1917] Barrows: Shipworyii in Smi Francisco Bay 29 



tically all of the timbers used in marine woodwork in the main part of 

 the bay are impregnated with creosote as a protection against ship- 

 worms as well as against Limnoria. 



The northern portion of San Francisco Bay is called San Pablo 

 Bay, and is separated from the main part of the bay by San Pablo 

 Strait. It is generally believed that shipworms do not live in San 

 Pablo Bay because of the reduction of the salinity of the water in this 

 bay by the influx of a large amount of fresh water from the Sacra- 

 mento and San Joaquin rivers emptying into the bay through Car- 

 quinez Strait. In spite of prolific activity on the part of shipw^orms 

 in the main part of San Francisco Bay, the records of the Mare Island 

 Naval Station, in the upper part of San Pablo Bay, do not mention 

 the presence of marine wood-borers, either Xylotrya or Limnoria, prior 

 to the discovery of shipworms in the Mare Island dikes in January, 

 1914. The experience of construction engineers in charge of numerous 

 wharves from Vallejo Junction to Port Costa, at the head of San 

 Pablo Bay, confirms this record for the practical absence of wood- 

 borers from this locality. 



Recent Appearance op Shipworms in San Pablo Bay 

 Dikes at Mare Island. — Mare Island is located at the head of San 

 Pablo Bay, opposite Carquinez Strait, through which empty the Sacra- 

 mento and San Joaquin rivers. Several wooden dikes have been built 

 in the channel which separates Mare Island from the mainland and 

 about the entrance to this channel in order to confine the currents arid 

 prevent the filling up of the channel by the deposition of silt. The 

 longest of these dikes reaches southward into San Pablo Bay along 

 the edge of the main ship channel through the bay for a distance of 

 over 8000 feet, and is designed to prevent a back current in the shallow 

 northern part of San Pablo Bay from carrying sediments into the 

 shipping channel by which the naval station may be reached. In 

 view of the usual limitation of the range of shipworms below San 

 Pablo Bay, it occasioned great surprise when an examination of cer- 

 tain of these wooden dikes in January, 1914, showed extensive damage 

 from the borers. It was found that the greatest damage had been 

 done in the long dike which offered the first exposure to tide water 

 coming up the bay, though damage had been done also in two shorter 

 dikes at the entrance to Mare Island Strait. 



In general the damage in the long dike was greatest on the side 

 which faced the ship channel, where the water is deepest and where 



