Mean current velocity'- of l8 cm. per second 



A current velocity of l8 cm. per second had diverse effects on the 

 seven size groups of clams tested. Clams of the smaller four size groups 

 (2-13 mm.) were dispersed fairly rapidly along the entire length of the 

 "flat". Movement was both by rolling and by sliding . At the end of 30 

 minutes, about 32,1 of the above size groups were in the sediment trap. 



Clams of the larger three size groups (lU-22mm.) showed a tendency to 

 jam together J orobably as a result of occupying more relative space on a 

 limited flat. They rolled more slowly than the smaller clams and remained 

 mostly along the upstream half of the "flat". Only 21% of the clams from 

 lii-22 mm, reached the sediment trap in 30 minutes : 



The graph in figure 3 indicates a burrowing activity sirrdlar to that 

 at 13 cm. per second (fig, 2), except the clams were somewhat slower in 

 starting to burrow in the 18 cm. per second current. A few clams of various 

 sizes which had obtained footholds veve finally dislodged after making pro- 

 longed efforts to burrow broadside to the current. Of the clams in the seven 

 size groups, only lj8,8:^ were able to burrow in 30 minutes^ vrhile 27=1^ 

 landed in the trao and 2l|..l/b remained en the flat but did not burrow. 



Mean current velocity of 25 cm. per second 



The effect of a current velocity'- of 2.5 cmc per second was severe on 

 all sizes of clams, movement being continuous and rapido The type of move- 

 ment was both rolling and sliding and was most rapid in the smaller size 

 groups, as in the other two current velocities. Indeed, many of the 2-ii mm. 

 clams spent part of their journey downstream out of contact with the flat, 

 their movement being a combination of rolling and jumping . Large numbers 

 of clams of all size groups were washed into the trap vjithin 1 or 2 minutes. 

 Within 5 minutes 89.ij.iS of the total number of clams were in the trap, and 

 at the end of 30 minutes 98;^ of the clams were in the trap. As in the 18 cm, 

 per second tests, the larger clams held one another stationary by clumping 

 together^ but in this case the "clumps" were washed into the trap en masse 

 in a very short time. The severity of the current was such that only 'l.,l% 

 of the total number of clams were able to burrow, 0.3^ remaining inactive 

 on the flat. 



Zero current velocit}'- (con troll 



The trials at zero current velocity were conducted the week following 

 the above trials. Although the clams were relatively slow in beginning to 

 burrow during the first 10 minutes, 68^ of those under lli mm, were burrowed 

 by the end of 20 minutes | at the end of 30 minutes %\i% were burrowed. The 

 lU-19 mm. clams burrowed at similar rates to those in the previous trials, 

 but the 20-22 mm. clams were somewhat slower. (Compare figures 2, 3, and 



