GROWTH AND MATURITY OF SALMON IN THE OCEAN 
73 
second year later. The full effect of the increased fishing effort during a given 
year will not, therefore, fall entirely upon that year but will be distributed over 
at least 3 years. Thus the outside fishing conducted in 1920 affected the run of 
mature fish into the river not only in 1920 but also in 1921 and 1922. Entirely 
apart from the fact that the young, immature fish produce an inferior product, this 
encroachment upon the runs of future years seems an especially vicious phase of 
this newly developed fishery. The full effect of the outside fishing is partially 
hidden; it is not immediately apparent in a decreased run into the river. It might, 
therefore, easily cause a very serious depletion before it became apparent that there 
was any danger of such an outcome. 
At the same time other factors that have undoubtedly tended to reduce the 
supply of salmon have been at work. Many of the tributary streams that were 
once used as spawning beds by thousands of salmon are now blocked by dams of 
one sort or another, and other streams are made barren by the removal of quan- 
tities of water during the irrigation season. Large numbers of young salmon on 
their seaward migration become lost in the irrigation ditches or impounded in the 
pools left in the main stream as the water is drawn off for irrigation — where they 
die as the water warms and evaporates. On the whole, there is no question that 
the available spawning area in the Columbia Kiver Basin has been materially 
reduced by such factors as these, and it seems probable that the encroachment on 
the spawning area will continue for some years to come. We have, then, a situation 
in which the continuance of the salmon is menaced on the one hand by a dimiaishing 
spawning area and on the other by an increased iatensity of fishiag. The various 
industrial and agricultural projects that are responsible for the erection of dams 
and irrigation ditches are of such importance that it is idle to suppose that they 
can long be opposed successfully by the iatercst of the salmon fishery. Regardless 
of right or wrong, it is inevitable that sooner or later the fisheries must disappear 
wherever they are directly and unavoidably opposed by the requirements of indus- 
trial and agricultural expansion. 
Efforts to counteract the effect of these various agencies, all of which tend 
toward the destruction of the salmon, fall into three general categories: (1) Legal 
restrictions. These may affect the type or amount of gear used, the area open to 
fishing, and the time during which fishing may be conducted. (2) Construction of 
fishways over dams and of screens to irrigation ditches. (3) Artificial propagation. 
The first of these is obviously designed to reduce the intensity of fishing. It is 
the oldest and still the most effective and indispensable of aU means for conserving 
fishery resources. Unless a sufficient number of mature fish are permitted to ascend 
the rivers to the spawning areas, depletion is certain to occur regardless of any 
efforts that may be made in maintaining spawning areas or in reducing the mortality 
of the young fish by means of artificial propagation. 
The construction and maintenance of fishways and screens to irrigation ditches 
is purely a palliative measure designed to offset in some measure the effect of en- 
croaching civilization and development. They merely lessen to some extent the 
effect which the building of dams and irrigation ditches has in destroying spawning 
areas, and can not be expected fully to counteract the effect of this one destructive 
agency. 
