674 
BULLETIN OF THE BUREAU OF FISHERIES 
Alaska. Records taken at the Bureau’s hatcheries on the Pacific coast also show the 
range of temperature tolerated by the adult salmon. Adult salmon have also been 
found migrating through estuaries, and streams fed by hot springs, whose tempera- 
tures were as high as 27° C. The California Department of Public Works (1931) 
reports a temperature range of 16° to 26° C. in the lower reaches of the Sacramento 
River during the months the salmon are migrating into the river. In July and August 
it is not uncommon for the temperature of the lower estuary of the river to hover for 
days around 24° C. Although the temperature of the streams in the native range may 
fluctuate at a high level during the spawning period it rapidly decreases with the onset 
of the winter season during which time the eggs pass through the incubation period. 
The studies of Donaldson (1936) have shown that the eggs can withstand temperature 
below 4° and above 11° C. for short periods of time but that the optimum lies between 
these limits. The mortality was extreme in eggs maintained constantly at tempera- 
tures below 4° or above 11° C. After hatching, the optimum range of temperature in 
fresh water, which controls the rate of growth and survival of the young, shifts to a 
level of 13° to 17° C. Constant temperatures above 17° C. retarded growth and 
increased the mortality of the young and at 20° C. the mortality was excessive. Con- 
stant temperatures below i3° C. retarded growth and at 3° mortality was excessive. 
In view of the results from these studies it may be assumed that temperature in the 
fresh-water habitats becomes a limiting factor in the early developmental period of 
the salmon. 
The eggs of the Pacific salmon are spawned free and, being of a higher specific 
gravity than water, sink to the bottom. Eggs of this type require a medium that will 
hold and cover them for protection and at the same time permit the free flow of well- 
aerated water for incubation. Such a medium is found in clean gravel beds, or in 
pockets among rocks, but not in mud or sand. The former conditions are invariably 
found in all of the native fresh-water habitats of the salmon. Spawning in the streams 
is usually confined to the comparatively shallow areas where the current is swift, 
and in the lakes to areas provided with flowing water from seepage or surface drainage. 
In the Pacific Coast States deforestation, agricultural developments, and mining 
operations have, in some areas, produced excessive erosion of the watersheds. This 
has always resulted in the silting of the streams and the subsequent destruction of their 
salmon populations. An excessive amount of silt in the water influences the normal 
respiration of the salmon and destroys the eggs by suffocating them with a blanket of 
mud. The character of the stream bed, therefore, becomes a very definite limiting 
factor in the distribution of the Pacific salmon. 
The environmental components of the marine habitats in the native distribution, 
which appear to be limiting factors, are ocean currents and associated temperatures 
and salinities (salt content). The mean directional drifts in the North Pacific from 
June through September are shown in figure 1. This period was selected because it 
is during these months that the salmon are known to be definitely migrating in the 
ocean. The adults are migrating from the open ocean to the streams to spawn and 
the young are migrating seaward from the streams. The currents in figure 1 were 
determined from the limits of the directional drifts during this period as given by 
Dali (1880), Schulz (1911), McEwen (1912), Mariner (1926), Hatai and Kokubo 
(1928), Uda and Okamoto (1930), Uda (1931 and 1933), Schumacher (1932), Zeusler 
