having the lowest ratio of areas of mesh opening 

 to mouth opening, and thus the lowest potential 

 for efficient straining; and (3) the filtration of 

 the liner mouth opening of trawl No, 5. 



Eight flow meters were mounted within the 

 pipe frame and one meter was mounted in the 

 cod end of the same trawl and net (trawl No. 1) 

 used in the flume. The eight meters were dis- 

 tributed peripherally and centrally within the 

 mouth opening of the trawl to sample variations 

 in straining by the net. Upstream and down- 

 stream tows were timed over a measured dis- 

 tance within a narrow estuarine channel and the 

 results of the two tows were averaged to adjust 

 for tidal currents. The experiment was repeated 

 using a No. 2 trawl with a liner mesh opening 

 of 2 mm, but flow meters could not be mounted 

 within the liner mouths. 



In constructing trawl No. 5, 1 made the dimen- 

 sions of the depressor, of exterior net and liner 

 equivalent to the mouth opening of the liner 

 in trawl No. 4. Both the exterior net and the 

 liner extended back from the trawl mouth. Six 

 flow meters were dispersed within the mouth of 

 the liner, one in the cod end, and two were 

 mounted within a small frame outside of the net 

 mouth. In this instance efficiency was consid- 

 ered to be the percentage of the average distance 

 of the upstream and downstream tows recorded 

 by flow meters inside and outside the liner. 



I did not weight the velocities recorded in the 

 field according to sectors of the mouth area 

 metered in the net and liner as in the case of 

 flume experiments. In the flume variations in 

 flow were largely vertical, but in the estuary 

 large lateral variations were known to occur. 

 The number of flow meters necessary to make 

 such integrations might have been sufficient to 

 physically affect the flow through the net. A 

 mean velocity recorded by the meters mounted 

 within the mouths of the net and liner was used. 



Results and Discussion 



The No. 1 trawl was highly efficient at the 

 flume velocities tested. The comparison of the 

 velocities measured in the nets to the flume ve- 

 locities (Table 2) gave an efficiency of approx- 

 imately 100%. Efficiency in the cod end approx- 



Table 2. — Flume and metered velocities for the Boothbay 

 Depressor Trawl No. 1. 



Net mouth 



Liner mouth 



Cod end 



—  cm/sec — 

 Metered velocity 205 309 

 Flume velocity 208 308 



— • — cm/ sec — • — cm/ sec 



156 238 314 156 238 309 314 

 156 231 317 126 200 248 262 



Table 3. — Vessel and metered velocities of the Boothbay 

 Depressor Traw^l No. 1 and 2. 



Table 4. — Metered distances of tow for Boothbay De- 

 pressor Trawl No. 5. 



Liner mouth 



Cod end 



Outside 

 Inside 



856 

 864 



763 

 821 



854 

 601 



763 

 763 



imated 83%. The release of dye and the align- 

 ment of tabs showed that the flow of water could 

 be traced horizontally through the net and the 

 liner and that this flow was not diverted by the 

 blade. Moving picture films of the trawl during 

 the test are available from the author. 



The results from field trials are similar to 

 those obtained in the flume (Tables 3 and 4). 

 Flow efficiencies at the net mouths of the pipe 

 frame and at the liner mouth were comparable 

 to those obtained in the flume. Efficiencies ob- 

 tained in the cod end of the nets fluctuated 

 around those (80-84%) from the flume. These 

 results suggested that the depressor blade and 

 the hanging of the liner within a larger net did 

 not affect filtration efficiency. The efficiencies 

 were independent of the velocity of towing and 

 the ratio of the mesh to mouth openings, as would 

 be expected with nets that were longer than 

 twice the diameter of their mouth openings 

 (Tranter and Heron, 1967). Further, it is un- 

 likely that the efficiencies would decrease during 

 sampling. Observations in the flume showed 

 that the net and liner would be cleansed con- 

 stantly during a tow by vigorous peristalsis of 



519 



