| Abstract |
Τhe identification of fish stock composition is important in fisheries science, as it is an
integral part of fisheries management and aims at sustainable production. Many methods have been
used in fish stock discrimination, including morphological and genetic indices and life history traits.
The aim of the present PhD thesis was the identification of natural stocks of Gilthead sea bream in
Greece through the use of morphological and genetic characters, as well as the study of the
phenotypic variability limits of otoliths and scales of the species, in response to the fish growing
environment. Specifically, a comparative study was performed on the otolith morphology (shape,
size and asymmetry), body shape and genetic diversity (microsatellite DNA) of Gilthead sea bream
(Sparus aurata) of different geographical origins. Samples were collected from the North, Westcentral
(Maliakos) and East-central (Kalymnos) Aegean, from different regions of the Ionian Sea
(Central Ionian, Corfu, Patraikos), as well as from the Missolonghi lagoon during 2014-2018. Fish
from two different aquaculture farms, one from the Ionian and one from the Aegean Sea, were used
(control samples). The rate of regenerated scales is greater in reared compared to wild fish, as the
former are succumbed into mechanical stress, which is provoked by aquaculture manipulations. The
degree of scale regeneration (SRD) was used to assign wild-caught fish as actually wild or,
potentially, aquaculture escapees, in order to estimate the actual morphological and genetic
diversity of the species natural populations. The wild-caught specimens were discriminated into
three categories based on their SRD levels, the increase of which increases the likelihood of the
presence οf aquaculture escapees: in the group of wild fish with low probability to include escapees
(L30: ≤30% SRD), in the group of fish with probability to include escapees (Μ30-60: 31-60%
SRD) and in the group of fish with high probability to include escapees (Μ60: >60% SRD). The
different morphological and genetic characters were examined in accordance to the natural
phenotypic variation in relation to the contribution of possible escapees in each geographically
distinct sample. Furthermore, the differences between wild-caught and reared fish were explored.
In the study of the phenotypic variability limits, the effect of temperature (17, 20 or 23°C)
during the short embryonic and yolk-sac larval period was tested on the otolith plasticity of
Gilthead sea bream experimental populations, during the period between the stage of
metamorphosis (56-58 days' post hatching, dph) and the juvenile stage (93-95 dph). Also, the effect
of different growth environment was examined, on the otolith morphology of reared sea bream
populations, in the period between the juvenile stage (122 dph) and adult stage (314-393 dph). To
evaluate the SRD as a reliable index for distinguishing aquaculture escapees, the evolution of SRD
levels in the reared populations was monitored, from the stage of metamorphosis to the end of the
rearing process.
An elliptic Fourier analysis and a geometric morphometric technique were used for the study
of otolith shape and body shape variation, respectively. For the examination of the asymmetry on
otolith shape and size characteristics, the variance of difference between the two sides of the body
and the mean absolute difference between the body sides were used. For the otolith size differences,
the standardized values of maximum length, maximum weight, surface and perimeter were
calculated. The shape differences between the different geographical groups were examined using
CAP (Canonical Analysis of Principal Coordinates). A Canonical Variate Analysis (CVA) was
performed to check the shape differences (otolith and body shape) between reared (control) and
wild-caught fish. In the remainder comparisons, the statistical significance of the differences
between groups were tested using parametric and non-parametric tests. The differences in genetic
diversity between groups, were examined by the indices of genetic (FST) differentiation.
The results of the present study, demonstrated three phenotypically sea bream stocks: the
Aegean, Ionian and Missolonghi lagoon stocks. However, the examination of genetic diversity
showed a genetically uniform stock in the area of Greece. In the categories of wild-caught fish, with
increasing possibility to include aquaculture escapees, the examination of shape (otolith and body)
variation did not show clear geographical differentiation between groups. In the geographical areas
where the presence of fish with high levels of SRD was increased, the fish with high possibility to
include escapees (M60) presented significant higher levels of otolith size and asymmetry compare
to their wild counterparts (L30). However, within the different geographical regions there was no
significant genetic differentiation between individuals with different SRD levels. Wild-caught and
reared fish differed significantly in their morphological and genetic characteristics.
The examination of the phenotypic variability limits on Gilthead sea bream showed that the
otolith shape and otolith asymmetry from the stage of metamorphosis to juvenile stage, changed
significantly depending on the early developmental temperature of the fish. Furthermore, the shape,
size and asymmetry of the otoliths differed significantly depending on the growth environment of
juvenile and adult fish. The evolution of the SRD levels demonstrated that scales are being lost
continuously, from the onset of squamation (11.2 ± 5.7% mean SRD, 58 days' post hatching, dph,
2.0 cm standard length, SL) until the end of the on-growing period (98.7 ± 3% to 99.8 ± 0.9% mean
SRD, 314-393 dph, 21.7-21.9 cm SL). At the transition between the hatchery period and the ongrowing
in sea cages, fish presented a wide distribution of SRD, ranging from 40 to 92% (122 dph,
5.1 cm SL). The importance of the results was discussed regarding the sea bream stock discrimination, the
correlation of SRD with the age of escaping reared fish, the distinction of potential aquaculture
escapees from wild fish, as well as the wild and reared differences. Additionally, was discussed the
otolith plasticity in response to different environmental factors during fish development. The main
results revealed the existence of three "phenotypic" stocks with similar genetic characteristics.
Otolith morphology is a highly plastic character and is a reliable tool in the study of the
environmental effects on fish physiology. Reared fish differ from wild-caught fish, both
phenotypically and morphologically. The SRD increased continuously with fish age and is a useful
index of distinguishing escapees based on morphology. However, it was not related to any
difference in the genetic composition between wild and possible escapees.
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