The salmon of each species almost invariably come back to spawn in the stream or lake in which they were hatched (parent stream). The most important discovery of recent years is this confirmation of the "parent stream theory" with the resultant conception of each species as made up of numerous self-contained colonies or races of interbreeding individuals. It results that if commercial fishing or other obstruction of a stream prevents so many individuals of any race from returning to their home spawning-ground that an adequate breeding stock is not main tained, that race is depleted or disappears and its racial ground is not afterward colonized by other races. It is then essential that commercial fishing be so conducted as to permit the escapement to the spawning ground of enough breeding fish of each colony to maintain that colony. In Alaska, the U.S. Bureau of Fisheries now requires a "spawning escapement" of 50%. This regulation was first imposed in 1924 and in 1926 the run of pink salmon, a two year fish, was the largest ever known. How large the spawning escapement must be to maintain a racial population in any locality is not yet definitely known. Actual counts made under the direction of Dr. Gilbert have shown that in the Karluk river, Alaska, three red salmon returned in 1926 for each parent fish of the 1921 spawning escapement. This ratio does not in clude the fish of the 1921 hatch that returned in earlier years or those still to return. If the proportion should prove to hold for all seasons an escapement of less than one-third would serve to maintain the race.
From studies made on the return of young salmon marked and released in fresh water, evidence is accumulating that arti ficial propagation may be made effective. It has been found that marked fingerlings of the Alaskan sockeye or red salmon, re leased in the Columbia river in the fall of their first year, fail to return, but if released in the spring of their second year, they return; success does not necessarily depend merely on the release of great numbers of large and vigorous fingerlings, but on their release at the right age, the age at which they naturally begin their seaward voyage. It has been found, further, that since each tributary (Columbia river) supports a separate colony it is inadvisable to transfer fingerlings from one tributary to another. Different racial groups start their spawning migrations at different times and each starts at such a time as to bring it to the spawning grounds when conditions there are suitable for laying of the eggs and development of the young of that race. If fingerlings are released on an alien ground they will return as adults to it, but they may then find the conditions on it unsuited to them at the time of their return. By releasing the fry or fingerlings of each race on its home ground and at the right age better results may be expected from hatchery operations than have been reached up to this time. Thus in an experiment on the Columbia river the number of returning adults, recaptured after having been marked as fingerlings and released on their home ground when of suitable age, was 3o times that necessary to produce the eggs from which they came (Holmes). Increased runs of Pacific salmon are also reported by Canadian officials as the result of hatchery operations in the Frazer river.
In the United States many of the natural oyster beds have been so exploited that they have disappeared and others are greatly depleted. The great size of the beds, the high cost of labour and
the relatively low price of the product make the method of pond culture currently employed in France and Japan impracticable. Culture consists at present wholly of rehabilitating the beds in natural waters and increasing their number (planted beds). About half the 30,000,00o bu. of oysters marketed annually are now obtained from planted beds and in the New England States about 9o% are from that source.
It has long been known that fertilized eggs could be obtained by cutting open oysters and stripping the apparently ripe milt and eggs from them, but recently oysters have been induced to spawn naturally in tanks of sea water by permitting the tempera ture to rise to about 7o° F and in this way better results have been obtained. But, for artificial culture, large numbers of the minute larval oysters must be raised to the fixed or spat stage. This has been hitherto impossible owing to the difficulty of retain ing them in tanks while supplying them with running water. Until they are several days old they pass readily through felt or the finest silk bolting cloth. A new filtering material, known as "filtros" has now proved entirely satisfactory and several lots of larvae have been reared to the spat stage by investigators of the U.S. Bureau of Fisheries. Seed oysters for planting have become increasingly scarce and costly, but if the method referred to becomes a part of commercial oyster culture they may be produced in quantity at a cost much below that of collecting them from the beds.
In the United States the supply of the species of fresh water mussels from which pearl buttons are made has declined in recent years with the result that much work has been done on methods of cultivating these forms. The larvae (glochidia), after expulsion from the gills of the female mussel, become parasitic on the gills of fish from which they draw their nourishment and from which they fall after transforming into young mussels. In the earlier attempts to increase the mussel supply, fish artificially inoculated with glochidia were released into natural waters and not only fish captured for the purpose, but great numbers of rescued fish (see above) were used. In 1926 fish rescued and released by the U.S. Bureau of Fisheries had been inoculated with nearly 3,000,000,000 glochidia taken from about 40,000,000 mussels. Later experiments showed that the inoculated fish could be held in troughs until the young mussels dropped from them. The latter are then cared for during the first season of their growth. But the number of glochidia that can be safely carried by a single fish is limited, and the inflammation caused by over-infestation results in many deaths. Hence, a method was sought of rearing the glochidia without permitting their attachment to a fish host. Although the inoculation of both fish to be held captive and of those to be released is still practised, it recently has been found that the glochidia may be reared to young mussels in an artificial medium which supplies the nourishment normally drawn from the fish host and makes the use of the latter unnecessary. With perfection of the technique it is practically certain that young mussels will soon be raised and distributed in large numbers without employing fish hosts.