Raft-Mounted Water Dredge 



The water dredge has several components. 

 The power unit consists of a 5.1 -cm. self- 

 priming centrifugal pump coupled directly to a 

 2-1/2-hp. 4-cycle gasoline engine. The engine 

 and pump are housed in a shallow waterproof 

 box with a removable top. The pump unit and 

 box are mounted on a twin sponson raft that is 

 1.8 m. long overall, 81.3 cm. wide, and 8.9 

 cm. deep. The individual sponsons, 1.8 m. 

 long, 30.5 cm. wide, and 8.9 cm. deep, are 

 hollow and made of 0.6-cm. plywood. Trans- 

 verse 2.5- by 15.2- by 91.4-cm. wooden 

 stringers tie the two sponsons together. 



A venturi tube (fig. 3) is mounted in the 

 forward portion of the raft between the two 

 plywood sponsons. The tube housing is con- 

 structed of two 45.7-cm. sections of 7.6-cm. 

 outside diameter aluminum irrigation pipe, 

 welded to form a 150° elbow. A moulded 

 plastic water jet nozzle is flange -mounted 

 inside a 15.2-cm. section of 5.1-cm. outside 

 diameter irrigation pipe. The nozzle assembly 

 is aligned and welded over a 5.1-cm. access 

 hole in the housing so that the water jet is 

 directed down the center of the elbow section, 

 which leads to the collecting bag. 



All pipe fittings are rigid 3.8-cm. PVC 

 (polyvinylchloride); union fittings at the intake 

 and discharge lines facilitate removal of the 

 pump unit for repairs (fig. 1). 



Coarse 0.6- by 0.6-cm. galvanized hardware 

 cloth is stapled across the center space on the 

 underside of the raft. This screening prevents 

 debris from clogging the finely screened water 

 intake in the after part of the raft. 



Hoses are used to link the sled, venturi 

 tube, and collecting bag. A 7.6-cm. inside 

 diameter flexible vinyl hose leads from the 

 suction head on the sled to the suction portion 

 of the venturi tube (fig. 1). Another length 

 of 7.6-cm. diameter hose leads from the dis- 

 charge end of the venturi, aft down the center 



TO PUMP — ► 



SCALE 



I 1 



?6cm 



Figure 3. — Schematic drawing of the venturi tube. 



of the raft into the collecting bag. The reusable 

 collecting bag, 60 cm. long and 30 cm. wide, 

 is made of 2.0-mm. bar mesh nylon netting 

 and has a drawstring at the top. Hose and 

 collecting bag connections are made with 

 10.2-cm. stainless steel hose clamps. The 

 raft-mounted water dredge weighs 43.6kg. 



SAMPLING PROCEDURE 



After the pump is started, the sled is drawn 

 across the substrate at a constant speed by 

 means of the hand-operated winch (fig. 4). 

 Penetration of the substrate is deepest at the 

 slowest speed. Pump, venturi tube, intake and 

 discharge lines, and collecting bag, which are 

 supported by the raft to facilitate mobility, 

 are drawn behind the sled by the sampling 

 hose. As the sled moves, marine organisms 

 and loose substrate straddled by the inner sled 

 runners are drawn into the suction head and 

 discharged into the collecting bag. The mesh 

 size of the bag can be varied, depending upon 

 the size of the organisms to be retained. 

 Sediment and organisms finer than the mesh 

 size of the bag are flushed through it. The 

 collecting bag with its contents is removed, 

 labelled, sealed, and placed in preservative. 

 The entire unit can be disassembled quickly 

 and transported between stations on a 5-m. 

 boat, although we found a 6-m. boat more 

 satisfactory (fig. 5). 



Although the sampler operates effectively 

 in deeper water, use of the device has been 

 generally limited to a depth of 1 m. or less, 

 where young pink shrimp are numerous enough 

 to provide a density index. The shrimp often 

 are concentrated in marine vegetation near the 

 low-tide mark in a band parallel to the shore- 

 line. Because this zone of concentration may 

 be either narrow or transitory in respect to 

 water level, season, or other factors, samples 

 taken parallel to shore do not always give a 

 realistic measure of shrimp abundance per unit 

 area at a station. For a more accurate measure, 

 we draw the sampler perpendicular to shore. 

 The sled's inner runners limit the width ofthe 

 swath to 8.0 cm., so that when the sled is 

 drawn 12.5 m. across the bottom, 1 m. 2 of 

 substrate is sampled. Possible environmental 

 changes caused by repeated sampling are 

 minimized since only a narrow strip of the 

 bottom is disturbed. 



The sampler may be used for several types 

 of collecting. We used it primarily as de- 

 scribed above with the suction head mounted 

 on a sled so that organisms may be sampled 

 from an elongated section of bottom. As 

 previously mentioned, Brett (1964) inserted 

 a free suction head into a cylinder which en- 

 circled a prescribed area of substrate. Suction 

 then removed all organisms and plants. The 

 suction head can be removed from the sled 

 and used in vacuum cleaner fashion to sweep 



