57 



borer life woukl continue to repel them, not takinj; into account the tact that such 

 conditions often change. 



FACTORS CONTROLLING INFESTATION 



The most imiiortaiit marine conditions which control infestation are salinity 

 ami current action (which ha\e been previously discussed under separate headings), 

 relative purit>' of water, depth of water and temperature. In a harbor such as San 

 Francisco Bay all degrees of these conditions are found, low and high salinity, slow 

 and rapid currents, pure and contaminated water, shallow and deep water and a 

 moderate range of temperature. Hence many \ariations exist in degree and character 

 of borer attack. 



RI'.LATUIC PIRITV OF WATER 



It has been obser\i'd that borer .iction is greatest in uncontaminated sea 

 water and that it has sometimes been reduced or e\en pre\ented by local contamina- 

 tion, such as sewage or factory waste. Such a condition has often led to the conclusion 

 that no borers were present or that no protection against them was necessary, and 

 structures ha\'e been built accordingly. But such contamination may disappear; 

 different kinds of pollution may be differently effectixe; and different kinds of borers 

 may be differently affected by the same contamination. 



The first condition is illustrated by the experience of the Howard Company of 

 Oakland who built a pier of untreated piling in Oakland harbor in 1900. The 

 untreated material proved so satisfactory that subsequent replacements were made 

 with it. It happened, however, that their structure was being protected by effluent 

 from a gas plant which was being discharged into the water nearby, depositing a 

 surface coating on the piles. This practice was later ordered stopped by the Fish and 

 Game Commission in cooperation with the Oakland Port authorities. With the elim- 

 ination of the effluent the water became similar to that pre\ailing elsewhere and borer 

 attack began. As a result it was soon necessary to replace the piling with protected 

 material. 



The degree of protection afforded by sewage has probalily been oxerestimated. 

 During this investigation Baiikia and Limnoria have been found under conditions of 

 considerable sewage pollution (see p. 267), while Teredo has been found in timbers close 

 to the mouths of sewers. 



In harbors such as San Diego, where large amounts of crude oil are constantly 

 being shipped, the spilling of oil coats the piles between tide levels and temporarily 

 protects them in tliis section. 



DEPTH OF WATER 



It has been found that borer action is greater in deep than in shallow water. A 

 majority of wharves and piers are built so that they project from the shore in shallow 

 water to deeper water at the outer end. In such a case shipworm attack is greatest 

 towards the mud line in the deepest water (see p. 292 and figs. 3 and 20), while struc- 

 tures built completely in shallow water are usually attacked only lightly by shipworms. 

 But limnoria is not thus deterred; his attack is indeed greatest between tide lexels, 

 although extending to considerable depths below that zone. 



TEMPER.ATURE 



Borers are known to attack timber in all latitudes. In general their activity 

 increases with the water temperature, although certain species, such as Bankia setacea, 

 are adapted to northern waters and thrive best under relatively low temperatures 



