I haven't done any ex-pisce collecting in recent years, but there was
a time when I was actively engaged in both recreational deep sea
fishing and conchology.  During that period I gained a dubious
reputation among the party boat regulars as the guy who collects fish
guts.  After a day of hauling in cod, pollack, flounder, and other
assorted ichthyological specimens, some of the fishermen would clean
(actually just gut, or pre-clean) their catch during the hour and a
half trip back to the pier.  The fish-cleaning group would sit at the
stern, and standard procedure was to throw the entrails to the swarm
of gulls which would appear out of nowhere and follow in the boat's
wake, looking for goodies.  Or to toss a fish liver into the air and
see if the gulls could catch it on the fly.  Until I arrived on the
scene.  I didn't clean my catch until I got home.  I started
examining the stomach contents, more out of curiosity than anything
else - and I started finding things.  There were many shells in
perfect condition.  In one cod I found a dozen perfect Calliostoma
bairdi, a species I had never seen until then.  Almost all the
gastropods still had their opercs in place (the whole animal was
present, not just the shell).  Well once I made this discovery, I
couldn't bear to watch the fishermen throwing fish stomachs to the
gulls, knowing that each little package was full of conchological
treasures.  So I started bringing along a one gallon wide-mouth
plastic jar.  I would sit over in one corner of the stern, and as the
guys gutted the fish, they would just slide everything over to me.  I
would put the stomachs in the jar, then send the rest off to the
waiting gulls.  When I got home, I would sometimes deal with the
material immediately, or if I was too tired, just put the jar in the
refrigerator, much to my wife's delight.  When it came time to
harvest my conchological catch, I would cut open each stomach with
scissors, and flush the entire contents into a plastic pail of water.
 Unlike the material Harry described, this material was very fresh,
and the smell was no worse than fish from the supermarket.  I would
rinse the pooled material by filling the bucket with water and
pouring it off several times.  A lot of fine organic matter with no
shell to weigh it down would be rinsed away.  Then I'd pick out the
obvious unwanted material, mostly fish, crabs, and shrimp.
Occasionally there would be squid or a small lobster.  At this point,
there would be nothing larger than a half inch or so in the mix.  You
can give the matertial a final rinse and proceed to pick through it
at this point.  However, I found another method that worked much
better, especially for retrieving the smaller shells.  I would pour
the whole mess into a regular kitchen screen strainer, to get rid of
the water.  At first, I tried saving the water to check for shells
that might go through the screen, but I never found any.  These were
fairly large fish, and presumably they didn't feed on mollusks under
a few millimeters in size.  Presumably smaller mollusks could be
obtained from small fish, but I haven't explored that possibility.
After removing the water, I would return the washed material to the
gallon jar, and fill the jar with alcohol (one time the lady at the
pharmacy asked me if I ran a massage parlor when I bought a 24-bottle
case of rubbing alcohol).  After a couple of days in alcohol,  the
material could be dried outdoors in the sun.  Once thoroughly dried,
it was nearly odorless, and could be picked through much more
conveniently than the fresh, wet material.  Also, it could be sifted
through screens of different meshes, to separate the smaller
material, which could then be examined under the stereoscope or other
magnifier.
The type of fish and its feeding habits are of course the best
indicator of whether the stomach is likely to contain mollusks.  You
won't find them in tuna, marlin, striped bass, bluefish, etc.  They
are not bottom feeders, and they eat mostly fish and squid.  One
local fish here, the tautog or blackfish, feeds on mollusks and
crabs, but it crushes them in its powerful jaws before swallowing
them.  The best sources here in the northeastern USA are cod,
pollack, hake, porgies, and flounder.  These species eat fish as well
as mollusks, but they are bottom feeders, and usually contain a good
number of mollusks.  Tomcod and cunners are two smaller varieties
that are also good.  I have found some good material in eels a couple
of times.  There are other bottom-dwellers that eat mollusks, like
sculpins, toadfish, sea robins, puffers, batfish, skates, etc.
(collectively known as "trash fish" by commercial fishermen).
However, these are not suitable for human consumption, and I don't
like to kill them just for their stomach contents, so I usually
release them.  Hmmm, I wonder if laxatives would work?
Shells from fish stomachs are usually in good shape (provided they
were swallowed whole) because fish stomachs are not highly acidic
like most mammalian stomachs.  Enzymes are highly pH dependent, and
mammalian gastric enzymes like pepsin only work at strongly acidic pH
levels.  However, fish digestive enzymes are designed to work at much
higher pH (that is, much lower acidity).  Shells in the stomach are
of course the freshest, but good shells can often be found even
farther down the GI tract.  They may not look quite as good as
stomach-collected specimens though, and the opercs may be lost since
the soft tissues of the mollusk are pretty well digested away by that
time.
Paul M.