FILTRATION EFFICIENCIES OF 

 BOOTHBAY DEPRESSOR TRAWLS 



Boothbay Depressor Trawls (Figure 1) are used 

 to obtain yearly estimates of the abundance of 

 larval herring, Clupea harengiis harengus Lin- 

 naeus, in the coastal water of the Gulf of Maine. 

 The Boothbay Depressor trawl is a relatively 

 new device (Graham and Vaughan, 1966), and 

 its use is contemplated by others. It differs 

 from other nets towed for collecting larval fishes 

 in that a large depressor blade is located below 

 the mouth opening and a liner is hung in a larger 

 meshed net some distance from the mouth. 

 These features of construction were examined 

 to determine whether they affected the filtration 

 of water through the trawls. Flow determina- 

 tions were made about the depressor blade and 

 trawl mouth, and the flow of water through the 

 liner mouth was compared to the ambient flow. 



Table 1. — Descriptions of liners used with Boothbay 

 Depressor Trawls. 



Figure 1. — Boothbay Depressor Trawl No. 1. Insert 

 shows wire cod end. 



Methods 



Nets with a mesh opening of 3.2 cm stretched 

 measure were lashed to pipe frames of 1 X 

 1.5 m, 1 X 2 m, 1.25 X 3 m, and 1.5 X 4 m 

 (height X width), trawl No. 5, 1, 4, and 2, re- 

 spectively. These large meshed exterior nets 



served to hang and protect the small-meshed 

 liners (Table 1) of nylon webbing which retain 

 the larvae. The diameter of the mouth opening 

 for a liner was selected empirically. It was first 

 hung throughout the length of its funnel-shaped 

 exterior net. To reduce friction the liner was 

 progressively shortened toward the cod end until 

 the ratio of wire out to the depth sampled ap- 

 proached 3:1, during repeated tows at 4 to 6 

 knots. Construction details for the trawls and 

 their respective nets are available from the 

 author. 



The filtration efficiency of a No. 1 trawl con- 

 taining a liner with a mesh opening of 4 mm was 

 determined in a flume (circulating water chan- 

 nel) at the U.S. Navy's David Taylor Model 

 Basin under experimental conditions as de- 

 scribed by Mahnken and Jossi (1967). Veloci- 

 ties in the mouths of the pipe frame and the liner 

 were measured by lowering a flow meter at in- 

 tervals equivalent to its diameter and recording 

 the velocity for each interval with a remote elec- 

 tronic counter. These velocities were then 

 weighted by the area of each interval to obtain 

 an average value for the entire mouth area. 

 Also, a meter was fixed in the center of the for- 

 ward opening of the wire cod end. Filtration 

 efficiency was measured as the velocity recorded 

 in the net divided by the velocity recorded in the 

 flume; the quotient was expressed as a percent- 

 age. Dye was released within the mouths of 

 the net and liner, and tabs were fixed to the blade 

 and nets to trace the direction of the flow of 

 water. 



Several tests were made in the field to com- 

 pare with the results from the flume. Field ob- 

 servations were carried out on (1) the same 

 trawl used in the flume; (2) the trawl No. 2 



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