In sea-fishing a much larger net, sometimes enclosing more than 10,000 square yards, is employed. This fishing is conducted chiefly at night, the electric lamp being then much more effective in attracting the prey, and lowered only a few inches below the surface. Many large destructive creatures, unfit for food, generally of a nature intermediate between fish and reptiles, haunt the seas. It is held unwise to exterminate them, since they do their part in keeping down an immense variety of smaller creatures, noxious for one reason or another, and also in clearing the water from carrion and masses of seaweed which might otherwise taint the air of the sea-coasts, especially near the mouths of large tropical rivers. But these sea-monsters devour enormous quantities of fish, and the hunters appointed to deal with them are instructed to limit their numbers to the minimum required. 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. It is necessary, however, to keep the fishes of tropical seas and streams in water of a very different temperature from that suited to others brought from arctic or sub-arctic climates; and this, like every other point affecting the natural peculiarities and habits of the fish, is attended to with minute and accurate care. The skill and science brought to bear on the task of breeding accomplish this and much more difficult operations with marvellous ease and certainty.