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AQUATIC BIOLOGY
Aquat Biol
Vol. 13: 275–284, 2011
doi: 10.3354/ab00373 Published online October 26
INTRODUCTION
Moray eels (Muraenidae) are a family of approxi-
mately 200 predatory reef fish species found in every
tropical and temperate seas, but they are not well
studied because of their cryptic habitats and occa-
sionally aggressive behaviour that make them dif -
ficult to collect (Reece et al. 2010 and references
therein). Consequently, basic information on their
biology and ecology is scarce. A few papers have
been published about behavioural ecology (Abrams
et al. 1983), spawning (Ferraris 1985), comparative
gonad morphology and sexuality (Fishelson 1992);
however, there are no data concerning spatial and
temporal features of spawning, or about age, growth
and reproductive biology of moray eels. Recently,
Jiménez et al. (2007) confirmed separation of Mu -
raena augusti from M. helena using a multidiscipli-
nary approach including biometric, ecological and
molecular analyses.
Moray eels have a long pelagic larval interval up to
2 yr (Bishop & Torres 1999, Reece et al. 2010) and,
being poor swimmers as juveniles and adults, they
maintain high site fidelity to a few square meters of
reef (Böhlke et al. 1989). They adopted biting as an
alternative prey capture strategy instead of sucking
and this could be related to their success as predators
in the confined spaces of reef crevices (Mehta &
Wainwright 2007, 2008). In general, adult moray eels
are night carnivores which mainly feed on benthic
© Inter-Research 2011 · www.int-res.com*Email: sanja@izor.hr
Mediterranean moray eel Muraena helena (Pisces:
Muraenidae): biological indices for life history
Sanja Matić-Skoko1,*, Pero Tutman1, Mirela Petrić2,
Daria Skaramuca3, Domagoj 0ikić4, Duje Lisi<ić4, Boško Skaramuca3
1Institute of Oceanography and Fisheries, PO Box 500, Šetalište Ivana Meštrovića 63, 21000 Split, Croatia
2University of Split, Center of Marine Studies, Livanjska 5/III, 21000 Split, Croatia
3Faculty of Aquaculture, University of Dubrovnik, Ćire Cari a 4, 20000 Dubrovnik, Croatia
4Faculty of Natural Sciences and Mathematics, Department for Animal Physiology, University of Zagreb, Rooseveltov trg 6,
10000 Zagreb, Croatia
ABSTRACT: This paper presents the first published data on age, growth and reproduction of the
moray eel Muraena helena in the Mediterranean Sea. A total of 193 individuals of M. helena were
sampled by longlines at depths from 5 to 80 m in the southern Adriatic Sea. The length and weight
of individuals ranged from 27.5 to 121.0 cm and from 116.1 to 3680.0 g, respectively. Age readings
from annual zones of M. helena otoliths revealed the presence of age groups 2 to 12. The von
Bertalanffy growth function (VBGF) for the sampled population had the following parameters:
L∞= 162.70 cm, k= 0.089 yr–1 and t0= −0.660 yr. Females dominated the total male:female ratio
(0.35:1.00) and, together with immatures, were more frequent in shallow coastal waters. The
greatest increase in gonadosomatic index occurred in summer, while condition factor was the
lowest in winter. The ovaries of moray eels mostly contained oocytes in a pre-vitellogenic or vitel-
logenic stage and males with testes at maturation stage were observed at the beginning of July.
The absence of males and ripe or spent females in the inshore coastal waters suggests that moray
eels leave this region in summer to spawn, most likely in the south Adriatic Pit at a depth below
600 m.
KEY WORDS: Moray eel · Muraena helena · Age · Growth · Reproduction · Adriatic Sea
Resale or republication not permitted without written consent of the publisher
Aquat Biol 13: 275–284, 2011
fishes, cephalopods and crustaceans (Göthel 1992).
Their diet probably depends on fish size and reflects
differences in prey availability, as was previously
found for the closely related species Conger conger
(Cau & Manconi 1983). Piscivores like moray eels
affect the post-settlement survivorship of some fish
species; thus such predation may have an important
role in structuring local reef-fish assemblages (Carr
& Hixon 1995).
Muraena helena is an eastern Atlantic-Mediter-
ranean moray eel whose range extends north to the
British Isles and south to Senegal and Cape Verde
(Smith & Böhlke 1990); it is also found in the Canary
Islands, Madeira and Azores (Randall & Golani
1995). This territorial species is found on rocky bot-
toms, commonly lurking in holes and writhing
through crevices or under rocks during the day. It is
more active during night, when moray eels leave
their shelter, but just for a short time and only in
vicinity of their own holes. The species is distributed
from shore to a depth of 800 m, rarely or infrequently
shallower than 50 m and commonly at depths of
100 to 300 m (Jiménez et al. 2007).
The eastern coastal region of the southern Adriatic
Sea includes shallow waters around the outer islands
but reaches depths of up to 1330 m in the south
Adriatic Pit (Fig. 1). It is characterized by rocky bot-
toms and therefore represents an ideal habitat for
Muraena helena. Two moray eel species are found in
the Adriatic: the very rare Gymnothorax unicolor and
the more common M. helena (Jardas 1996). In the last
decade, M. helena has become very frequent and is
now one of the species with pronounced dominance
in the last decade among the fish communities of the
outer islands in the middle and southern Adriatic, as
is evident from longline catches of artisanal fishers
(Mati´c-Skoko et al. 2009).
Although it is well known and probably plays a sig-
nificant ecological role in Mediterranean rocky ben-
thic communities as a part-time resident top predator
(Hixon 1991), Muraena helena is still a poorly studied
species and there are no published data on its biol-
ogy and ecology. The present study was conducted to
examine age, growth and reproductive biology of the
Mediterranean moray eel M. helena from specimens
captured along the eastern Adriatic coast.
MATERIALS AND METHODS
A total of 193 individuals of Muraena helena were
caught on monthly basis between October 2009 and
December 2010 in the southern Adriatic with bottom
longlines at depths of 5 to 80 m (shallow costal waters
around Dubrovnik: 5 to 15 m; Elafiti Archipelago: 15
to 30 m; islands of Mljet and Sveti Andrija: 30 to 80 m),
depending on the coastal configuration (Fig. 1). Ap -
proximately 16 individuals (range 12 to 46) were
examined per month. For each fish, the total length
(TL) was measured (precision 0.1 cm), the total body
weight (W) was recorded (precision 0.1 g), and the
sagittal otoliths were removed, cleaned and stored
dry for further examination. Before the
age reading process, otolith length
(OL) was measured (precision 0.1 mm)
at the longest axis and otolith mass
(OM) was weighed (precision 0.1 g).
For testing the sex ratio a simple
Chi-square test was used. The size
frequency distributions (5 cm length
classes) between sexes were deter-
mined. All data were tested for normal-
ity and homogeneity of variances.
The length–weight relationship was
described by the equation W= aTLb
(Ricker 1975). The variation in bvalue
from 3 was tested by the 1-sample
t-test. Analysis of covariance (ANCOVA)
was used to test for differences in size–
weight relationship between the sexes,
considering fish length as a covariate.
The condition factor (CF) was calcu-
lated from the equation: log W= blog
TL + log a(Ricker 1975).
276
Fig. 1. The study area in the southern Adriatic Sea with sampling locations
classed according to depth: (j) 5−15 m; (m) 15−30 m; (d) > 30 m
Mati´c-Skoko et al.: Life history of Muraena helena
Age analysis
Age was determined by interpreting the growth
rings on the otoliths. Otoliths were fastened to a slide
and then sanded using 30 µm sandpaper and 0.3 µm
Buehler Alpha MicroPolish alumina and polished to
facilitate the observation of the opaque and translu-
cent zones. All otoliths were observed immersed in
water against a black background using a stereo mi-
croscope at 3.75 magnification. Age was determined
by counting growth rings on the posterior otolith re-
gion. All otoliths were read independently by 3 expe-
rienced researchers, with age assigned when at least
2 of the readers were in agreement. The index of av-
erage percent error (IAPE) (Beamish & Fournier 1981)
as well as the mean coefficient of variation (CV)
(Chang 1982) was calculated to estimate the relative
precision between readings. Low values of these in-
dices indicated a good precision of age estimation.
The sample available for age and growth studies
consisted of 179 individuals (92.8% readable oto -
liths). A ring mark was considered as the outer edge
of the opaque zone. Opaque and translucent zones
exhibited an alternating pattern. The total number of
translucent zones was recorded in order to assign an
estimated age to the Mediterranean moray eel speci-
mens. The periodicity of opaque zone formation was
examined by edge-type analysis (Palazón-Fernandez
et al. 2010). The marginal edge of each otolith was
examined and classified as opaque or translucent
and percentages of otoliths with opaque and translu-
cent margins were plotted by month of capture in
order to determine periodic trends in zone formation.
The marginal increment was measured along the
long axis of each otolith (precision 0.01 mm) and used
to validate the ageing.
Relationships between OL or OM and fish age (t)
were determined applying a simple linear regression
model. The main objective was to obtain a predictive
equation that would allow simple determination of
fish age from otolith morphometry.
Length-at-age was described by the von Berta-
lanffy growth function (VBGF), TL = L∞(1 − e−k(t−t0)),
where TL is total length at age t, L∞is asymptotic
length, kis the body growth coefficient and t0is the-
oretical age at zero length (Beverton & Holt 1957).
The von Bertalanffy growth parameters were esti-
mated using a non-linear least square procedure,
through a Gauss-Newton algorithm. The multivari-
ate Hotelling’s T2-test was used to compare growth
parameters obtained for males and females. Kolmo -
gorov-Smirnov 2-sample test was used to analyze
both age and size frequency distributions of both
sexes. The mean lengths of males and females in the
same age classes were tested using the t-test, after
testing data for normality (Kolmogorov-Smirnov test
of normality). Taylor’s (1960) equation was used
to calculate longevity from parameters obtained by
VBGF: A0.95 = [2.9957/k] + t0, where A0.95 is the life -
span or age required to reach 95 % of the final length
(L∞), and t0 and kare von Bertalanffy growth parame-
ters, in order to confirm that obtained ages from
otolith reading were realistic.
Reproductive analysis
Sex and maturity stages (immature, premature and
mature) of the Mediterranean moray eel were deter-
mined by direct visual observation of the gonads.
Individuals without developed gonads were classi-
fied as immature. Premature specimens were dis -
tinguished by wrinkled or smooth gonad surface, a
morphological structure distinctive for females and
males, respectively. Mature females were easily rec-
ognized by big, orange eggs and mature males by
big, white and tight testes. The gonadosomatic index
was calculated as GSI = (Wg/W)*100, where Wgis
the gonad weight. For the estimation of mean length
at 50% maturity, a percentage of the mature individ-
uals by size class was used. For histological analysis
of the gonadal tissue, a subsample of 39 individuals
(22 male, 12 female and 5 immature) collected sea-
sonally throughout a 1-yr period was used. Tissue
sections were fixed in Bouin solution and processed
for routine histological preparation. The tissue was
dehydrated in a graded ethanol series (from 70 to
100%), cleared in xylene and embedded in paraffin.
Histological sections were cut at 5 µm, stained with
haematoxylin-eosin and then mounted permanently
for further microscopic analysis. The terminology
used in the histological description of gonad develop-
mental stage followed criteria of Wallace & Selman
(1981) and Grier (1981).
Significant differences in CF and GSI were deter-
mined by 1-way analysis of variance (ANOVA) fol-
lowed by a Tukey test.
RESULTS
A total of 193 individuals of Muraena helena were
sampled, namely 48 immature, 106 female and 39
male specimens. The male:female ratio for all fish
combined was 0.35:1.00 and it differed statistically
significant from the expected 1:1 (χ² = 35.5; p < 0.05).
277
Aquat Biol 13: 275–284, 2011
TL of all fish ranged from 27.5 to 121.0 cm (Fig. 2),
with an average TL of 73.51 cm (±14.965 SD). W
ranged from 116.1 to 3680.0 g, with an average Wof
975.26 g (±666.811). The length frequency distribution
of the total sample exhibited one mode at 70 cm
(Fig. 2). Immature individuals ranged from 31.5 to
78 cm, with an average TL of 62.55 cm (±9.425). Adult
males ranged from 60.0 to 121.0 cm, with an average
TL of 80.00 cm (±14.138), while females ranged from
27.5 to 113.1 cm, with an average TL of 76.38 cm
(±14.841). The size frequency distributions of males
and fe males were not significantly different (2-sample
t-test, p = 0.203). Females were found at all sampling
locations (at depth of 5 to 80 m), unlike immatures and
males, which dominated in inshore (depth ~5 m) and
offshore waters (depth > 30 m), respectively (Fig. 3).
The calculated length–weight equation for the
whole sample was W= 0.0056 × TL2.776 (R2= 0.817).
This relationship for males was described by the
parameters a= 0.0062 and b= 2.764 (R2= 0.800) and
for females by the parameters a= 0.0116 and b=
2.612 (R2= 0.764). The variations in bvalues from
3 were statistically significant both for males (t-test,
p = 0.001) and females (t-test, p = 0.001). Analysis of
covariance revealed significant differences between
sexes (ANCOVA, p < 0.0005). The relative CF ranged
from 0.88 to 3.16 for the female moray eels. There
were no significant mean differences among seasons
(1-way ANOVA, p > 0.05); however, CF was the
lowest in winter (Fig. 4).
Age analysis
Otoliths of Muraena helena displayed well-defined
alternating opaque and translucent zones under
reflected light (Fig. 5). The IAPE of ring counts for
each reader did not differ greatly, and was slightly
lower for the first author (2.36) than for the second
(2.48) and third (2.51). The precision of the age esti-
mates (CV) was 1.1. A distance measurements analy-
sis revealed that the first regular, continuous ring
appeared at a distance of 1.08 mm (±0.047 SD) from
the otolith nucleus. The formation of growth in -
crements followed a seasonal pattern (Fig. 6); the
opaque zones began to develop during winter
months and they were completely formed in summer,
which was coincident with estimated spawning pe -
riod. The proportion of otoliths with opaque margins
was the highest (>60%) in August and September.
The translucent zones were laid down mainly in the
autumn–winter period, and our data suggest that 1
opaque and 1 translucent zone are laid down per
278
20 Males
Females
Tot a l
10
15
Frequency (%)
0
5
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100105110115 120
Length classes (cm)
Fig. 2. Muraena helena. Length (cm) frequencies of males,
females and the total sample of Mediterranean moray eels
from the Adriatic Sea
50
60
70 % Imm % mal % fem
20
30
40
Presence (%)
0
10
20
5 –15 m 15– 30 m 30– 80 m
Sampling locations
Fig. 3. Muraena helena. Vertical distribution of immature,
male and female Mediterranean moray eels from samp -
ling locations classed according to depth in the southern
Adriatic Sea
GSI CF
Sprin
g
Summer Autumn Winter
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
Fig. 4. Muraena helena. Seasonal variation of the relative
condition factor (CF) and gonadosomatic index (GSI) of
Mediterranean moray eels from the Adriatic Sea
Mati´c-Skoko et al.: Life history of Muraena helena
year. Moreover, the monthly mean marginal incre-
ment showed a single maximum in August (Fig. 6)
and confirmed the formation of 1 annulus per year.
The age analysis revealed 11 age classes and the
presence of age groups 2 to 12, with only 2 individu-
als in the 2+group. The oldest males and females of
Muraena helena were estimated to be 12 yr old. Pre-
dominance of age classes 6+and 7+in the total catch
(44.1% individuals) was observed. There was some
overlapping of individuals with the same lengths,
especially for the individuals with higher TL. The
pooled length-at-age data for M. helena is given in
Table 1. Females were the most numerous in the
middle 3 age classes, represented by 16.2, 17.2 and
24.2% for classes 5+, 6+and 7+, respectively. Males
were the most numerous in 6+and 7+age classes
(equally represented by 28.6%) and their number
declined in all subsequent classes. The younger age
classes (2+and 3+) were represented by only few
female specimens. Age frequency distributions of M.
helena are presented in Fig. 7. The mean lengths-at-
age of males and females showed that there were no
significant differences for any of the age classes (2-
sample t-test, p = 0.259). Overall, the age frequency
distributions of males and females did not differ
significantly (Kolmogorov-Smirnov 2-sample test,
p > 0.05).
Significant linear relationships were found be -
tween OL and fish age (t) (OL = 2.636 + 0.22t; R2=
0.491, p = 0.001) and between OM and t(OM =
−0.002 + 0.0014t; R2= 0.744, p = 0.001).
The VBGF for the sampled population had the fol-
lowing parameters: L∞= 162.7 cm (SE = 0.489), k=
0.089 yr−1 (SE = 0.003) and t0= −0.660 yr (SE = 0.219)
(R2= 0.928). The estimated non-linear least squares
parameters for the total sample, males and females
are given in Table 2. A significant difference was
found between the von Bertalanffy growth parame-
ters of males and females, using a Hotelling’s T2-test
(T 2= 231.4 > T02= 11.97), with females growing
at slightly slower rate than males. The calculated
asymptotic length value was higher than the maxi-
mum observed length (121 cm). According to the von
Bertalanffy growth equation, obtained longevity
(A0.95) is estimated at 35 yr. Furthermore, during the
first 4 yr of its life, M. helena grows rapidly; after-
wards, its growth rate slows down considerably.
Reproductive analysis
All specimens above 78 cm TL were
sexually mature. The smallest female
with ovary containing ripe eggs had a TL
of 68.6 cm. According to the estimated
data, length at 50% maturity was found to
be L50 = 76 cm TL for females and L50 =
79.0 cm TL for males. The percentage of
mature males and females per length
classes is shown in Fig. 8. GSI varied from
0.02 to 12.50 and varied signi ficantly
among seasons (1-way ANOVA, p < 0.05),
namely between summer and autumn
(Tukey Test, p < 0.05) (Fig. 4). GSI varied
279
Fig. 5. Muraena helena. Otoliths of from the Adriatic Sea:
(A) a 4 yr old individual (length: 59.0 cm; weight: 420.4 g);
(B) a 7 yr old individual (length 78.0 cm; weight: 803.6 g).
White dots indicate yearly growth rings. Magnification: 3.75
0.16
0.18
0.2
80
90
100
% Opaque % Translucent MRI
0.08
0.1
0.12
0.14
30
40
50
60
70
% Edge type
0
0.02
0.04
0.06
0
10
20
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mar
g
inal increment (mm)
Months
Fig. 6. Muraena helena. The monthly frequency of opaque and translucent
margins and (j) the monthly means of marginal increment (MRI) of
otoliths of Mediterranean moray eels from the Adriatic Sea
Aquat Biol 13: 275–284, 2011
from 0.02 to 12.50 for females, and from 0.02 to 0.22
for males.
Histological examination of the gonads revealed
that the Mediterranean moray eels in the Adriatic
Sea mature in warmer months and spawn during
the late summer. The autumn–winter period is their
rest phase, when only individuals with cells in
growth stage were observed. In July, males were
found to be at their maturation stage (Fig. 9A). An
increase of the number of mature spermatozoa was
noted and low numbers of other germ cells. Imma-
ture males, with early spermatogonia, were found in
the winter/ spring period, as well as females with
immature ovaries containing only primary growth
stage oocytes. The ovaries of vitellogenic females
showed numerous vitellogenic oocytes together with
a small number of oocytes at the primary growth
stage of development (Fig. 9B). These females were
observed from February to May, while females with
fully mature oocytes and post-ovulatory follicles
were not re corded among histologically sampled
individuals. However, females with ripe eggs were
noticed during spring months on the field. Inter -
estingly, in females, GSI values less than 1 corre-
sponded to ovaries containing pre-vitellogenic pri-
mary oocytes.
280
Length Age (yr) Total
interval (cm) 2 3 4 5 6 7 8 9 10 11 12
27 1 1
30 1 1
33 0
36 0
39 0
42 2 2
45 1 1
48 1 1
51 1 3 4
54 4 4
57 9 9
60 6 6 12
63 1 10 4 15
66 6 8 14
69 5 13 4 22
72 6 9 5 1 21
75 4 5 9
78 1 4 15 20
81 1 7 4 12
84 1 6 4 11
87 2 7 9
90 12 3
93 53 8
96 12 3
99 31 4
102 11
105 12 3
108 0
111 0
114 11
117 11
120 11
Total 2 3 25 35 43 44 22 9 6 3 1 193
% 1.12 1.68 13.41 16.76 21.79 22.35 12.29 5.03 3.35 1.68 0.56 100
Mean TL (cm) 29.5 44.3 55.7 65.8 70.2 78.1 86.4 94.9 102.0 110.3 118.0
SD TL 2.828 4.932 4.116 5.381 5.067 4.927 5.186 3.832 6.504 7.587 0.00
Mean W (g) 277.8 156.2 338.1 644.0 768.0 1018.1 1432.686 1933.5 2274.6 3460.0 3680.0
SD W 144.250 66.192 101.761 220.045 174.683 264.464 382.516 580.305 591.620 117.757 0.00
Table 1. Muraena helena. Length-at-age data for Mediterranean moray eels from the Adriatic Sea aged using otolith readings.
Figures show the number of individuals classed by age within each length interval. TL = total length, W = weight
25.5–28.4
28.5– 31.4
31.5– 34.4
34.5– 37.4
37.5– 40.4
40.5– 43.4
43.5– 46.4
46.5– 49.4
49.5– 52.4
52.5– 55.4
55.5– 58.4
58.5– 61.4
61.5– 64.4
64.5– 67.4
67.5– 70.4
70.5– 73.4
73.5– 76.4
76.5– 79.4
79.5– 82.4
82.5– 85.4
85.5– 88.4
88.5– 91.4
91.5– 94.4
94.5– 97.4
97.5–100.4
100.5–103.4
103.5–106.4
106.5–109.4
109.5–112.4
112.5–115.4
115.5–118.4
118.5–121.4
Mati´c-Skoko et al.: Life history of Muraena helena
DISCUSSION
As previously mentioned, there is a lack of the age
data for most species of the Muraenidae family, and
in this study we decided to use otoliths, since they are
the most frequently used hard part for fish ageing
(Sparre & Venema 1992). Muraena helena otoliths
exhibited a well-defined and consistent mark pattern
consisting of one opaque and one translucent zone.
The present results showed that growth zones in
otoliths of the Mediterranean moray eel are laid
down annually, the opaque zone being generally
deposited in summer, which corresponds to the ob -
served spawning period, and the translucent zone in
the autumn/winter period.
In general, otoliths of Muraena helena were deter-
mined to be valid structures for age and growth stud-
ies due to the great consistency among repeated age
readings. However, determination of the first annu-
lus was problematical, especially in older individuals.
281
25
30 % Males
% Females
% Total
15
20
Frequency (%)
0
5
10
023456789101112
Age
Fig. 7. Muraena helena. Age distributions of males, females
and the total sample of Mediterranean moray eels from the
Adriatic Sea
L∞(cm) k(yr−1)t0(yr)
Males 163.6 ± 0.628 0.090 ± 0.008 –0.630 ± 0.659
Females 159.1 ± 0.553 0.089 ± 0.003 –0.887 ± 0.279
Totals 162.7 ± 0.489 0.089 ± 0.003 –0.660 ± 0.219
Table 2. Muraena Helena. Growth parameters (±SE) for
males and females and the total sample of Mediterranean
moray eels from the Adriatic Sea, estimated by non-linear
regression from otolith readings
80
90
100
40
50
60
70
% mature
0
10
20
30
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100105110115 120
% M % F
30 35 40 45 50 55 60 65 70 75 80 85 90 95
Length classes (cm)
Fig. 8. Muraena helena. Length at 50% maturity of (Δ)
male and (j) female Mediterranean moray eels from the
Adriatic Sea
Fig. 9. Muraena helena. Photomicrographs of histological
sections of gonads. (A) Testis showing seminiferous lumen
filled with spermatozoa (sz) with spermatocytes (sc) and
spermatids (sd) lining the lobular germinal epithelium. (B)
Ovary with oocytes in different developmental stages: peri -
nucleolar (pn) and in advanced vitellogenic (v) stage with
migratory germinal vesicle (gv)
Aquat Biol 13: 275–284, 2011
We tried to solve the problem by measuring the dis-
tance of each annual growth ring from the otolith
nucleus. Results showed that the first growth ring
(that corresponds to the fish’s first year of life) can be
determined as the first regular, continuous ring that
is found at a similar distance from the otolith nucleus
in the majority of otoliths. In <10% of the specimens,
1 or 2 rings were visible at different distances to the
otolith nucleus, closer than the determined first growth
ring. However, these rings were usually irregular
and incomplete. Their formation could be attributed
to a very long pelagic larval interval (around 1 yr),
different migration routes chosen by each specimen
and oceanographic conditions during settlement
from pelagic to benthic phase.
According to our results, ages of the Muraena
helena from the Adriatic Sea ranged from 2 to 12 yr.
The only other investigation of M. helena age comes
from the Canary Islands, where Jiménez et al. (2007)
reported ages for M. helena and the closely related
species M. augusti from 3 to 15 yr and 4 to 12 yr,
respectively. Our results are also consistent with
those obtained for conger eel Conger conger (family
Congridae) from the Atlantic Iberian waters (Correia
et al. 2009) and Irish coastal waters (Sullivan et al.
2003). C. conger is an anguilliform fish and thus
related to M. helena; moreover, current investigation
of the biology of both species points to similar repro-
ductive strategy, age and growth patterns as well
as similar trophic ecology in the Adriatic Sea. Based
on these findings, we can assume that the average
maximum age of both species is up to 12−15 yr.
The length of Muraena helena caught by bottom
longlines in the Adriatic Sea ranged from 27.5 to
121.0 cm, while the length range of the Canary
Islands specimens, collected using bottom longlines,
bottom traps and hand lines, was from 41.9 to
134.0 cm (Jiménez et al. 2007). We consider it is likely
that the use of different gears in our study biased the
size of sampled individuals. Shorter individuals could
not be captured due to the hook size and the small
length range is probably a consequence of the oper-
ational depth of the gear used. Studies investigating
gear selectivity revealed that conger eel individuals
caught by bottom longlines are longer than those
caught by other gears (Hood et al. 1988, Sullivan
et al. 2003). Unlike conger eels, which can be seen
swimming over the bottom, moray eels rarely leave
their holes, maintaining high site fidelity (Böhlke et
al. 1989) and are thus less susceptible to other fishing
gears, like trammel nets. So, for practical purposes,
bottom longlines are the only fishing gear which
allows collection of large numbers of this species.
Both females and males of Muraena helena were
captured in Adriatic coastal waters at depths of
around 30 to 80 m although females dominated in the
total sex ratio (0.35:1.00). The majority of sampled
males were taken from the deeper sites located at a
greater distance from the coastline. Jiménez et al.
(2007) also reported predominance of females for
moray eels off the Canary Islands. Sex ratio fluctua-
tion, with male predominance in deeper waters, is
also known for Conger conger in the Sardinian chan-
nel (Cau & Manconi 1983). In contrast, in the case of
the American conger eel C. oceanicus both sexes
were captured by longline in inshore waters (Hood et
al. 1988), as in our study. However, since M. helena
males occur in deeper water, our sex ratio could be
partially the result of fishermen lowering longlines
to preferred depths and due to bathymetric sexual
segregation.
The Mediterranean moray eels of the Adriatic Sea
showed negative allometric growth and bvalues
were significantly different from 3 for both sexes.
However, the value of bdetermined by Jiménez et al.
(2007) for Muraena helena from the Canary Islands
was 3.314, indicating positive allometric growth.
This difference could be due to different sample
sizes (they collected 750 individuals from commercial
catches using different fishing gears at different fish-
ing ports in the Canary Islands), but it could also
reflect the sex ratio and percentage of mature indi-
viduals in the total analyzed sample (Ricker 1975).
The relative CF for M. helena in the present study
was lowest in the winter and highest in the spring
months, most likely as a consequence of sea temper-
ature changes that affect metabolic rates. Further-
more, lower winter CF values could be attributed
to lower food availability and the mobilization of
somatic energy reserves for reproductive develop-
ment, as suggested for Conger conger by Sullivan
et al. (2003).
Growth parameter estimates for Muraena helena
from the Adriatic Sea gave results of L∞= 162.7 cm,
k= 0.089 and t0= −0.660. The obtained asymptotic
length is higher than maximal length observed in
sampled individuals. The mean lengths correspond-
ing to first years of life are probably overestimated
due to the lack of smaller specimens as a result of
bottom longlines selectivity. Parameters of the VBGF
for moray eels from Canary Islands were very similar
to ours with L∞= 170.0 cm, k= 0.078, t0= −0.355 and
L∞= 105.9 cm, k= 0.178, t0= 1.103 for M. helena
and M. augusti, respectively (Jiménez et al. 2007). It
seems that both species, as well as Conger conger
(Sbaihi et al. 2001, Sullivan et al. 2003, Correia et al.
282
Mati´c-Skoko et al.: Life history of Muraena helena
2009) have relatively slow growth and a prolonged
life span. Population density, water temperature or
the quantity of available food could affect growth of
M. helena because of its restricted range of move-
ment. The oldest sampled M. helena specimens were
11 and 12 yr old, female and male respectively. We
consider that the species’ life span could be around
15 yr, which is supported by the estimated longevity
value of 35 yr. Jiménez et al. (2007) estimated a lifes-
pan of 38.1 yr for their moray eel specimens aged
from 3 to 15 yr. Longevity is a theoretical value based
on parameters obtained by VBGF and is very sensi-
tive to the sample size, so it can be far from the real
life span; i.e. if organisms are large, the asymptotic
length increases while the growth coefficient de -
creases, and if organisms present a large kvalue, its
asymptotic length is reached in a shorter period of
time (Barr et al. 2008). Our VBGF estimates appear
realistic values since moray eels are known to grow
to large sizes up to 2 m (Jardas 1996) and have a
relatively slow growth rate. For conger eels, Sbaihi et
al. (2001) reported body growth increment of 10 cm
yr−1, with the growth rate slowing down from year
10 upwards. This is in accordance with our findings,
at least for the first 5 yr of life.
Comparison of the female GSI values with the his-
tological examination of the ovaries showed that
smaller GSI (<1) corresponds to the presence of pre-
vitellogenic primary oocytes. This observation was
also reported by Hood et al. (1988) and Sbaihi et al.
(2001) for the related species Conger oceanicus and
C. conger, respectively. Based on the present results,
we suppose that Muraena helena spawns in summer
with peak in July, probably with size at maturity at
around 79 cm and at 6 to 8 yr of age. A similar size
and age at first maturity of around 75 cm and 7.1 yr,
respectively, were reported for M. helena by Jiménez
et al. (2007) in the Canaries. However the spawning
period was prolonged from January to July, which is
probably related to the higher sea temperatures in
the Canary Islands compared to the Adriatic Sea.
The majority of specimens collected were either
immature or in a developing stage. Ovaries con-
tained either oocytes in a pre-vitellogenic or in early
vitellogenic stage and ripe females with mature
hydrated oocytes were never found. The lack of fully
mature, ready to spawn individuals in the inner
waters indicates that moray eels do not reach full
sexual maturity in the coastal waters of the south
Adriatic Sea. In females in the present study, increas-
ing GSI was attributed to deposition of adipose tissue
surrounding the oocytes. The same phenomenon was
recorded for Conger conger (Sullivan et al. 2003,
Correia et al. 2009). However, some morphophysio-
logical changes observed in wild conger eels related
to advanced stages of sexual maturation, like regres-
sion of the digestive tract, calcium teeth and bone
resorption or body coloration (Cau & Manconi 1983,
Hood et al. 1988), were not found in Muraena helena
in our study.
Some uncertainties related to the range of ages in
which 50% of individuals firstly become matured
appeared in the present study. Similar uncertainties
are also reported for the European conger eel, which
is believed to reach sexual maturity at the age of 5 to
15 yr (Correia et al. 2009 and papers therein). The
wide ranges reported for both species may be due to
the lack of specimens of moray and conger eels in
a fully developed stage of maturity. The estimation
of accurate maturity schedules requires accurate
descriptions of stages of maturity, as well as basing
estimates only on samples collected during the iden-
tified spawning season. Using females with fully
developed oocytes in their ovaries to assess maturity
and sampling during the spawning season, to reduce
the possibility of confusing post-spawning females
with immature or inactive females, has proven to
give greater precision in size at maturity estimates
(Arocha & Bárrios 2009).
On the basis of data presented herein, we suggest
that Muraena helena individuals reach sexual matu-
rity at ages ranging from 3 to 10 yr (when 100% of
the population has reached maturity) and then they
migrate to spawn in the south Adriatic Pit, at a depth
below 600 m, most likely in the summer. A similar
scenario with migration towards the deep sea spawn-
ing areas was previously proposed for Conger conger
(Cau & Manconi 1983, Sullivan et al. 2003, Correia et
al. 2009). The assumption of the south Adriatic Pit as
the most likely spawning place of M. helena and C.
conger in the Adriatic Sea is further supported by the
length and otolith analyses of their leptocephali col-
lected in that particular area (S. Mati´c-Skoko pers.
obs.). Moreover, it should be taken into consideration
that the nearest reported spawning ground for C.
conger is in the Sardinian channel at a depth of
around 600 to 800 m (Cau & Manconi 1983), which
seems too far away to be the spawning area for the
moray and conger eel species of the Adriatic Sea.
In conclusion, more information about the repro-
ductive biology (exact time and place of spawning,
larval period duration, time of the settlement from
pelagic to benthic habitats), population structure and
distribution of Muraena helena in the Mediterranean
and the Adriatic Sea is needed in order to improve
scientific knowledge of this species and to inform
283
Aquat Biol 13: 275–284, 2011
fisheries management in Mediterranean coastal zones.
Future studies should also attempt to estimate to
what extent habitat conditions modify the growth
pattern of resident moray eels and to incorporate ju -
venile growth into studies of adult population growth.
Moreover, determination of ontogenetic feeding strat -
egy of this predator fish together with its trophic sta-
tus is needed to clarify its specific growth pattern and
its position in complex littoral benthic communities.
Acknowledgements. We thank all those involved in the col-
lection of material, particularly local fishers Mato Lujo, Mato
Oberan and Hajro Turkovićand student Ivan Ban. Special
thanks to Josipa Ferri for help in otolith readings and prepa-
ration of the manuscript. The present work was supported
by the Ministry of Science, Education and Sport, Croatia
through projects 275-0010501-0856 and 001-0013077-0844.
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Editorial responsibility: Asbjørn Vøllestad,
Oslo, Norway
Submitted: January 19, 2011; Accepted: August 26, 2011
Proofs received from author(s): October 14, 2011
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