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9 November 2001
A Dredge Net for Studying Aquatic Life
by Shawn Carlson
Orginally published in The Amateur Scientist, September 1996.To examine bottom-dwelling aquatic life, you'll need a dredge net. Purchase 54 inches of three-quarter-inch diameter plastic polyvinyl chloride (PVC) pipe from a plumbing supplier and cut it into three 18-inchlengths. Next, you'll need nylon fish netting. Some fabric stores and bait-and-tackle shops carry it(look under "Netting" in the Yellow Pages). The size of the mesh is not critical. I buy three-quarter-inch web (diamond-shaped netting) for 32 cents per square foot. It comes in bolts 20 feet wide by 300 feet long, although you will need only a 60- by 60-inch square. Retailers are usually happy to trim it. From the square, cut out an equilateral triangle 60 inches to each side.
Using an old paintbrush, liberally coat one of the PVC pipes with one-hour epoxy. Place the pipe at one base of the triangular netting and carefully draw in the netting along both sides, leaving clear one inch of pipe at either end. Roll the netting around the pipe twice. Stitch the netting in place with a couple of twist ties so the pipe won't unroll. Then hang the pipe over some old newspapers and pour on more of the one-hour epoxy, thoroughly covering the pipe.
Split lengthwise three 14-inch sections of garden hose. These protect the net while it is being dragged. Slip one of the split lengths of hose over the pipe. Hose clamps will clasp the assembly tight while the epoxy sets, but they are a pain to attach. I prefer to smother the assembly under plastic trash bags filled with sand. Repeat the same procedure with the two other pipes, rolling them up on the other sides of the triangular net. You will end up with a dredge net about 28 inches deep and 18 inches to aside. Next, fill one of the pipes with sand and cap the ends with cotton wadding soaked in epoxy. This weighted side drags along the bottom.
Now you need to link the pipes together to form a rigid frame. From a plumbing supply store, purchase a short length of one-half-inch flexible (L soft) copper tubing and six unthreaded bell reducers. They are fittings that join two different size pipes-in this case, they should connect three-quarter-inch pipe to one-half-inch pipe. Cut a two-and-a-half-inch section off the copper tubing and thread two bell reducers over the ends of the cut piece so the fittings are separated by about a half inch. Epoxy the bell reducers into place with low-viscosity aluminized epoxy-available from Devconin Danvers, Mass.; call (508) 777-1100 for the nearest distributor. Before gluing, be sure to roughen the ends of the tube and the inside surfaces of the bell reducers with coarse sandpaper.
Once secured, bend the tubing to form a 60-degree angle. You can make the bent tube rigid by filling it completely with epoxy and letting it set. Use a knife to score the ends of two adjacent sections of pipe and epoxy the bell reducers over the ends (again, use aluminized epoxy). Repeat the entire procedure twice to finish the rest of the frame. Paint the copper tubings to prevent corrosion.
The dragline completes the assembly. Tie eight inches of nylon cord to each point where a bell reducer meets a PVC pipe, then tie the opposite ends to form three pairs. Melt these ends together with a soldering iron. Tie these three points with nylon cord so they come together about two and a half feet in front of the assembly. Finally, tie this point off to at least 100 feet of nylon line. Make sure to adjust the cords so that the opening of the net tips backward about 10 degrees when dragged.
