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4. MANAGEMENT OPTIONS

4.1 Life and Death in the Wild

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.

4.2 Captive Rearing: Increasing Nature's Yield

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.

4.3 Captive Breeding

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.

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