before. They were, however, always given a normal one-day ration on the day
preceding that on which they were weighed, to eliminate, as far as possible, errors
due to varying amounts of food in the gut. The shrimps were dried on blotting
paper and then weighed in a tube of water. So long as the amounts used were small
they were counted as well as weighed, in order to obtain figures which might later
be compared with data on the population of Gammarus in a river. The tanks were
examined every day, and any dead Gammarus were removed, dried on blotting
paper and weighed. Live Gammarus were not removed until the day on which the
fish were weighed. On this day all the tanks were thoroughly cleaned, the water
siphoned out and all Gammarus, alive or dead, were removed, dried on blotting
paper and weighed. By this method the amount of food eaten per week was
estimated.
In order to check this estimation, at one period during the experiment one tank
was left for 3 weeks without a fish in it; a weighed quantity of Gammarus was put
in each day, dead ones picked out and weighed daily, and the surviving Gammarus
recovered and weighed at the end of each week. In each of the 3 weeks the
weight of Gammarus (living and dead) recovered slightly exceeded that put in
(Table I).
Table I
Week ending
4 April 1935
11 April 1935
18 April 1935
Weight of Gammarus Weight of Gammarus
recovered (g.)
put in (g).
6710
10-014
5-270
6865
10-612
5-312
The differences are perhaps small enough to be due to experimental error; if
they have any significance they represent the amount of growth made by Gammarus
during the experiment. Clearly there was no loss of food during this time, and in
the feeding experiments the difference between the weights of food put in and
recovered can be taken to be the amount eaten by the fish.
Six fish, those in the odd-numbered tanks, were given an excess of food in order
to obtain data on the relation between the amount of food eaten and the growth
made in fully fed fish. The rations of the other six were carefully regulated in an
attempt to keep the fish at constant weight from week to week, in order to estimate
the amount of food required for the bare metabolic needs of the animal, allowing
no excess to be used for growth. It will be shown that it is impossible to foretell
exactly how much food will be required for this purpose, but a fair measure of
success was achieved.
The fish were first placed in the tanks and fed on Gammarus on 12 October 1933,
and, after some preliminary trials, routine weighings were begun on 2 November.
Relation between Growth and Food Consumption in the Brown Trout 449
They were continued without intermission until 31 August 1934. Trout 10 died
on 24 November and no. 2 on 24 December. They were replaced, no. 10 by a fish
of approximately the same size (no. 10 A), and no. 2 by one a year older and weighing
12 g., but this fish was lost after 1 week and another fish (2B) weighing 14 g. was
substituted. Owing to tank 7 developing leaks large enough to allow Gammarus
to escape, the data provided by fish 7 were unreliable until the leaks were finally
stopped on 12 April 1934. On 14 June 1934 all the fish except nos. 1, 2B and 3
were killed accidentally, and until the end of August the experiment was continued
with the three survivors only.
The experiment was then discontinued for a period of 7 weeks. During this
time, as at all other periods when the work was discontinued, the fish were retained
in the tanks, fed on Gammarus and the temperatures were recorded daily but neither
the fish nor food were weighed.
On 18 October the experiment was restarted. Fishes 1 and 3 were given an
excess of food and no. 2B a maintenance ration as before, and six more fish were
placed in tanks 7-12. These fish will be referred to as nos. 7A, 8A, 9A, 10B, 11A
and 12 A. They were approximately 8 months old and their weights ranged from
1 to 9 g. The rationing of these fish was different from that of the corresponding
numbers in the original experiment; nos. 7A and 10B received maintenance rations,
nos. 8 A and 12 A were given an excess of food, and nos. 9 A and 11A were given
about 1 -7 and 0-75 g. of food per week respectively. These amounts were roughly
twice the maintenance requirements of fishes of their size, and it was hoped to see
in the early weeks of the experiment whether fish on an intermediate ration used
their food more efficiently for purposes of growth than did those receiving an excess
of food (cf. Dawes, 1930-1), but insufficient data on this point were obtained. As
the fish grew this constant amount of food became a smaller proportion of the body
weight until eventually it was not sufficient to allow any further growth, and it was
hoped by keeping the amount of food constant to obtain variations in the weight
of fish corresponding to variations in the amount of food required for maintenance.
Fish 10 B died after 2 days and was replaced by another somewhat larger trout (10 C).
The experiment was suspended from 21 December to 10 January 1935, when it
was continued with the same fishes and the same rationing system until 18 April.
On 7 March it was impracticable to continue to weigh fishes 1 and 3 (which now
weighed over 50 g.) on an accurate balance, and thenceforward they were weighed
to the nearest gram as described on p. 447.
On 28 March, tank 12 was found to be empty, the fish having escaped. The
weighings were again suspended from 18 April to 9 May. Then they were recommenced and continued without intermission until 8 August. On 6 June fish 1
jumped from the trough in which it was confined whilst the tank was being cleaned
and died. When the experiment began in November 1933 this fish weighed 3 g.
and at the time of its death it weighed 91 g. On the same day (6 June) a trout fry
about 3 months old was weighed and placed in tank 12. This fish (12B) was fed
on young Gammarus of such a size that it could easily swallow them until it grew
sufficiently to take shrimps of the ordinary size. On 24 June fish 3 was found dead,
F. T. K. PENTELOW
45°
presumably owing to the effect of the high water temperature on a comparatively
large fish in a small tank. At the beginning of the experiment this fish weighed
3 g., and at the time of its death 99 g.
On 8 August the experiment was again suspended until 19 September. From
then until 7 November, when the experiment was concluded, no more interruptions
occurred. Fish 2B was found dead on 17 August, but there were no further losses.
During 1936 eleven experiments were carried out to determine the loss of weight
of fish kept without food for a week. The fish were taken from a stock pond, dried
on a towel, weighed and kept, each usually in a separate tank, for a week without
food and then reweighed. At first a number of fish were weighed daily, but this
involved too much handling and all these fish died within the week.
During the final experiment in November the fish, after being weighed at the
end of a week's starvation, were replaced in the tanks and starved for another week.
They all survived, but the loss of weight in the second week was considerably less
than in the first. The implication of this will be discussed later.
THE GROWTH OF THE FISH
To avoid any more handling of the fish than was necessary no measurements
of length were made, and the only data obtained on the rate of growth were the
figures for increase of weight. Similar series of determinations of the rate of growth
of trout do not appear to be available elsewhere, and they are worth a brief discussion
purely as measurements of growth.
The.growth of fully fed fish
For the study of the growth of fish which were at all times supplied with an
excess of food, and where therefore the amount of food available was not a factor
limiting the rate of growth, the data given in Table II are available.
Table II
Fish no.
7
12A
12B
5
9
11
Duration of
observations
days
70
154
iS4
224
224
224
Initial weight
Final weight
Total increase
g-
g-
g-
2-4
6-7
15-0
8-4
4-3
"•5
77
203
i7'3
i4'3
3'5
07
30
2'4
21
l6-7
7-8
8A
385
18
8I-I
1
581
3°
91-0
3
595
30
990
5'7
793
880
960
The data are clearly insufficient to allow any statistical investigation of growth
in brown trout, yet, as Minot (1891) has pointed out, the regular measurements of
individuals over a considerable period of time often reveal features of growth which
are masked or misinterpreted when the figures are based on the average of a large
number of determinations.
Relation between Growth and Food Consumption in the Brown Trout 451
There are considerable variations in the rate of growth expressed as increase in
body weight per unit time. The fish which grew most rapidly was 8A, which
increased its original weight by nearly 80 g. in 385 days, whilst no. 11, which showed
the poorest growth, only increased by 57 g. in 224 days. Since, as will shortly be
shown, this rate of growth is continually changing, determination of the average
amount of growth per day from thesefiguresgives an entirely misleading comparison
between one fish and another and cannot therefore be used.
Surber (1935) found that American brook and rainbow trout fed solely on
Gammarus fasciatus gave the growth in weight as shown in Table III.
Table III. Brook trout (SalveUnus fontinalis)
Fish no.
1
2
3
4
Duration of
observations
days
iSi
iSi
iSi
iSi
Initial weight
g-
Final weight
g-
2O-O
109-0
II2-O
ioo-o
820
23-5
I9-0
22-0
Total increase
g.
890
88-s
8i-o
6o-o
Rainbow trout (Salmo gcrirdneri)
Fish no.
Duration of
observations
days
Initial weight
g-
Final weight
g-
Total increase
g-
1
2
3
4
iSi
ISi
iSi
iSi
30-0
23-0
130
970
84-0
67-0
102-0
67-0
6i-o
S4-o
91-0
II-O
It will be seen that the fish used by Surber grew considerably faster than the
brown trout. There are several possible explanations of this fact. First, the American
species may be faster growing than the British one. Secondly, in Surber's experiment the water temperature throughout the experiment was constant at 54° F. In
the experiments described in this paper the temperature varied considerably, and
it will be shown later that the increase in weight in unit time was greater when the
water temperature was between 50 and 6o° F. Thirdly, the rainbow and brook
trout were considerably larger when measurements began than were the brown
trout, and when we come to discuss the weekly increments in weight it will be seen
that they increase, within limits, with the size of the fish.
It may here be noted that the brown trout used in these experiments appeared
to grow as quickly as fish of the same age in the Itchen at Alresford. Wild fish in
this area are generally from 6 to 8 in. long when they are 2 years old, and in this
experiment fishes 1 and 3 were both 8 in. long when they died in June 1935, at
approximately z\ years old, and fish 8 A was 8 in. long in November 1935 when it
was only if years old.
The complete records, showing the increase of weight from week to week have
been filed and are available to anyone interested. Many are illustrated graphically
J E B • XVIIV
30
F. T. K. PENTELOW
452
in Fig. 2. The curves are not smooth, and, since the weighings were not begun until
the early phases of growth were past and were discontinued before the fish became
mature, they are by no means complete, but in form they resemble those for increase
in weight in man, guinea-pig, mouse and carp given by Ostwald (1908) and for the
plaice by Dawes (1930-1).
100-
9080-
t
1
zo\
^
to-
T
M&U
F/'sh
Date
2 7 4 11
J 3 7 5 Z 6 4 1 6 3
XI XII I II III IV V VI VII Ml IX X XI XII I
&J3 I9U
7 7 4 Z 6 4 1 S3
7
II III IV V VI VII VIII II X XI
1933
Fig. 2. The growth of fully fed fish.
They show that, except at certain periods, increasing size was accompanied by
increasing increments in weight, so that when the figures are plotted a curve concave
to the ordinate is produced. It may be assumed that, had the measurements continued, at some point this relation would no longer hold, increasing size being
accompanied by diminishing increments and the curve would become.convex, thus
producing the S-shaped curve typical of growth. There is no indication that this
point of inflexion was reached in these experiments.
Relation between Growth and Food Consumption in the Brown Trout 453
All the fish used grew very slowly from November 1933 to about the middle of
February 1934, and from then onwards the rate of growth increased, though it was
more marked in some fish, e.g. no. 5, than in others, e.g. no. 11, until 14 June, when
four of the six fish were killed. The rate of growth of fishes 1 and 3 appeared to
diminish somewhat in July but increased again in August and continued to be rapid
until the beginning of November when it declined, and indeed on 30 November
both fishes weighed less than they did the week before. After this, however, rapid
growth recommenced, to be interrupted again in the middle of January, an interruption that was more marked and persistent in no. 3 than in no. 1. Thereafter th*e
growth rate of both fish continued high until both fish died in June. The growth
of fish 8 A was more regular. Growth was rather slow when thefishwas first brought
into the experiment in October 1934, but it increased fairly steadily up to the middle
of July. Then growth practically ceased for a fortnight and was slow until the end
of August. Then the rate increased and remained high until the experiment was
discontinued on 7 November 1935. It is noteworthy that all three fish showed a
period of less rapid growth at midsummer as well as during the winter.
The variations in growth rate are, however, very much more easily realized and
measured if successive increments of weight are plotted against time and a curve
drawn through the points so obtained. This is the acceleration curve. It rises with
an increasing growth rate, is parallel to the axis when the rate is constant, and falls
when the growth rate diminishes. The weekly increments have been plotted and
curves drawn for fishes 1 and 8 A in Fig. 3. The other fish show similar features and
have been omitted in order to avoid undue complication of the graph.
The figure shows that the actual growth rate constantly varied, periods of rapid
growth alternating at frequent intervals with periods of slow growth, and that it
was only occasionally that the rate was the same even for a fortnight. A similar
phenomenon has repeatedly been observed when a growing mammal is weighed
at regular intervals (Davenport, 1908, footnote on p. 288) but it does not appear
in the data for the growth of plaice given by Dawes (1930-1), either because in this
animal the growth rate is more regular or because the fish were not weighed
frequently enough to record it. The variations of the growth rate deduced from the
study of Fig. 2 represent therefore changes in the average rate over a period of
some weeks, for it is only when the differences are considerable and continued for
several successive weeks that, owing to the scale of the diagram, the slope of the
curve is appreciably affected.
The growth offish on restricted rations
It has already been explained that a number of fish were given a restricted
amount of food so adjusted as to maintain their weight approximately constant.
The growth of these fish from week to week cannot therefore profitably be discussed
except as part of the study of their food requirements. At intervals, however, the
experiment was suspended and during these periods thefishwere given an unweighed
and presumably ample ration and were therefore free to grow. Their growth rate
at these times is worth a brief examination.
30-2
F.
454
T.
K.
PENTELOW
4th
•60$,
•4
Food eaten Fish N° I
Growth Incrtmtnts Ftsh
•—«
-
-
-
8A
Ttirptralurt
kkfj, Ending g 7 4 11
J J 7 J 2 6 4 1 6 3 7 7 4 2 6 4 I J 3 7
xi XII i ii III iv v vi M vti ix x xi XII i ii in iv v vi va vm ix x xi
m
1954
I9H
Fig. 3. Increments of growth, food eaten (fishes 1 and 8 A) and temperature.
Fish 2B was given unweighed quantities of Gammarus from 31 August 1934 to
18 October 1934. During this period its weight increased from 15-8 to 27-6 g., an
increment of 11-8 g. During the same period fish 1 showed an increase from 24-6
to 32-7 g., i.e. 8-i g., and fish 3 from 27-9 to 35-6 g., i.e. 7-7 g. The experiment was
again suspended from 20 December 1934 to 10 January 1935. During this period:
Fish 2B increased from 27-4 g. to 35-0 g., i.e. 7-6 g.
».
» 9'3 g- .. I I - I g- .. 1>8 g»» 7 A
.. 10C
„
„
8-2 g. „ 87 g. „ o-5g.
During the same period the fish which had at all times received ample food showed
the following increases:
Fish 1 from 42-0 g. to 45-1 g., i.e. 3-1 g.
„ 3
„ 42-4 g. „ 45-6 g., „ 3-2 g.
„ 8A „ 3-5 g. „ 4-4 g., „ 0-9 g.
„ 12A „
7-6 g. „ 9-1 g., „ 1-5 g.
Relation between Growth and Food Consumption in the Brown Trout 455
The experiment was suspended again from 18 April to 9 May. The fish usually
kept on restricted diets showed the following increases :
Fish 2B from 31-7 g. to 41-9 g., i.e. 10-2 g.
» 7 A .. I I - 5g- »» i5"6g-> .. 4 - I g„ 10C „
97 g. „ 13-4 g., „ 37 g.
whilst the fully fed fish showed the following growth:
Fish 1 from 74 g. to 78 g., i.e. 4 g.
„ 4g.
,, 3
.. 6 5g- » 69 g.
„ 8A „ 12-4 g. „ i6-8g., „ 4-4 g.
During the period between 8 August and 19 September the following increases
were obtained:
Fish 7A from 12-8 g. to 22-1 g., i.e. 9-3 g.
„ 10C „ • n - 6 g . „ 19-9 g., „ 8-3 g.
During the same period the other fish grew as follows:
Fish 8 A from 447 g. to 56-0 g., i.e. 11-3 g.
„ 12B „
27 g. „ 5-0 g., „ 2-3 g.
These data show that whenever its food supply was uncontrolled fish 2 B increased
its weight more over the same period than did nos. 1 and 3. There is here some
indication that a fish, whose growth has been retarded by lack of food, can, when
food becomes abundant, grow more rapidly than those whose previous growth has
been unrestricted. Fishes 7 A and 10 C, when their food supply was unregulated,
grew at approximately the same rate as no. 8 A and do not appear therefore to
support this view, but these fish were never kept on maintenance rations for more
than 14 weeks without a break, whereas no. 2B, from the time it came into the
experiment, was rationed for 34 weeks continuously. As during the intermissions
the fish were free to grow it is clear that nos. 7 A and 10 C were never so far removed
from the normal weight for their age as was no. 2B, and we should not expect
therefore such striking effects on their growth rate.
Minot (1891) found that if any of his guinea-pigs showed for a period a growth
rate lower than normal, e.g, on account of illness, the loss was very soon made up
and, as he says, "each individual appears to be striving to reach a particular size".
It seems to be a reasonable hypothesis that any young animal has the power to
compensate for a period of reduced growth.
Temperature and growth
The average weekly water temperature has been plotted in Fig. 3, and it is quite
clear by inspection of this curve and the weekly growth increments of the fish plotted
on the same graph that there is no simple and direct relation between the amount
of growth made in any one week and the temperature of the water during that week.
This does not mean, of course, that differences in temperature have no effect on
the growth rate of trout; the experiments were not designed to study the effect of
temperature, and there were too many other uncontrolled variables to enable it to
F. T. K. PENTELOW
456
MO-
160-
l
%
§80
SO
60
7emperature V
Fig. 4. The relation between temperature and growth.
70
Relation between Growth and Food Consumption in the Brown Trout
457
Table IV. Summarized data for fully fed fish
Fish no
Temp.
0 x?
r.
4 weeks from 9 Nov. 1933
3
,,
, , 7 Dec. 1933
4
,,
,, a8 Dec. 1933
3
,,
„ 25 Jan. 1934
4
,,
,, 15 Feb. 1934
5
..
.. '5 Mar. 1934
4
>,
,, 19 Apr. 1934
S
„
„ 17 M a y 1934
7
„
,, ai June 1934
4
„
, , 9 Aug. 1934
(")
(*)
Weight
offish
Food
45
a-8
38
42
2-8
2-9
40
3'i
42
46
36
5-J
S2
8-o
59
66
125
18-0
22'I
62
eaten
38
78
126
149
217
362
579
765
339
36'O
"5
48
39-5
212
„ 17 Jan. 1935
. . 7 Feb- 1935
41
468
112
44
172
•S3
3
3
„
45-3
46-5
492
81
"3
—
104
+ 9
57-2
278
+ 60
85-2
491
+ 72
2-6
3-o
3-3
37
4'4
5'9
257
143
+ •48
+ 27
2'2
+
+
+
+
25
26
2-8
33
4-2
6-2
38
+ 100
+ 190
87
134
777
529
+ 77
3-9
55'
+ 110
34°
+ 62
+ 118
48
67
301
410
+ 87
97
180
II-2
277
188
+ 44
+ 73
+ 44
46
798
2 weeks from 35 Oct. 1934
4
„
, , 8 Nov. 1934
3
„
, , 6 Dec. 1934
5°
2'0
44
48
2-3
3'i
447
17 Jan. 1935
41
44
41
48
5'7
6
4
3
,,
..
..
,, 16 M a y 1935
.. 27 June 1935
.. 35 July 1935
3
3
..
„
. . 3 Oct. 1935
„ 24 Oct. 1935
11
172
182
214
33i
510
445
,, 28 Feb. 1935
„ 21 Mar. 1935
2
+ 25
9
42
40
42
2-8
8A
231
393
242
206
.46
J04
>59
194
22
35
37
5i
7
47
+ 136
+ 27
+ 9
+ 28
45
35
49
53
94
+ 108
+ 128
2-4
12A
+ 54
+ 79'
+ 75
+ 116
390
+ 25
+ 36
— 3
+ 24
+ 30
+ 5°
45°
+ 75
820
. +129
12B
+ 111
47
7-1
9-8
438
55
67
24'7
39'1
657
667
64
439
464
+ 56
+ 26
2-3
53
49
653
755
352
286
+ 64
+ 5i
6-4
77
134
+ 63
+ 107
+ 98
187
+ 42
28
28
21
58
565
106
213
315
+
+
+
+
790
38-4
406
247
eaten
to
Growth
made
162
367
838
59
5'i
7-2
(*)
Food
10-7
+ 47
+ 67
55
52
4-3
+ 127
+ 130
„
,,
„
18
37
39
68
526
417
„
3
5
57
(a)
Weight
offish
775
6
Feb. 1935
+
+
+
+
+ 135
+ 110
280
„
160
172
154
237
305
374
8-i
13-2
19-2
660
, , 7
221
35
3-8
167
3i3
533
47
„
3'2
+ 4
4-6
4'
„
+ 4i
i95
38 Feb. 1935
„ 31 Mar. 1935
3
218
165
41
„
3
made
+ 36
+ 60
+ 119
3
5
Fi«h no.
eaten
3-6
37
+
+
+
+
Fish no.
4 week* from 9 Nov. 1933
3
„
, , 7 Dec. 1933
4
„
„ 38 Dec. 1933
3
„
„ 35 Jan. 1934
4
..
.. 15 F e b - 1934
5
„
„ 15 Mar. 1934
4
..
>> 19 Apr. 1934
5
,,
„ 17 May 1934
to
Growth
3'2
50-4
S6-i
16 May 1935
(ft
Food
— 20
+ 33
+ 17
+ 42
5°
„ 25 Oct. 1934
, , 8 Nov. 1934
, , 6 Dec. 1934
(0)
Weight
offish
+ 4
+ '4
+ 43
+ 59
44
„
,,
„
to
Growth
made
553
356
207
2
4
3
5
3
1
+ 169
(a) Mean body weight in g.
(ft) Mg. of food eaten per week per g. of mean body weight.
(c) Mg. of variation of body weight per week per g. of starting weight.
i'5
975
783
+ 196
+ 142
497
+ 83
+ 64
341
F. T. K. PENTELOW
458
be clearly shown in the primary data. If, however, the experiment is divided into
periods during which the temperature remained approximately constant, the average
temperature for these periods calculated, and the growth data from all fully fed
fish used, then some idea of the optimum temperature conditions for the growth
of brown trout can be obtained. Since these periods were not all of the same length,
and the fish differed in size, it was necessary to reduce the growth data to some
common unit. The method adopted was to calculate the variation in weight (increase
or decrease) in milligrams per week per gram of the weight of the fish at the beginning of each period (known for convenience as the starting weight). This method
of presenting the data is similar to that used by Minot (1891), and it has been
criticized by D'Arcy Thompson (1917) on the grounds of unnecessary complexity
and by Gray (1929) because it is based on the assumption that the amount of growing
tissue in an organism is directly proportional to the whole weight of the organism.
As, however, no better way of comparing the growth of one fish over a given period
with that of another of a different size over a different period has been found, this
method has necessarily been used, though its limitations have been fully realized.
In Table IV are shown, then, the average temperatures of the various periods
into which the experiment was divided, the weight of the fish at the beginning of
each period, the average increase or decrease in weight per week expressed as
milligrams per gram of the starting weight, and the average amount of food consumed per week by fully fed fish also expressed as milligrams per gram of the mean
body weight. Discussion of this last column will be deferred for the present.
From these data a dot diagram (Fig. 4) has been prepared to show the relation
between the increase in weight per week and the average temperature. It is immediately apparent that no curve can be drawn to express this relation, for there were
large variations in the amount of growth at all temperatures, but it does appear
that on the whole growth increased with increasing temperature from 38 to 500 F.,
reached a maximum between 50 and 6o° F. and declined when the temperature
exceeded 60° F. This is more clearly shown if the data are arranged in a correlation
table in which the temperatures have been grouped at 50 intervals and the increases
(or decreases) of weight at 20 mg. intervals and the number of times each growth
class was associated with each temperature class recorded (Table V).
Table V. Growth in mg. per week per g. of starting weight
Temp.
36-40
41—45
46-50
51-55
56-60
61-65
66-70
0
1—
20
40
00
mg.
mg.
mg.
mg.
-19-
3
21-
2
6
2
5
10
7
5
6180
mg.
1
5
3
3
1
1
1
•
3
3
81-
101-
100
120
12:140
141160
161180
200
mg.
mg.
mg.
mg.
mg.
mg.
1
1
6
.
1
1
•
2
3
•
4
181-
1
1
1
It appears then that increasing temperature up to about 6o° F. was accompanied
by an increase in the growth rate of these fish, but that above 6o° the growth rate
Relation between Growth and Food Consumption in the Brown Trout 459
tended to diminish. It has previously been noted that in both years of the experiment there was a period in the summer when the growth of fully fed fish was slow,
and it seems likely that when the water temperature exceeds 6o° F. the optimum
conditions for trout growth have been passed. From the known facts of the
geographical range of the brown trout (Tate Regan, 1911) we should expect that
high water temperatures would not be the most favourable for the growth of these
fish, for they are natives of northern lands and inhabit the colder parts of streams,
the head-water or spring-fed portions. In the southern half of their range they do
not occur in the lowland rivers which become warm in summer, and although they
have been introduced in the tropics it is only where, as in Kashmir or Kenya, the
effects of latitude are modified by the considerable altitude of the streams that they
have been successful.
The relation between food and growth
A fish, like any other organism, needs food to provide the energy for its vital
processes such as respiration, digestion and excretion, and for its various activities
involving movement of the body. If it obtains more food than is necessary for these
metabolic needs the surplus is available for increasing the body substance, that is,
growth. Before, therefore, the relation between food and growth can be discussed,
it is necessary to determine what proportion of the food is required for the maintenance of the ordinary activities of the trout. It should be noted that as this study
was primarily ecological no attempt has bfcen made to assess the energy value of
Gammarus as food nor to discuss its composition in physiological terms. It must
also be understood that the relation between the arnount of food required to maintain
the fish at constant weight and the basal metabolism of the fish cannot be determined from these data, for, within the limits of their tanks, the fish were free to
move as they liked and clearly their activity is a factor of unknown magnitudeaffecting the data. Yet, since all the fish, both those on maintenance rations and
those fully fed, were kept under uniform conditions it is reasonable to suppose that
the figures obtained from the different fish are approximately comparable.
The maintenance requirements of trout
To determine the "maintenance" requirements a number of fish were given
restricted rations which were adjusted from week to week in an attempt to maintain
the weight of the fish constant. The full weekly data have been filed for reference;
they show that the experiment met with a very fair degree of success, for, although
in only one instance was the weight of the fish the same in two successive weeks,
the variations were very small and the amount of food given in any week was a
reasonable approximation to the maintenance requirements of the fish.
It was at once apparent that, as would be expected, the maintenance requirements depend on the size of the fish and they vary seasonally, being high in the
summer and low in the winter. In order to eliminate, therefore, the factor due to
the varying sizes of fish used in the experiment and to investigate the seasonal
changes in food requirements, in Table VI the data have been grouped and
460
F. T. K. PENTELOW
Table VI. Summarized data for fish on maintenance rations
Fish no
3
Temp.
0
F.
4 weeks from 9 Nov. 1933
3
„
, , 7 Dec. 1933
4
,,
„ a8 Dec. 1933
3 „
„ 35 Jan. 1934
4
„
„ 15 Feb. 1934
5
..
>, 15 Mar. 1934
4
„
„ 19 Apr. 1934
5
„
„ 17 May 1934
7
„
„ 31 June 1934
4
„
, , 9 Aug. 1934
45
38
a
4
3
„
..
„
3
3
3
5
6
4
3
(a)
Weight
offish
(6)
Food
eaten
I-I
100
11
79
95
to
43
•143
14-8
14-7
103
103
46
147
"3
53
I4'3
130
59
66
6a
138
145
333
262
I5'4
186
+ 3
+ ia
-'7
+ 4
- 3
+ I
-'5
+ 4
+ 7
+ 15
„ 35 Oct. 1934
. . 8 Nov. 1934
, , 6 Dec. 1934
50
263
267
37-3
93
-35
80
64
+ 8
+ 4
,.
„
„
„
„ 17 Jan- 1935
, , 7 Feb. 1935
, , a 8 Feb. 1935
„ 31 Mar. 1935
41
58
-40
60
47
3i'7
30-9
31-4
3.6
51
+ 3
+ 8
- 6
,,
„
„
„
„
,,
55
67
64
38-0
35-8
347
50
-17
66
no
— 31
45
38
a-o
16 May 1935
37 June 1935
35 July 1935
43
4°
44
48
44
41
a-i
4°
a-o
a-o
42
a-i
46
3-1
53
59
a-o
••9 .
43
Fish no.
79
89
75
98
88
no
104
105
139
a8a
53
0
ta-5
— 11
a-3
55
76
+ 13
3-3
33
+ 6
10a
1
3-3
"3
— 10
a-a
89
88
"7
373
+ 3
a-3
365
81
—
7A
to
Growth
made
a-i
95
— 13
a-1
83
81
-13
96
97
+ 8
+ 3
103
153
314
— 13
2'O
2-0
a-o
a-o
19
i-8
0
0
+ 1
13
— 3
+ 6
a-3
65
96
83
3'3
100
76
76
+ '5
+ 3
-16
a-4
a-3
a-a
109
— 11
+ 1
3-3
357
-
-36
- 7
3-4
3-4
-19
+ 8
+ a
— a
10C
0
—
1
8-4
86
78
+ 4
8-3
80
84
74
54
5»
- 7
+ 13
8-7
8-8
+ 7
9-1
0
95
84
77
66
57
5°
— 2O
130
71
-34
ia-i
n-6
54
69
"4
19a
197
73
54
9-1
9-1
9-3
107
80
3
3
3
5
..
,,
„
„
,. 17
, , 7
,,28
„ 21
4i
io-8
44
II-I
6
4
3
„
..
..
„
..
..
16 May 1935
37 June 1935
as July 1935
55
67
64
143
130
ia-6
"5
+ 10
3
3
..
„
..
3 Oct. 1935
, , 3 4 Oct. 1935
53
49
ao-s
ao-8
74
54
+ 8
Fish 3 B.
- 7
+ 15
- 7
+ 5
+ 9
- 6
-'7
+ 33
(*)
Food
eaten
10
5°
44
48
47
10a
1-4
„ 25 Oct, 1934
n
8 Nov. 1934
, , 6 Dec. 1934
n-4
11-4
96
0
„
»
„
41
116
to
(a)
Growth Weight
made
offish
+ 3
2
4
3
J«n. 1935
Feb. 1935
Feb. 1935
Mar. 1935
•5
•5
•5
5
•6
•6
'•5
•5
8
Fish no.
4 weeks from 9 Nov. 1933
3
„
, , 7 Dec. 1933
4
„
„ 38 Dec. 1933
3
„
,, as Jan. 1934
4
„
„ 15 Feb. 1934
5
..
.. IS Mar. 1934
4
,.
„ 19 Apr. 1934
5
„
„ 17 May 1934
5'
6
4
(a)
(*)
Growth Weight- Food
eaten
offish
made
—
0
—
—
-
3
6
+ 3
+1
+ 31
+ 6
-
6
-33
-
3
+ 7
+ 8
f Fish 10A.
(a) Mean body weight in g.
(b) Mg. of food eaten per week per g. of mean body weight
(c) Mg. of variation of body weight per week per g. of starting body weight.
— 10
— 10
4
Relation between Growth and Food Consumption in the Brown Trout 461
recalculated. The experiment has been divided into periods of differing lengths,
but of approximately constant temperature, the mean temperature and the mean
body weight of each fish have been calculated and the amount of food eaten has
been expressed as milligrams per week per gram of the mean body weight and the
variations in weight as milligrams per week per gram of starting weight, for each
period.
Since in only a few cases was the weight of a fish the same at the end as at the
beginning of a period, it is impossible in most cases to state the exact maintenance
requirements of the fish but, so far as can be judged from the amount of food eaten
and the gain or loss in weight, the requirements of each fish in any one period were
roughly the same and, indeed, show much less variation than might have been
expected. In particular it may be noted that the body weight of the fish did not
appear to influence its relative maintenance requirements; for example, fish 2B
when it weighed about 14 g. gave much the same values as no. 4 which only weighed
i g., and later on when no. 2B weighed about 26, 31 and 35 g. it gave figures similar
to those of no. 7 A, which during these same periods weighed only 9, 11 and 13 g.
respectively. Dawes (1930-1) found that there was a definite diminution in the
maintenance requirements of the plaice with increasing body weight, but no evidence
of a similar phenomenon was shown in these experiments with trout.
It seems then that, as the variations in weight of the fish from week to week
were so small, if the average food consumption per fish expressed in milligrams
per week per gram of mean body weight be calculated for each period a measure
Table VII
Period
4 weeks from 9 Nov. 1933
7 Dec. 1933
3 ,
4
3
4
.
.
>
5 „.
4
5
7
4
,
.
.
.
2
,
4
.
3
3
3
3
5
6
4
3
3
3
.
.
.
.
,
.
.
.
.,
,
,
,
,
,
,
,
Dec. 1933
Jan. 1934
Feb. 1934
March 1934
April 1934
May 1934
June 1934
Aug. 1934
Oct. 1934
Nov. 1934
Dec. 1934
Average
temp.
°F.
No. of
fish used
45
3§
6
6
6
6
6
6
6
6
,
,
,
28
25
is
15
19
17
21
9
25
8
6
,
17 Jan. 1935
4i
7
28
21
16
44
,
,
,
,
,
27 June 1935
25 July 1935
„
3 Oct. 193s
24 Oct. 1935
Feb. 1935
Feb. 1935
March 1935
May 1935
42
40
42
46
Mean body
weight
g18
2'O
41
42
4'2
4'2
4-0
Average maintenance
requirements in
milligrams per week
per g. of mean
body weight
84
83
85
IO2
93
94
62
66
1
1
I5'4
127
270
262
186
SO
2
177
100
44
48
3
3
3
14-7
82
149
171
74
75
3
3
3
16-9
70
173
57
53
52
59
41
47
55
67
64
53
49
3
3
3
2
2
39
i7'S
217
51
20-3
19-6
19-8
20-2
69
"3
74
54
462
F. T. K. PENTELOW
of the maintenance requirements is obtained, which, whilst not quite accurate, is
a good approximation to the true value and which can be applied, without serious
error, to all the fish used in the experiment. The values so obtained are given in
Table VII. It will be seen that the amount required varied from 51 to 270 mg./g.
of body weight, but extreme values were not numerous and in most cases the figure
was between 70 and 102 mg. The median is at 83-5 and the semi-interquartile
range is ± 16-0.
Dawes (1930-1) found that the maintenance requirements in plaice varied
seasonally and also according to the body weight of the fish. The values determined
by him vary between 28 and 315 mg. per week per g. of mean body weight but most
of them lie between 70 and 140 mg. That plaice fed on mussels and trout fed on
Gamrnarus should give approximately the same values for maintenance requirements
is curious and can, at present, only be regarded as a coincidence.
The variation in maintenance requirements
The variations in the maintenance requirements of trout are obvious in Table VI
and have been illustrated graphically in Fig. 5. During the first 32 weeks of the
experiment, when six fish were under observation, the points lie approximately on
a reasonably smooth curve which follows closely the curve of temperature variations.
Later, when the experiment was continued with one, two or three fish the relation
between food requirements and temperature was not nearly so regular or well
marked. It is to be expected that in a cold-blooded animal the amount of food
required would be greatly influenced by the external temperature, and the results
of the earlier months of the experiment were in accord with this expectation.
Whether the discrepancies in the subsequent data were due to some unrecorded
factor, or whether they were merely a result of the less satisfactory experimental
conditions, the small number of fish and the periods when observations were
suspended, is not known. Dawes (1930— 1) found that the maintenance requirements
of plaice were influenced by the temperature of the water and that during the winter
"a plaice requires only a fraction (£—|) of the daily ration which is required to
maintain the weight constant during the summer months".
The loss of weight of starved fish
During 1936, as has already been recorded, experiments were carried out to
determine the loss in weight of trout kept without food for a week, in the hope that
the data so obtained could be correlated with the amount of food required for
maintenance. This hope was not fulfilled but the experiments are worth a brief
discussion.
The methods used have already been described on page 450 and the data obtained
have been summarized in Table VTII. In this table the loss of weight has been
expressed in mg./g. of starting weight, and the observations have been grouped
according to the temperature, and to the size of the fish. The average values obtained
have been given for 2, 10 and 40 g. intervals, and the number of fish in each group,
Relation between Growth and Food Consumption in the Brown Trout 463
is shown by the index figure. The full data on which this table is based have been
filed and are available to anyone interested.
These figures show only that, as would be expected, the loss in weight is
considerably greater at temperatures between 55 and 6o° F. than it is below 500.
There is not, however, any simple relation between the temperature and the weight
70^
1
60*
\
V
\
.. 111111111111111 • 1111111
_ „ 9 7 4 I
I
3 3
ill
111111
7 3 Z 6 4
11
I
6 3
111111111111111111111111 i n 111
7 7 4 - 2 6 4 1
•Date xi XII i n III iv v w w/ w a x xi XII i u in iv v w wi m
IS03 1934
1955
3 J 7
ix / XI
Fig. 5. Variations in maintenance requirements of brown trout.
lost, for, although there are sequences in the averages, they are not progressive from
a low loss at a low temperature to a high one at a high temperature. The proportionate loss in weight does not appear to bear any relation to the total weight of the
fish, a result which accords with the finding that the relative maintenance requirements of trout do not appear to vary with the body weight, but it is doubtful
whether this evidence is of any value, for the figures for loss of weight in Table VIII
bear no apparent relation to the amounts required for maintenance as shown in
Table VII.
464
F. T. K. PENTELOW
Table VIII. The loss in weight of brown trout starved for one week.
Index figures denote number of fish
Starting weight
of fish
g-
6T t o 6
Loss in weight in mg./g- of starting weight at
37°
4i°
42°
44°
59
62«
90*
55
113
26
36
8-10
10-12
14—16
16-18
18-20
20-22
22-24
24-26
26-28
28-30
30-32
32-34
86
47°
89'
74
92
82'
45
51
146
17
48°
4
~°;
44
s
3~6
29
88
27
22'
40
34
69
40-42
57°
58°
59°
133
10s'
123
ii7*
i37|
89"
6o°
113 9
163
i
138
105
154*
59*
94
67
1^0'
"3
104
9
173
147
140'
8o«
58
129
35*
l
4-14
14-24
24-34
34-44
59
40;1
27
65'
69«1
69
85*
146 1
4-44
42'
67'
94'
37*
35*
32*
85 4
70'
76'
32*
37 10
78 10
3i'
1/
4 o«
116'
134*
in*
149'
89 1
1601
137*
128'
1041
122*
121 11
137*
124*
It is probable that these unsatisfactory results were due to deficiencies in the
experimental procedure. The fish were taken direct from the stock pond, and no
information on how recently they had fed was obtained. Some, therefore, probably
had food in their stomachs, whilst others were already empty. The digestion of an
unknown weight of food must clearly produce a diminution in the weight of the
fish which is quite unrelated to the normal wasting due to starvation. Another
probable source of error was shown in the first series of experiments. It has
previously been recorded that an attempt was made to weigh some of the fish every
day, but that all those subjected to this procedure died at or before the end of the
week. Table IX shows that in every case the weight of the fish increased before it
died although it received no food. The most reasonable explanation of this is that
when the fish became weakened by starvation and excessive handling the normal
mechanism controlling the water intake into and output from the body broke down,
with the result that an excess of water entered and the weight therefore increased.
It is not unreasonable to suppose that a similar variation in water intake may have
occurred in some, at any rate, of the starved fish which were not weighed every
day. In this connexion it may be noted that the fish which were starved for a
fortnight (see p. 450) without exception showed a greater loss of weight in the first
week than in the second, and it is not certain whether this was due to less loss of
body substance in the second week, or to greater water absorption during this time,
and the data will not therefore be further discussed.
Relation between Growth and Food Consumption in the Brown Trout
465
Table IX. Daily variation in the weight of starvedfish(g.)
Date
Fish A
FishB
FishC
FishD
216
8-9
8-7
5-S
5-4
5-3
S-4
5-9
181
4 Feb. 1936
5
>
6
7
8
9
10
11
19-5
19-6
86
IO-2
8-S
8-S
8-3
8-8
188
19-2
,
,
195
2O-7
17-8
17-7
17-3
171
17-3
I7-S
182
(died)
—
(died)
(died)
(died)
The growth of fish on a constant ration
From 18 October 1934 until the experiment ended on 7 November 1935,
fishes 9 A and 11A were given respectively approximately 17 and 0-75 g. of
Gammarus per week, except at periods when weighings were suspended, when they,
like all the other fish, received an unknown, but probably ample, meal of Gammarus
every day. At the beginning 1 -7 and 0-75 g. were, for these fish, rations considerably
in excess of their maintenance requirements, for they represented 238 and 269 mg./g.
of body weight respectively, while the maintenance requirements (see Table VII)
were at this time about 100 mg./g. of body weight. As can be seen from Table X
Table X. Summarized data for fish on a constant ration
Fish no.
9A
(a)
Temp. Weight
of fish
2 weeks frc>m 25 Oct. 1934
, 8 Nov. 1934
4
, 6 Dec. 1934
3 .
,
3
3
3
5
.
6
4
3
,
.
.
3
3
.
'.
44
48
76
8-6
89
44
n-4
n-9
50
,
,
17 Jan. 1935
7 Feb. 1935
28 Feb. 1935
21 Mar. 1935
,
,
,
16 May 1935
27 June 1935
25 July 1935
55
67
64
2O'O
,
3 Oct. 1935
34 Oct. 1935
S3
49
41
47
(b)
Food
eaten
'5+
189
187
146
143
134
11A
to u (a)
Growth Weight
offish
made
+ 43
+ 14
+ 24
+ 18
+ 11
(b)
Food
eaten
Growth
made
32
37
181
200
+ 100
+ 22
183
+ 43
S-4
S-8
6-4
128
126
US
103
+ 33
+ 34
"9
+ 34
+ 30
7-1
98
92
8-6
76
11
— 20
I9-S
87
85
85
+ 2
- 4
- 4
25-0
2S-2
68
68
0
I2-O
13-3
61
60
—
12-8
14-2
I9-S
+ 5
+ 43
+ 30
-
5
-
17
2
+ 16
(a) Mean body weight in g.
(b) Mg. of food eaten per week per g. of mean body weight.
(c) Mg. of variation of body weight per week per g. of starting weight.
(the full data have been filed for reference) the fish accordingly grew steadily.
Obviously as the fish grew the proportion of the weight of food to body weight
declined and eventually growth stopped. In both fishes this point was reached in
early summer, and as at this time, owing to the high water temperature, the maintenance requirements continued to increase, the fish began to lose weight, for the
amount of food they received was below their maintenance ration. Then in the
466
F. T. K. PENTELOW
autumn, when the temperature- and the amount of food needed to keep the body
weight constant fell, these rations again became sufficient to allow growth to
recommence. These facts are summarized in Table XI below, which is abstracted
from the data in Tables VII and X.
Table XI
Period
2 weeks from 25 Oct. 1934
,
8 Nov. 1934
4 ,
,
6 Dec. 1934
3 >
3 ,
, 17 Jan. 1935
3 >
7 Feb. 1935
3
,
, 28 Feb. 1935
, 21 Mar. 1935
0
;
, 16 May 1935
4 ,
, • 27 June 1935
3 ,
, 25 July 1935
3 ,
3 Oct. 1935
3 ,
, 24 Oct. 1935
Fish no. 9 A
Fish no. 11A
Approximate
maintenance Mg. of food Growth Mg. of food
Growth
requirements eaten per
+ or eaten per
+ or (mg./g. of week per g. per week week per g. per week
in mg./g.
in mg./g.
starting
of mean
of mean
bodv
of starting
body
of starting
weight) from
Table VII
weight
weight
weight
weight
74
75
154
189
187
146
+ 42
+ 14
+ 24
+ 18
70
H3
+ 11
183
128
126
57
53
134
119
+ 34
"5
5i
87
8S
8S
68
68
+ 2
- 4
- 4
76
79
100
82
69
113
74
54
+ 3°
0
+ 5
181
200
103
86
61
60
+ 100
+ 22
+ 42
+ 23
+ 24
+ 43
+ 20
-
5
— 20
- 17
— 2
+ 16
The weight variations of these two fish therefore agree reasonably well with
expectations based on the amount of food required to keep fish at a constant weight.
The food requirements of grouting fish
The growth of those fish which at all times received an excess of food has already
been described, and it only remains to discuss the amount of food eaten and the
relation of food eaten to growth made.
The data are available in full for reference and have been summarized and
reduced to a common form in Table IV. Those for fishes 1 and 8 A are illustrated
in Fig. 3.
In the last two months of the first year (fishes 1, 3, 5, 9 and 11) the amount of
food eaten per week in the winter was small, less than 1 g., and in all fish except
no. 1 was lower in December than in November. From the beginning of January
onwards the amount increased week by week with an almost unbroken regularity
until by the beginning of June the fish were eating between 20 and 30 times as
much as they were during the first week of December. All the fish save nos. 1 and 3
were then lost, but the data for the two survivors were continuous until the end of
August. The amount of food eaten by fish 1 continued to increase until early in
July when it reached 12 g. per week, but then it showed a marked decrease and
remained approximately constant for the rest of the summer. The amount eaten
by fish 3 was at its maximum (13-5 g.) in the middle of June. It then fell markedly,
but there occurred a small increase in July, another fall early in August, and another
increase towards the end of that month.
Relation between Growth and Food Consumption in the Brown Trout 467
In October 1934 the experiment was restarted using fishes 1 and 3 of the original
batch and 8A and 12A, newcomers to the tanks. During the first week of the
experiment the food requirements of all these fish were fairly high, but in all of
them the amount eaten was considerably lower in November and increased again
in December. It fell off again from the middle to the end of January 1935, an
increase occurred in February, but there was another period of low consumption
in March, and then the increase was continued somewhat irregularly through April,
May and early June. Fishes 1 and 3 died in June, and the marked decrease in the
amount.eaten by no. 3 in the last fortnight of its life is probably of little significance
from the point of view of normal feeding. Fish 8 A continued to eat large quantities
(up to 30 g.) each week until early in August, when suddenly its requirements fell
to less than hah0. In the autumn this fish again fed heartily, though during October
its appetite was not quite so good as in either September or November.
From 6 June onwards tank 12 was occupied by a fish only a few months old.
This young fish ate more food in proportion to its body weight than did the others,
and in the week ending n July ate more than its own body weight, i.e. i-8 g.,
a relative consumption which was never approached by any of the others.
It is clear from an inspection of Fig. 3 that there is no simple relation between
the amount of food eaten and the amount of growth made in the same week, for
although there are many cases where a large increase in weight accompanied a large
food intake, there are also many where comparatively little growth was made
although a large amount of food was eaten. It appears then that the efficiency of
digestion varies greatly from week to week.
If, however, the figures be grouped into longer periods according to the temperature, as has been done in Table IV, then a more simple picture of the relation
between food and growth is obtained, for the short period fluctuations are smoothed
out. In Fig. 6 the data given in Table IV are illustrated graphically. Points have
been plotted to show the amount of growth made (in mg./g. of starting weight per
week) according to the amount of food eaten (mg./g. of mean body weight) over
periods of approximately constant temperature. Dots represent observations made
at temperatures below 500 F., circles those between 50 and 6o° F., and crosses
those at a temperature of over 60 ° F.
It will be seen that the dots are grouped reasonably evenly about the straight
line A-B, indicating that at temperatures below 500 F., on the average, the growth
made is roughly proportional to the amount of food eaten, and that z\ units increase
in the food eaten produced on the average one unit of extra growth. It will also be
observed that the point at which A-B cuts the abscissa and which therefore indicates
the amount of food required to keep the body weight constant, is 85 mg. of food
per gram of body weight per week, a value which is in fair agreement with those
given in Table VII. The circles, indicating the relation between food intake and
growth at temperatures between 50 and 6o° F., are very scattered and no curve can
be drawn to which they approximate. It is obvious, however, that within this range
it requires more food to produce a unit increase of body weight than at the lower
temperature, but since generally speaking the fish ate more than when the water
JEB-XVliv
31
468
F. T. K. PENTELOW
was colder, the amount of growth made was high. Some part of the lower efficiency
of food utilization at this temperature is clearly due to the increased maintenance
requirements of the fish, but from the irregularity of the points other factors, on
which no information was obtained, are also involved. When the temperature rose
above 6o° F. there was a further marked decline in the efficiency of food conversion,
and since this was accompanied by no increase, and often indeed a decrease, in
I
•• Observations be/on 50 F.
obtth/ttn JO A 60 F
x.
above 60F
1/00
200
300
400
JOO
600
700
800
900 WOO
Food eaten mq~ per gramme of Mean Body h/eiqht per HWA
-20
Fig. 6. The relation between food eaten and growth made.
appetite, the growth rate fell markedly. Here again, the points are irregularly
distributed and no further deductions can be made from them. It is clear, however,
that the check in the growth rate of all the fully fed fish used in the experiment
during the summer (see p. 453) is due partly, if not entirely, to the low efficiency
of food conversion, coupled in some cases with a decline in appetite. It may be
noted in passing that the crosses in the right-hand top corner of the graph, which
are exceptions to the statements made above, were given by fish 12B, and it has
Relation between Growth and Food Consumption in the Brown Trout 469
previously been stated that at this time this fish was growing very fast and eating
proportionately far more food than any of the others.
It is perhaps worth while to examine the question of the efficiency of food
conversion in somewhat more detail. An index of efficiency can be obtained simply
by dividing the amount of food eaten in any period by the amount of growth made,
and it then represents the amount of food required to produce a unit increase in the
weight of the fish over that period. This may be called the "crude efficiency".
Since, however, a proportion of the food eaten is used for maintenance, and is not
available for fresh growth, this figure is only valuable when a fish is growing
reasonably fast, for when it is eating only a little more than its maintenance ration,
and therefore growing slowly, very high crude efficiency figures are obtained.
A better idea of the use made of the food is obtained if we subtract from the total
amount of food eaten the amount estimated to be required for maintenance and
then divide the amount available for growth by the amount of growth made. This
figure will be called the "net efficiency". It is true that the maintenance requirements for all individual fish at all times are not known, but we can take those given
in Table VII as reasonable approximations and so obtain figures comparable with
those given by Dawes (1930-1) for plaice feeding on mussels.
Since, as has already been explained, there were from week to week wide
fluctuations in the relation between food eaten and growth made, in Table XII the
grouped data from Table IV have been used instead of the detailed figures for food
eaten and growth made per week.
Even with this grouping there is a good deal of variation, but of the 78 values
of "net efficiency" calculated, 6 lie between 1 and less than 2, 22 between 2 and
less than 3, 22 between 3 and less than 4, 17 between 4 and less than 5, and only
11 at 5-0 and over. The median is at 3-35 and the semi-interquartile range ±0-9.
There is no regularity about the variations, and although of the 11 values greater
than 5, 8 were recorded when the temperature was 500 F. or more, there is no
correspondence between the more normal values and the temperature. The figures
do show, however, that for brown trout, Gammarus is a highly nutritious food, for
it may take less than 2 g. of food over the maintenance requirements to produce 1 g.
increase in weight, and it very seldom takes more than 4g. Dawes (1930-1),
feeding plaice on mussels, found a similar inconstancy in "net efficiency" from
one period to another to that here recorded, but his figures show that mussel is a
less efficient food for plaice than Gammarus is for trout, for the highest "net
efficiency" he records is 2-4, and the figures are more generally in the region
of 5Surber (1935), in his experiments with brook and rainbow trout feeding on
G. fasciatus, did not determine the maintenance requirements of his fish and gives
therefore figures of "crude efficiency" only. He found that for brook trout it
required 6-05 g. (wet weight) of food to produce 1 g. increase of weight and for
rainbow trout 6-63 g. These figures show good agreement with many of the values
for the "crude efficiency" of brown trout feeding on G.pulex given in Table XII.
31-2
33
33
33
4
34
34
34
34
34
34
34
34
34
5
35
35
35
35
35
5
S
5
Table X I I . Efficiency offood conversion
Efficiency from Table IV
Approximate
maintenance
requirement Temp.
0
F.
(m) for all
fish from
Crude
Table VII
i.e.
blc
84
83
85
45
38
42
102
40
42
46
52
59
66
93
94
127
270
262
186
100
82
62
50
Fish no.
1
J
Net
i.e.
—
—
90
4'1
29
•'3
36
21
3°
2-3
129
33
4'5
6-8
60
6-3
6-8
2-9
3-2
i'9
16.
44
48
75
41
51
70
44
4'9
57
S3
41
4'i
1-7
2-9
2-6
47
55
67
64
53
49
5-5
7-5
4-5
6-3
5i
69
"3
74
54
Crude
Net
Crude
Net
Crude
Net
Crude
Net
5-3
33
20-5
7'9
4'9
4-2
25
i'9
32
32
7-3
3'3
4'1
23
48
3-5
5-8
63
3'4
2-6
4-S
4-0
3-S
4'7
2-S
41
61
4-°
8-2
5-6
—
6-6
6-5
S-8
60
64
4-9
32
5'4
S-3
49
3-7
36
61
8A
Crude
Net
Crud
69
S'5
S-6
50
4-2
35
4'3
3-7
29
28
4-1
3'2
25
3-8
39
119
33
36
10-7
17-8
13-5
5-5
5-6
4-3
4'5
(b-m)lc
—
—
4'3
7-o
.
1I
9
41-2
8-9
5-3
4'3
46
4-2
60
I I'2
62
6-9
n-8
3'9
5-6
34
3'2
2-7
2 2
2'5
16
23
3-5
2-4
—
2-4
34
39
43
43
V 7/
j
4-S
n-6
46
6-8
17
5-2
3-8
6-i
3-8
4'3
Relation between Growth and Food Consumption in the Brown Trout 471
The average live weight of Gammarus pulex
During the course of this experiment, whenever the quantity of Gammarus was
reasonably small, the amount given to the fish was counted as well as weighed.
The method of weighing has already been described, and although it was not exact,
as they were weighed alive with an indeterminate amount of water adhering to them,
it is possible to make an estimate of the average weight and so express the amount
of food eaten in numbers as well as weight.
In all 31,763 Gammarus were counted and weighed and the mean weight was
0*026 g. As the shrimps were generally weighed several at a time, and only rarely
individually, it is impossible to give the actual size range of those used, for it
happened that all those that were weighed one at a time were all very near the mean
weight. The smallest average weight for any group was 0-008 g. and the largest
0-067 gIt is probable that the average weight of 0-026 g. is lower than the actual average
weight of the Gammarus population of the River Itchen at Alresford, for a deliberate
attempt was made to feed small fish on small Gammarus, and as it was generally
for the small fish that the rations were counted as well as weighed (for when the
fish became larger and could take fully grown shrimps their food requirements were
too high for numbers to be counted) it is certain that an undue proportion of small
Gammarus were included in the 31,763.
Since, however, it is reasonable to suppose that the smaller and younger fish
prey principally on the smaller shrimps, for a full-grown Gammarus is rather a large
mouthful for a fish less than 10 gr. in weight, this average of 0-026 g. can be used
without serious error to calculate the numbers of shrimps eaten by thefishto produce
a given increase in weight.
In Table XIII are shown, for various periods, for which the data are available
the increase of weight of the fish, the total amount of food eaten, the estimated
number of Gammarus this quantity represents and the average number of shrimps
required to produce 1 g. increase in weight of the fish.
It will be seen from Table XIII that, although the numbers vary considerably,
in order to increase its weight by 1 g. a brown trout, generally speaking, ate between
200 and 300 Gammarus. Surber (1935) estimated the average live weight of each
individual G.fasciatus to be 0-017 g., and the average number required to produce
1 g. increase of weight to be 356 for brook trout and 390 for rainbow trout.
These fish were, of course, restricted to a diet of Gammarus, and as, under wild
conditions, other foods are available, a fish in the river will not eat so many shrimps
for the same increase in weight. Yet in the Itchen at Alresford, Gammarus is the
dominant invertebrate, and as it has been repeatedly shown (Neill, 1938; Slack,
1934; Pentelow, 1932) that trout feed on the most easily obtainable food, there is
little doubt that taking all seasons of the year Gammarus will comprise at least half
the food eaten. Therefore, as a general statement, it may be said that for every
gram increase of weight of the trout in the Itchen at Alresford at least 100 Gammarus
F. T. K. PENTELOW
472
are consumed. These figures may, in the future, be useful in assessing the stock of
trout that a given length of stream can profitably hold.
Table XIII
Fish
no.
Period
i
2 Nov. 1933—
30 Aug. 1934
3
5
,,
2 Nov. 1933-
9
14 June 1934
„
,,
II
i
3
8A
12A
1
2
8A
12A
1
j
18 Oct. 193420 Dec. 1934
Increase
in weight
g.
Total
weight of
food eaten
24-6
216
279
24-9
183726
201529
7,066
7,75i
327
312
30
2-4
203
17-3
I4-3
5-7
117-689
9I-549
45013
4,527
262
245
301
93
6-8
55-754
2,144
2,155
2-1
167
78
i-8
3-5
76
10 Jan. 193518 Apr. 1935
45-i
45-6
4-4
740
650
289
)>
,,
19-4
159634
121-554
12-4
80
34-983
10 Jan. 193521 Mar. 1935
9 May 1935—
91
150
5'9
78-0
91-0
69-0
6 June 1935
9 May 193520 June 1935
12B
6 June 19358 Aug. 1935
19 Sept. 19357 Nov. 1935
»
Gammarus
Average
no. eaten
per gram
increase in
weight
3-0
30
42-4
3-5
„
,,
Estimated
number of
eaten
32-7
35-6
>,
9 May 19351 Aug. 1935
12B
Final
weight
g-
420
8A
8A
Starting
weight
g.
3.521
i,73i
706
229
321
208
170
6,140
212
4,675
i,346
241
170
25-988
1,000
167
130
105-470
4.057
312
990
300
250687
9,642
321
168
44-7
279
261-414
10,054
360
0-7
2-7
2O
12670
487
252
56-0
5-o
811
25-1
3'4
157-564
6,060
815
241
236
8-4
1-7
4-i
56-027
9-275
i8-349
21-199
357
SUMMARY
1. The growth of brown trout (Salmo trutta), fed on Gammarus pulex, in their
first and second years has been studied.
2. The growth in weight varies considerably from week to week but, generally
speaking, it increases with increasing size of the fish. It is assumed that in these
experiments the second point of inflection of the normal S-shaped growth curve
was not reached because the fish were too young.
3. In all the fishes studied there was a period of slow growth during the winter
and during the summer. Growth is at its maximum at temperatures between 50
and 6oc F.
4. By careful adjustment of the rations it was possible to keep the body weight
of the fish approximately constant from week to week. The amount of food required
for this purpose varied from 51 to 270 mg./g. of body weight per week, but was
mainly between 70 and 102 mg. and was apparently affected by the water temperature, being higher when the water was warmer.
Relation between Growth and Food Consumption in the Brown Trout 473
5. Starved fish lost more weight at higher temperatures than at lower, but the
loss of weight could not be related to the amount of food required to maintain the
body weight constant at a given temperature.
6. The appetite of fully fed fish increases as the temperature rises to 6o° F.
but generally declines at temperatures higher than this. Between 40 and 500 F.
the amount of growth made is roughly directly proportional to the amount of food
eaten, but above 50° no such simple relation exists.
7. G. pulex is a very efficient food for trout; generally speaking about 5 g. of
this food produce 1 g. increase in weight. If from this amount the quantity required
to maintain the body weight constant is subtracted, it is found that 1 g. increase
in weight is produced by about 3 g. of food available for growth.
8. The average weight of the Gammarus used as food in this experiment was
0-026 g., and it is estimated that for every gram increase of weight each fish consumed between 200 and 300 Gammarus.
The author wishes to express his very sincere thanks to Dr E. S. Russell,
Director of Fishery Investigations, Ministry of Agriculture and Fisheries, for his
constant interest and advice in this work, to Mr T. Edser and the staff of the Statistical
Branch of the Ministry for invaluable help in the treatment of the data, to the
Department for permission to publish these results, and to his colleagues Dr J.
Grindley and Mr H. Stokes for their assistance in the preparation of the diagrams.
REFERENCES
DAVENPORT, C. B. (1908). Experimental Morphology. New York.
DAWES, B. (1930-1). J. Mar. biol. Ass. UJC. 17, 103.
(1930-1). J. Mar. biol. Ass. U.K. 17, 877.
GRAY, J. (1929). Brit. J. exp. Biol. 6, 248.
MINOT, C. S. (1891). J. Physiol. 12, 97.
NEILL, R. M. (1938). Trans, roy. Soc. Edinb. 59, 481.
OSTWALD, W. (1908). Vortr. EntwMech. Org. Heft 5.
PENTELOW, F. T. K. (1932). J. Anim. Ecol. 1, 101.
REOAN, C. T. (1911). British Freshwater Fithes. London.
SLACK, H. D. (1934). J. Anim. Ecol. 3, 105.
SURBER, E. W. (1935). Trans. Amer. Fish. So€. 65th Annual Meeting.
THOMPSON, D'ARCY W. (1917). Growth and Form. London.