Paul, that was a very informative reply that must have taken a bit of effort.  Thanks.  You apparently have spent considerable time dredging and sorting the samples or hauls.

I have one suggestion:  Tie a rope to the end of the bag about twice the depth of the water and then attach a float to the free end.  When the dredge hangs up, as it almost always does, detach the tow line from the boat, attach a float to it and throw the float overboard.  Now retrieve the float atached to the bag line and then pull the dredge a short distance in the direction opposite to the tow.  That almost always pulls the dredge free without tearing the net much, if at all.  If you decide to continue the tow, retrieve the tow line and set a course perpendicular to the original direction.

Another suggestion: Make a set of screens that can be nested so that the innermost screen is the largest.  I use 1", 1/2" hardware cloth or sometimes 1/4" and 1/8" if I've targeted small animals. 

1.  Cut the bottoms out of galvanized pails leaving about a 3/4" ledge.  2.  Cut out circles of hardware cloth the diameter of the bottom of the bucket.  They should be snug when you press them down into the bucket but not so tight that they warp.  3.  Use sheet metal screws through washers to fix the screen to lip around the bottom. I usually attach them at least 8 points.

You'll need another container at least as deep as your stack of sieves and a little wider.  Fill it about full of water.  Place your stack of sieves into that container, fill it about 1/2 full of dredged material.  Lift out the inside sieve leaving the other(s) behind.  Rinse the material by shoving the sieve straight down into the other sieve fast enough to shake the material loose and then lift the sieve so the water will wash out the mud and sand - but raise sieve so that the bottom is no higher than the lip of the next sieve.  You might lose some of your smaller shells if they don't fall into the next sieve.  Repeat until most of the mud and sand has been washed out.  NEVER "TWIST" THE HANDLE TO AGITATE THE MATERIAL - WHATEVER GETS BETWEEN THE SEDIMENT AND THE SCREEN WILL BE DESTROYED.  When I was doing a clam survey on some local beaches, we would twist the bucket as the water ran out to speed up the process.  For the first few days, I was mystified that we were finding only a very few juvenile clams.  Finally I figured out what was happening to them when one of my workers asked me to identify a small mass of shredded flesh with a few bits of attached shell.  As soon as we stopped the twisting, we began to find the smaller animals. 

This might sound tedious but you can sort through washed material very quickly with a much higher recovery rate since the animals won't be covered with mud. 

I use two screens usually because the large material slows down the process when I leave it in with the rest of the material.  Anyway, why wash the same material over and over.

As soon as the material in the first sieve looks clean, give it so someone at the sorting board so they can get started.  Then sieve out the next screen and hand it off.  If you have no plans for the material left in the large container, dump it, restack the sieve, reload with fresh dredge material and start over.

Keep us posted on your progress this summer!  Sounds very interesting