Biology Reference
In-Depth Information
is perhaps reported for the fi rst time (Fig. 27). The trend shows that the egg
production rate increases upto about 300 g body weight but beyond which
it declines, clearly indicating that (i) the process of reproductive senescence
is progressively expressed more strongly from 300 g to 461 g and (ii) the egg
(and possibly oocytes + OSC) production rate also decreases from about 2.0
eggs/g/day in a 227 g fi sh to 0.1 egg/g/day in a female weighing 461 g. Of
13 larger females, nine (i.e., 69%) have not spawned even once during the
80 day period, though they are expected to spawn once every 56 days. Egg
size is known to increase in larger females (see Pandian, 2010). A possible
alternate is that the four larger females that spawned have produced larger
eggs at the cost of reduced number of eggs, as is true of some marine fi shes.
In fact the follicle number per gram ovary is reported to decrease with
increasing follicle (egg) size in many marine fi shes including
G. morhua,
Clupea harengus, Pleuronectes platessa, Scomber scombrus
and
Sebastes marinus
(Fig. 28). The egg size of
T. zilli
also shows a trend of increase but only upto
200 g size (Fig. 27). Clearly,
T. zilli
undergoes a terminal size related period
of reproductive senescence.
Fig. 28.
Calibrated follicular diameter of some marine fi sh as function of relative fecundity
(expressed in follicle number/g ovary) (from Witthames et al., 2009. Fish Bull, 107: 148-164)
The annual spawner
Hoplostethus atlanticus
is a long-living fi sh. It
attains maturity at the age of 27.5 years (4,000 eggs/year), reaches the peak
fecundity at age 125+ (16,000 eggs/year) and subsequently its fecundity is
levelled around 6,000 eggs/year at age 150+; however, older females of upto
the age of 175+ have been recorded (Minto and Nolan, 2006). The reason
why Coward and Bromage (1999) have not found a signifi cant correlation
between age and fecundity is due to the presence of old senescent tilapia
