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International Journal of Marine Science 2014, Vol.4, No.31
http://ijms.biopublisher.ca
3
1.4 Age determination
Scales were taken from all specimens collected during
the sampling procedure, cleaned, mounted between
two glass slides and used for age determination. The
total radius of the scale (R) and the radius of each
annulus were measured to the nearest 0.01 mm.
Regression analyses of the scale radius - total length
was done using the method of least squares. The TL-R
relationship was described by the equation TL = a +
bR, where L is the total length, R is the scale radius, a
is the intercept and b is the slope.
The back-calculated length, i.e., the body length at a
specific age estimated by back-calculation rather than
by measurement (Francis 1990), was examined to
estimate the total length at age using the Fraser-Lee
equation (Duncan 1980): L
n
= a + (L - a) R
n
/R where
L
n
is the length at the formation of the n
th
annulus, a is
the intercept in the L-R linear function, and R
n
is the
scale radius of the n
th
annulus.
1.5 Length-weight relationship
To estimate the relationship between the total length
(TL) and the total weight (W), the variables were
log-transformed to meet the assumptions of normality
and homogeneous variance. A linear version of the
power function: W = a L
b
was fitted to the data.
Confidence intervals (CI) were calculated for the
slope to see if it was statistically different from 3.
1.6 Growth
Growth curves were fitted to the back-calculated data
using the von Bertalanffy (1938) growth function
(VBGF) (Chen et al., 1992):
L
t
= L∞ (1 - e
-K (t - t0)
), where L
t
is the predicted length
at age t, L∞ is the mean theoretical maximum length,
K is the Brody’s growth coefficient, and t
0
is the
theoretical age at 0 length.
The von Bertalanffy growth parameters (K and L∞)
were estimated using the Ford- Walford method.
While t
0
was estimated from the following rearranged
formula of the von Bertalanffy equation:
- ln [1 - (Lt/L∞)] = - Kt
0
+ Kt
2 Results and Discussion
2.1 Age composition
Scales (Figure 2) were used for age determination of
H. harid
and
C. sordidus
from Hurgada fishing area.
Scales as a reliable and valid method for ageing these
species have been proven. Body length – scale radius
relationship (Figure 3) showed a strong correlation
between the body length and scale radius. Also, the
increase of fish size is accompanied by an increase in
the number of annuli on the scales. On the other hand,
back-calculated lengths are accord with the observed
lengths for the different age groups.
Figure 2 Scales of
Hipposcarus harid
(TL 35 cm; age 4 yrs)
and
Chlorurus sordidus
(TL 29 cm age 4 yrs)
Figure 3 Body lengths – scale radius relationship of
Hippo-
scarus harid
and
Chlorurs sordidus
from Hurgada.
Based on the number of annuli on the scales, the
oldest individuals were 8 and 5 years old for
H. harid
and
C. sordidus
respectively (Figure 4). It is found
that the age group two was the most dominant age
group for
H. harid
forming 34.7 % of the total
collected samples, while for
C. sordidus
the fourth age
group was the most frequent one representing 45.6 %