Assiut university researches
Biological and Taxonomic Studies on some
Species of Genus Siganus (Family:
Siganidae) from the Red Sea, Egypt
Biological and Taxonomic Studies on some
Species of Genus Siganus (Family:
Siganidae) from the Red Sea, Egypt
Biological and Taxonomic Studies on some
Species of Genus Siganus (Family:
Siganidae) from the Red Sea, Egypt
‫درا سات ب يول وج ية وت ص ن ي ف ية ع لى ب عض ان واع‬
‫ س يجان يدى) من ال بحر االح مر‬:‫ج نس س يجان (عائ لة‬
‫ب م صر‬
Yaser Hasen Ahmed Obady
‫ي ا سر ح س ين أحمد ع بادى‬
Mohamed Tawfik Wahba, Imam Abd El-Ghany Ahmed Mekkawy
‫ إمام ع بدال غ نى أحمد م كاوى‬،‫محمد ت وف يق وه بة‬
Abstract:
The present work is a part of a project executed in Assiut
University to study the biology and fisheries of the Red Sea
fishes at Hurghada. So, using more advanced techniques and
statistical methods, the present study on the siganid species
namely, S. rivulatus, S. luridus, S. stellatus and S. argenteus
aimed at: 1) Applying the concept of size and shape on the
morphometric characteristics of the four Siganus species, S.
rivulatus (Forsskal, 1775), Siganus luridus (Ruppell, 1829),
Siganus stellatus (Forsskal, 1775) and Siganus argenteus
(Quoy and Gaimard, 1825) to elucidate intra- and interspecific variations and to determine their validity in stock unit
identification of these species; can one consider these four
species as one stock unit in fisheries management?. These
morphometric characters include those of the body, skull and
urohyal bone. The meristic characters were also considered.
2) Estimation of the biological and population parameters,
age composition, length distribution and age-length key of the
four Siganus species. 3) Studying and modeling population
dynamics, stock assessment and fisheries status of siganid
species considered. 4) Collecting qualitative data on the teeth
of Siganus species, S. rivulatus, S. luridus, S. stellatus and S.
argenteus, to illustrate their taxonomic importance. 5)
Identifying the electrophoretic patterns of general proteins of
eye-lens nucleus of siganid species to emphasize on the
nature of their intra- and interspecific differences. The general
body and skull (neurocrinum) characteristics of the four
Siganus species studied were described in terms of the
parameters of linear regression equation of raw morphometric
measurements relative to two reference lengths, the standard
and head lengths and of raw skull measurements relative to
three reference lengths, the standard, head and dorsal skull
lengths. The size variations in the Siganus species were only
reflected by few morphometirc and osteometric characters.
The type of allometry (i.e. isometry, negative allometry and
positive allometry) of the body morphometric and osteometric
(skull) characters in terms of size and shape were
determined. Such allometric growth was considered as a
taxonomic tool in identification of Siganus species. The body
morphometric and osteomitric indices exhibited a great
variability in their behavior among Siganus species studied,
except for some characters. The four Siganus species had
size-free and size-related characters with variable
percentages of distributions. The interspecific relationships
between Siganus species were also evaluated on further
patterns of size and shape using stepwise discriminate
function analysis. The body morphometric and osteomitric
indices were treated by cluster analysis and Mahalanobis D2.
As regard the meristic counts, the results of the present work
indicate that there is no relationship between the gill raker
count and the size of Siganus species studied. The ventral fin
was constant in all species studied. On the other hand, the
dorsal fin spines, dorsal fin rays, anal fin spines, anal fin rays
and vertebral counts exhibited only a slight variations in their
counts with variable percentages of distributions. The level of
genetic differentiation among Siganus species in terms of
electrophoretic patterns was revealed by determination of
polymorphism or heterogeneity in eye-lens nucleus protein
patterns. Qualitative data on the teeth were collected to
illustrate their taxonomic importance in identification of
Siganus species studied. The teeth are tricuspid in the lower
jaws for S. rivulatus, S. luridus and S. argenteus, while the
teeth for S. stellatus are bicuspid. On the other hand, the
upper pharyngeal bones of S. rivulatus, S. stellatus and S.
argenteus have very small number of the teeth rows, while S.
luridus has a large number of teeth rows. Analysis of the
patterns of variations collectively recorded in the present
investigations quantitatively and qualitatively in the Siganus
species revealed different patterns of affinities between these
species. The degree of close relationship between Siganus
can be arranged in an descending order as follows: S.
rivulatus & S. argenteus, S. argenteus & S. stellatus, S.
argenteus & S. luridus, S. stellatus & S. luridus, S. rivulatus &
S. luridus and S. rivulatus & S. stellatus. Another pattern of
relationship, if one considers these species as a separate
group of three is as follow: S. rivulatus + S. argenteus + S.
luridus and S. rivulatus + S. stellatus + S. argenteus in a
descending order. However, each of the Siganus species
according to other patterns of variations is represented as a
separate cluster in a descending percentage of occurrence as
follows: S. stellatus, S. luridus, S. rivulatus and S. argenteus.
Such general picture of relationships led to the conclusion
that different genetic mechanisms influence such a wide
spectrum of Siganus species characteristics; the expression
of some of such mechanisms may be controlled partially by
certain environmental factors acting on the species studied.
In the present work, the growth in weight of S. rivulatus, S.
luridus, S. stellatus and S. argenteus relative to length was
best described by the power function equation. Such growth
was allometric for all Siganus species except for S. luridus.
The estimated length-weight equations (W in g and SL in
mm) were found to be: S. rivulatus W = 1X 10-5SL3.1044 S.
luridus W = 2X 10-5SL3.0393 S. stellatus W = 5X 106SL3.3326 S. argenteus W = 1X 10-6SL3.5008 The condition
factor of the four Siganus species studied showed no
particular trend toward increase or decrease with the variation
of fish size. The condition factor (Kc) of S. luridus, S. stellatus
and S. argenteus was higher than those of S. rivulatus in all
length groups and months. The relationship between SL and
vertebral radius (V) was found to be linear for S. rivulatus, S.
luridus, S. stellatus and S. argenteus. Such a relationship
was best described, according to the least square method, by
the following equations: S. rivulatus SL= 63.969 + 11.715V S.
luridus SL= 35.035 + 14.269V S. stellatus SL= 60.183 +
12.168V S. argenteus SL= 28.936 + 16.637V The least
increment of the vertebral margin were recorded in June and
September and in June and October for S. rivulatus and S.
luridus respectively. This means that the time of annulus
formation on the vertebrae of S. rivulatus and S. luridus take
place in June and September and June and October
respectively. The four Siganus species attained their highest
growth rate (annual increment) in length during the first year
of life. Interspecific variations in increments in weight with age
were revealed in such Siganus species. S. rivulatus and S.
stellatus attain the greatest part of its weight in the first year
of life. On the other hand, S. luridus and S. argenteus attain
the greatest part of its weight in the 5th year of life. The
siganid and total teleost catches and effort (number of boats)
are considered and analyzed through 1977-2000 period.
Generally there is an increase in the catch and the effort.
Maximum sustainable yield (MSY) estimated by Fox model
was to be 75.722 tons and the corresponding effort is 9050
boats. The population parameters of Siganus species were
estimated and discussed with those of other authors. Such
parameters exhibited the status of Siganus species fisheries
in different localities and times. S. rivulatus of the present
work exhibited the highest exploitation rates, so slightly
overfishing was happened. The optimum exploitation is
recorded by S. luridus while S. stellatus and S. argenteus
populations are underexploitation. The fishing mortality and
total mortality of S. rivulatus and S. luridus were higher than
those of S. stellatus and S. argenteus. Furthermore, S.
argenteus has higher values of Lm, Lopt and Lmax, while
higher Tmax-value was represented by S. stellatus. Also, a
low Tc-value was recorded in S. rivulatus. The parameters of
the Von Bertalanffy growth model were estimated using least
square method. Their estimates are: S. rivulatus )L∞ =33.75
cm, K =0.122 and To =-2.875 year( S. luridus )L∞ =31.34 cm,
K =0.196 and To = -0.425 year( S. stellatus )L∞ =33.90 cm, K
=0.135 and To =-2.651 year( S. argenteus )L∞ =41.55 cm, K
=0.154 and To =-1.031 year) As regard Von Bertalanffy
growth model, the curvature parameter, K, indicated that S.
luridus )K= 0.196( approach their asymptotic length )L∞(
faster than S. rivulatus (K= 0.122), S. stellatus (K= 0.135) and
S. argenteus (K= 0.154). Also, the ogive selection curve as
reflected by Lc25, Lc50 and Lc75 shows variations among
Siganus species catched at the same period. The yield per
recruit of S. rivulatus, S. luridus, S. stellatus and S. argenteus
were estimated by means of Gulland’s )1969( equation. The
results show that in S. rivulatus, at the present level of fishing
mortality (F), age at first capture (Tc) and natural mortality
(M), the yield per recruit was estimated to be 44.68 g. The
maximum sustainable yield per recruit (MSY/R) is 48.69g at
3.3 of fishing mortality. The reduction of current M from 0.6 to
0.2 led to increase of MSY/R from 41.98 to 57.36g at fishing
mortality of 3.3. At the present level of fishing mortality (F),
age at first capture (Tc) and natural mortality (M), the yield
per recruit was estimated to be 29.97g for S. luridus. The
maximum yield per recruit (34.3 g) was estimated with a
fishing mortality equal to 4.3. The present level of fishing
mortality is lower than that which gives the maximum yield per
recruit by about 83.7% and by increasing the level of fishing
mortality from 0.7 to 4.3 (83.7%), the yield per recruit will be
increased from 29.97 to 34.3g; i.e., by 12.6%. To evaluate the
effect of changing the natural mortality “M” on the yield per
recruits, M was increased to be 0.7 and decreased to be 0.3.
So, it is concluded that the maximum yield per recruit
increases with the decrease of natural mortality. In S.
stellatus, at the present level of fishing mortality (F), age at
first capture (Tc) and natural mortality (M), the yield per
recruit was estimated to be 35.04 g. The maximum
sustainable yield per recruit (90.08 g) was corresponding to
higher values of fishing mortality (F= 6). This means that the
present level of fishing mortality (F= 0.12) is lower than that
which gives the maximum yield per recruit by about 98% and
by increasing the level of fishing mortality from 0.12 to 6
(98%), the yield per recruit will be increased from 35.04 to
90.08g. By increasing of M to be 0.6 and decreasing to be
0.2, MSY/R of 71.09 and 128.5g respectively can be obtained
at higher value of fishing mortality (F= 6). For S. argenteus, at
the present level of fishing mortality (F), age at first capture
(Tc) and natural mortality (M), the yield per recruits was
estimated to be 14.1g. MSY/R was estimated to be 84.6 at
fishing mortality of 1.3. By increasing M to be 0.6 and
decreasing to be 0.2, the maximum sustainable yield per
recruits were 69.61 and 117.08g respectively at fishing
mortality of 1.3. The effect of age at first capture (Tc) and age
at recruitment (Tr) on the yield per recruits of the four Siganus
species was studied. The maximum sustainable yield per
recruits shows variability in the better condition of each
species. The yield isopleths exhibited that the maximum
sustainable yield per recruit of S. rivulatus lies between 0.15
and 0.45 of Lc/L∞, while in S. luridus, S. stellatus and S.
argenteus, the maximum sustainable yield per recruit is
noticed to be varied between 0.15 and 0.5 of Lc/L∞. MSY of
S. rivulatus, S. luridus, S. stellatus and S. argenteus revealed
by Length-based Thompson and Bell analysis were 24.2,
11.7, 3 and 2.9 tons at a multiplication factor (X factor) of 2.2,
2.3, 9.6 and 4.1 respectively. On the other hand, to reach
MSE, the multiplication factor should be 0.7, 0.9, 1.8 and 1.7
for S. rivulatus, S. luridus, S. stellatus and S. argenteus
respectively. The highest yields of S. rivulatus, S. luridus, S.
stellatus and S. argenteus at lowest relative effort and its
corresponding better mesh size were found to be 25, 12, 3
and 3 tons respectively. For the three latter species the
current mesh size was the most suitable one. The decisionmaking process in fisheries management is a complex
system that includes many essential steps that begins with
determination, selecting and formulation of objectives and
terminates by implementation of the best strategy (Lackey,
1974). Research especially the basic one is needed to
develop effective prediction models and to evaluate a
potential management solution within a broader decisionmaking framework. Accordingly, one must ask the following
question: Can one lump or treated the four species under
consideration together for management purposes and define
the aggregate as a unit stock? Such a situation must fulfill
that the values of the statistics of growth, mortality and
recruitment do not vary substantially between these Siganus
species. However, this would be a very gross assumption to
make, but perhaps a necessary one. Moreover, pursuing the
idea of similar exploitation rates and the assumption of like
vital statistics of the population parameters and the
morphological characters, another possibility for unit stock
would be based on the general ecological behaviour of
species. So, due to the results of the present work and the
behavioural considerations of the Siganus species studies
mentioned by Woodland (1983a), each of these Siganus
species must be treated by a single separate fishery
management strategy since each species in Hurghada
represents a single unit stock according to the pattern of
variations recorded in the present work