Their average increase is to be destroyed each year.

If at any time it appear that, for whatever cause, the total number left alive is falling off, the chief of this service suspends it partially or wholly at his discretion.
We now came to the entrance of a vast enclosure bordering on the river, the greatest fish-breeding establishment on this continent, or indeed in this world.

One of its managers courteously showed me over it.

It is not necessary minutely to describe its arrangements, from the spawning ponds and the hatching tanks--the latter contained in a huge building, whose temperature is preserved with the utmost care at the rate found best suited to the ova--to the multitude of streams, ponds, and lakes in which the different kinds of fish are kept during the several stages of their existence.

The task of the breeders is much facilitated by the fact that the seas of Mars are not, like ours, salt; and though sea and river fish are almost as distinct as on Earth, each kind having its own habitat, whose conditions are carefully reproduced in the breeding or feeding reservoirs, the same kind of water suits all alike.