but certain individuals make much longer journeys (maximum: 100 km in
30 days). Tags carried by S. mossambicus juveniles in Lake Sibaya (Bruton
and Boltt 1975) showed that most of the returns found within two days
were in a radius of 400 m from the tagging point; one individual was recaught
400 m away from the tagging point after two hours and another at 3,100 m
after 14 days. Lowe (McConnell) (1956b) recorded important movements in
S. esculentus in Lake Victoria (56 km in 40 days; 10 km in 3 days) but most
of the individuals were recaptured at the marking place after 228 to 700 days,
which suggested wellcircumscribed populations, fixed to a home range.
Marking experiments made by Holden (1963, cited by Lowe-McConnell
1975) showed great spatial stability of S. niloticus and S. galilaeus in the
pools of the Sokoto River, Nigeria.
The first case of homing in the Cichlidae was reported by Fryer (1961a)
in Lake Victoria, Jinja region. Eight S. variabilis, captured and marked on
the brooding grounds, were recovered after 45 to 805 days on the same
brooding ground or in its immediate proximity. In Lake Sibaya (Bruton and
Boltt 1975), three S. mossambicus marked during a breeding season were
captured during the following season: two individual males exactly on the
breeding site of the previous year (homing) and one individual on another
spawning ground three kilometers away.
Ecological Effects of Introductions and TransfersAFRICA INCLUDING MADAGASCAR AND MAURITIUS
(see Tables 2 and 3)Introductions into natural waters not containing any tilapias or offering a
vacant ecological niche for tilapias are often crowned with success when one
uses a species for which the natural distribution area is near: e.g., S. nilo-
ticus in Uganda (Koki Lakes) and in Madagascar; S. spilurus niger in Kenya;
S. macrochir and T. rendalli in Zambia (e.g., Lake Lusiwashi); S. macrochir
in Lake McIlwaine; T. zillii in Lakes Naivasha, Kyoga and Victoria; S. alcali-
cus gmhami in Lake Nakuru and T. rendalli in certain South African lakes.
On the other hand, many long-distance introductions, to a different
climatic zone or to a different altitude, have been considered complete
failures: S. macrochir, S. andersonii, S. spilurus niger and T. rendalli in cer-
tain regions too cold in South Africa; S. spilurus niger in Madagascar; S. mos-
sambicus in the north of Egypt and in Malta and S. spilurus niger and S.
esculentus in lakes at altitude in Uganda. This is explicable for the most part
by the unsuitable temperature conditions. A certain number of other results
disappointing for the control of vegetation (T. rendalli in the Sudan), for
improving% fishery (S. niloticus in certain lakes at altitude in Uganda) or for
fish culture are explained by 1) poor survival (e.g., fish kills of S. mortimeri
and S. mossambicus in man-made lakes of the Zimbabwe plateau, Jubb
1974) and slow growth (for example in T. rendalli of Lake Itasy, 1,200 m
altitude, Madagascar, Moreau 1975) observed in the species transferred into
regions characterized by a well-marked cold season and 2) excessive repro-