Hi Dan,

Following is the text of an article I wrote for American Conchologist
several years back. Perhaps it will help.  Momentarily I'll also post a
followup article I wrote in response to some questions generated by the
original article ......

EXTEND YOUR REACH BEYOND THE BEACH - DREDGING FOR SHELLS

     What collector, while turning rocks during a minus tide, hasn't gazed
wistfully seaward, and imagined the treasures that might reward his efforts
if only the tide would ebb another ten feet - another fifty feet!  And then,
with a sigh of resignation, accepted the inevitable intertidal boundaries
imposed by Mother Nature, and overturned another rock.  More than a few
collectors though, have taken up the challenge, and learned to extend their
reach beyond the beach, either by taking up scuba diving, or by
experimenting with dredging.  The notation "dredged at 30 fathoms" on a
specimen data slip often invokes images of large commercial vessels
outfitted with complex and expensive apparatus, and we might thereby assume
that the deep water shells offered by dealers are necessarily products of
such operations.  Sometimes they are;  but, an individual with a small boat
and a few dollars worth of simple equipment can successfully collect shells
from depths well over 30 fathoms.

     Shells have been successfully dredged by dragging an old section of
sewer pipe or a metal pail behind a boat.  However, a more typical and far
more efficient dredge consists of a simple metal frame, behind which is
attached a bag of netting or metal screen, extending back 3 to 4 feet.
Three strips of heavy gauge flat steel stock, 2 to 3 inches wide, can be
welded together at the ends, to form an equilateral triangle.  Some
collectors prefer a rectangular frame.  In this case, one pair of parallel
sides should be three to four times as long as the other pair of parallel
sides.  When the rectangular dredge is on the ocean bottom, either long side
may lie on the substrate.  In the case of the triangular model, any one of
the three sides may make contact.  The leading edge of all contact sides
should be bent downward 20 to 30 degrees, forming a lip which will bite into
the sediment.  The trailing edges of ALL sides are drilled with a series of
1/4" holes, about an inch apart, to facilitate attachment of the bag.  The
mouth of the dredge need not be, and in most cases should not be very large.
A 15 to 18 inch contact edge is plenty, and even a 10 inch dredge can pick
up a surprising number of shells.  A large dredge combined with a small boat
is a recipe for trouble.  An oversize dredge will collect material until it
is too heavy to tow.  At that point it is no longer a dredge, but an anchor,
and quite possibly too massive an anchor to haul back up to the boat.  You
then have two options - cut it loose and go home, or don't go home.

     The dredge bag, in which the specimens accumulate, should be formed of
rigid screen rather than soft netting, as the latter often snags on
underwater projections, resulting in tearing or loss of the dredge.  Mesh
size is very important.  The larger the mesh, the more shells will be lost
through it.  The smaller the mesh, the more likely it is to fill with
sediment, rendering it both ineffective and extremely heavy.  Ideally, most
of the sediment should pass through the screen, while most of the shells are
retained.  Heavy galvanized steel screen with a mesh of 1/4 inch is good for
general use.  However, if you want to collect the smallest shells, a finer
mesh may be required.  The parts of the bag are cut out with metal shears,
and "stitched" together with wire.  A conical bag for the triangular dredge
is made by piecing together three long screen triangles.  For the
rectangular frame, an elongate rectangle of screen is bent around and
attached to both long edges of the frame, with two side pieces then cut to
fit (see accompanying illustration).  The bag is attached to the frame by
looping wire through the mesh of the screen, and through the drilled holes
in the frame.

     A solid steel ring is secured to each corner of the frame, and to each
ring is attached a slender steel rod or strap, approximately four feet long.
Rope or chain can be substituted, but the dredge maintains optimum
orientation better when these "leads" are rigid.  The free ends of all four
leads (three in the triangular model) are attached to a single large steel
ring.  A 4 to 6 foot length of heavy chain is also attached to the ring.
This weight is necessary to ensure that the cutting edge of the dredge
maintains contact with the substrate.  The tow line is attached to the other
end of this chain.  Don't skimp on your tow line - remember, if it breaks,
all is lost.  Cheap cotton clothesline just won't do.  Good quality manila
or nylon is preferable.  The rule of thumb is: tow line length = 3X water
depth.  To dredge successfully in 100 feet of water, you will need 300 feet
of line.  If you don't have a fathometer to calculate the depth, use a
sawed-off salt water fishing rod and reel, with a 1 pound lead weight, and
braided nylon line, marked with colored fluorescent paint at regular
intervals - a different color for each increment of 5 fathoms for instance.

     Any boat can be used for dredging, provided it is adequate for safe
navigation of the waters you will be working, and the dredge size is matched
to the boat size.  Even a skiff with oars can bring up some treasures of the
deep, if the collector has sufficient personal stamina.  However, dredging
requires some physical exertion, even without the additional work of rowing,
so a motor is vastly preferable.  A larger motor will get you to the dredge
site faster, but it is no great advantage during actual dredging, because
you must dredge at a very slow speed.  If you go too fast, the dredge will
lift off the bottom and skim along above the sediment instead of biting into
it.  With a little experience, you will be able to tell whether the dredge
is on or above the bottom, and even the type of bottom sediment, by feeling
the vibrations of the tow line with your hand.

     After dragging for a few hundred yards or so, it's time to retrieve the
dredge.  This is where most of the physical labor is involved.  If your
dredge is not excessively large, and is not filled with rocks, you can pull
it up by hand, but it's a lot of work, and a tow line of less than a half
inch diameter doesn't provide much of a handhold.  A small, hand operated
mechanical winch is a big help here.  If your boat and your pocketbook are
large enough, a small electric winch could be used.  Emptying a dredge into
a boat can make an awful mess, and trying to pick through material on the
deck is exhausting and tough on the back.  The solution is a sorting board.
This is a rigid wood plank, 1 to 2 feet wide (depending on the size of the
boat and dredge) which spans the entire width of the boat, and overhangs
each of the boat's sides by several inches.  A block is attached to the
underside at each end, to rest against the inner surface of the boat side,
preventing the board from sliding.  Each of the long sides has a vertical
lip, a smaller board about 2 inches high, extending the full length of the
sorting board.  The two ends overhanging the water are open.  The dredge is
dumped onto the sorting board, allowing people to sit or stand (depending on
the size of the boat) along the board, and pick through the dredged material
in relative comfort.  Uninteresting material is simply pushed off the open
end of the sorting board, back into the water.  Rubber gloves should be
worn, to protect against possible spines, stings, etc.  The dredge will
seldom contain just shells.  A typical haul is likely to include a mixture
of mollusks, crustaceans, echinoderms, coelenterates, annelids, fish, and
other organisms, as well as vegetation, rocks and assorted trash.  The
suspense over what the dredge may contain is of course part of the
excitement of collecting by this method.

     Dredging isn't just for deep water.  You can snorkel over a sand or mud
bottom a few feet deep, and not see a thing; yet a small dredge towed along
the same path may yield many fine specimens which were concealed just below
the surface of the sediment.  Dredging in such shallow water doesn't even
require a boat!  The dredge can be towed by hand.  Make a large loop in the
tow rope, to fit around your waist.  Or, tie the rope to the center of a
stout pole, and two people can push on the pole together, like a team of
oxen.

     Some dredging trips have, of course, been far more rewarding than
others.  However, I have never come back empty-handed, and the intrigue of
seeking out the elusive creatures of the deep, and bringing back something
new for my collection which I could never have found by any other method,
has always made for an enjoyable and exciting outing.

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