Natural hybridization between immigrant narrow-barred Spanish
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Animal Biodiversity and Conservation 44.2 (2021) 219
Natural hybridization between
immigrant narrow–barred Spanish
mackerel Scomberomorus commerson
(Lacepède, 1800) and endemic
West African Spanish mackerel
Scomberomorus tritor (Cuvier, 1832)
in the Egyptian Mediterranean coast
S. A. Bakhoum
Bakhoum, S. A., 2021. Natural hybridization between immigrant narrow–barred Spanish mackerel Scomberomorus
commerson (Lacepède, 1800) and endemic West African Spanish mackerel Scomberomorus tritor (Cuvier, 1832)
in the Egyptian Mediterranean coast. Animal Biodiversity and Conservation, 44.2: 219–227, Doi: https://doi.
org/10.32800/abc.2021.44.0219
Abstract
Natural hybridization between immigrant narrow–barred Spanish mackerel Scomberomorus commerson (Lacepède,
1800) and endemic West African Spanish mackerel Scomberomorus tritor (Cuvier, 1832) in the Egyptian
Mediterranean coast. Immigrant narrow–barred Spanish mackerel, West African Spanish mackerel and specimens
with an external appearance somewhere between these putative parents were collected from Abu Qir Bay, East
Alexandria, Egypt. The hybrid index results and univariate and multivariate analysis indicated a natural hybridization
between these two species. The discriminant function analysis successfully classified individual fish in the data
to one of the three fish groups. Squared Mahalanobis distances extracted from the groups indicated the three
groups were clearly distinct from each other. Moreover, distances between the hybrid and Scomberomorus tritor
were longer than those of the hybrid and S. commerson. The mean values of the condition factor for the hybrid
were significantly higher than those of S. commerson. Natural mortality of the hybrid was significantly lower than
that of the exotic parent (S. commerson), indicating that the environmental conditions in the examined region are
more suitable for the hybrid type species than for the invasive parental species.
Key words: Immigrant, Scomberomorus commerson, Native Scomberomorus tritor, Natural hybridization, Egyptian
Mediterranean coast
Resumen
Hibridación natural entre la carita estriada del Pacífico Scomberomorus commerson (Lacepède, 1800) y la carita
oeste–africana Scomberomorus tritor (Cuvier, 1832) en la costa mediterránea egipcia. Se capturaron ejemplares
inmigrantes de carita estriada del Pacífico y de carita oeste–africana y otros ejemplares con un aspecto externo
intermedio entre estos progenitores putativos en la bahía de Abu Qir, al este de Alejandría, en Egipto. Los re-
sultados del análisis univariado y multivariado del índice de hibridación indicaron que existía hibridación natural
entre las especies parentales previstas. La función discriminante permite determinar si un determinado ejemplar
del que se tienen datos pertenece a uno de los tres grupos y calcular su tasa de éxito. Las distancias cuadradas
de Mahalanobis obtenidas entre los grupos analizados indicaron que los tres grupos estaban claramente dife-
renciados y que las distancias entre los híbridos y Scomberomorus tritor eran superiores a las de los híbridos y
S. commerson. Los valores medios del factor de condición de los ejemplares híbridos fueron significativamente
mayores que los de S. commerson; además, la mortalidad natural de los híbridos fue significativamente inferior
a la de los progenitores exóticos (S. commerson), lo que indica que las condiciones ambientales de la región
estudiada son más adecuadas para el tipo híbrido que para las especies parentales invasoras.
Palabras clave: Inmigrante, Scomberomorus commerson, Scomberomorus tritor autóctono, Hibridación natural,
Costa mediterránea de Egipto
Received: 19 III 21; Conditional acceptance: 31 V 21; Final acceptance: 22 VI 21
Shnoudy A. Bakhoum, National Institute of Oceanography and Fisheries, Alexandria, Egypt.
Corresponding author: S. A. Bakhoum. E–mail: shnoudybakhoum@yahoo.com
ORCID ID: 0000-0003-4691-9218
ISSN: 1578–665 X © [2021] Copyright belongs to the authors, who license the
eISSN: 2014–928 X journal Animal Biodiversity and Conservation to publish the
paper under a Creative Commons Attribution 4.0 License.220 Bakhoum
Introduction were taken to the nearest mm) and eight meristic
counts. To minimize any variation resulting from allo-
The Suez Canal was opened in 1869, connecting the metric growth, all morphometric measurements were
Red Sea to the Mediterranean and allowing numerous standardized according to Reist (1985):
Indo–Pacific species to invade the Mediterranean
(Golani, 1998). This process of immigration through X'i,j = log Xi – b . (log TLj – log TLi)
the Canal increased alien species of Indo–Pacific
origin from 12 species in 1882 to 92 in 2010 (Keller where X'i,j is the standardized measurement of the i
1882; Zenetos et al., 2010). morphometric character; log Xi is the mean logarithm
In 1985, Ben–Tuvia recorded that the narrow–barred of i morphometric character measurement; TLj is the
Spanish mackerel S. commerson (Lacepède, 1800) had total length of the individual j; log TL is the logarithm
reached the eastern part of the Mediterranean Sea. of the mean total length of pooled individuals and b
Golani et al. (2002) later reported that this Lessepsian is the slope of the log X against logTL plot.
migrant has expanded its distribution to the Aegean Sea. To elucidate the differentiation of the species and
Several migrant Lessepsian fishes are now well– expected hybrid we used forward stepwise discrimi-
established in the eastern Mediterranean. Halim and nant analysis (DA) on the characters, based on the
Rizkalla (2011) published a checklist of 42 immigrant generalized Mahalanobis distance to determine the
Erythrean fish in the Egyptian Mediterranean, 17 of similarity between groups and the ability of these
which are commercially exploited. variables to identify the specimens correctly (Hair et
Permanent change of habitat has a great effect al., 1998). We used univariate analysis of variance
on the biometric characters of aquatic organisms (one way ANOVA) for meristic and size–adjusted
(Abd El–Gawad et al., 1995; Bakhoum, 2017). In the data sets and multivariate discriminant function
Egyptian Mediterranean waters, the immigrant Brush– analysis to select the important variables (Henault
tooth lizardfish showed a sufficiently high degree of and Fortin, 1989).
biometric differences to recognize the Mediterranean The hybrid index was calculated according to
brush–tooth lizardfish (Saurida undosquamis) as a Witkowski and Blachuta (1980):
distinct group from that of the Red Sea Fish (Bak-
houm, 2000). Hybrid index = H – M1 / M2 – M1
Hybridization is defined as the crossing of geneti-
cally distinguishable groups or individuals. It includes where, M1 is the numerical value of the same cha-
crosses both between lineages of the same species racter of S. tritor, M2 the numerical value of a cha-
(intraspecific) and between individuals of different racter of S. commerson, and H the numerical value
species (Pinheiro et al., 2019). of hybrid characters.
The invasion of closely related fish species may Characters of the hybrid approach 50 are interme-
disturb a habitat and lead to an increasing incidence of diate, while characters close to 0 or 100 indicate a
interspecific hybridization. It may also facilitate hybrid character state close to that of a parent species. The
zones where the likelihood for interbreeding between commonly used length–weight relationship was applied:
native and exotic species is seriously increased (Cos-
tedoat et al., 2005; Almodóvar et al., 2012). W = a . L. b
The aim of the present study was to detect hybrids
between the native species West African Spanish where L is total length (cm), W is weight (g), and a
mackerel S. tritor (Cuvier, 1832) and the exotic species and b are constants.
S. commerson (Lacepède, 1800 ), and to evaluate the The coefficient of condition (Fulton condition factor,
degree of hybrid adaptation in Egyptian Mediterranean K) was calculated from the equation:
waters east of Alexandria.
K = 100 . W / L3
Material and methods where W is the gutted weight in grams, L the total length
in cm. This factor is often used as an approximation
Study area even when the allometric factor is theoretically more
appropriate (Bagenal and Braum, 1971; Ricker 1975).
Between November 2019 and January 2020, 102 spe- The natural mortality coefficient 'M' was calculated
cimens (27 S. tritor, 33 S. commerson and 42 possible by the method described by Ursin (1967). Data of
hybrid fishes) were collected with the help of local condition factor and natural mortality were statistically
fishermen using daytime purse seine fishing gear from analyzed using ANOVA.
Abu Qir Bay in the Egyptian Mediterranean coast, All statistical analyses were performed using the
east of Alexandria (latitude from 30º 5' to 30º 20' N SPSS PC ver. 16 software packages.
and longitude from 31º 15' to 31º 25' E).
Methods Results
The biometric characters examined included twenty– Comparing morphological features of the parental
one morphometric measurements (all measurements species with expected hybrid showed that the backAnimal Biodiversity and Conservation 44.2 (2021) 221
A
B
C
Fig. 1. Photographs of parent species and putative hybrids: A, Scomberomorus commerson; B, Scomberomorus
tritor; C, Hybrid form.
Fig. 1. Fotografías de las especies parentales y los híbridos putativos: A, Scomberomorus commerson;
B, Scomberomorus tritor; C, Hybrid form.
colour of S. tritor was bluish–green. Sides were distinct from both in other characters. In contrast, the
silvery, with about three rows of vertical elongated hybrid index revealed that hybrid specimens contained
spots and lateral line gradually curving down towards 4 individual characters with a hybrid index > 100 and
the caudal peduncle. Conversely, the back of S. < 0, 2 intermediate characters (hybrid index 45–55),
commerson was iridescent blue–green, and sides 3 close to S. commerson (hybrid index > 55) and 19
were silver, with numerous wavy vertical bands. The close to S. tritor (hybrid index < 45) (see table 1).
lateral line abruptly bent downward below the end
of a second dorsal fin. Biometric characters
The hybrid back was blue–green and its sides con-
tained vertical elongated spots and numerous wavy Table 1 summarizes a comparison of 21 morphometric
vertical bands inherited from both parental species. measurements and eight meristic counts between
Moreover, the lateral line gradually curved downwards parent species and hybrid groups.
below the end of the second dorsal fin (fig. 1).
Univariate analyses
The hybrid index One–way ANOVA indicated significant differences
of meristic counts and morphometric measurements
For some characters, the average values of hybrids (p < 0.001) between the hybrid groups and parental
were close to either S. commerson or S. tritor but species. Tukey's honest–significant difference (HSD) of222 Bakhoum
Table 1. Hybrid index of meristic and morphometric measurements (mean ± SD) of Scomberomorus
tritor, Scomberomorus commerson and hybrid specimens collected from Abu Qir Bay coast, east off
Alexandria, Egypt: Hi, hybrid index.
Tabla 1. Índice de hibridación de las mediciones merísticas y morfométricas (media ± DE de los ejemplares
de Scomberomorus commerson y Scomberomorus tritor y los híbridos capturados en la bahía de Abu
Qir, al este de Alejandría: Hi, índice de hibridación.
S. tritor S. commerson Hybrid Hi
Biometric characters
First dorsal fin 15.78 ± 1.093 (15–18) 16.17 ± 0.983 (15–18) 15.86 ± 0.727 (14–17) 20.51
spines and rays
Second dorsal fin 16.56 ± 0.882 (15–18) 16.67 ± 1.633 (15–19) 16.43 ± 0.676 (15–18) –118.18
spines and rays
Anal fin spine 17.78 ± 1.302 (16–20) 17.67 ± 1.633 (16–20) 17.50 ± 1.044 (16–20) 254.55
and rays
Pectoral fin spine 19.56 ± 1.509 (18–22) 22.17 ± 1.169 (21–24) 21.76 ± 1.338 (20–24) 84.29
and rays
Ventral fin spine I±5 I±5 I±5 –
and rays
Dorsal finlets 8.56 ± 0.726 (7–9) 9.33 ± 0.516 (9–10) 9.62 ± 0.590 (8–10) 137.66
Anal finlets 8.89 ± 0.333 (8–9) 9.50 ± 0.548 (9–10) 9.52 ± 0.512 (9–10) 103.28
Vertebrae (total) 45.78 ± 0.667 43.00 ± 1.265 44.19 ± 1.436 57.19
(45–47) (42–45) (41–46)
Morphometric measurement (mm)
Total length 275.38 ± 44.201 394.17 ± 43.407 312.95 ± 56.743 31.63
(240–375) (310–430) (262–455)
Forked length 248.63 ± 44.622 359.33 ± 50.162 275.10 ± 52.371 23.91
(215–350) (262–395) (233–410)
Standard length 241.63 ± 42.671 349.83 ± 48.967 275.00 ± 50.838 30.84
(212–340) (264–392) (231–398)
Predorsal length 62.75 ± 8.631 85.17 ± 10.610 67.76 ± 11.036 22.35
(55–80) (64–92) (51–97)
Prepectoral length 58.63 ± 7.347 (51–72) 79.00 ± 10.621 (62–90) 65.29 ± 9.660 (56–91) 32.70
Preanal length 132.38 ± 16.370 194.83 ± 32.591 147.90 ± 26.760 24.85
(113–138) (145–225) (123–210)
Prepelvic length 61.63 ± 6.948 (55–75) 88.67 ± 4.590 (83–95) 67.71 ± 10.340 (59–92) 22.49
Body depth 47.38 ± 7.671 (38–60) 64.67 ± 7.202 (60–73) 53.90 ± 7.409 (45–74) 37.71
Caudal peduncle length 17.88 ± 3.523 (15–25) 25.67 ± 3.724 (20–30) 21.57 ± 3.501 (18–30) 47.37
Caudal peduncle depth 10.50 ± 2.204 (9–14) 14.17 ± 1.602 (12–16) 11.19 ± 3.371 (8––13) 18.80
Dorsal rays height 24.25 ± 2.816 (21–28) 35.50 ± 7.007 (23–40) 27.81 ± 27.81 (24–40) 31.64
Anal base length 23.00 ± 5.732 (19–35) 36.17 ± 7.808 (22–44) 26.62 ± 5.220 (21–39) 27.49
Head length 56.38 ± 5.927 (52–69) 78.00 ± 12.000 (58–88) 63.48 ± 8.418 (55–86) 32.84
Head width 23.25 ± 3.655 (18–29) 32.33 ± 3.933 (26–36) 25.76 ± 4.122 (22–36) 27.64
Head depth 36.50 ± 4.811 (31–46) 49.00 ± 4.817 (42–54) 40.76 ± 4.959 (30–53) 34.08
Head depth passing
through eyes 18.13 ± 2.850 (15–23) 30.83 ± 3.764 (25–35) 27.43 ± 4.261 (20–36) 73.23
Eye diameter 11.88 ± 0.835 (10–13) 16.00 ± 2.608 (12–19) 12.62 ± 1.564 (17–11) 17.96
Interorbital width 17.38 ± 1.996 (15–21) 24.17 ± 2.483 (20–27) 19.33 ± 3.055 (27–16) 28.72
Snout length 21.75 ± 2.188 (20–26) 31.67 ± 3.559 (26–35) 24.52 ± 3.710 (34–21) 27.92
Upper jaw length 32.50 ± 3.295 (29–39) 43.83 ± 3.251 (39–47) 35.76 ± 4.493 (48–30) 28.77
Lower jaw length 31.88 ± 3.227 (29–39) 45.00 ± 3.225 (39–48) 37.86 ± 4.607 (34–49) 45.58Animal Biodiversity and Conservation 44.2 (2021) 223
Table 2. Significant differences between S. tritor (St), S. commerson (Sc) and hybrid (H) specimens
based on ANOVA followed by the Tukey HSD test (honestly significant difference). (Significance level
* p < 0.05, ** p < 0.01).
Tabla 2. Diferencias significativas entre los ejemplares de S. tritor (St), S. commerson (Sc) y los híbridos
(H) basadas en el ANOVA y el test HSD de Tukey (diferencia honestamente significativa). (Nivel de
significación * p < 0,05; ** p < 0,01)".
Tukey HSD test
F–value St vs. Sc St vs. H Sc vs. H
Biometric characters
First dorsal fin spines and rays 56.723** 0.000** 0.891 0.000**
Second dorsal fin spines and rays 4.158* 0.091 0.758 0.018*
Anal fin spine and rays 1.634 0.203 0.933 0.313
Pectoral fin spine and rays 18.37** 0.000** 0.000** 0.683
Dorsal fin lets 21.742** 0.000** 0.000** 0.855
Anal fin lets 13.401** 0.001** 0.000** 0.990
Vertebrae (total) 64.642** 0.000** 0.000** 0.000**
Morphometric measurement (mm)
Total length 20.126** 0.000** 0.100 0.000**
Forked length 7.137** 0.003** 0.015* 0.549
Standard length 19.874** 0.000** 0.000** 0.220
Predorsal length 0.084 1.000 0.934 0.941
Prepectoral length 0.658 0.665 0.529 0.996
Preanal length 45.093** 0.000** 0.000** 0.186
Prepelvic length 24.289** 0.000** 0.001** 0.001**
Body depth 46.902** 0.000** 0.000** 0.892
Caudal peduncle length 43.119** 0.000** 0.000** 1.000
Caudal peduncle depth 5.052** 0.010** 0.700 0.041*
Dorsal rays height 11.288** 0.000** 0.001** 0.714
Anal base length 10.572** 0.007** 0.000** 0.769
Head length 4.776* 0.025* 0.031* 0.891
Head width 7.850* 0.008** 0.002** 1.000
Head depth 3.115 0.065 0.123 0.803
Head depth passing through eyes 87.776** 0.000** 0.000** 0.089
Eye diameter 12.91** 0.003** 0.000** 0.711
Interorbital width 25.627** 0.000** 0.000** 0.483
Snout length 28.661** 0.000** 0.000** 0.385
Upper jaw length 0.922 0.947 0.400 0.666
Lower jaw length 37.766** 0.000** 0.000** 0.869
meristic counts indicated that the significant differences species. The hybrid type differed in 15 and 3 measu-
between parental species were found in five charac- rements from S. tritor, and S. commerson, respectively
ters. The differences between hybrid and S. tritor were (table 2).
observed in four meristic counts, while hybrid type
varied from S. commerson only in first and second Multivariate analysis
dorsal fin spines and rays and vertebrae (p < 0.001). Canonical variate analysis (CVA) was performed on
Adjusted morphometric measurements revealed meristic counts and twenty morphometric ratios to
17 of 21 significant characters between parental assess the shape variation between the two species224 Bakhoum
Table 3. Discriminant analysis classification Table 4. Discriminant analysis classification
showing the percentage of specimens classified showing the percentage of specimens classified
in each species, based on meristic counts: St, in each species, based on morphometric ratios:
Scomberomorus tritor; Sc, Scomberomorus St, Scomberomorus tritor; Sc, Scomberomorus
commerson; H, hibrid. commerson; H, hibrid.
Tabla 3. Clasificación según el análisis Tabla 4. Clasificación según el análisis
discriminante en la que se muestra el discriminante en la que se muestra el porcentaje
porcentaje de ejemplares clasificados en cada de ejemplares clasificados en cada especie,
especie, según los estudios merísticos: St, según las proporciones morfométricas: St,
Scomberomorus tritor; Sc, Scomberomorus Scomberomorus tritor; Sc, Scomberomorus
commerson; H, híbrido. commerson; H, híbrido.
Predicted species Predicted species
Species St Sc H Species St Sc H
St 94.40 0.00 5.60 St 100.00 0.00 0.00
Sc 0.00 100.00 0.00 Sc 0.00 100.00 0.00
H 0.00 0.00 100.00 H 0.00 0.00 100.00
and hybrid form. Consequently, standardized ca- The mean values of condition factors of S. tritor
nonical discriminant function coefficients extracted fish (0.803 ± 0.279) were higher than those of S. com-
two meristic factors and seven morphometric ratios. merson (0.354 ± 0.193) and hybrid (0.660 ± 0.083)
The discriminant function successfully identified the fish. Analysis of variance indicated significant differen-
membership of individual fish in the data to one of ces in the mean values of condition factor between
the three fish groups. The percentage of correctly parental species (F = 7.277, p < 0.01), S. tritor and
identified specimens based on meristic counts for the hybrid fish (F = 61.921, p < 0.01) and between S.
hybrid was 100 % and for S. tritor and S. commerson commerson and hybrid fish (F = 26.110, p < 0.01).
individuals it was 94.4 % and 100 %, respectively
(table 3). Identification of fish groups based on se- Natural mortality
ven landmark morphometric ratios classified 100 %
parental species and hybrid (table 4). The hybrid fish had the least mean value of natural
CVA extracted two CVs, accounting for 100.00 % mortality coefficient (0.177 ± 0.024) followed by S.
variations and showed three groups with no overlaps tritor (0.203 ± 0.015). S. commerson had the highest
between groups, with a position of hybrid specimens mortality value (0.382 ± 0.126). Statistical analysis
lying between expected parental species. Moreover, revealed significant differences in the natural mortality
CVA plots suggested that hybrids were resultant due of S. commerson compared with S. tritor (F = 34.151,
to crossing between S. tritor and S. commerson p < 0.01) and the hybrid (F = 46.011, p < 0.01).
(fig. 2). Analysis of variance revealed no significant diffe-
Squared Mahalanobis distances extracted among rence in the mean values of natural mortality between
groups of S. tritor, S. commerson and hybrid speci- S. tritor and the hybrid (F = 2.020, p > .05).
mens based on meristic and morphometric characters
were highly significant (P < 0.0001), indicating that
all groups were clearly distinct from each other, and Discussion
squared Mahalanobis distances between hybrid and S.
tritor were longer than those hybrid and S. commerson. Nature hybridization in most cases is a temporal
phenomenon occurring at different scales over an
Length–weight relationship and condition factor extended time frame (Avise and Walker, 2000).
Hybrid species, especially those leading to genomic
The relationship between total length (cm) and total introgression, may be an evolutionarily constructive
weight (W) was represented by the equations: process, as occurred in the family Salmonidae (Ar-
For S. tritor nold, 1997; Dowling and Secor, 1997). Extensive
W = 0.0245 . L . 2.6548 (r2 = 0.9655) hybridization and introgression occurs more com-
S. commerson monly in fish than in other vertebrates of comparable
W = 0.0092 . L . 2.1036 (r2 = 0.9666) levels of genetic divergence (Epifanio and Nielsen,
Hybrid form 2001). Almodóvar et al. (2012) recently described a
W = 0.0192 . L . 2.3997 (r2 = 0.9455) case of natural hybridization between invasive bleakAnimal Biodiversity and Conservation 44.2 (2021) 225
A B
1 100 1
5.0 2 2
3 3
Centroid Centroid
50
2.5
Function 2
2
Function 2
1 2
1
0.0 0 3
3
–2.5
–50
–5.0
–100
–5.0 –2.5 0.0 2.5 5.0 –100 –50 0 50 100
Function 1 Function 1
Fig. 2. Plots of canonical variate analysis based on meristic counts (A) and morphometric ratios (B) for
S. tritor (1), S. commerson (2) and putative hybrid (3) showing frequency of specimen distribution in
respective group on the first two axes.
Fig. 2. Gráficos del análisis de variables canónicas según los caracteres merísticos (A) y según las pro-
porciones morfométricas (B) de S. tritor (1), S. commerson (2) y los híbridos putativos (3) en los que se
muestra la frecuencia de la distribución de los ejemplares en el grupo respectivo en los dos primeros ejes.
Alburnus alburnous and endemic calandino Squalius Many researchers use biometric characters to
alburnoides complex following a short period of identify natural hybrids in fish (Reist et al., 1992;
contact. This study deals with the characterization Bakhoum, 2009; Jacquemin and Pyron, 2016). In
of the hybrids and their distinction and similarities the present study, the hybrid index revealed that
with the parental species. most characters studied in hybrids were closer to
Hybridization in nature is generally the result of either S. commerson or S. tritor, and one interme-
interference by humans, through the construction of diate character revealed the hybrid nature of these
reservoirs, for example, or introduction of exotic species, specimens. Canonical variate analysis (CVA) allowed
or modification of rivers and Seas (Crivelli and Dupont, parent species to be distinguished from hybrids. No
1987; Pouyaud and Agnèse, 1995; Agnese et al., 1998; overlapping between groups was revealed, and clas-
Bakhoum, 2019). The Suez Canal is an artificial water- sification in the respective group was 100 % correct.
way connecting the tropical Red Sea and the subtropical CVA placed hybrids in a position between groups
Eastern Mediterranean Sea. This example of human of S. commerson and S. tritor, indicating that hybrids
intervention caused a global change in the distribution were resultant from crossing between the two species.
of native and non–native fishes in Mediterranean waters Squared Mahalanobis distances indicated that the hy-
(Ben–Tuvia, 1985; Golani et al., 2002). brid form was relatively closer to S. commerson than
Natural hybridization occurs when reproductive to S. tritor. This may attribute to the back–crossing
barriers break down. These barriers may be phys- between the first generation of hybrid specimens with
iological, behavioural or geographic. Hybridization parental S. tritor fish.
between S. commerson and S. tritor in the Suez Canal The condition factor is used to compare condition,
is an example. fitness or wellbeing of fish. It is based on the hypoth-
Reproductive barriers between species, both esis that heavier fish of a particular length are in a
pre– and post–zygotic, appear to be incomplete for better physiological condition (Bagenal and Tesch,
many fishes (Simon and Noble, 1968; Rosenfield et 1978). It is strongly influenced by both biotic and abi-
al., 2000; Hendry et al., 2000). The parental species otic environmental conditions and can be used as an
examined have pelagic eggs and larvae (Collette, index to assess the status of the aquatic ecosystem
1986) and the spawning season of S. tritor from July to in which fish live (Anene, 2005).
August interfered with the spawning of S. commerson, The mean values of condition factors of endemic
which extends from October to July (Collette and Nauen, S. tritor fish were higher than those of immigrant S.
1983; Collette and Russo, 1984), thus providing great commerson and hybrid fishes due to their adaptation
opportunity for crossbreeding. to environmental conditions over many years. The226 Bakhoum
mean values of the condition factor for the hybrid Sciences, 5: 43–47.
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possibly because exotic species are strongly influ- tion. Oxford University Press, New York.
enced by both biotic and abiotic situations in the Arnold, M. L., Hodges, S.A., 1995. Are hybrids fit or
new habitat. Moreover, recent studies have provided unfit relative to their parents? Trends in Ecology
evidence for hybrid fitness when compared to parental and Evolution, 10: 67–71, Doi: 10.1016/S0169-
species (Arnold and Hodges, 1995; Reyer, 2008). 5347(00)88979-X
Natural mortality (M) is an essential parameter in Avise, J. C., Walker, D., 2000. Abandon all species
determining fish stock and an indicator of the adaptation concepts? A response Conservation Genetics, 1:
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maturity is achieved determines the proportion capable history. In: Methods of Assessment of Fish pro-
of reproduction and the relative reproductive potential. duction in fresh waters, IBP Handbook 3: 165–201
In the present study, the natural mortality of the hybrid (T. Bagenal, Ed.). Blackwell Scientific Publication,
and S. tritor were significantly lower than that of the Oxford.
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habitats indicated that the natural mortality coefficient tooth lizardfish Saurida undosquamis (Richard-
for the emigrant to Mediterranean fish was comparative- son), from the Red Sea and Mediterranean Sea
ly higher than that of endemic Red Sea fish (Bakhoum, coasts of Egypt. Oebalia, International Journal of
2000). This may be attributed to variation in environ- Marine Biology and Oceanography, 26: 35–48.
mental conditions. El–shenawy et al. (2006) recorded – 2009. Biometric characteristics and some biological
that the range of water temperatures in the Egyptian features of natural hybrids between Nile tilapia
Mediterranean coast was wide(17.14–26.31 ºC), with Oreochromis niloticus and blue tilapia Oreochromis
narrow fluctuations in salinity (37.51–39.710 ‰). In aureus in Lake Edku, Egypt. International Journal
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fluctuated between 20.8 ºC and 28.1 ºC and salinity – 2017. Shape and behavior of aquatic organisms.
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research into the importance of natural hybridization iterranean ecosystem. In: Mediterranean Marine
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