For hatching the larger and heavier demersal eggs of trout and salmon primitive apparatus, still in use in parts of Europe, con sists of long wooden troughs in which the eggs lie on gravel in running water. In more modern hatcheries, in order to economize space and utilize the water supply to the best advantage, the troughs are divided into "boxes" by transverse partitions and the boxes are filled with trays or baskets of eggs. The partitions are so constructed that the water which flows along the trough from box to box is given an upward or downward movement through the eggs in each box. For hatching trout eggs ten shallow wooden trays with bottoms of wire gauze each supporting a single layer of eggs are stacked in each box, covered by an empty tray and latched down by a cross-bar. A trough of five boxes carries from 500,000 to 750,000 eggs. The trout fry remain on the trays until a week or ten days before the yolk sac is absorbed and are then planted or transferred to rearing ponds. Four Pacific salmon baskets of wire screen are half filled with eggs (about 30,000), and one basket is placed in each box with the top pro jecting above the water so that no eggs can escape. The newly hatched fry fall through the meshes in the bottom of the basket and remain on the bottom of the box until the heavy yolk is so far absorbed that they are able to swim upward. They may be then planted or held and fed in artificial ponds near the hatch eries to be planted when a year or more old.
For the buoyant eggs of cod, use is commonly made of the McDonald or tidal hatching box which is so constructed that a jet of water gives a rotary motion to the eggs while an inter mittent siphon causes the water in the box to rise and fall at regular intervals, so that the rotating eggs are carried up and down much as they would be by the waves of the open sea.
The hatching of the eggs, whether of fresh or salt-water fishes presents no serious difficulties, if suitable apparatus is employed; but the rearing of fry to an advanced stage, without serious losses, is less easy, and in the case of sea-fishes, with pelagic eggs, the larvae of which are exceedingly small and tender, is still an unsolved problem, although recent work carried out at the Plymouth laboratory of the Marine Biological Association, in England, is at least promising. It has been found possible to grow pure cultures of various diatoms, and by feeding these to various larvae kept in sterilized sea water, great successes have been attained. In fresh-water culture there may be little ad vantage in artificial hatching, unless it is followed by a successful period of rearing. Thus the Howieton Fishery Company (Eng land) recommended their customers to stock their streams either with unhatched ova sown in redds or with three-months-old fry. Their experience is "that there is no half-way house between ova sown in redds and three-months-old fry. Younger fry may
do, but only when ova would do as well and at half the cost." In marine hatcheries, on the other hand, it is the invariable prac tice to hatch the eggs, although the fry have to be put into the sea at the most critical period of their lives, in an environment which teems with predacious enemies.
Some of the advances made in the United States in recent years in the cultivation of fish and shell-fish are shown by the examples which follow. They suggest the importance of a detailed knowledge of the life-history of the forms to be cultivated and the advantages of rearing to a suitable stage.
Decline in the Pacific salmon fishery led to the establishment of hatcheries until in 1921 there were 90 such establishments maintained by the United States, by the individual States, and by British Columbia. But in spite of the great hatchery output, the fishery continued to decline alarmingly in certain localities. This apparent failure of the hatcheries to rehabilitate the fisheries led the authorities of the United States and British Columbia to promote a co-operative and detailed study of the life history of the Pacific salmon in the hope that this would reveal better means of artificial propagation or enable fisheries restrictions to be intelligently applied, while in 1924, by act of Congress, full control of the Alaskan fisheries was placed in the hands of the Department of Commerce and has since been exercised by its Bureau of Fisheries.
There are five species of Pacific salmon (Oncorynchus) all of which ascend from the sea into the rivers to spawn and are captured in commercial nets on the upward journey. Unlike the Atlantic salmon, they invariably die after spawning, but the young hatched from their eggs return to the sea after a longer or shorter stay in fresh water. Investigations of the past ten years have added many important details of the life history. The method of investigation consists, in part, in marking the young by removal of some of the fins, releasing them in fresh water at various ages and in various places and recovering the marked fish when they return from the sea to fresh water to spawn. In part it consists of the microscopic study of the markings of the scales of returning adults. From these the age, rate of growth and period of time spent in fresh water may be determined. From these investigations it is now known that the fry may begin their return to the sea soon after hatching (pink and chum salmon), or may remain in fresh water for a year (silver salmon) or from one to four years (red salmon), or, as in the chinook, they may either start for the sea soon after hatching or when 12 or 18 months old. The age at return to fresh water also varies. In the pink it is invariably two years, in the chum from two to five or six years, in the silver three years, in the red three to seven years, in the chinook two to seven or eight years.