Effects of light spectrum on growth and stress response

of rainbow trout Oncorhynchus mykiss reared under

recirculating system conditions

Nafsika Karakatsouli a, Sofronios E. Papoutsoglou a,*,Georgios Panopoulos a, Eustratios S. Papoutsoglou a,

Stella Chadio b, Dimitris Kalogiannis b

a Department of Applied Hydrobiology, Faculty of Animal Science and Aquaculture,

Agricultural University of Athens, Iera Odos 75, 118 55 Athens, Greeceb Department of Anatomy and Physiology of Farm Animals, Faculty of Animal Science and Aquaculture,

Agricultural University of Athens, Iera Odos 75, 118 55 Athens, Greece

Received 4 April 2007; accepted 24 October 2007

Abstract

Fish growth and physiology may be affected by light spectrum, which can be easily manipulated in indoor aquaculture facilities,

and especially recirculating water systems, with little cost. Since data related to light spectrum and widely reared fish are still few,

the present study aimed to investigate the effects of coloured light on growth performance and stress response to confinement of

rainbow trout Oncorhynchus mykiss. Fish (145.3 � 1.5 g) were reared under white (full spectrum, fluorescent lamps), red (605 nm)

and blue (480 nm) light (lamps covered with appropriate filters) for 111 days under recirculating water systems (150 lx, 12L–12D).

At the end of the experimental period and for each light treatment, fish were either subjected for 1 h to confinement stress or

remained undisturbed (control groups). Total length of the fish reared under red light was greater than that of the other regimes,

whereas other growth parameters showed a similar trend but were not significantly different from one another. Carcass proximate

composition was not affected by light spectrum. Fish exposed to confinement showed typical primary (high cortisol) and secondary

(high glucose and haematocrit, liver lipid mobilization, osmotic and ionic disturbances, blood acidosis, etc.) stress responses.

Nevertheless, in fish reared under blue light, stress-induced cortisol increase was lower and liver lipids mobilization was absent

compared with white light (significant interaction). Present results indicate that if stressors are kept to a minimum then red light

could be suggested for the intensive rearing of rainbow trout.

# 2007 Elsevier B.V. All rights reserved.

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Available online at www.sciencedirect.com

Aquacultural Engineering 38 (2008) 36–42

Keywords: Light spectrum; Oncorhynchus mykiss; Growth; Confinement; Stress

1. Introduction

Intensively reared fish may often be subjected to

several chronic and acute stressors of endogenous (e.g.

* Corresponding author. Tel.: +30 2105294401;

fax: +30 2105294401.

E-mail address: sof@aua.gr (S.E. Papoutsoglou).

0144-8609/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.aquaeng.2007.10.006

social interactions) or exogenous (e.g. unfavourable

water quality, high densities, grading, handling) origin

that may have detrimental effects on fish welfare and

growth especially when imposed together (Huntingford

et al., 2006). Among other advantages, the use of

recirculating water systems in fish farming, through a

major control of fish as well as their environment, can

help to minimize the occurrence of stressful conditions

and is strongly encouraged (Papoutsoglou, 1993, 1996;

N. Karakatsouli et al. / Aquacultural Engineering 38 (2008) 36–42 37

Blancheton, 2000; Funge-Smith and Phillips, 2001).

Nevertheless, since an artificial environment has to be

involved, the knowledge of applying appropriate

rearing conditions for each species is necessary for

the most effective and productive operation of these

systems.

Lighting is one of the rearing conditions that can be

easily manipulated in recirculating water systems and

most of the related research papers are focused on

photoperiod and intensity (Boeuf and Le Bail, 1999).

Despite the fact that fish visual system has been

adequately described for several species and proven to

be sufficiently equipped to respond to different

wavelengths (Neumeyer, 1992; Flamarique and Hawry-

shyn, 1996; Cheng and Flamarique, 2004), light

spectrum effects on farmed fish have only recently

been investigated. Depending on fish species natural

habitat characteristics and their specific visual abilities,

light spectrum has been reported to affect multiple

physiological aspects such as growth, neuro-hormonal

system, reproduction, behaviour, etc. (e.g. Downing,

2002; Bayarri et al., 2002; Naor et al., 2003; Ruchin,

2004; Karakatsouli et al., 2007).

To our knowledge only a limited number of studies

are related to coloured light effects on fish stress

response either to chronic stressors such as daily

disturbance (Head and Malison, 2000) and high density

(Van der Salm et al., 2004) or to acute stressors such as

confinement (Volpato and Barreto, 2001) and chasing

(Barcellos et al., 2006). Although mechanisms involved

are not yet elucidated, the above-mentioned studies

indicate that light spectrum can differentiate fish stress

response (evaluated by plasma cortisol levels) and may

be a useful means to alleviate adverse effects of stress.

It has been previously reported that rearing of

rainbow trout Oncorhynchus mykiss for 11 weeks under

blue light (480 nm) had negatively affected growth and

led to increased brain neurotransmitters indicating that

blue light was perceived as stressful (Karakatsouli et al.,

2007). In that experiment light intensity was 300 lx and

although red light seemed to improve fish performance

it was not possible to distinguish significant differences

compared to white light. In another experiment, the

same authors demonstrated that O. mykiss reared under

red light of 150 lx grew better than under red light of

300 or 600 lx (unpublished results). Thus, it seems that

the effect of light spectrum could be modified under

specific rearing conditions.

The present study aimed to investigate whether light

of specific spectral composition could favour rainbow

trout growth and alleviate stress response, so that an

appropriate light colour for intensively reared trout

under recirculating water system could be definitely

suggested. For this reason, fish were reared under white

(full spectrum), red and blue light at light intensity of

150 lx and then an acute stressor (confinement) was

imposed to evaluate fish stress response.

2. Materials and methods

Rainbow trout O. mykiss purchased from a commer-

cial Greek hatchery and raised under laboratory

conditions for at least 12 months were used. One

hundred and twenty fish of mean initial body weight

(�S.E.M.) 145.3 � 1.5 g and total length 23.6 �0.09 cm were randomly distributed (10 fish per group)

in 12 tanks (glass, length � height � width: 83 cm �42 cm � 49 cm, volume capacity 171 l, rearing density

59 fish m�3). Experimental tanks were part of an indoor

recirculating freshwater system provided with mechan-

ical and biological filters, UV-sterilization, compressed

air supply and cooling water apparatus. Water flow

rate was 1.8 l min�1 and all tanks were thorou-

ghly cleaned once a week. Water physiochemical

properties were monitored daily and water quality

was maintained as follows: temperature, 16.6 �0.03 8C; dissolved oxygen, 9.1� 0.01 mg l�1 (93.4�0.08% saturation); pH, 7.18 � 0.005; total ammonia

nitrogen, 0.503 � 0.0139 mg l�1; unionized ammonia

nitrogen, 0.0024 � 0.00008 mg l�1; nitrite nitrogen,

0.187 � 0.0084 mg l�1; chloride, 37.3 � 0.88 mg l�1;

total hardness, 216.8 � 2.44 mg l�1 CaCO3.

Fish were acclimated to experimental tanks for 5

days under room ambient lighting. After the acclima-

tion period white, red and blue light colour was applied

(four tanks per treatment) and fish remained in these

conditions for 111 days. Light colour was achieved by

covering light source (Cool White fluorescence lamps,

OSRAM DULUX D/E 26W/840 G24Q-3) with

coloured filters (red #024, blue #165; LEE Filters,

Andover, Hampshire, England, UK) while no filter was

used for white light colour (full spectrum). Light

spectrum was specified using a constant slit Kruss

spectroscope equipped with a graduated scale that was

wavelength calibrated. Red filter had peak transmission

at approximately 605 (90% relative transmission) and

blue filter at 480 nm (83% relative transmission). In

order to avoid complications due to room lighting, all

tank sides were covered with opaque covers and light

source was placed, through appropriate opening in top

cover, above each tank, at approximately 10 cm from

water surface. All experimental populations were

subjected to photoperiod 12L–12D (with half hour

dawn and dusk simulation) and light intensity, in all

N. Karakatsouli et al. / Aquacultural Engineering 38 (2008) 36–4238

Fig. 1. Mean weight (�S.E.M.) during rainbow trout (Oncorhynchus

mykiss) rearing (tank-acclimation period included) under white, red

and blue light.

treatments, was adjusted to 150 lx. Light manipulation

was controlled with winDim 4.0e PC software and light

intensity measured by means of digital light meter (RS

180-7133, RS Components Ltd., Corby, Northants,

UK).

Fish were fed by hand a commercial pelleted diet

(moisture, 7.9%; crude protein, 44.0%; crude lipid,

11.0%; ash, 9.5%; nitrogen-free extract, 27.6%) twice

daily from Monday to Saturday, while no food was

given on Sunday. All fish populations were fed at

feeding level 1.5% b.w.t. throughout the experiment and

were individually weighed every 2 weeks.

At the end of the experimental period and following

48 h fasting, half fish populations (two tanks from each

light colour) were subjected for 1 h to confinement

stress by lowering water level at 10 cm height (rearing

density 246 fish m�3). The other half fish populations

remained undisturbed and used as controls. All fish

were anaesthetized, in their tanks, using 2-phenox-

yethanol at a dose of 0.018 ml g�1 l�1. After complete

anaesthetization (within 1 min) blood sampling took

place from the ventral aorta. About 0.3 ml of each blood

sample was immediately used for measuring haemato-

crit (12,000 � g for 10 min) and blood electrolytes

(Na+, K+ and Ca2+), pH and pCO2 taking into

consideration fish temperature (Rapidpoint 400 Blood

Gas Analyzer, Bayer, Tarrytown, NY, USA). The

remaining blood was centrifuged (12,000 � g for

10 min) for plasma separation. Plasma was used for

the determination of glucose, triacylglycerides, choles-

terol (enzymatic colorimetric methods, Elitech diag-

nostics, Sees, France), osmolality (cryoscopic

osmometer, Gonotec Osmomat 010) and cortisol, which

was measured by radioimmunoassay, using a commer-

cially available kit (Coat-A-Count Cortisol, DPC, Los

Angeles, CA, USA) that has been previously validated

for fish (Ainsworth et al., 1985). In the present study, the

sensitivity of the assay was 0.2 mg dl�1 and intra- and

inter-assay coefficient of variation was 3.2 and 6.5%,

respectively.

All fish were individually weighed and total length

was measured to calculate condition factor

[K = 100 � (body weight, g) � (total length, cm)�3].

Liver was removed from each fish and used for the

determination of moisture, total lipids (Folch et al.,

1957), as well as for the calculation of hepatosomatic

index. Spleen was also isolated, weighed and expressed

as percent body weight. Sampled fish of each population

were minced (without viscera) in two groups of five

specimens each, and lyophilized for carcass proximate

composition determination according to Kjeldahl and

Soxhlet methods (AOAC, 1984). Specific growth rate

[SGR = (ln Wfn � ln Win) � 100 � t�1, Wfn mean final

body weight (g), Win mean initial body weight (g), t is

the days of rearing], weight gain (% initial body weight)

and food conversion ratio [FCR = (food consumed,

g) � (weight gain, g)�1] were calculated for the whole

fish population in each tank.

Data concerning carcass and liver composition,

organosomatic indices and blood–plasma parameters

were analyzed by two-way analysis of variance

(ANOVA) with light colour and stress as factors. For

growth related data (final weight, total length, condition

factor, SGR, weight gain, FCR), no significant effect of

stress or significant interaction were detected and

therefore these data were analyzed by one-way analysis

of variance with light colour as factor. There was no

significant difference (P > 0.05) between duplicated

tanks, so data for each species concerning replicate

treatments were pooled. Where P values were

significant (P < 0.05) multiple comparisons were

carried out using the Duncan test. Wherever necessary,

data were transformed (logarithm or square root) in

order to obtain normal distribution and/or homogeneity

of variance. All values presented in the text and tables

are untransformed means � S.E.M. (Sokal and Rohlf,

1995).

3. Results

Light colour did not have a significant effect on fish

weight, SGR, weight gain or food utilization, although

these parameters presented their best values for fish

under red light (Fig. 1; Table 1). On the other hand, red

light led to significantly higher total length compared

with blue or white light, while condition factor was not

N. Karakatsouli et al. / Aquacultural Engineering 38 (2008) 36–42 39

Table 1

Growth performance of rainbow trout Oncorhynchus mykiss reared under white, red and blue light for 111 days

Light colour P

White Red Blue

Final weight (g)a 370.9 � 9.2 396.2 � 9.7 378.9 � 9.0 NS

Total length (cm)a 29.9 � 0.2 a 30.7 � 0.2 b 29.8 � 0.2 a *

Condition factora 1.37 � 0.01 1.36 � 0.01 1.42 � 0.03 NS

SGR (% day�1)b 0.82 � 0.02 0.87 � 0.01 0.83 � 0.03 NS

Weight gain (%)b 155.14 � 4.58 172.33 � 1.42 160.47 � 9.44 NS

FCRb 1.69 � 0.05 1.57 � 0.02 1.66 � 0.07 NS

All values are means (�S.E.M.); P: significance level; NS: non significant; *P < 0.05. Means with the same superscripts are not significantly

different.a n = 40 fish.b n = 4 tanks per colour.

differentiated among experimental light treatments

(Table 1). No differences were observed for carcass

proximate composition apart from a significant inter-

action for ash content (Table 2). However, differences

were small and probably not biologically important.

Liver water content was increased in confined fish

(Table 2). Also, fish under white light had significantly

higher liver water content than fish under red light,

while fish under blue light presented the lowest levels. A

significant interaction was detected for liver total lipids

(Table 2). For fish under white or red light liver total

lipids were reduced when fish were subjected to

confinement, response that was not observed when fish

were reared under blue light (Table 2). Confinement

also resulted in increased hepatosomatic and decreased

spleenosomatic indices irrespective of light colour

(Table 2).

A significant interaction was also observed for

plasma cortisol levels. Cortisol was increased when fish

were confined but this increase was less in fish under

blue light (Table 2). Confinement resulted in elevated

plasma glucose, osmolality, blood haematocrit, reduced

blood pH and increased blood pCO2, sodium and

potassium irrespective of light colour (Table 2). On the

other hand, white light resulted in lower levels of

plasma glucose than red or blue light (Table 2). Plasma

osmolality levels were higher in fish under red light

followed by those under blue and white light (Table 2).

Also, plasma triacylglycerides and cholesterol levels

were highest in fish under blue light compared with

white light, while fish under red light presented

intermediate values (Table 2). No significant effects

of light colour were detected for the other blood and

plasma parameters, apart from blood calcium levels

(interaction term P < 0.05, Table 2). Unconfined

(control) fish under red light had significantly lower

calcium levels compared with white or blue light. When

fish were subjected to confinement, calcium levels were

increased only in fish under red light (Table 2).

4. Discussion

Present results showed that growth of rainbow trout

was favoured when reared under red light, although

significant differences were detected only for fish total

body length. Similarly, Head and Malison (2000)

reported that yellow perch Perca fluviatilis under red

light grew more in body length than fish under blue or

white light. Also, larval rearing of Wallago attu under

red light resulted in increased survival and total biomass

compared with white light (Giri et al., 2002). Present

growth performance of rainbow trout under red light

confirms what has been previously reported as a trend

(Karakatsouli et al., 2007). In the latter study, a negative

effect of blue light was detected which was not observed

in the present study and this may be related to

differences in experimental design and mostly to the

combination of fish initial size and light intensity.

Karakatsouli et al. (2007) used a light intensity of

300 lx, while in another experiment of the same authors

studying red light effects of 150, 300 and 600 lx on

rainbow trout growth, better growth was observed at

150 lx (unpublished results). Thus, the possibility that

the effect of light wavelength itself may be differ-

entiated under different light intensities cannot be

excluded. It should also be emphasized that these two

light characteristics combined with fish biological stage

have been indicated to interact for larval phototactic

behaviour (Gehrke, 1994), first feeding success (Down-

ing and Litvak, 2001), survival (Downing, 2002) and

reaction distance to prey (Utne-Palm and Bowmaker,

2006).

In the present study, fish subjected to confinement

showed typical primary (high cortisol) and secondary

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Table 2

Carcass proximate and liver composition, organosomatic indices, blood and plasma parameters of rainbow trout reared under white (W), red (R) and blue (B) light for 111 days and subjected to

confinement

Unconfined fish (U) Confined fish (C) Two-way ANOVAa

White Red Blue White Red Blue Stress (S) Light colour (LC) S � LC

Carcass composition (% wet weight)b

Water 72.1 � 0.2 72.2 � 0.2 71.5 � 0.1 72.3 � 0.2 71.9 � 0.3 72.0 � 0.2 NS NS NS

Protein 19.3 � 0.1 19.2 � 0.1 19.6 � 0.3 19.3 � 0.2 19.1 � 0.1 19.3 � 0.1 NS NS NS

Lipid 6.3 � 0.2 6.3 � 0.2 6.5 � 0.1 6.0 � 0.2 6.5 � 0.2 6.3 � 0.3 NS NS NS

Ash 2.1 � 0.1 a 2.3 � 0.1 a 2.6 � 0.1 b 2.3 � 0.1 ab 2.2 � 0.1 a 2.2 � 0.0 a NS NS **

Liver parametersc

Water (%) 79.2 � 0.2 79.0 � 0.2 78.0 � 0.2 79.8 � 0.2 79.1 � 0.2 78.7 � 0.2 U < C** B < R <W*** NS

Total lipids (% wet weight) 3.09 � 0.04 bc 3.10 � 0.05 bc 3.16 � 0.05 c 2.91 � 0.05 a 2.99 � 0.05 ab 3.21 � 0.05 c U > C*** W = R < B* *

Organosomatic indices (% body weight)c

Liver 0.982 � 0.030 1.004 � 0.028 0.954 � 0.031 1.146 � 0.037 0.999 � 0.031 1.049 � 0.051 U < C** NS NS

Spleen 0.174 � 0.015 0.148 � 0.008 0.155 � 0.009 0.111 � 0.006 0.100 � 0.008 0.111 � 0.008 U > C*** NS NS

Blood and plasma parametersc

Cortisol (ng ml�1) 2.5 � 0.4 a 2.1 � 0.3 a 3.2 � 0.3 a 36.8 � 4.8 c 35.2 � 4.5 bc 26.6 � 3.4 b U < C*** NS *

Glucose (mg 100 ml�1) 73.12 � 1.14 80.13 � 2.72 82.30 � 2.22 92.22 � 3.47 102.95 � 3.83 100.06 � 4.32 U < C** W < R = B*** NS

Osmolality (Osmol kg�1) 0.313 � 0.001 0.324 � 0.001 0.318 � 0.001 0.326 � 0.002 0.337 � 0.002 0.333 � 0.002 U < C*** W < B < R*** NS

Triacylglycerides (mg 100 ml�1) 202.11 � 8.07 241.34 � 14.85 258.13 � 11.90 239.52 � 14.92 250.59 � 13.31 272.09 � 23.63 NS W � R � B; W < B** NS

Cholesterol (mg 100 ml�1) 234.09 � 7.58 238.63 � 7.88 261.60 � 6.10 241.79 � 7.24 252.73 � 7.51 255.89 � 6.33 NS W � R � B; W < B* NS

Haematocrit (%) 31.1 � 0.5 30.9 � 0.7 30.7 � 0.6 37.0 � 0.8 35.7 � 0.8 35.9 � 0.6 U < C*** NS NS

pH 7.576 � 0.019 7.531 � 0.085 7.592 � 0.082 7.304 � 0.145 7.332 � 0.139 7.326 � 0.112 U > C*** NS NS

pCO2 (mmHg) 7.1 � 0.3 8.2 � 0.3 7.7 � 0.2 12.3 � 0.6 12.2 � 0.5 12.8 � 0.4 U < C*** NS NS

Sodium (mmol l�1) 143.7 � 1.6 139.4 � 1.8 140.5 � 1.3 145.9 � 0.9 146.6 � 2.0 146.2 � 1.6 U < C*** NS NS

Potassium (mmol l�1) 3.88 � 0.16 4.08 � 0.20 3.75 � 0.15 4.70 � 0.34 5.09 � 0.25 4.96 � 0.20 U < C* NS NS

Calcium (mmol l�1) 0.82 � 0.06 b 0.60 � 0.04 a 0.83 � 0.05 b 0.95 � 0.05 b 0.92 � 0.05 b 0.87 � 0.04 b U < C* R � B �W; R <W* *

a Significant treatment effects are shown as symbols; < or >, means are significantly lower or higher; � or =, means are not significantly different; significant interaction is shown as letters;

*P < 0.05; **P < 0.01; ***P < 0.001; NS, non-significant.b Results represent means � S.E.M.; n = 4 groups of 5 fish each.c n = 20 fish.

N. Karakatsouli et al. / Aquacultural Engineering 38 (2008) 36–42 41

(high glucose and haematocrit, liver lipid mobilization,

osmotic and ionic disturbances, blood acidosis) stress

responses (Mommsen et al., 1999; Barton, 2002), which

are in accordance to what has been previously reported

for rainbow trout under confinement or other acute

stressors (Pottinger et al., 1992; Kakizawa et al., 1995;

Davidson et al., 2000; Kulczykowska, 2001). Focusing

on our objective, confined fish under blue light showed a

better stress response as it is indicated by the lower

increase in plasma cortisol which may also explain the

absence of liver lipid utilization. Volpato and Barreto

(2001) subjected Nile tilapia Oreochromis niloticus for

48 h to confinement under blue, green or white light and

reported lower stress-induced cortisol increase in fish

under blue light. Nevertheless, their results are

considered with certain reservations since they used

different light intensity for white and coloured lights.

Contrary to present results, jundia Rhambia quelen

juveniles subjected to chasing for 60 s after rearing for

10 days under 480 nm (which corresponds to our blue

filter) presented the highest cortisol increase compared

with light of 580 and 436 nm (Barcellos et al., 2006). On

the other hand, no effect of lighting spectrum on plasma

cortisol was detected for yellow perch P. fluviatilis

subjected to daily routine disturbances for 87 days

(Head and Malison, 2000) and for red porgy Pagrus

pagrus subjected to crowding for 30 days (Van der Salm

et al., 2004).

Although involved mechanisms remain still to be

elucidated, light spectrum effects have been generally

related to fish natural habitat and specific visual

abilities. Clean fresh water is a medium that permits

the penetration of longer wavelengths and visual

pigments of rainbow trout are sensitive to long

wavelength (Hawryshyn and Harosi, 1994). Lighting

conditions under red light may be perceived by rainbow

trout as more natural and thus favouring growth. On the

other hand present light spectrum effects on growth as

well as on stress response are probably related to neuro-

hormonal mechanisms and the interaction among

several hormones affected by both acute stress and

lighting conditions cannot be excluded. For example,

plasma or pineal melatonin has been reported to be

influenced by light spectrum (Bayarri et al., 2002; Naor

et al., 2003; Ziv et al., 2007) but also by acute stress in

rainbow trout (Kulczykowska, 2001). Furthermore,

thyroid hormones, somatolactin, growth hormone and

a-melanocyte-stimulating hormone have been sug-

gested to be affected by lighting conditions (Boeuf

and Le Bail, 1999; Szisch et al., 2002; Van der Salm

et al., 2004) and by acute stressors (Kakizawa et al.,

1995; Reddy et al., 1995; Ruane et al., 1999).

Present light spectrum treatment had also some other

physiological effects irrespective of confinement.

Higher liver water content observed in fish under white

light is in contrast to previously reported results for

rainbow trout (Karakatsouli et al., 2007). Differences in

experimental design may explain this discrepancy.

Nevertheless, since osmolality was lower in fish under

white light while no differences were detected for

plasma electrolytes, a probable effect of light spectrum

on osmotic regulation is indicated. Plasma glucose

results are also in contrast with previously reported for

rainbow trout (Karakatsouli et al., 2007) or red porgy P.

pagrus (Van der Salm et al., 2004). However, higher

glucose levels in fish under blue or red light seem not to

be indicative of a stress effect of light spectrum, since

cortisol levels in unconfined fish were similar and low

among light colours. On the other hand, glucose levels

especially in fish under blue light, along with the

observed higher levels of plasma cholesterol and

triacylglycerides could reflect a better nutritional status

(Congleton and Wagner, 2006) and may be related to the

better stress response these fish exhibited when

confined.

In conclusion, present results indicated that red light

favoured growth and blue light favoured fish acute

stress response. Providing that stressors are kept to a

minimum then red light (605 nm, 150 lx, 12L–12D)

could be suggested for the intensive rearing of rainbow

trout. Light spectrum effects on fish physiological

condition require further research especially in the field

of neuro-hormonal mechanisms involved. However, it is

clear that light spectrum should not be neglected when

artificial light in indoor fish farming facilities and

recirculating water systems is concerned.

Acknowledgments

We are most grateful to G. Konstantinou, X. Vrettos,

P. Louizos, H. Louizos, J. Garofalakis for their

laboratory and technical assistance and to Mr. P. Dimou

for providing the fish.

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