250 



the width and depth of the channel at different points, and b>- differences in the seasonal 

 discharge of the rivers, which is ordinarily more than five times as great during the 

 first six months of the year as during the last six months (see p. 36, etc.)- In the 

 Carquinez Strait the annual range of variation in salinity is as great as from to 27 

 parts per 1000. Salinities at several points in the upper bay during 1920-1922 are 

 graphically shown in the appended charts (figs. 19, 93, 94). 



The middle bay exhibits a set of conditions nearly the opposite of those just 

 described. Its greater depth and volume and proximity to the open sea make for a 

 condition of high and fairly constant salinity and for a minimum temperature fluctua- 

 tion. Salinities at two localities in this portion of the bay during 1921-1922 are charted 

 in figures 93, 94. These are not, unfortunately, the localities of maximum salinity, 

 being considerably to the northward of the Golden Gate. Records of the Albatross 

 investigations during 1912-1913 show the mean annual salinity at certain points in 

 this area of the bay to be as high as 30 and even 31 parts per 1000 (Sumner, et al., 

 1914, p. 4). 



The lower bay represents an environment intermediate between the two extremes 

 just described. A great unbroken expanse of comparatively shallow water (mean 

 depth 7.8 fathoms, as compared with a mean depth of 14.8 fathoms for the sector 

 of the bay between Goat Island and Point Richmond), receiving no streams of any 

 importance, the lower bay represents a complex of conditions peculiar to itself. The 

 salinity is somewhat lower than that of the waters more proximal to the open sea, 

 though at the same time it probably is subject to less daily and seasonal variation. 

 Unfortunately, hydrographic investigations in this region have been extremely limited. 

 The Albatross records show a mean salinity of from 28.47 to 29.14 at eight different 

 stations south of Goat Island, the lowest figure recorded during a one-year period being 

 24.85 as compared with a maximum of 31.36 at the same station. For a very large 

 stretch of water at the southern end of the bay no salinity records are available, but 

 we are fairly safe in assuming that they would be consistent with the above figures, 

 with possibly a slight increase in the mean during the summer, owing to the effect of 

 evaporation (Sumner, et al., 1914, p. 85), and the opposite effect of seepage during 

 and after the maximum run-off. 



The temperature range is notably greater here than in the middle bay, owing 

 largely to the influence of air temperatures on so large an expanse of relatively shallow 

 water; but less, on the other hand, than in the upper bay, whose temperatures depend 

 in considerable measure on those of the rivers tributary to it. 



Temperature ranges in the three divisions of the bay can best be compared by 

 reference to figure 95, where they are graphically represented. 



Seasonal Distribution of Teredo with Reference to 

 Fluctuating Conditions in the Upper B.\y 



The upper portion of San Francisco Bay, particularly the region of Carquinez 

 Strait and beyond, affords an environmental complex of peculiar interest as regards 

 Teredo navalis. Here the organism is subjected to conditions of stress — low and 

 fluctuating salinity, much turbidity, and a wide range of temperature variation. 

 Somewhere in this region occurs the combination of conditions which marks the 

 limit of the activities of the animal. 



The breeding season of this species in San Francisco Bay is practically co-extensive 

 with the autumn period of minimal river discharge. In consequence, the free-swimming 

 larvae make their appearance at just the time when salinities in the upper bay are 

 highest, and the conditions most favorable for their survival. The tidal movement is 



