Commercial crocodile farms in Rhodesia (now Zimbabwe) may have a role in the conservation of Nile crocodile Crocodylus niloticus, a declining species over much of Africa. Legislation stipulated that when crocodile eggs are collected from the wild, a quota of juveniles equivalent to 5% of the number collected must be released at 3 years of age into suitable habitat. Hatching success and juvenile growth was compared with wild populations, and subsequent survival of captive-reared releases assessed.
At the time of study three crocodile rearing stations were present in Rhodesia, Kariba, Binga and Victoria Falls (located in the northwest of the country) operating under permits issued by the Department of National Parks and Wildlife Management.
Data of eggs collected and hatched from 1967 to 1973 by these rearing stations were collated; hatching success and juvenile growth was compared with wild populations. Survival and growth of captive-reared releases was assessed by mark and recapture surveys of individuals releasedinto the wild at Lake Kariba.
Hatching success: Hatching success on rearing stations was better than for wild populations.Over the study period 22,679 wild eggs were collected, of these 16,697 (73.6%) hatched. Data available on hatching success in the wild varies greatly, for example: nest predation recorded at 49.4% three weeks after laying (Rhodesia); in a Kenyan study, six clutches had a hatching success of 68.6-96.4%, but another five failed; in a Ugandan study, 55.1 % of clutches were predated and some lost to flooding and damp, thus eggs in only 30% of nests hatched.
Growth: Juvenile growth rate on rearing stations was variable but around twice that of wild individuals.
Survival: Juvenile mortality on the rearing stations averaged around 50% to 3-years of age; there was a clear trend indicating that as operators became more experienced this high loss was reduced. No comparative information is available for wild populations but mortality rates are thought to be very high.
Of 355 juvenile crocodiles returned for release, 53 were marked and liberated at Lake Kariba. Twenty (37.7%) were caught at least once in twice-yearly recapture exercises undertaken since 1970. This survival rate was considered good; growth slowed substantially.
Juveniles for release have proved valuable for restocking suitable areas and supplementing natural recruitment.
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