218 



BULLETIN OF THE BUREAU OF FISHERIES 



Below the algal zone one finds a scattered growth of barnacles and incrusting 

 Bryozoa on almost all ships that are lightly fouled, but on such ships these growths 

 usually are very sparse, especially on the more perpendicular sides of the hull. How- 

 ever, on all parts not so perpendicular as aft (on the "quarter" or near the "run," 

 etc.) these growths often were noticeably more abundant. As previously noted, 

 some ships that were otherwise clean had small amounts of growths only in the seams 

 formed by the overlapping of the steel plates. (See fig. 24.) On most ships barnacles 

 and Bryozoa were found here, if at all. On some, as the Paul Luckenbach (June 12, 

 1924), large clusters of worm tubes (Hydroides) were found in these seams. 



The third vertical zone would include those growths that occur on the more 

 horizontal portion of the hull — the true bottom of most ships. In the case of heavily 

 fouled ships, this portion was also the most heavily coated. Hydro ids are found in 

 great abundance, while mussels, Ascidia, and often barnacles also are found here in 

 great quantities. In the case of moderately fouled ships, this region is again most 

 heavily coated, as a rule, with sessile barnacles, hydroids, and Bryozoa, and if from 

 certain routes, with stalked or goosenecked barnacles. In the case of but lightly 

 fouled ships, the growths here were of secondary importance to the algal zone but 

 were always most severe in the region directly under the bilge keels and in the "run" 

 of the ship. The factors that determine this distribution are numerous, no doubt, 

 but some may be pointed out at this time, of which several will be discussed under 

 separate headings. 



The presence of the algal zone only at the upper limit of growth is determined 

 rather largely by the fact that these organisms are dependent upon sunlight for 

 continued existence and growth. Light also may play a part in determining the 

 activities of the larva? at the time of setting, and so determine the location of later 

 growths. The distribution of animal life is affected by the factors that determine 

 the place of attachment of the young larval forms as well as by the conditions provid- 

 ing the food necessary for continued growth. The effect of too strong a current of 

 water, as when a vessel is cruising, probably may cause many of the more tender 

 growths to be torn off. This undoubtedly accounts in part for the presence of growths 

 in the seams behind the overlap of the steel plates in vessels that are in constant 

 service. It is a fact that most barnacles, hydroids, and tunicates attach in largest 

 number below the bilge keel and on other shaded parts of the bottom. This would 

 indicate that relative light intensity plays some part in determining the place of 

 attachment on the bottom. 



In view of these considerations, it will be seen that the contour of the vessel is 

 an important factor in the matter of fouling. Flat-bottomed ships of shallow draft 

 often are more foul than boats of similar design but greater draft; while vessels 

 designed so as to permit the effective sweep of the water while cruising to play on 

 the entire surface usually are more free from fouling under similar conditions than 

 are vessels with deep "runs." 



Directly associated with the type of hull and the contour of the ship is the 

 factor of speed of the ship while cruising. That this factor has some effect on the 

 amount of fouling can not be doubted, but evidence on this point has been very 

 difficult of obtainment without complications. The tremendous pressure exerted on 

 the sides and prow of a vessel as it progresses at the rate of 30 knots undoubtedly 



