مرکزی صفحہ Aquaculture Effect of light and darkness on incubation of eggs, length, weight and sexual maturity of sea trout...

Effect of light and darkness on incubation of eggs, length, weight and sexual maturity of sea trout (Salmo trutta L.), brown trout (Salmo trutta fario L.) and rainbow trout (Salmo irideus Gibbons)

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جلد:
2
سال:
1973
زبان:
english
DOI:
10.1016/0044-8486(73)90162-2
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Aquaculture,

@ Elsevier

2 (1973) 299-315
Scientific Pubkhmg

299
Company,

Amsterdam

~ Printed

in The Netherlands

EFFECT OF LIGHT AND DARKNESS ON INCUBATION OF EGGS,
LENGTH, WEIGHT AND SEXUAL MATURITY OF SEA TROUT (Sulmo
trutta L.), BROWN TROUT (Salmo trutta fario L.) AND RAINBOW
TROUT (Salmo irideus Gibbons)

KRZYSZTOF

BIENIARZ

Agricultural Academy,
(Received

Institute of Applied Zoology, Al. Mickiewicza 24128, Krakdw (Poland)

April 9, 1973)

During 5-year investigations
on sea trout, brown trout (both from the
moment of egg fertilization
until absorption of the yolk-sac) and rainbow
trout (from the moment of egg fertilization
until the age of 1050 and
1410 days) the fish were kept in three types of lighting conditions:
-natural light with the circadian and annual rhythms preserved;
-continuous
complete darkness, and
-continuous
artificial illumination.
After S-year experiments
carried out in hatching apparatus in wooden
hatcheries and ponds, the following conclusions were made:
1. The negative effect of light of an intensity exceeding 330 lx, on the egg
incubation of rainbow trout;
2. An additional period of increased mortality following the moment of
eggs eyeing compared with the eggs not exposed to light;
3. A definitely
worsened growth of trout maintained
in complete darkness;
4. No changes in pigmentation
and sexual maturation
of rainbow trout
kept in continuous
artificial illumination
and continuous
complete
darkness were observed.

INTRODUCTION
Among the data regarding effects of light on fish, information
is lacking
on the influence of continuous
illumination
and darkness on many physiological and morphological
properties of fish. In general, in higher vertebrates, light influences such physiological processes as the secretion of the
melanophore
hormone, maintenance
of water equilibrium,
formation of

300
blood, rhythm of eosinophilic
leucocytes,
fluctuation
of the cholinoesterase and sugar levels in the plasma, formation of secondary sexual features, sexual cycle, thyroid activity and gr; owth.
Literature regarding the influence of light on certain physiological and
morphological
traits in taxonomically
higher groups of vertebrates
has
been reviewed among others by Spode (1952--19531,
Surowiak (1967)
and Surowiak and Tilgner ( 1966, 1967).
In many species of fish light plays a big role in their orientation
in
space, inducing vision and phototaxis
(Blaxter and Parrish, 1958; Hoar,
1954; Gibson and Keenleyside,
1966; Rivas, 1953 and Steven, 1963) and
their orientation
in time, inducing photoperiodism
(Aronson,
19.5 1;
Rosenthal,
1952; Henderson,
1963; Goryczko,
1972 and Hoar, 1956,
1958).
Light affects the eggs of many fishes during their incubation (Lescinskaja, 1954; Semonov,
1957; Stedronsky,
1957; Lubickaja,
195 1, 1956;
White and Perlmutter,
1963; Hamdorf,
1960 and Eisler, 1957, 196 1).
Literature
is lacking on the influence of prolonged lighting and darkness
on fishes.
That is why in 1964 at the Department of Fisheries, Cracow Agricultural Academy, studies were undertaken
on the effects of darkness and of
prolonged lighting on some physiological and morphological.properties
of
salmon trout, brown trout and, in particular, rainbow trout.
MATERIAL

AND METHODS

Investigations
on sea trout and brown trout were conducted simultaneously in the hatching apparatus of Californian type and covered the
period from the moment of egg fertilization
until absorption of the yolksac. (For schematic firesentation see Fig. 1.) For the incubation period and
growth of larvae under various lighting conditions three types of lighting
were used:
(1) Natural lighting in the hatchery (l-2
lx during the day, 0.0 lx
during the night).
(2) Artificial continuous light (80- 100 Ix, day and night).
(3) Artificial continuous light (120- 140 lx, day and night).
Studies
ments:

conducted

on rainbow

trout

included

three

series of experi-

301

NATURALLIGHTINGIHATMRY)

q CL2VSIINIARTlflUALLIGHTING
INOIWDUALS

Fig.].
Scheme of experiment
number of males and females
the material.

initiated
in 1964 (brown
giving origin to individuals

trout and sea trout).
under study. Arrows

In circles is given
denote transfer of

First series
These were started in the spring of 1965 and covered the period from
egg fertilization
until 4 years of age. Experiments
were conducted
in the
hatching apparatus of Californian type in wooden containers and ponds.
Material for experiments
came from the trout kept for a few generations
in the Fisheries Experimental
Station at Mydlniki. (For schematic presentation see Fig.2.)
The following types of lighting were used:
continuous light (330.--350 lx, day
(1) 1?r the hatch& np&attrs_artificial
and night), continuous
darkness (0.0 lx, day and night), natural
lighting in the hatchery (l--2 lx, during the day and 0.0 Ix during the
night).
Zrz
wooden containers, artificial continuous
light (120- 140 Ix, day
(2)
and night), continuous
complete
darkness (0.0 lx, day and night),
natural lighting in the hatchery (l-2
lx during the day and 0.0 lx
during the night).
complete
darkness (0.0 lx,. day and night),
(3) Z/z ponds; continuoud
natural lighting with the circadian
and annual rhythms preserved
(200-500
lx during the day and 0.0 lx during the night).

302

1965
year

COMPLETEDARKNESS
NATURALLIGHTING
CONSTANTARTIFICIAL
LIGHTING
NATURAL LIGHTING
/HATCHERYl

Fig.2.

Scheme

of experiment

initiated

in 1965

and

1967

(rainbow

trout).

For designations

see

Fig.1.

Second series

These were started in the spring of 1966 and covered the period from
the egg fertilization until 3 years of age. Experiments were conducted in
hatching apparatus of Californian type in hatcheries and ponds. Test trout
came also from the Fisheries Experimental Station at Mydlniki, kept there
for many generations.
The following types of lighting were used:
(1) In hatching apparatus; artificial continuous light (230-250 lx, day
and night), and the other two types as in series one.
(2) In wooden containers; three types of lighting as in series one.
(3) In ponds; artificial continuous light (120-160 lx, day and night)
and the other two types as in series one (see Fig.3).
Third series

These were started in 1967 and covered the period from egg fertilization until 2 years of age. Experiments were conducted in the hatching
apparatus of Californian type and in wooden containers. Trout used in
this portion of the study originated from 2-year-old rainbow trout taken
for the first series of experiments and kept in ponds in complete darkness.
(For the schematic presentation see Fig.2.)

303

7966year

I

COMPLETEDARKNESS

q NATURALLIGHTING
o

q

CONSTANTARTIFICIAL
LIGHTING

NR OF INDIVIDUALS

NATURAL LIGHTING
/HATCHERY]

Fig. 3. Scheme

of experiment

initiated

in 1966 (rainbow

trout).

For designations

see Fig.1.

(1) In the hatching apparatus; three types of lighting as in series two
were used.
(2) In wooden containers; two types of lighting were used: continuous
artificial light (120-140
lx, day and night), continuous complete darkness (0.0 lx, day and night).
During the experiments
aiming at finding variations between the fish
maintained in different lighting conditions, darkness included, the following parameters were considered:
(1) Estimates of damaged eggs during incubation, based. on daily counts
and sorting out dead eggs. Results were listed as %CJ of the initial
number of eggs.
(2) Estimates of lost larvae from the moment of hatching until absorption of the yolk-sac. Loss was estimated by daily counts and discarding
of dead fish.
Results were given as %O of the initial number of eggs.
(3) The length of the hatching period (the number of days from beginning to end of the hatch).
(4) Length and weight of the body.
(5) Sex ratio, weight of gonads.

304
(6) Weight of digestive tract, liver and spleen. Experiments
were conducted on 180-day trout kept in hatcheries in complete darkness, in
continuous
artificial lighting and natural lighting. Results are given as %
of body weight.

RESULTS
Estimates of losses of the eggs and hatch

Losses of the eggs and fry in the apparatus constantly illuminated with
a light intensity of 330-3.50 Ix were nearly three times greater than those
observed in the remaining apparatus placed in conditions of natural light
and in total darkness. (see Fig.4).

n
n

NATURAL LIGHTING

COMPLETEDARKNESS

ia

LARVAE

LARVAE
F24 EGGS

EGGS

0

CONSTANT LIGHTING

0

LARVAE
EGGS

=

900
800
700
600
500
400
300
200
100
SEA TROUT

Fig. 4. Mortality

BROWN TROUT RAINBOW TROUTOF1965 RAINBOW TROUTOf1966 RAINBOW TROUTOF1967

of eggs and larvae in %o of initial number

of eggs.

In investigations
of series two and three, where artificial continuous
light of 230-250
lx was used, no relationship
between light’ conditions
and losses of the eggs and fry has been observed.

Period of hatching

The length of the period of hatching was 10 days for sea trout, 5-7
days for brown trout, 2 days for rainbow trout of 1965 (series one), 4
days for rainbow trout of 1966 (series two), and 3 days for rainbow trout
of 1967 (series three).
No correlation
between light conditions and the beginning and the end
of the hatching period was observed.

Course of losses of the eggs and larvae

The course of losses of the eggs and hatch, until the moment of absorption of the yolk-sac for sea trout, brown trout and rainbow trout of 1966
and 1967 were similar. Two periods of increased mortality were observed
during incubation.
For 1965 rainbow trout two periods of increased mortality were also
observed. Excepting the third period, increased mortality
was observed
between
ages of 28 and 35 days from the moment
of fertilization
(immediately
after the eyeing of the eggs) only in the illuminated
apparatus (see Fig.5).

Fig. 5. Mortality
of rainbow trout (1965) eggs and larvae, 1, under constant
artificial lighting, in
two apparatus;
2, under natural lighting, in one apparatus;
and 3, in complete
darkness, in three
apparatus;
a, beginning of formation
of eyes pigment; and b, beginning of hatching.

Length and weight of Rainbow trout reared in containers

A greater average length and weight of trout kept in illuminated
wooden containers was observed in three series of investigations (see Fig.
6).
Length and weight of Rainbow trout reared in ponds

From the first year onwards, the development
of trout reared in ponds
in conditions
of constant darkness was much more retarded when compared to development
of trout reared in the remaining experimental
ponds. Such results were obtained in investigations
of series one and two
(see Fig.6).

306
WElGHl
rq1

WEIGHT Cgl
,

240
220

ca

2w
180

0

COMPLElE
DARKNESS
NATURAL
UGHTING
CONSTANT
LIGHTING

120

110
F r1

160
140
120
KM

WElGHl igl

80
60
40
20
B

0

E

Fig. 6. Weight of rainbow trout. A, 660 day-old specimens of 1965 kept in containers (series one);
B, 150 day-old specimens of 1966 kept in containers (series two); C, 660 day-old specimens of
1967 kept in containers (series three); D, 1050 day-old specimens of 1965 kept in ponds (series
one); E, 1410 day-old specimens of 1966 kept in ponds (series two).

The mean weights and lengths of trout reared in conditions of constant
light were greater than those of trout kept in total darkness, but less than
those for trout reared in natural light conditions. These results, however,
are not completely
reliable on account of damage to the electric system
that took place in the illuminated pond and caused death of the largest
individuals. The differences in average length and weight between rainbow
trout kept in light conditions and rainbow trout kept in complete darkness were statistically significant in all cases described.

Sex ratio

The percentage of males in adult specimens reared in containers ranged
from 58.3 to 65%, and that of females from 35 to 54.2 %. The percentage
of males in adult specimens reared in ponds from 41.7 to 5 1.8%, and that
of females from 48.2 to 58.3%. In all investigations
no perceptible correlation between sex ratio and light conditions was noted.

Weight of gonads

Average weights of gonads in the percentage
of the body weight for
adult females ranged from 15.08% (56.60 g) to 18.60% (109.50 g).
Average weight of gonads in the percentage of the body weight for adult
males ranged from 1.97% (3.20 g) to 4.03% (16.50 g).
No perceptible
correlation
between weight of gonads and light condi tions was observed.

307
Weight of digestive tract, liver and spleen

The heaviest digestive tracts, livers and spleens were found in fish in
conditions
of continuous
light. The lowest mean weights of alimentary
canals, livers and spleens were stated in trout in conditions of total darkness (see Fig. 7).
DIGESTIVE
TRACT

LIVER

SPLEEN

n COMPLETE DARKNESS
FzI NATURAL LIGHTING
c]

CONSTANT LIGHTING

%
0.12 0.11-

%
10

0.10 -

9

%

oil9 -

8

LEO

0.08-

7

L70

0.07-

6

L60

a06 -

5

1.50

0.05-

4

140

ao4-

3

L30

0.03-

2

I.20

a02 -

1

1.10

0.01-

0

1.00

CL00

Fig.7.
number

Weight

of digestive

of specimens;

vertical

tract,

liver

and

bars, standard

spleen

as “lo of body

weight.

Figures

over columns,

deviation.

Only the differences between the average weights of spleen of rainbouw
trout kept in light conditions and rainbow trout kept in complete darkness were statistically significant. The differences between average weights
of digestive tracts and livers were not statistically significant.

DISCUSSION

OF RESULTS

Results obtained in the present study concerning the influence of light
upon the eggs and larvae of sea trout, brown trout and rainbow trout
generally correspond
to the data from literature. In nearly all the experiments an adverse effect of light, exceeding a certain intensity, upon the
development
of embryos was stated. An increased mortality,
premature
hatching and a higher percentage of monstrosities
were observed.
Such results were obtained by Semonov (1957) for the eggs and hatch
of sturgeon, Lescinskaja (1954) for Engraulis encrasicholus maeoticus,
Stedronsky
(1957), Lubickaja (195 1, 1956) and White and Perlmutter

308
(1963) for the eggs and hatch of salmonids; Gibor (1958) in his study on
the influence of light upon the eggs of Oncorhynchus
nerka Walbaum and
Oncorhynchus kisutsch Walbaum demonstrated
a considerable
species
variability in responses to light. The same dose of light provoked a mortality in the eggs of Oncorhynchus kisutsch which was twice as high as in
those of Oncorhynchus nerka.
Extensive studies on the influence of light upon eggs and hatch were
conducted
by Hamdorf
(1960) on Salmo irideus Gibbons and Eisler
(1957,
196 1) on Salmo clarkii Richardson,
Oncorhynchus kisutsch,
Oncorhynchus tschawyscha and Salmo gardnerii Richardson.
From his
investigation,
Hamdorf
came to the conclusion
that the illuminated
embryos increased their sensitivity
to radiation until the heart started
action, after which time sensitivity of embryos to light decreased.
In his experiment,
Eisler, like Hamdorf, irradiated separate developmental stages of embryos with light of various intensities, from 157 ftcandles to 20 ft-candles. He found that the eggs in the period prior to
eyeing are most sensitive to the action of light. Light of 157 ft-candles
killed any amount of the eggs until the moment of hatch. Light of a lower
intensity illuminating the eggs until the moment of eyeing later caused an
acceleration
of the process of hatching, reduced fingerling growth and
changes in the liver. The eggs illuminated in later developmental
periods
were characterised
by a greater resistance to the action of light or displayed higher mortality,
and larvae hatched out of these eggs were distinguished by arrested growth rate. Illumination
of larvae did not cause any
visible alterations.
In our own experiments,
when light of an intensity of 330-350
lx was
applied, two moments of a particularly
high mortality of the eggs were
observed immediately
after eyeing and immediately
after the hatch,
appearing in other stages of development
than those cited by the authors
mentioned
above. It is not impossible, however, that it could have been
the result of an adverse effect of light in the period prior to the eyeing of
the eggs.
From the above mentioned
studies it is obvious that the use of various
methods as well as of various light intensities and water of changing
chemical composition (perhaps some physicochemical
properties of water)
can influence
to some degree the sensitivity of the eggs and larvae to
radiation.
It can be expected that the unfavourable
physicochemical
properties of
water (low 02 content, high temperature)
increasing the sensitivity of the
eggs and larvae to light, do not permit a full comparison of results. Mention should be made that a definitely negative influence on the course of
the incubation of eggs was noticed only when a strong light was used. This

309
contradicts
the opinion prevailing since the time of Willer ( 1928) that the
eggs of salmonids should be protected
against light irrespective
of its
intensity.
On the basis of the results obtained in the present work, it can be
admitted that the threshold of sensitivity to light of the eggs of rainbow
trout in the conditions
of Mydlniki is within 250 and 330 Ix (25533
ft-candlers) and this is probably the reason that no differences were observed in the action of light with an intensity of l-280 lx.
Contrary to the results of investigations
of the authors cited above, no
interdependence
was observed in any of the experimental
groups between
the time of the beginning and *end of hatching of larvae and light condi tions.
However, the exact time of hatching might have been overlooked, since,
in the opinion of the authors cited above (Eisler 1957, 1961 and Hamdorf, 1960), light accelerated it by several hours only.
Control of the eggs was executed only in 24 hours during investigation,
as light was necessary even with the entirely darkened apparatus.
In Hamdorf’s (1960) opinion, the reason for the negative intluence of
light on fish embryos is the decomposition
of lactoflavin, a component of
respiration ferments, by light. One proof of this is the fact that eggs more
intensively pigmented with a greater number of carotenoids
were more
resistant to the action of light.
Sparrow and Rubin (195 1) carried out among others extensive investigation on the mechanism of infraoptical action of different kinds of radiation upon living organisms. They are of the opinion that radio and infra
red waves very seldom induce thermal reaction, but are responsible for
direct chemical changes. The energy of visible radiation can affect the
changes of chemical structures,
although
it is not strong enough to
produce ionization
which is most frequently,
in the opinion of Sparrow
and Rubin, the cause of radiobiological
effects.
In Eisler’s ( 196 1) opinion, visible light influences physiological processes by means of its photochemical,
photoelectrical
and thermal properties.
He explains the more rapid hatching of illuminated
embryos in early
developmental
stages (quoting Clarke, 1922) by thermal effect of light.
Luminous energy absorbed by the body can increase the movement
of
molecules and as a result can raise the temperature.
The alterations observed in mortality
and histological and anatomical
features, caused by
visible light of great intensity, are very similar to changes resulting from
ionizing radiation, that might suggest photoelectrical
properties of visible
radiation.
However,
the results obtained in the present work, concerning
the
lengths and weights attained by rainbow trout in different conditions of

310
light and darkness, provide a basis for maintaining that in the majority of
cases the growth of rainbow trout in conditions of total darkness is considerably worse in comparison with that of trout reared in conditions of
natural and constant light.
Activity of the rainbow trout is greatest during day time as well as
during
intake
of food,
as demonstrated
by the investigation
of
Edmundson
et al. (1968) and Jenkins (1969). Therefore, the rainbow
trout belongs to fish for which light plays a very important
role, i.e.
during feeding. Its worsened growth in conditions of total darkness should
probably be ascribed to this fact.
Differences between the average lengths attained by trout kept in darkness and illuminated
containers are less distinct, but they too indicate a
worsened growth of trout reared in darkness.
Trout reared in conditions of total darkness in ponds during the entire
period of rearing exhibited a decidedly worsened growth than trout reared
in the remaining ponds.
A short circuit resulting from damage to the mercury lamp in an illuminated pond caused the death of the largest individuals among trout
reared in it for 1.5 years. It is difficult, therefore,
to characterize
the
growth of trout in conditions of constant illumination on the basis of the
survivors. It was stated, beyond doubt, that they exhibited a more accelerated growth than in a darkened pond. One might infer that, had the
damage not taken place, they would have attained the same average
dimensions as trout reared in conditions of natural light.
Trout were reared in containers with a surface 40 times smaller than
that of fish ponds. The location of food, provided ad libitum by means
other than a light receptor, was undoubtedly
easier on such small surfaces,
and the differences
in the mean values of length and weight for trout
reared in darkened or illuminated
containers are not so distinct as for
trouts in ponds.
Data concerning mean values for weights of digestive tracts and livers,
expressed as percentage of body weight, confirm the hypothesis presented
above, that rainbow trout in conditions of total darkness feeds much less
well than when exposed to light. Eisler (1957) obtained quite similar
results concerning the growth of Oncorhynchus tschawytscha Walbaum in
darkness and in light.
The possibility of the influence of light upon the growth of fish has, to
a large extent, been ignored. Although its influence upon the endocrine
organs is quite certain
(Hoar,
1939; Rasquin,
1949; Hoar,
1957;
Sathyanesan,
1965 and Brown 1957).
Generally speaking, the fish can be divided, on the basis of literature
data, into three groups (see Table I).

311
TABLE 1
Classification of fish according to their response to light
---___
Groups
-____-_
I Fish growing poorly
in darkness

-.___

Species

Authors

Salmo gairdneri Richardson
Salmo trutta m. fario L.
Salmo salarL.
Clupea harengus L.

Edmundson et al. (1968)
Jenkins (1969), Hoar (1942)

Carassiusauratus L.
Thymallus arcticus baicalensisDyb.
Perca fluviatilisL.
Lepomis macrochirus Rafinesque
Lepomis gibbosus L.
Fundulus diaphanus
Pleuronectes platessa L.
Astyanax mexicanus

Musinic (1931), Suskina
(1939), Batlle and McNiarn
(1936), Blaxter (1964, 1968).
Hirata (1957), Hirata and
Kobayashi (1956)
Olifan (195 1)
Keast and Welsh (1968)

Blaxter (1968)
Rasquin and Rosenbloom
(1954)

Blennius pholis
Centronotus gunellus
Lepomis cyanellus Rafinesque

Qasim (1955)
Gross et al. (1965)

II Fish growing well
in darkness

Oncorhynchusgorbuscha Walb.
Salmo clarki Richardson
Salmo trutta lacustris L.

Smith (1916)
Tryon (1943), Smith (1916)
Seguin (1957)

III Fish growing well
both in darkness
and when exposed
to light

Lebistes reticulatus Peteos
Etheostoma lepidum
Carassiusauratus L.
Lepomis machrochirus Rafinesque
Esox lucius L.

Rosenthal (1952)
Hubbs and Strawn (1957)
Bjorklund (1958)
Anderson (195 9)
Lillelund (1966)

Data quoted from literature are not always consistent.
Thus, on the
basis of the investigations
of Hirata and Kobayashi
(1956) Curussius
aurutus L. ought to belong to the fish for which light conditions play a
significant role, but Bjorklund (1958) maintains that this species does not
react to different
conditions
of light. The division presented above is
therefore an arbitrary one and does not take into consideration
the differences between conditions in which the experiments were carried out. The
studies and observations
of the authors cited above were different. The
light intensity was not always taken into consideration
in the works comparing the growth of fish in darkness and light. One can suppose, therefore, that fish needing for their growth light of a certain intensity,
in
conditions
of as over-intensive
illumination,
display a more reduced
growth than in conditions
of complete darkness. This can be supported

312
among others by Brown’s investigation ( 1946) on the growth of a 2-year
old brown trout. This species of trout, which in Jenkin’s (1969) opinion
feeds better during the day-time than at night, when illuminated
for
12- 18 hours daily, exhibited a more retarded growth than that of fish
illuminated for 6 hours only.
Mention must be made here of the observations
of Steucke (1968) on
Sulvelinus mzmycush artificially hatched and reared in ponds. He observed
an epidemic occurrence of cataract of the eye. The covering of ponds gave
sensational results. It turned out that an excess of light was responsible for
this disease.
It can be stated, therefore, on the basis of our own experiments,
as well
as of those of the above mentioned authors, that in a considerable number
of fish total darkness renders growth more difficult either by impeding the
search of food or by a certain decrease in the intensity of the internal
biological rhythm.
It is of interest to note that such a long period of breeding rainbow
trout in continuous
artificial illumination
and in continuous
complete
darkness did not cause any changes in pigmentation
and sexual maturation. It is difficult to explain why the weight of spleen of rainbow trout
kept in artificial continuous
light was twice as high as that of rainbow
trout kept in continuous
complete darkness. It suggests that it is connected with some changes in the blood components,
but hematological
investigation shows that light does not influence the blood components
of
fishes (Bieniarz and Masfowska, 1970). Such results were obtained in
hematological
investigation
(Bieniarz, in pre-print) with the use of the
same specimens used in investigations described above.

CONCLUSIONS
(1) Negative effect of light of an intensity exceeding 330 lx, on the egg
incubation of rainbow trout;
(2) An additional period of increased mortality following the moment of
eggs eyeing compared with the eggs not exposed to light;
(3) A definitely
worsened growth of trout maintained in complete darkness;
(4) No changes in pigmentation
and sexual maturation
of rainbow trout
kept in continuous
artificial illumination
and continuous
complete
darkness were observed.

313
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