However, for females and males, there was no significant DL effect on plasma sexual steroids and thyroid hormones levels. Likewise, no DL effect on N and P retention and loss was found. From
this study, it appears that manipulating DL is a promising tool for improving rearing performance of Eurasian perch. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Eurasian perch; Perca fluÕiatilis; Daylength; Growth; Weight variability; Gonad development; Food efficiency
1. Introduction
Ž .
The Eurasian perch Perca fluÕiatilis L. has been described recently as a new
Ž .
candidate for intensive freshwater aquaculture Kestemont and Dabrowski, 1996 .
However, this species shows large variations in growth between fish of the same cohort Ž
. Ž
Melard et al., 1995 and significant mortality rates due to cannibalism Kestemont et
´
. Ž
. al., 1995 . These led to economic losses in aquaculture Smith and Reay, 1991 . Growth
Ž heterogeneity is considered to be caused by initial size heterogeneity Loadman et al.,
. 1986; Katavic et al., 1989; Van Damme et al., 1989; Folkvord, 1991; Brabrand, 1995 ,
Ž .
Ž density Baardvik and Jobling, 1990; Melard et al., 1996 , social interactions Abbott et
´
. Ž
al., 1985; McCarthy et al., 1992; Jobling et al., 1993 or genetic factors Bry and Gillet, .
1980; Kindschi and MacConnell, 1989 . It was recently reported for Eurasian perch that Ž
. Ž
. rearing conditions Fontaine et al., 1996a and temperature Melard et al., 1995, 1996
´
can also affect variation in growth. Considering that Eurasian perch show a typical diurnal feeding activity when fed on
Ž .
demand with self-feeders Anthouard and Fontaine, 1998 , one might hypothesise that daylength will influence its growth and heterogeneity, as it has already been shown in
Ž . Ž
other perciform species such as juvenile sunfish Lepomis cyanellus
Gross et al., .
1965 . Other effects of daylength may relate to growth heterogeneity and cannibalism, but
whether this latter parameter is a cause or a consequence of the former is not known. Ž
. Indeed, daylength can also act on gonad development De Vlaming, 1972 , which in turn
Ž may induce changes in body and muscle composition Norton and Macfarlane, 1995;
. Jørgensen et al., 1997 . In addition, daylength may interfere with feeding activity rhythm
Ž .
Ž and feed intake Barlow et al., 1995 and, in turn, may affect nutrient retention Boujard
. Ž
. and Leatherland, 1992; Boujard and Luquet 1996 and loss Thorpe and Cho, 1995 .
Consequently, the purpose of our study was to investigate the influence of three daylengths on growth and growth heterogeneity in Eurasian perch juveniles reared in
recirculation systems, with particular attention to physiological variables, such as feed efficiency, plasma thyroid and sexual steroid levels, and N and P balance.
2. Materials and methods
2.1. Fish, facilities and water quality Ž
. In April 1997, a ribbon of perch eggs was harvested at Lindre pond Moselle, France
Ž .
and incubated at the University of Nancy Laboratory of Animal Sciences . After
hatching, larvae were introduced to a 500-l tank and fed Artemia nauplii for the first 30 Ž
. days, then weaned and fed a pelleted diet Biomar, Aquastart 15.50 no. 01 for 2
months. Juvenile perch were then transfered to two 1750-l tanks and reared at 20–228C Ž
. Ž
. under 12 h light 100 lux at the water surface :12 h dark photoperiod LD 12:12 . At the
Ž beginning of the experiment, the population 2660 perch, 150 days old, mean weight:
. Ž
y3
5.1 g was divided into nine batches of 280–310 juveniles initial biomass: 1 kg m ,
. initial coefficient of variation in weight CVi: 25–30 and then randomly allotted to
Ž nine blue-walls tanks of 1750 l each, functioning as recirculation systems Fontaine et
. al., 1996b . The experiment lasted 16 weeks from September 1997 to January 1998.
During the experiment, the physico-chemical parameters of the water were checked three times a week. Water temperature was maintained at 23.0 0.48C, optimum for
Ž .
perch growth according to Melard et al. 1996 . Dissolved oxygen content and pH were
´
y1
Ž
y1
. Ž .
Ž .
always above 7.0 mg l 7.6 0.3 mg l
mean range and 7
7.1 0.1 , respectively. The N–NH
q
and N–NO
y
levels were measured with indophenol blue and
4 2
Ž .
sulfanilamide methods, respectively Cary I spectrophotometer, Varian . The values remained between 0.00 and 0.13 mg N–NH
q
l
y1
, and between 0.00 and 0.15 N–NO
y 4
2
mg l
y1
. 2.2. Lighting conditions
Ž The lighting of each tank was provided by incandescent white bulbs OSRAM L, 18
. W, type 12-950 . Each tank was separated from the others by a plastic frame. After
switching off, light intensity was nil. Light intensity, measured at the centre of the square tank and at the water surface, was set to 100 lux. This value was in the range of
Ž optimal values for a similar species, the yellow perch Perca flaÕescens Hinshaw,
. Ž
. Ž
1986 . Perch were subjected to three different daylengths DL : a 12-h L group light on .
Ž .
at 0800 h , a 18-h L group light on at 0500 h and a 24-h L group. Each group comprised three replicates. In the 12- and the 18-h groups, DL was controlled by
electrical clocks and a switch phase of 30 min, when light intensity was reduced to 10 lux. This light intensity was provided by a 15-W bulb located above the water surface of
each tank. This phase simulated dawn and dusk, known as the most adequate period for
Ž .
Eurasian perch feeding in their natural environment Thorpe, 1977; Dabrowski, 1982 . 2.3. Feeding
Ž Perch juveniles were fed with an extruded diet Biomar, Ecolife 15.50 no. 2, 46
. proteins, 15 lipids according to the manufacturer . Each tank was equipped with an
Ž .
electronic self-feeder Anthouard and Wolf, 1988 . The following procedure was
applied: during the first week, the lever was hit artificially every 15 min from 0800 to 1900 h. Then, the number of artificial hits was reduced by 25 every 2 days. However,
this level of artificial hits was kept until day 56 in tanks where fish did not activate the rod themselves, i.e., the 12-h group. During the first week of the learning phase, perch
Ž .
Ž .
y0 .24
were fed at optimal feeding rate as defined by Melard et al. 1996 : R s 3.3 = W
´
Ž .
W: individual mean weight, g . The feed reward was set at 0.7 0.1 g per trigger activation, corresponding to 25–30 pellets. This reward level was kept during the whole
period of the experiment and checked every week. The amount of feed remaining in the feed dispenser was checked twice daily at 0800 and 1800 h, and special care was taken
to always maintain filled feed dispensers.
2.4. Growth, sex determination and gonado-somatic indices Mortality was recorded daily and each dead fish was weighed. An assessment of
causes of mortality was performed through observation of external symptoms, such as necrosis, injuries, enucleation or regurgitation. A growth control was performed at day
Ž . D 56 and 84 by individually weighing 100 fish to the nearest 0.1 g for each replicate,
Ž
y1
. after anaesthesia 2-phenoxyethanol C H O ; 0.3 ml l
. Mean body weights and
8 10
2
Ž .
coefficients of variation in weight were calculated. At the end of the experiment D112 , all fishes were counted and individually weighed. Survival rate, growth, growth hetero-
Ž .
geneity variation in individual weights among fish of the same replicate and feed efficiency were calculated as follows:
Ž .
Ž .
y1
Survival rate Sr, s 100 = N y N = N
i f
i
Ž .
Ž .
y1
‘‘Lost’’ fish L, s 100 = N y N y N = N
i f
d i
Ž .
Ž .
y1
Relative growth rate RGR, s 100 = W y W = W
f i
i
Ž
y1
. Ž
.
y1
Specific growth rate SGR, day s 100 = lnW y lnW = D t
f i
Ž .
y1
Final coefficient of variation in weight CVf, s 100 = SD = W
f
Ž .
Ž .
y1
Feed efficiency FE s B q B y B = Feed supply
f d
i
where N and N are the initial and final fish number, W and W are the initial and final
i f
i f
mean wet body weight, SD is the final standard deviation for weight, B and B are the
i f
initial and final fish biomass, and N and B are the number and the biomass of dead
d d
fish collected during the course of the experiment, respectively. At the end of the experiment, the fishes were killed with an excess of 2-pheno-
xyethanol. Sexes were determined for an average of 30 fish per tank. For each batch and Ž .
both sexes, the average final value of gonado-somatic index GSI was calculated: GSI s 100 = GW = W
y1
Ž . Ž .
where GW is gonad weight g and W is body weight g . 2.5. Assays of sexual steroid and thyroid hormones
Ž .
On D112, a 1-ml blood sample was taken from the hearts of 10 fish per tank . Using Ž
. a heparinized syringe, the plasma was separated 4000 rpm, 25 min then stored at
Ž . Ž
. y258C until assayed. Plasma concentrations of testosterone T , 17b-estradiol E
and
2
Ž .
11-keto-testosterone 11 KT were assayed by means of the RIA analysis after two Ž
. extractions with cyclohexanerethyl acetate according to Fostier and Jalabert 1986 . The
Ž .
inter-assay coefficients of variation were 10.8, 7.9 and 7.7 for T n s 5 , E
2
Ž .
Ž .
n s 5 , and 11 KT n s 5 , respectively. The intra-assay coefficients of variation were Ž
. Ž
. Ž
. 14.2, 12.1 and 8.9 for T n s 5 , E
n s 5 , and 11 KT n s 5 , respectively.
2
Ž .
Ž .
Plasma concentrations of triiodothyronine T and thyroxine T
were measured by
3 4
Ž .
RIA using commercial kits T RIA kit, Monobind, USA and T RIA kit, Pantex, USA .
3 4
Ž .
The inter-assay coefficients of variation were 10.9 and 11.3 for T n s 10 and T
3 4
Ž .
n s 14 , respectively. The intra-assay coefficients of variation were 9.9 and 10.6 for Ž
. Ž
. T
n s 10 and T n s 14 , respectively.
3 4
2.6. N and P balance study An initial pooled sample of 100 g of fish, and a final sample of approximately 20 fish
from each replicate, were crushed, homogenized and freeze-dried before composition analyses. Analyses of diet and whole-body samples were made following the usual
Ž .
procedures: dry matter after drying at 1058C for 24 h; nitrogen protein s N = 6.25 by the Kjeldahl method after acid digestion, total phosphorus by spectrophotometric
analysis of the phosphovanadomolybdate complex after mineralization and acid diges- Ž
. tion ISOrDIS 6491 method .
N and P retention and loss were deducted from N and P supply, carcass and feed N and P content:
Ž . ŽŽŽ
. Ž
. Ž
Retention s 100 =
B q B = final carcass nutrient content y B = initial
f d
i
..
y1
carcass nutrient content = Nutrient supply ;
Ž
y1
. Ž
Total loss g = kg fish gain s 1000 = Nutrient supply y B = final carcass nutri-
f
. Ž
..
y1
ent content y B = initial carcass nutrient content = Gain
i
Ž
y1
. Ž
ŽŽ Faecal and metabolic loss g = kg fish gain
s 1000 = Nutrient supply y B q
f
. .
Ž .
Ž
y1
B = final carcass nutrient content y B = initial carcass nutrient content = Gain
d i
Ž
y1
. Loss due to mortality g = kg fish gain
s Total loss y Faecal and metabolic loss Ž .
with Nutrient supply g s Feed supply = Feed nutrient content, and Gain s B y B .
f i
2.7. Data analysis Ž
Each parameter was analysed by a one-way ANOVA with daylength effect DL: .
df s 2; error term: df s 6 . For parameters which had values fluctuating with time, a one-way ANOVA was performed at each time to verify the hypothesis of the normality
of the variance; then, the model used to analyse data was a hierarchical mixed model Ž
. procedure MIXED, SAS Software; Little et al., 1996 :
Y s m
qDL qTime
qTimeDL qTank
qTimeTank
i jk l i
j i j
k Ž i. jk Ž i.
df :35 s 2
3 6
6 18
Ž DL and time effect were considered as fixed effects and the hierarchical effect tank
w x.
DL as the reference means square in testing the DL effect. The data were analysed
with a split plot ANOVA. Finally, influence of daylength and sex on mean weights and hormonal levels were tested through a hierarchical model with a split plot ANOVA:
Y s m
qDL qSex
qDLSex qTank
qSexTank
i jk l i
j i j
k Ž i. jk Ž i.
df :17 s 2
1 2
6 6
Ž DL and sex are fixed effects sex is considered as being naturally randomised between
. fish within each tank . Consequently, the analysed variables corresponded to Ls means
Ž .
SD of measures carried out per experimental unit, adjusted by effects of other Ž
factors. For one-way ANOVAs, the means were tested by a Newman–Keuls test fit for .
balanced design . For the split-plot ANOVAs, the Ls means were tested by a Fisher test with the contrast method. In all tests, a significance level of P - 0.05 was used.
3. Results
