A female crocodile might, on average, lay a clutch of 40 eggs every year for forty
years or more. If this applies to every adult female in a wild population, and if the
sexes are present in equal numbers, there will be a production of 800 eggs per crocodile.
This is irrespective of how many females associate with one male. If a male has two or
more mates then, of course, production per breeding crocodile will be higher. What
happens, then, to all these eggs?
The dynamics of wild animal populations are never simple but for numbers to remain
constant each animal must only be replaced by another when it dies. If production works
out at a thousand or so eggs per adult then the vast majority can not survive to reach
maturity, let alone live for the full lifespan. Otherwise, there would be spectacular
fluctuations in numbers.
Many eggs never hatch. In some circumstances flooding can destroy them en masse. Others
may be discovered and eaten by monitor lizards (Varanus spp), pigs or people. The chances
of survival are lessened if human disturbance prevents the parent crocodile from guarding
her nest. After hatching, the young crocodiles have even more enemies, including birds
(e.g.storks and herons) and predatory fish (Pooley 1969). As they grow, so the danger from
such predators becomes less until eventually only man and other crocodiles can threaten
them.
Those who have studied crocodiles agree that most losses occur either before hatching or
during the few months afterwards. This is the main reason why hunting crocodiles for skins
has always led to a decline in the resource; hunting kills the survivors after the others
have already been lost.
This is not to say that there are never any surplus adult crocodiles. Where crocodiles are
plentiful competition between breeding adults could prevent some from finding a territory
or a mate. Such individuals may be forced into poorer habitats where they are unable to
breed or even survive. But at best, from a harvest point of view, the sustainable yield of
well-grown animals can only be a minor proportion of their numbers in the wild and a
minute percentage of the total production. Hunting large crocodiles for skins is therefore
an extremely inefficient management practice, either yielding a small annual crop or
leading to the destruction of the resource.
There is reason to believe that when crocodile numbers have been reduced by hunting,
the survival rate of young ones improves. In other words, survival depends partly on the
numbers, or density of crocodiles, and the losses are greater at higher densities. Such
density dependence is quite probable but it does not alter the fact that the greatest
losses, whatever the figures may be, still occur in the egg and hatchling stage. It
follows, therefore, that nature could sustain the greatest harvest of eggs and/or young if
these losses could be prevented.
Protecting breeding animals and reducing disturbance during nesting could result in more
eggs being hatched but it would not guarantee that more young would survive. They might
just serve as food for more predators. A better rate of survival can certainly be achieved
if the eggs and/or hatchlings are collected and cared for in a safe place. In areas where
flooding is a problem major losses are only to be avoided by collecting eggs.
On a commercial scale the collection of eggs or young for captive rearing is known as
"ranching" to distinguish it from "farming" in which eggs are produced
by captive animals. Ranching has been practised in Africa for twenty years and in Zimbabwe
the policy includes a potential contribution to the survival of wild stocks (Blake &
Loveridge 1975. Blake, 1982). A number of crocodiles, equal to 5% of the eggs collected,
must be made available for release into the wild when they are considered large enough to
be safe from most predators. Initially this was set at three years of age and available
evidence indicates that 5% of eggs collected will be far more than would have survived for
three years in the wild. Such a policy could be particularly useful in restocking areas
which have been depleted of crocodiles.
It would not be good policy to release crocodiles with the intention of catching them
again when they have reached commercial size. With a good food supply growth in captivity
can be twice as fast as in the wild (Coulson & others 1973, Blake & Loveridge
1975, McNease & Joanen 1981) and selective culling of captive animals is more
efficient in every way than harvesting wild stock.
Probably everyone involved in rearing crocodiles will wish to breed them in captivity.
There is a special satisfaction involved because successful breeding is generally
considered to be a criterion of successful husbandry. The commercial advantages of being
independent of the wild resource are obvious.
In practice, however, it is rarely possible to assemble a large breeding stock in a short
time. It usually takes a number of years, and substantial capital, to build up the stock,
enclosures and large pools that are needed for captive breeding on a commercial scale. A
captive breeding programme is often developed gradually as an extension of a ranching
operation; selected animals being kept for future breeding stock instead of being culled
for skins.
Farmers with access to wild stock for ranching have an enormous advantage compared with
those who must rely from the start on captive breeding. The latter will have no income
from the sale of skins for a very long time. Clutch sizes will be small and a proportion
of eggs may be infertile when the crocodiles first start breeding. After three or four
seasons they are likely to be breeding much more successfully but it may also take three
to four years for any hatchlings to reach optimal culling size. Clearly, a supply of
crocodiles breeding, feeding and accomodating themselves in the wild is a most valuable
asset. Wild stocks not only facilitate the development of new captive breeding centres,
they can supplement and safeguard their operations for as long as they exist.
