through November, but the maximum rate of set- 

 tlement was observed in May and August. How- 

 ever, in the subtidal zone,S. crenatus settled from 

 April to November, but with a peak in late July 

 and early August. Others, like B. cariosus 

 and Chthamalus dalli, settles sporadically from 

 May to November and peaked in August and 

 September. 



Settlement by the bay mussel occurred primar- 

 ily from August to November, although there was 

 some settlement during April and May. It appears 

 that prior settlement by diatoms, algae, and bar- 

 nacles is necessary for the establishment of a mus- 

 sel colony. Cleaned test plates were exposed in 

 both the intertidal and the subtidal zones and 

 were compared with plates already having an es- 

 tablished community of diatoms, algae, and bar- 

 nacles. Only on those plates which were already 

 fouled was there any settlement by mussels. Coe 

 (1932) reported the same phenomenon and con- 

 cluded that the smooth quality of nonfouled sur- 

 faces was not suitable for attachment by the bys- 

 sus of young mussels. 



Vertical Distribution in 

 the Subtidal Zone 



At both subtidal test sites there was a distinct 

 vertical pattern to the fouling of the test plates. 

 The greatest number of species settled at the 

 near-surface (1-m) level. At that level there were 

 colonial diatoms of the genera Melosira, Navicula, 

 and Fragilaria, and three species of the acorn bar- 

 nacle, Balanus crenatus, B. glandula, and B. 

 cariosus. Subdominant genera included the green 

 algae, Ulothrix, Ulva, and Cladophora. Small 

 numbers of the bay mussel were also found at the 

 surface level. At the middle depth (6.1 m) the 

 species composition of the fouling organisms 

 changed. Green algae became rare and diatoms 

 were less dense. Mytilus edulis, B. glandula, and 

 B. cariosus were absent. Balanus crenatus in- 

 creased in density with increasing depth at Skagit 

 Island, but not at the Kiket Island site. 



The 15.3-m level was very different from the two 

 upper levels. The plates had no algae or diatoms. 

 Balanus crenatus was the dominant species. Con- 

 sistently higher densities of 5. crenatus were ob- 

 served at the Skagit Island test site. The ratio of 

 densities between Skagit and Kiket Island for B. 

 crenatus at the 15.3-m level ranged as high as 50 to 

 1 for the wood and concrete test plates. 



FISHERY BULLETIN: VOL. 74, NO. 2 



DISCUSSION 



Marine fouling presents one of the most serious 

 long-term operational problems for power 

 generating stations using saline waters for cooling 

 (Powell 1933; Dobson 1946; Holmes 1970). Foul- 

 ing accumulations reduce the carrying capacity of 

 cooling system conduits by increasing the fric- 

 tional resistance and by reducing the pipeline 

 diameter. In addition, marine fouling reduces the 

 heat transfer efficiency of steam condenser sys- 

 tems and promotes severe corrosion of the con- 

 denser system components. The accumulation of 

 fouling debris, such as dead shells, adds to the 

 inefficiency by clogging the condenser tubes. 



Data are needed by design engineers in order to 

 determine the probable construction require- 

 ments for the control of fouling in a power plant 

 cooling system. Because marine fouling varies 

 considerably from one location to another, an on- 

 site determination of the population dynamics of 

 fouling organisms is desirable. Each site should be 

 studied in order to determine: 1) the species com- 

 position of sessile organisms colonizing specific 

 construction materials at various subtidal levels, 

 2) the types of construction materials least likely 

 to be fouled, 3) the seasonal variations in settle- 

 ment and abundance, and 4) the times of the year 

 when antifouling procedures must be considered. 

 The present study was, in a sense, an attempt to 

 study all these factors, and although the power 

 plant for which the study was intended may never 

 be built, this report should be a useful guide to 

 future studies of power plant siting. 



Data for the present study were collected from 

 test plates suspended at various depths in the wa- 

 ter. However, caution must be used in extrapolat- 

 ing studies carried out with these small static test 

 plates. Graham and Gay (1945) reported that 

 plates, 9.8 cm x 9.8 cm, were found to give results 

 just as reliable as larger ones. Holmes (pers. com- 

 mun.), however, considers that "edge effects and 

 top-to-bottom gradients could be very important 

 in biassing results from such small panels." Al- 

 though no effort was made in the present study to 

 determine the reliability of the small plates, a 

 3-cm border zone surrounding the 49-cm2 exami- 

 nation area was considered sufficient to eliminate 

 any edge effect. There was consistently less than 

 10% variation in the dry weights of the removable 

 material and in the density of barnacles taken 

 from different plates of the same media at the 

 same water level. 



382 



