The necessary qualities of a good artificial bed are, a good supply of clean water which is not liable if there is a spate to deposit sediment on the eggs, protection from light, and protection from the many creatures which prey upon the ova. The hatching beds may be so arranged that the young fish may escape as soon as they like after hatching out, but it is best to watch and protect them for at any rate the first few weeks after they have begun to feed, and while continuing the feeding, to allow those of the fish that wish to escape. The rearing of young salmon and sea-trout is practically the same as that of the common trout, except that they require more water. If kept in rearing ponds they grow more quickly than they do when left to find food for themselves. While young, the salmon is marked with transverse bars of a darker colour than the rest of the body. During the time it bears these marks it is known as a parr.[3] In about fifteen months it loses these marks and becomes quite silvery, being now known as a smolt. Shortly after assuming the smolt dress, the young salmon takes its departure to the sea. In some cases the young salmon do not appear to go down to the sea till over two years after being hatched out, but they should always be set at liberty in March, April, or May in the year following that in which they were hatched out, according to how far they have developed the smolt or silver appearance. [3] All the trouts go through this stage, which is distinguished by "finger marks" upon the sides. If spring water is obtainable, particularly if the water, as is usually the case, is of an even temperature throughout the year, the troubles of the fish culturist are considerably lessened. Without a building for the hatching troughs it is almost impossible in many places to guard against frost unless such a spring is available. Sediment may be avoided by putting frames covered with flannel at the inlets to the hatching beds, these will, if kept clean, prevent any sediment from coming into the ponds, and will allow plenty of water to flow in. If hatching trays are not used, the bottom of the artificial bed should be covered with clean gravel. The time which elapses from the impregnation of the eggs to their hatching out varies according to the temperature of the water, a fairly average time is about ninety days. The ova should be watched during this time, and the dead ones removed. For a short time after they are impregnated they are fairly hardy, but from then till shortly before they hatch out the very slightest concussion will kill or seriously injure them. The management of sea-trout ova is similar to that of salmon, and the ova are obtained in the same way. As in the case of the salmon it is best to rear the little fish artificially, till they are ready to go down to the sea; they will thus escape dangers likely to cause the loss of about eighty per cent. of their number.