Abstract European hake (Merluccius merluccius) were reared from eggs up to 245 days of age in temperature and salinity controlled stable conditions. The lapillus and sagitta of one of these fish were examined for microstructural features. The age derived from the increments support the daily nature of the hake sagittal increments starting the formation at day 8, probably related to the start of exogenous feeding. The lapillus shows a later increment formation. The sagitta otolith increments show rhythmical groupings and banding that are similar to wild fish. The accessory growth centres appear to be related to pectoral fin development and demersal habitat. The growth rates obtained are discussed

Beatriz Morales-Nin

Erlend Moksness

R M Bjelland

CiteSeerX

Fisheries Research 74 (2005) 300–305
Otolith microstructure of a hatchery reared European
hake (Merluccius merluccius)
Beatriz Morales-Nina,∗, R.M. Bjellandb, Erlend Moksnessc
a CSIC/UIB-IMEDEA, Miquel Marqu´es 21, 07190 Esporles, Mallorca, Spain
b Institute of Marine Research, Austevoll Research Station, N-5392 Storebø, Norway
c Institute of Marine Research Arendal, Flødevigen Marine Research Station, N-4817 HIS, Norway
Received 7 October 2004
Abstract
European hake (Merluccius merluccius) were reared from eggs up to 245 days of age in temperature and salinity controlled
stable conditions. The lapillus and sagitta of one of these fish were examined for microstructural features. The age derived from
the increments support the daily nature of the hake sagittal increments starting the formation at day 8, probably related to the start
of exogenous feeding. The lapillus shows a later increment formation. The sagitta otolith increments show rhythmical groupings
and banding that are similar to wild fish. The accessory growth centres appear to be related to pectoral fin development and
demersal habitat. The growth rates obtained are discussed.
© 2005 Elsevier B.V. All rights reserved.
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eywords: Merluccius merluccius; Rearing; Otoliths; Increment validation; Sagitta; Lapillus
. Introduction
European hakeMerluccius merlucciusL. is a
ajor demersal finfish in the marine ecosystem of
he North Atlantic and Mediterranean Seas. This
pecies is heavily exploited and in recent years
ssessment has raised concern about the state of the
tocks (ICES, 2003). Knowledge of hake’s biological
raits (especially growth and maturity) is considered
nsufficient to improve the assessment of stocks.
∗ Corresponding author. Tel.: +34 971 611 721;
ax: +34 971 611 761.
E-mail address:beatriz.morales@uib.es (B. Morales-Nin).
The age interpretation of European hake is unus
complex due to the presence of a multiplicity of grow
rings, that at least in the Mediterranean are not for
with seasonal periodicity (Morales-Nin et al., 1998).
Despite the effort carried out to develop stand
ageing criteria for North Atlantic hake (Piñeiro and
Sáınza, 2003) a recent mark and recapture experim
has raised doubts about the actually accepted gr
rates, indicating that possibly growth is much fa
than expected (Pontual et al., 2003).
In the Mediterranean, a series of studies on E
pean hake juvenile age and growth have used o
daily growth increments (DGI) and length freque
analysis (Morales-Nin and Aldebert, 1997). The daily
165-7836/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.fishres.2005.03.001
B. Morales-Nin et al. / Fisheries Research 74 (2005) 300–305 301
nature of the increments has been validated indirectly
for juvenile hake in the Adriatic (Arneri and Morales-
Nin, 2000) and increments have been used to de-
termine growth rates and hatch date distribution for
juveniles (Morales-Nin and Moranta, 2004) and larvae
(Palomera et al., 2005).
A pioneer project carried out at Austevoll Aqua-
culture Research station (Norway) successfully reared
for the first time European hake from eggs to 1 year
of age. The eggs incubated at 12◦C hatched approxi-
mately after 4 days of fertilization, within 6 days af-
ter hatching the larvae started to feed and at around 30
days the larvae metamorphosed. The rearing conditions
and larvae development are described inSkiftesvik and
Bjelland (2004).
The aim of the present work is to take advan-
tage of this successful rearing to carry out the first
direct validation of European hake daily growth in-
crement periodicity. Although the rearing conditions
may affect fish development and only a single fish
was available for study, the information provide in-
sights in the otolith formation and age attribution on
a relevant species with otoliths particularly difficult to
interpret.
2. Material and methods
Adult hake caught on the spawning grounds west
of FlorØ (Norway) were used to obtain gametes
( y
r rvae
t hey
w hey
w um.
T s on
d
ped
f ated.
S was
m on to
h -
p
l was
p
days
c sex
w The
sagitta and lapillus otoliths were extracted, cleaned and
stored dry until preparation.
The otoliths were mounted in slides with a thermo-
plastic resin, polished on the external side until the core
was reached and then mounted with the external side
downward. The internal side was then ground to ob-
tain thin sections along the frontal plane. All mounts
were ground and polished with frequent microscopic
control, until the core plane was reached. The otoliths
were viewed using a microscope equipped with a high-
resolution video camera and a PC with an image anal-
ysis system. Otoliths were viewed on the screen using
an immersion oil objective at×1000 magnification and
GI were selected by fine focussing for their definition
and continuity. GI were enumerated from the otolith
core to the edge following for sagitta the dorsal prism,
which provides the most complete increment sequence
(Morales-Nin and Aldebert, 1997) and the maximum
radius for lapillus. GI were counted twice from the nu-
cleus to the dorsal margin and vice versa. The mean
of the readings was considered the specimen’s age in
days.
3. Results
3.1. Sagitta
The otolith showed the typical prismatic growth
common to hake otoliths (Fig. 1a) with accessory
g pri-
m r of
4 e-
t
t d of
a ere
e ent
c
i ly in
t
( ari-
a
T nce
p ure-
m ent
a n of
n
Skiftesvik and Bjelland, 2004). The experimentall
eared fish hatched on August 31, 1999. The la
ransferred from the incubator to a tank at day 45. T
ere feed Artemia and rotifers until day 55, when t
ere weaned to formulated feed and feed ad libit
hey were transferred to progressively larger tank
ays 210 and 226.
The water supplied to the rearing units was pum
rom a depth of 110 m, sand filter, heated and aer
alinity was a constant 34.5. Water temperature
easured daily, mean temperatures from incubati
atching were 12.3± 0.5◦C and afterwards the tem
erature was maintained at approximately 12◦C. The
ight was maintained constant and no seasonality
resent in the environmental parameters.
One European hake was sacrificed after 245
orresponding to a total length of 135 mm. The
as not identified due to undifferentiated gonads.rowth centres laid down around an irregular
ordium which had an anterior–posterior diamete
61.28m. The primordium continues growing b
ween the prisms (Fig. 1b box) with overlapping in
he increments (19 GI). The increments compose
bright unit and a dark wide discontinuous unit, w
numerated in the primordium till the first increm
oincident with the accessory growth centre (Fig. 1b
nbox), after the increments were enumerated on
he prism.
Hake otoliths have a central core of 1.94m
Fig. 2a) surrounded by narrow GI which have a v
ble thickness (mean = 2.24m, S.D. = 0.88;Fig. 2b).
his central area is composed by 64 GI. A lumina
rofile along the same radius where the GI meas
ents were taken (Fig. 2a–c) shows that the transluc
nd opaque bands are not related to the formatio
arrower GI.
302 B. Morales-Nin et al. / Fisheries Research 74 (2005) 300–305
Fig. 1. (a) Dorso-ventral area of the frontal sagitta section showing
the primordium and the prismatic growth (×50). (b) Magnification
of the transition zone (square in a) between the primordium and the
prisms (×1000). Arrows, accessory growth centres; P, prisms.
The primordium last increments are thick and dense
with a clear discontinuity, around which accessory cen-
tres are seen (Figs. 1a, b and 3a). These accessory cen-
tres gave origin to prisms, characteristic of hake otoliths
(Morales-Nin and Aldebert, 1997), in which the in-
crements are laid down in regular patterns with some
rhythmical grouping (Fig. 3b).
The GI in the prisms initially are narrower (mean
4.68m, S.D. 1.6,n= 21), getting more wide in the in-
termediate zone (mean 5.33m, S.D. 1.82,n= 23) and
had decreasing width towards the otolith edge (mean
2.45m, S.D. 0.79,n= 44).
Fig. 2. (a) Core and central area of the sagitta otolith. The line in-
dicates the measurement radius. (b) Evolution of the daily growth
increment (DGI) width along the radius. (c) Luminance profile along
the radius. Note that the DGI were measured only in the inner area
before the sharp transition in luminance (arrows a and c).
The total GI number on the prismatic zone was 173
added to the 64 of the primordium gave an age of 237
days.
3.2. Lapillus
The lapillus had a maximum radius of 193.04m
with regular GI around a circular core (Fig. 4). There is
B. Morales-Nin et al. / Fisheries Research 74 (2005) 300–305 303
Fig. 3. Prismatic sagitta growth. (a) Accessory growth centre (black
arrow) and edge of the primordium (white arrow). (b) Growth pat-
terns of wide DGI. (c) Marginal otolith area with narrow DGI. Scale
bar 50m.
not a complex structure similar to the sagitta. The width
of the increments decreased from 1.25 (S.D. = 0.06) to
0.92m (S.D. = 0.01) towards the edge. Some six GI
groupings were present in part of the lapillus. The total
number of increments was 208.5.
4. Discussion
The GI had a poor contrast in the lapillus and in
some sagitta otolith areas, probably introducing some
Fig. 4. Growth increments in the lapillus. Scale bar 50m.
error in the increment enumerations. The most diffi-
cult area was the central part of the sagitta, where the
discontinuity between the primordium and the prisms
complicate both the otolith preparation and interpreta-
tion. Moreover, the increments depending of the focus
plane (Fig. 5) show a different appearance with a very
wide discontinuous zone in which in some cases nar-
row increments can be discerned introducing error. The
lapillus is more easy to prepare and observe, but there is
discrepancy in the number of GI in both otoliths, being
superior in the sagitta. The correspondence between
the number of days in the rearing experiment and the
number of increments suggests that the lapillus start
increment formation later than the sagitta (29 days),
while the GI are first formed in the sagitta at the age
of 8 days. The start of feeding at 6 days of hatching,
suggest that the exogenous feeding might be related to
the start of the increment formation.
The sagitta hake otoliths show a great variability in
translucency that make difficult to obtain good uniform
images. These changes have been associated to the
presence of organic material (Morales-Nin, 1986) or
to structural changes (Arneri and Morales-Nin, 2000).
In the reared fish, similar banding was found. Another
common feature on hake otoliths is the presence of 7,
14 and 28 GI groupings, with subtle changes in translu-
cency and increment width, which have been related
with the vertical migrations and with tidal influences
(Morales-Nin, 1987). These groupings were also found
in the reared specimen; although the number of in-
crements more frequent was 7 and 9 (Fig. 6), there
304 B. Morales-Nin et al. / Fisheries Research 74 (2005) 300–305
Fig. 5. (a and b) Differences in focussing in a prismatic zone (×400).
Fig. 6. Plot of the frequency in increment groupings in the prismatic
zone of the sagitta.
were groupings with fewer increments that suggest that
the lack of an external signal had weakened the inner
growth rhythm. In the lapillus, some grouping of six
DGI was also observed.
The accessory centres or primordia in hake have
been related to the transition to the demersal mode
of life; they are absent in pelagic larvae of 1.1 cm
standard length (Palomera et al., 2005) but they are
present in demersal fish of 1.6 cm total length (Arneri
and Morales-Nin, 2000). Our results show that they
are related to the metamorphosis and to the formation
of the pelvic fins, corresponding to a bottom dwelling
behaviour in the juveniles (Bjelland, 2001). Probably
these changes on the otolith correspond to the necessity
to detect different sound signals (Lombarte and Popper,
2004).
A comparison of otolith features of the reared fish
with wild Mediterranean hake (Table 1) shows some
particularities that probably are related to the different
environmental conditions. The Mediterranean hake re-
cruits on the outer part of the shelf between 100 and
150 m depth, where the water temperature at the recruit-
ment depth is almost constant (13.5–14◦C) and similar
to the rearing one. However, although there was not sea-
sonality in the rearing conditions, the light regime was
quite different from the natural one.
The reared specimens (n= 4) reached a length of
12 cm at 200 days post hatch representing a monthly
increase in length of about 1.8 cm. A growth rate of
1.15 cm month−1 has been reported for juvenile hake
i t,
1
L s
w ngth
f 8 cm
w s ob-
t
a r-
r t
g ith
Table 1
Otolith features from the reared European hake and from wild fish (n= 15 ,
2000)
Otolith feature Core (m) First increments
mean width (m)
Number of DGI
the primordium
Reared hake 1.94 1.39 64
Wild hake 8 1.4 62.7n the NW Mediterranean (Morales-Nin and Aldeber
997) and between 0.7 and 1.2 cm month−1 for the
igurian Sea (Orsi Relini et al., 1989). These estimate
ere obtained by modal progression analysis of le
requencies and refer to specimens larger than 6–
hose absolute ages were unknown. Growth rate
ained from otolith readings in the Adriatic (Arneri
nd Morales-Nin, 2000) and Catalan Sea (W– Medite
anean) (Morales-Nin and Moranta, 2004) how grea
rowth plasticity at individual and seasonal levels, w
0) caught on the Western Mediterranean (Arneri and Morales-Nin
in Number of
accessory centres
DGI mean width
in the prisms
Fish growth
(cm month−1)
4 3.9 1.2
2–6 2.5–4 1.2–2.5
B. Morales-Nin et al. / Fisheries Research 74 (2005) 300–305 305
intense growth rates depending of the season but with a
mean growth superior to 1.6 cm month−1. These quite
fast growth rates are directly confirmed in the Atlantic
hake by a recent tagging experiment (Pontual et al.,
2003). Thus, the reared hake were growing at simi-
lar rates than wild fish or possibly at slower rates. Al-
though food was provided ad libitum, direct observa-
tions showed that the fish eat once a day until gorging.
A hake age determination has a great degree of un-
certainty and there is a lack of standard ageing criteria
for North Atlantic hake (Piñeiro and Sáınza, 2003). The
first annulus is identified based on its relative size be-
cause the presence of multiple macrostructure rings ob-
scures the identification. These rings are present in the
reared specimen and do not seem to be caused by nar-
row GI (at least in the first one, probably corresponding
to the first false ring).
The results provided, although only based in a single
fish, support the growth rates from wild fish. However,
a larger sample size is necessary before drawing firm
conclusions on which otolith provides the better esti-
mate of hake age and on the daily periodicity of the
GI.
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Otolith microstructure of a hatchery reared European hake (Merluccius merluccius

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Otolith microstructure of a hatchery reared European hake (Merluccius merluccius

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