magnitude may initially appear extraordinarily high, but they are not
uncommon in the shallow marginal waters of tropical and equatorial lakes,
lagoons and river pools of Africa. These high, temperatures, however, are
generally not stable throughout the day but, due to the shallowness of the
water, are subject to die1 oscillations often in excess of 15°C per day. It has
been noted by many authors (e.g. Welcomme 1964; Donnelly 1969; Fryer
and Iles 1972; Bruton and Boltt 1975; Caulton 1975) that a variety of
tilapias react to such temperature oscillations by moving inshore during the
day and offshore at night. A graphical example of such a movement by
juvenile T. rendalli is illustrated in Figure 2.
Time of day (hr)
Figure 2. The daily inshore (10 cm depth)/offshore (40 cm depth) movement of young
Tilapia rendalli in Lake McIlwaine, Zimbabwe. (1.10.74). Catch per unit effort (CPUE)
is given as the number of fish caught over a 15 min time period using a 2 m cast net. The
numbers of fish caught are shown by the cross hatch areas. Upper lines show the change
in temperature (after Caulton 1975a).
With the knowledge that tilapias are strongly thermophilic, it may not be
surprising that such daily inshore/offshore movements do occur. In the
example illustrated, the movement of young T. rendalli (up to 10 cm SL)
can be followed from the deep (> 1.5 m) homothermal water, where they
stay overnight, into water 40 cm deep, beginning at 8:00 AM. or when the
temperature at this depth exceeds that of the homothermal waters. The