Dredging on grassy beds with a small, lightweight dredge is not very
productive, because the dredge simply bends the grass forward and slides
over it.  The only shells you are likely to get with a dredge on a grass
bottom are small snails that may cling to the grass blades, well above the
bottom.  You rarely get a shell that was on the substrate among the bases of
the plants, because the dredge simply can't get down to that level.
Consequently, the dredge does very little damage to the grass beds.  I
usually avoid grass beds though, for the reasons already stated, unless I am
specifically after a species that commonly climbs up the grass, or lies on
top of it, like some Pectens.
On a rocky bottom, a small dredge is again unlikely to take many mollusks,
or to do much damage (except to itself).  A small dredge works best on a
fairly smooth sand or fine gravel bottom, where it does a minimum of damage,
and can take quite a few species.  Muddy bottoms are a bit more difficult to
dredge, but here too a fair number of species can often be taken.

Paul M.