1917] Barrows: Shipworm in 8mi Francisco Bay 39 



even if they should become attached to woodwork during the season 

 when the salinity is high. If any larvae are brought up alive from the 

 lower portion of the bay after the flood season and do become attached, 

 they must usually be killed within a few months by the recurrence of 

 the next flood season. Adult shipworms embedded in the wood might 

 occasionally be able to endure the reduced salinity of a single ebb tide 

 by tightly closing the entrance to their bore, as they are able to do, 

 but they could not endure prolonged reduction of salinity. 



On the other hand, the increased salinity of the water of San 

 Pablo Bay during a year of unusually small discharge of fresh water 

 from the Sacramento-San Joaquin river system may be such as to 

 permit not only the continuance of living Teredo in this bay through- 

 out the entire year but also much damage on account of increased 

 numbers and activity of the borers under the conditions of increased 

 salinity. 



It is just possible that an infection of marine timber in the upper 

 part of San Pablo Bay may have occurred in the fall of 1911, and 

 that the shipworms entering the wood then may have continued to live 

 through the months of minimum fresh-water runoff in the winters of 

 both 1911-12 and 1912-13, and until the discovery of the damage 

 found in December, 1913; but so early an infection seems improbable 

 because the discharge of fresh water during the previous year was 

 nearly 28 per cent more than the average annual discharge. 



It seems on the whole more probable that the first infection of the 

 attack in question occurred in the summer of 1912, because the falling 

 off of the discharge of fresh water unusually early in the spring of 

 that year must have lengthened the summer period during which San 

 Pablo Bay may have been open to invasion by Teredo larvae, per- 

 mitting thus an unusually heavy infection to gain a foothold. If this 

 infection occurred then, the shipworms settling at tliat time must have 

 been able to withstand a reduction of salinity of about ten parts per 

 thousand during the spring freshets of both 1912 and 1913. Another 

 infection in the breeding season of 1913 may have caused an acceler- 

 ation during the fall of that year in the damage being done to the 

 structures attacked. 



The lowest surface salinity recorded during 1912 was 9.08 parts per 

 1000 and the lowest bottom salinity 10.89. The average annual surface 

 salinity for the three stations referred to in the upper part of San 

 Pablo Bay ranged during the period of observation from 13.21 to 

 15.46 parts per 1000 and the average annual bottom salinity from 



