FISHERY BULLETIN: VOL. 77. NO. 4 



ence of adults may influence or be associated with 

 spat growth and survival. 



METHODS 



In 1976 a study site was chosen in southern 

 Puget Sound on a narrow estuary called Little 

 Skookum Inlet (Figure 1). The site was on inter- 

 tidal land from which Manila clams were com- 

 mercially harvested. There was easy access to the 

 beach, but because of its private ownership, the 

 probability of people tampering with the experi- 

 mental plots was low. 



Plot Construction 



To study the possible influence of adult clams 

 on the growth and survival of spat, I constructed 

 plots in which I experimentally manipulated the 

 beach material to yield different concentrations 

 of adult clams. Each plot contained four treat- 

 ments, as follows: 



Treatment 1. No adult clams and new sub- 

 strate. All beach material was removed to a 

 depth of 15 cm to ensure removal of adult clams. 

 Gravel from the high intertidal ( +3 m) was then 

 carried down to fill the excavated hole to the 

 existing beach level. 



Treatment 2. No adult clams and old sub- 

 strate. All beach material was removed to a 

 depth of 15 cm and then sifted through a 12 mm 

 mesh screen to remove the adult clams. All 

 screened material and any large rocks retained 

 on the screen were then returned to the excavated 

 hole and enough additional screened material 

 was added from residue of Treatment 1 to bring 

 the treatment level to the same height as the 

 beach. 



Treatment 3 . Moderate adult clam density (con- 

 trol). A shovel blade was inserted vertically, 15 

 cm into the substrate on the edge of the treat- 

 ment. The handle was pulled back and forth 3 or 4 

 times until the surface above the blade was 

 loosened. The shovel blade was inserted and agi- 

 tated around the perimeter of the treatment until 

 the entire surface area of the plot had been dis- 

 turbed. No clams were added to or subtracted 

 from the treatment, and no counts of naturally 

 occurring clams in the treatment were made prior 

 to the settling experiments. 



Treatment 4. High adult clam density. This 

 treatment was disturbed in the same manner as 



SEATTLE 



kilometers 



meters 



Figure l. — Diagrammatic representation of the study site for 

 the Manila clam on Little Skookum Inlet in southern Puget 

 Sound, Wa.sh., on a minus tide (MLLWI. Plots A, II, and III were 

 established in July 1976. Plots R, S. and T were established in 

 July 1977. Plots A, R,S, and T were 2  2 m squares divided into 

 16 sections each and Plots II and III each contained four 1 .5 x 1 .5 

 m sections in a row. 



Treatment 3. In addition, enough adult clams be- 

 tween 2.0 and 5.0 cm in length were placed on the 

 surface of the treatment to create a surface density 

 of approximately 480/m^. This density was the 

 number of clams necessary to completely cover the 

 surface area. 



In all cases, the day following the construction 

 of Treatment 4, no clams remained on the beach 

 surface, and a large number of new siphon holes 

 were apparent. In no case were any adult clams 

 found in Treatments 1 or 2, and it was thus 

 assumed that the clams added to Treatment 4 

 buried within the treatment and did not move to 

 adjacent treatments or outside the plot. I as- 



892 



