A - manilo tow line 



B - woter hose 



C - haul bock cable 



D ■ tow bar 



E - sled runner 



F - water m onifold 



G - knife 



H - rear cage 



I - chain bog 



Figure 2.--Diagramatic sketch of a jet dredge 



Sampling Stations and Procedure 



Since specific knowledge of the location 

 and distribution of the hard clam beds in 

 Nantucket Sound was not available a grid 

 pattern of sampling stations was employed. 

 The stations were located 1 /2 mile apart 

 north and south and 1 mile apart east and 

 west. During the time limit of the vessel 

 charter, 214 samples were taken. The 

 sample stations were distributed through- 

 out most of the area where clams were 

 believed to exist and where the dredge could 

 be operated. Commercial fishermen in- 

 dicated that many parts of the Sound were 

 unsuitable for jet dredge operations be- 

 cause currents are fast and bottoms hard. 

 These areas were therefore eliminated from 

 the survey. Echo sounder traces showing 

 the bottom topography and charts showing 

 the bottom composition and current speeds 

 were also used to delimit unsuitable dredg- 

 ing areas. 



To obtain comparable results sampling 

 tows of 20 minutes duration were made at 

 each station. The area sampled was ap- 

 proximately 4 feet wide and 2,640 feet long 

 or about 10,500 square feet. Loran bearings 

 to deternnine position and echo sounder 



traces for depth and bottom topography 

 were taken at each station. Otherwise, the 

 jet dredge was operated according to stand- 

 ard commercial practices. The cutting 

 blade of the dredge was set at 5 1/2 inches 

 throughout the survey. 



Since the sannpling tows were based 

 upon the dredge's being on the bottom 20 

 minutes and since the standard unit of catch 

 employed by the fishermen is the numbers 

 of bushels of clams caught per 1 -hour tow, 

 it is necessary to convert the sample catches 

 to amounts comparable to those obtained in 

 the fishery. The fishermen customarily tow 

 the dredge on the bottom for 50 minutes 

 and the rennaining 10 minutes of the 1 -hour 

 tow is employed in raising, dumping, and 

 resetting the dredge. Sampling tows were 

 converted into bushels per commercial 1 - 

 hour tows by the following calculations: 

 the number of hard clams caught at each 

 sample station was nnultiplied by 2 1/2 to 

 convert to 50 minutes fishing time and then 

 divided by 83, the average number of clanns 

 of the sizes caught in a bushel. The term 

 bushels per 1 -hour tow will be used through- 

 out this paper to mean bushels per com- 

 mercial 1 -hour tow. 



