GROWTH IN STRAINS OF GILTHEAD SEABREAM

The lsraeli Journal of Aquaculture
-
Bamidgeh 49(2), 1997, 43-56.
43
GROWTH IN STRAINS OF GILTHEAD SEABREAM
SPARUS AURATA L.
W. Knibb*, G. Gorshkova and S. Gorshkov
lsrael Oceanographic and Limnological Research, National Center for Mariculture,
PO Box 1212, Eilat 881 12, lsraeL Tel. 972-7-637-3154 Fax 972-7-637-5761
(Received 29.1 1.96, Accepted 12.4.97)
Abstract
Five independent data sets were analyzed to assess whether growth and survival differ among
different strains of gilthead seabream , Sparus aurata, under captive culture conditions. Differences
were detected and we deduced (a) inferior performance of a wild strain relative to a hatchery strain;
(b) lack of evidence for substantial heterosis in our particular strains; (c) a slight trend among the larger fish to undergo sex reversal from male to female, and that sex effects should be considered in
future growth analyses. There were no detectable detrimental effects of PIT (passive integrated
transponders)tagging on a range ol S. aurata size classes.
Introduction
Commercial mariculture of gilthead seabream (Sparus aurata) is expanding rapidly in
the Mediterranean basin (Stephanis, 1996). To
date, there are no published data on whether
gilthead seabream from different wild locations
or different hatcheries differ in their performance under the same captive culture conditions. Understandably, information on strain differences is of immediate commercial interest.
Moreover, information on strain differences
should be obtained before embarking on withinstrain selection programs, as choice of the best
strain(s) could equal the genetic gains made by
years of selection on inferior strains (Gunnes
and Gjedrem, 1978).
*
Corresponding author
Intraspecific crossing (between strains) may
yield heterosis as has been noted for some
freshwater aquacultural teleosts, including
common carp (Cyprinus carpio, Moav et al.,
1975; Wohlfarth, 1993) and channel catfish
(lctaluridae, lctalurus; Dunham and Smitherman, 1983; Smitherman et al., 1983, Dunham,
1987). However, relatively weak heterosis was
reported for crossbred Atlantic salmon (Salmo
salar;Gjerde and Refstie, 1984), a species with
an anadromous life history. There are no published data concerning intraspecific crosses of
fully marine fish strains.
Here we analyze independent data sets for
evidence of differences in performance under
Knibb et al.
44
captive culture conditions among seabream
strains and crosses.
.
Materials and Methods
Hatchery strain. From 1972 until 1974 sev-
eral hundred wild-caught juvenile S. aurata
were taken from the Bardawil lagoon on the
Sinai coast of the Mediterranean Sea. They
were raised at the National Center for
Mariculture (NCM) in Eilat on the Red Sea.
Potentially, from the time of the first collection
untilthe present analysis, seven captive generations could have passed. However, precise
equal number from each subgroup. In addition, starting when the average weight of the
fish was 1 g, we anaesthetized the groups to
minimize differences in net avoidance behavior. After tagging with passive integrated
transponders (PlT tags), sampling was randomized by randomly selecting PIT tag numbers.
Spawning, larval and nursery rearing.
Females from broodstock tanks in pre-repro-
ductive condition received slow release
implants of [D-Alao-Prog-Net]-LHRH (Luteiniz-
breeding records were not kept.
Wild strain In 1988, approximately 60 individuals ol S. aurata weighing 10-20 g were captured from the Mediterranean Sea near Haifa
(approximately 280 km nofthwest of Bardawil
lagoon) and raised at the NCM. First generation
ing Hormone-Releasing Hormone;Zohar et al.,
1990). Approximately 20 males and 40 females
were held in each spawning tank. During the
natural spawning season (winter), each tank
typically produced between one and two million
eggs per day (females are daily spawners with
each female spawning up to 3 months; see
progeny were used for growth comparisons
Zohar et al., 1995, and references therein).
(hereafter referred to as wild fish).
Since 10,000-30,000 eggs can be obtained by
Crosses /-3. Cross 1 fish resulted from
crossing wild females with hatchery males.
Cross 2 fish resulted from crossing hatchery
force stripping a female, it seems that most
females spawned daily. For this study, only
females with wild males. Cross 3 fish resulted
from crossing males of an imported Cyprus
hatchery strain with the Eilat hatchery females.
Fish husbandry. The fish were fed dry pellets at 1-7% of their body weight per day,
depending on age. Pellets were 46% protein
overnight spawnings of at least 5 x 10s fertilized
eggs (estimated minimum of 15 females per
cross or strain) were used. Eggs were transferred to hatchery rearing tanks for 35 days,
then to the nursery for approximately eight
months. However, exact numbers of spawning
males and females were not determined
and 12/o lipid, manufactured according to a
because of mass spawning.
closed commercial formula
Experimental designs and analyses for
strains. In brief, five independent data sets
(Table 1) were analyzed to assess growth and
by
Matmor
Feedmill, lsrael. All tanks received the same
food ration, calculated periodically according
to the average tank biomass of all the replicates and strains, using the standard NCM
feeding tables designed forthe hatchery strain
(J. Lupatsch, pers. comm.). The fish biomass
of the tanks did not exceed 7 kglms. Sea water
was pumped from the Gulf of Eilat and salinity
and temperature followed those of the Gulf
(40+1 ppt and 22+4oC, respectively). Before
handling, fish weighing over 20 g were anaes-
thetized by immersion in 20 ppm quinaldine,
followed by immersion in 200 ppm ethylenglycol-monophenylether. After handling, the fish
were treated with 5 ppm nitrofurazone. We
attempted to randomize our sampling by dividing allthe individuals in a given group into several different subgroups, and by taking an
(usually) survival differences: l. communal rear-
ing in tanks in 1991-2, ll. separate rearing in
tanks in 1991 -2, lll. communal rearing in tanks
in 1993-4, lV. separate rearing in tanks in 19934, and V. separate rearing in sea cages in 1994.
Under communal rearing conditions, fish of different strains were individually tagged with passive integrated transponders (PlT tags) and
mixed in the same tanks. Under separate rearing conditions, fish of the different strains were
reared separately and not PIT tagged. The data
sets are independent and not contemporaneous. Therefore we did not attempt formal statisticalcomparisons among them, or use statistical
models to assess heterosis (see Klupp, 1979,
for example). However, each data set included
Growth in strains of gilthead seabream, Sparus aurata
45
Table 1. Data sets and experimental designs.
Data
Rearing
sef
condition
| (1ee1-2)
Communal
Strains Replicate
'
wird
Fish
containers per
strain
Type
of container
Times when samples
Date of
hatching were weighed
per strain
(in days post-hatching)
20 m3
tank
Feb. '15,
1991
352, 445, 482
150
5 m3
tank
Feb. 15,
1991
496, 6031, 645
102
5 m3
tank
March 1,
'1993
277, 388, 449,
o6
(G1)
Hatchery
Cross
ll
(1991-2)
Separate
wird
1
(G1)
Hatchery
Cross
lll (1993-4) Communal
1
Hatchery
515,6962
Cross 2
Cross 3
(1993-4)
lV
Separate
Hatchery
100
0.6 m3
tank
March 1 ,
1993
Cross 2
V
(1994)
Separate
Hatchery
275, 407, 466,
5223
3
150,000
Cross 2
1
m3
cage
,000
Feb. 1,
1
993
3794, 401, 477, 523,
560,596,634
1 During the interval day 496-603,
2
3
+
one cross 1 replicate suffered moftalities from an infection of
Cryptocaryon irritants. On day 603, five replacement fish were added to the replicate, and numbers of individuals in all other replicates were randomly reduced so that there were 39-40 fish
per replicate.
On day 696, hatchery fish only (26 from tank 1 ,27 fromtank2 and 25 from tank 3) were sexed
by biopsy and weighed. Day 696 data were used only in analyses assessing effects of sex.
During days 466-522,35 fish were lost from the second cross 2 replicate through infection by
C. irritants. Data from this tank during this time were discarded from all except survival analySES.
Samples of 100 fish per cage were group weighed after each time period.
the hatchery strain which may be considered
the reference strain.
For data set | (communal rearing 1991-2),
the following ANOVA model (a) was used to
assess strain effects on weight at a given time of
weighing:
Yi=t+s;+€4
where Y4is the weightforthelh fish in the fth
strain, m is the mean, si is the fixed contribution
forthe lth strain, and e4is the errorterm of thelh
fish in the fth strain. Note:weights did not depart
significantly from normal (using KolmogorovSmirnov goodness of fit test for single samples)
and, consequently, weight data were not transformed.
Model (b) was used to analyze data set lll
(communal rearing 1993-4), and was the same
as model (a) except it contained an additional
fixed factor for the tank as well as the strain x
tank interaction term. Models (c) and (d) were
the same as (a)and (b), respectively, exceptthe
covariate of the first recorded weight for the individualfish was added to assess the effect of initial weight. For situations where the results of
the ANOVA and ANCOVA analyses were simi-
lar. we concluded there was no substantial
Knibb et al.
46
effect
of initial weight, which
permitted
a
straightforward interpretation of the ANOVA
results. When the results of the ANOVA and
ANCOVA analyses varied, we concluded there
were confounding effects of initial weight, and
the ANOVA results should be considered cautiously. ANCOVA analyses should not be used
by themselves for tests of strain differences, as
statistical correction for initial weight may
remove both genetic and environment components of weight differences (Wohlfarth and
hatching at initial weighing for a given period.
For a discussion of this and other growth rate
formulas, see Ricker (1979).
Effect of PIT tagging. To test whether tagging with P|T-tags influenced growth (presently
unknown for seabream), experiments were car-
ried out on four size groups of hatchery
seabream (averaging approximately
1
2, 24, 52
and 146 g). On June 5, 1991, ten fish were
selected at random from the 12 g group and
Sex effecfs. Assessment of sex effects
required keeping the fish to sexual maturity
implanted with 1 1 x2Jmm PIT tags in the anterior dorsal part of the body. Ten control fish of
the 12 g group were chosen for similarity of their
individual weights to the ten individually tagged
fish and transferred, with the tagged fish, to a
single 600 ltank. This procedure was repeated
by transferring ten randomly chosen tagged fish
and ten individually chosen non-tagged fish of
the 12 g group to a second 600 | tank. This
experimental protocol was also carried out for
fish of the24 9,52 g and 146 g groups. Twentyseven and 78 days after implantation, weights
and tag numbers of individual fish were recorded using a portable radio-frequency identity tag
reader (BioSonics, Inc., Model SM1301).
Differences in survival among strains were
assessed from the first weighing until the final
weighing, using arc sine square-root trans-
because S. aurata is a protandrous hermaphro-
formed survival proportions (Sokal and Rohlf,
dite (initially male, then female). This informa-
1e81).
Moav, 1972, 1993). However, it should
be
emphasized that this issue is controversial, and
the use of covariate or regression procedures to
control for stafting weights remains acceptable
(see Trippel and Hubert, 1990; Nilsson 1992;
Jarayabhand and Thavornyutikarn, 1 995).
In data sets ll, lV and V (separate rearing in
tanks or sea cages), the identity of the individual
fish was unknown (fish were not PIT tagged).
Models were similar to (a) and (c) except that
tank or cage mean weights were used. Mean
weights were used to permit ANCOVA analyses
on untagged fish, although this approach results
in a substantial loss of available degrees of freedom.
tion was available for only a subset of the total
data sets: some individually PlTtagged hatchery fish from data set lll were kept after completion of the growth trials (day 515 post-hatching)
untilthey reached sexual maturity (day 696). On
day 696, 64.3% of the sampled hatchery fish
were male and35.7'/" were female, according to
biopsy (Zohar et al., 1978). Here it was apparent
that sex differences in growth rates at different
times, rather than final weights, were of primary
interest. Consequently, we considered both
weight and growth rate data, presented graphically for different growth periods.
The specific growth rate (designated GR)
was calculated as follows: GR = 100 x (ln wz - In
wr)/(tz - t1) where w2 is the weight in grams at
final weighing for a given period, wr is the weight
in grams at initial weighing for a given period, t2
is the time in days after hatching at final weighing for a given period, tr is the time in days after
Results
Weights and weight gain. Three general patterns were evident f rom growth gains of different
strains and different experiments (Table 2).
First, under both communal and separate rearing conditions, progeny of wild fish showed infe-
rior growth gains relative to the hatchery strain
which was propagated for many generations in
captivity. Second, growth gains for crosses
were usually only slightly superior to the hatchery strain or mid-parent mean (not tabulated).
Third, and within the limitations of the data sets
(non-contemporaneous weighing times, only
modest differences among strains/crosses),
there were no obvious magnification effects
under communal rearing conditions.
Fuller statisticaltreatments by ANOVA and
ANCOVA of the final weight data (Table 3) indi-
cate some strain effects were not entirely
Growth in strains of gilthead seabream, Sparus aurata
i
47
Table 2. Initial weight averages and weight gain averages (g) for five different strains and five
ndependent experiments.
Character
Data set
Hatchery
Cross
1
Cross 2
Cross
3
Test period
(in days
post-hatching)
ll (separate
Wt. gain
133.4a
96.8b
Initialwt.
181
.9b
166,6a
211.0c
Wt. gain
175.0a,b
143.6a
207.1b
rearing)
| (communal
rearing)
Initial wt.
126.8a
132.1a
80.2a
99.2b
352-482
496-645
lll (communal rearing) Initialwt.
48.9a
49.8a
54.7b
Wt. gain
234.9a
248.0b
229.1a
rearing)
lV (separate
Initial wt.
Wt. gain
V (separate
rearing)
wt.
gain
49.9
277-515
49.9
275-466
185.4
184.1
Initial
144.7
141.7
Wt.
274.0
299.1
379-634
Values with the same superscript are not significantly different (at the 0.05 probability level), using
Student-Newman Keuls post-hoc tests and records of individual fish for communal rearing data
sets, and tank or cage averages for separate rearing data sets.
Table 3. Analysis of variance and covariance of final weights for different data sets.
Data set
ANOVA
source
I communal
rearing
strain
residual
ANCOVA
df
2
263
MS
16447
1
F
source
8.8***
873
strain
2
covariate (initial weight)
't
residual
ll separate rearing
strain
2
residual
6
8782 47.8**'
strain
184
covariate (initialweight)
residual
lll communal
rearing strain
tank
strain x tank
residual
*
*"*
2
2
4
288
6839 2.3
407 0.1
519 0.2
2948
MSF
df
262
2
1
5
strain
2
tank
strain x tank
4
covariate (initialweigh|
residual
287
1
1
7240 10.4***
15 470.4***
3281
698
1085
15780
1
7.6.
10.2***
143
16939
623
459
11
.5***
0.4
0.3
405557 275.1***
't474
p<0.05
p<0.001
For communal rearing analyses, weights of individual fish were used; for separate rearing analyses, weight averages from tanks were used. Relatively few degrees of freedom were available for
data sets lV and V and analyses are not tabulated.
Knibb et al.
48
accounted for by initialweights. More obvious,
however, were the very substantial effects of
initial weights (see covariate mean squares of
Table 3). Overall, there was a pattern of initial
weight differences between strains, or even
replicates, which persisted over the different
weighing times (data not tabulated). Weights of
individual fish (from communal rearing data
sets) were, as expected, strongly correlated
during the different time intervals (lowest: day
277 to 515, data set lll, r= 0.68, p<0.001; highest: day 449 to 515, data set lll, r = 0.97,
p<0.001). Growth rates
of individual fish
between different time periods were also significantly positively correlated (data not tabulated).
Suruival. Survival from initial to final weighing was assessed jointly for data sets I to lV by
considering the 25 occasions (tank/strain
groups) for which survival proportions could be
calculated. In general, survival was high with
an overall average of 90.8+3.4%. Athree-way
ANOVA (reclassifying strains as parental or
cross) assessing strain, rearing condition and
year effects indicated lower survival for the
crosses (F 1r.r 7l= 5.7, p<0.05), lower survival
under separate rearing conditions (F 1r.rz1 =
6.4, p<0.05), a strain x rearing interaction (F
11.171=
5.4, p<0.05), , a rearing x year interac-
tion (F
l.nl=
5.1, pcO.05), but no effect of year
(1991-2 or 1993-4). These results reflect outbreaks of Cryptocaryon irritans (a marine ciliate parasite) and losses of some cross 1 and
cross 2 fish under separate rearing conditions
in 1991 -2 and in 1993-4. (Note: details of the
model are not specified, and results are not
tabulated).
Coefficients of variation. For the hatchery
strain, and considering all 35 instances where
individual fish were weighed from tanks in data
sets I to lV, coefficients of variation for individual
fish weight ranged from 17o/" Io 35%. As the
hatchery strain was included in all data sets,
hatchery coefficients of variation at different
times and rearing conditions could be used as
references to compare other genetic groups.
Ratios of coefficients of weight variation (hatch-
ery coefficient as denominator) revealed that
coefficients for the wild strain usually were comparable to the hatchery strain, but crosses tend-
ed to have lower coefficients (Fig. 1). Moreover,
coefficient of variation values of the crosses, relative to the hatchery strain, tended to decline
with time, suggesting that the low variances of
crosses at slaughter (>250 g) were not entirely
due to initial sampling errors and starting variances.
Sex effects. To assess potential confounding effects of sex in the protandrous hermaphro-
dite, S. aurata, some individually P|T-tagged
(hatchery fish from data set lll) were grown to
reproductive maturity and sexed. Thus, earlier
weight records of individual fish were assigned
a sex. At spawning (= day 696), sex differences
in weights were slight and not statistically significant (i.e., p>0.05, Fig.2a). However, in younger
fish (approximately 50 g), where all individuals
should be gonadally male (Zohar et al., 1978),
the group which eventually became females
tended to be significantly larger than the group
which stayed male. Slightly larger weights for
the group destined to be females persisted until
slaughter weight (= day 515). These sex differences for weight translate into initially higher
(before records were kept), then lower, growth
rates for fish destined to be females, especially
after sex reversal and during reproduction (Fig.
2b). Sex differences for growth were evident in
other data sets (Knibb et al., 1997, and unpublished).
Effect of PIT tags. Ettects of PIT tags were
assessed using four size groups of S. aurata
(averaging approximately 12, 24,52 and 146 g),
two replicate communal rearing tanks per
group, and two growth periods. Paired f-test
analysis of the 16 possible paired groups of
tagged and non-tagged fish weights showed not
even a slight trend across days and sizes for
detrimental effects of PIT tags (t1rs1 = -.44; data
not tabulated). Of the six fish which died during
the experiment, all were from one tank of the
smallest size group from day 27 to day 78. lt
would seem that an equal number of tagged and
nontagged fish were lost, assuming no loss of
tags during this time interval. The level of tag
loss was low, and the majority of lost tags (three
of five) were f rom the smallest (12 g) size group.
Thus PIT tagging S. aurata individuals, as small
as 12+2 g, had no detectable detrimental effect
on subsequent growth.
Growth in strains of gilthead seabream, Sparus aurata
Communal rearing
a
o
o
O
o
v
0.6
vo
VO
89
E
c.)
L=
b
otr
o
S 0.8
Separate rearing
!
Y
trl
49
o
V
VV
r
o
v
0.8
o
vv
o
Yv)
k
0.6
0.4
OO
a
T
I
I
I
ll
TI
c.)
U
B
0.4
250
350
450
550
200
300
400
Days
500
600
700
Days
Q
wito,
tgst-z
v
Cross 2,1993-4
!
Cross
l.l99l-2
O
Cross 3,1993-4
Filled symbols indicate p<0.05 (or less)
Fig. 1. Coefficient of variation ratios. Ratios were calculated by dividing the coefficient of variation for
weight of cross 1, 2, 3, or the wild strain by the coefficient of variation for the hatchery strain using contemporaneous data from the same data set. For communal rearing data, ratios were calculated for each replicate tank. For separate rearing, numerator values were calculated for each replicate tank, and denominator
values from the average hatchery value over replicate tanks. Probability levels for ratios were calculated
according to Lewontin (1966).
Discussion
Even though few seabream strains were
analyzed, growth differences under captive culture conditions were detected. This was evident, mostly, as inferior per{ormance of the first
ences already existed among the ancestralwild
populations, as have been noted for salmonid
species (Gunnes and Gjedrem, 1978; Refstie
and Steine, 1978, Gjerde and Refstie, 1984;
generation wild fish relative to the hatchery
Withler et al., 1987) and channel catfish
strain which was propagated in captivity. These
differences could have resulted from natural/
domestication selection for performance in culture conditions as has been suspected in channel catfish, carp and salmonid species (Doyle,
1983). The other possibility is that genetic differ-
(Smitherman et al., 1983; Dunham, 1987). Our
two parental strains were collected at different
sites: the wild strain was collected, approximately 280 km northeast of the original collection site for the hatchery strain.
Dobzhansky (1952), Lerner (1954), Haldane
Knibb et al.
50
700
600
bo
bo
.(D
200
r00
1.1
1
0.9
0.8
(D
0.7
0.6
B
0.5
0.4
0.3
0.2
0.1
0
277-388
449-sr5
388-449
5
l5-696
Days
male
female
!
Fig. 2. (a) Mean weights and (b) specific growth rates with standard deviations for sexed hatchery fish
and for each tank from communal rearing 1993-4. (Sex in young fish represents their ultimate sex identity).
+ p<0.1, * p<0.05, *** p<0.001. Probabilities were for F ratios assessing sex effects and were calculated using model (b) ANOVA (but replacing strain with sex and, for growth analyses, replacing weight with specific growth rate). ANOVA analyses used records of individualfish for each weighing time or gromh interval.
Tank effects and tank by sex interactions were not
significant.
,
Growth in strains of gilthead seabream, Sparus aurata
(1955) and others have suggested that
increased growth and survival, and reduced
phenotypic variances, may be expected
51
electrophoresis, etc., are indicators of the
genetic variation responsible for commercially
in
desirable heterosis and strain differences in
Crosses through increased heterozygosity (also
see McFarquhar and Robertson, 1963; Tachida
culture (Kinghorn, 1983; Bentsen 1991, 1994).
The Eilat hatchery S. aurata broodstock, as
in other seabream hatcheries around the
Mediterranean basin, was established 20 years
and possibly 7 generations ago. The extreme
fecundity ol S. aurata females (several millions
of eggs are produced per female per season,
Zohar et al., 1 995), combined with less than certain breeding records in captivity (effective population sizes were unknown), created the potential for high levels of inbreeding in the hatchery
and Mukai, 1985; Koljonen, 1986). Overall,
growth of the S. aurata crosses was only marginally superior to the best parental strain
(where information was available). Survival
was relatively low in some of the crosses. Thus
the finding that crosses exhibited lower variation for weight at maturity than the parental
"pure" strains was perhaps unexpected,
although this feature may be desirable under
certain commercial production systems. Little
heterosis has been reported for Atlantic salmon
(Gjerde and Refstie, 1984). By contrast, substantial heterosis for growth has been reported
for some, although not all, crosses of freshwa-
ter species such as carp (Moav et al., 1975),
and channel catfish (Dunham, 1987) while
moderate heterosis has been repofted for rain-
bow trout (Oncorhynchus mykiss; Ayles and
strain (Taniguchi et al., 1983; Sugama et al.,
1988). Inbreeding, especially high levels, has
been reported to lower survivability, growth
rates and size in a variety of fish species
(Kincaid, 1983, for review). Our present data
showing superior growth and equivalent survival of the hatchery strain relative to the wild
sample (and our unpublished data on DNA fingerprints) suggest that substantial inbreeding
Baker, 1983; Gjerde, 1988; but see Klupp,
1979; Horstgen-Schwark et al., 19BO). lt
did not occur in the hatchery strain.
remains to be determined whether low levels of
intraspecific heterosis in marine fish are a general phenomenon. Few impediments to gene
slight trend for the larger fish to undergo sex
Available data on sex effects indicate a
reversal into females, followed by slowed
flow among wild populations, and relatively
short histories of captive propagation, might
growth, most pronounced during reproduction
(Kadmon et al., 1985). Clearly, however, other
factors such as the social and hormonal envi-
retard intraspecific genetic differentiation for
marine fish, at least in comparison with those
ronment should be of much greater impoftance
in sex reversal (Zohar et al., 1995). For the pre-
reshwater species with geographically isolated
populations or with long histories of domestication (see Macaranas and Fujio, 1990).
Conflicting data are provided from population
sent analyses and data sets, sex effects
f
genetic surveys of allozyme, mitochondrial
DNA, DNA fingerprinting and DNA microsatel-
lite polymorphisms among wild (and farmed)
marine populations. Whereas little variation
(fixed and frequency allele differences) is evident among tuna (Katsuwonus pelamis) and
flatfish (Pleuronectes platessa) populations
and strains (Purdom, 1993), some variation is
evident for the European sea
bass
(Dicentrarchus labrax; Martinez et al., 1991;
Patarnello et al., 1993), and the red sea bream
(Pagrus major;Taniguchi and Sugama, 1990;
Takagi et al., 1995). Moreover, it is unknown
whether the genetic differences observed using
appeared too slight to confound strain performance (especially past slaughter weight), yet it
seems prudent for future analyses to include
information on sex, especially if sex ratios vary
among groups.
In conclusion, the differences in performance under captive culture conditions detected among the relatively few seabream strains
tested in this study indicate a need for further
and more extensive strain assessments.
Acknowledgements
We thankfully acknowledge the financial
contributions of the Rothschild Foundation (Yad
Hanadiv) and the Leichtag Family Foundation.
We are indebted to ltal lvri and Natan Wajsbrot
for their excellent technical assistance. to
R.
Knibb et al.
52
Avtalion, G. Hulata, G. Kissil and D, Popper for
comments on the manuscript, and especially to
M. Soller for general and statistical advice. We
especially thank G. Pagelson who contributed
the sea cage rearing data.
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