253 



considerable in this region, the influence of the tides being obser\ed far up the Sacra- 

 mento and San Joaquin rivers. Through Carquinez Strait in particular the tidal 

 currents rush with great velocity. Thus on each recurring high tide the larvae of 

 Teredo have an opportunity to be carried far into the waters of Suisun Bay, the 

 uppermost limit at which they will establish themselves depending on the conditions 

 during the year in question. 



With the recurrence of the flood season of winter and spring tlujse organisms 

 which have become established farthest up stream are killed olT by the fresh water 

 and the frontier of teredo activity is pushed back to a point which again varies with 

 the conditions of diiTerent years. It becomes of interest therefore to note for a series 

 of years the uppermost limit to which teredo extends during the autumn, and the 

 uppermost point at which the organisms are able to sur\ive during the following winter 

 and spring, until the return of favorable conditions. 



In the autumn of 1920, following two years of reduced rainfall, tiie acti\ity of 

 Teredo extended as far up as Antioch on the San Joaquin river. Reports of its occur- 

 rence at Rio Vista on the Sacramento river were not confirmed by acutal specimens. 

 With the return of the winter flood period, however, all borers were apparently ex- 

 terminated, not only in Suisun Bay, but also well down the Carquinez .Strait. Exami- 

 nation of piling at several localities from January to April, 1921, showed no living 

 Teredo above Crockett, which is thus established as the uppermost limit of sur\ival 

 during the winter of 1920-1921. 



This winter was one of approximaleW' normal rainfall and ri\er clisciiargc, and 

 the following autumn (1921) was not favoral)le to the extension of Teredo into Suisun 

 Bay. The initial settlement of larvae at Crockett and Port Costa was delayed a month 

 as compared with the previous year, and no organisms were found farther up than 

 Benicia, where a few small specimens, up to 1 cm. in length, were found in test boards 

 on Novemlier 14. At Crockett, in 1922 as in 1921, Teredo survived the fresh water 

 period of winter and spring, but in minimal numbers only. It is estimated that 

 90 percent of the teredos at this locality were killed off between January and July, 1922. 



In the autimin of 1922 Teredo is known to ha^'e gone upstream as far as the wharf 

 of the Mountain Copper Company at Martinez, where some specimens reached a 

 length of 10 cm. before being killed off by the returning flood waters. The organism 

 did not reach Roe Island (opposite Bay Point), as a number of test boards planted 

 at the Roe Island Light Station showed no attack during this season. There is e\idence 

 that some individuals survived the winter of 1922-1923 at Port Costa, although the 

 great majority were killed off. One living specimen 29.2 cm. in length was found at 

 this locality on December 4, 1923, which, from its size, is believed certainly to have 

 been a survivor of the fall brood of 1922. 



The autumn of 1923 was marked by a recurrence of conditions of increased salinity 

 in the upper bay, and a renewed extension of the activities of Teredo in this region. 

 On November 28th specimens 4 cm. or more in length were found in test timbers at 

 the wharf of the Associated Oil Company at Avon, and there is evidence that some 

 indi\'iduals established themscKes during this season as far up as Pittstjurg. No 

 surviving specimens were found at the latter locality when a search for them was made 

 on June 23, 1923, indicating that the limit of survival during the winter of 1922- 

 1923 was at some point between Avon and Pittsburg. At Avon some specimens had 

 reached a length of 7 cm. liy June 1st, and appeared to be in healthy condition at that 

 time. By the end of July it was found that they had seriously damaged the piling 

 at this locality, as shown in figure 96. 



It appears then that the conditions of low salinity lethal to Teredo recur each 



