I'm no dredger, so I don't know all of the mechanics of working an ocean bottom dredge, BUT, wouldn't it make sense to put a single quarter inch steel crossbar over the end of the dredge tube to block any rock that is over half the diameter of the flexible tubing so as it doesn't get jammed with an over size rock?
Just makes sense to me as there are no huge gold nuggets being found.
If I remember right, the dredges they used during the summer had two bars in an X shape to prevent exactly that kind of thing. And yes, it would make sense to do so.
And if they should happen to find a gold nugget too big to fit into the opening, I think they would make the effort to pick it up manually anyway. So it really isn't like they would lose anything.
My guess is; then they would have to work harder. Not only would they have to remove big rocks from the end of the nozzle, but now they would have to remove smaller rocks -- which normally would have passed through.