Very nice Slraep, enjoy your booby traps. On a duller note, couldn't they simply cover the mussel beds with a net, that when tampered with, sounded an alarm at the police station. Or have the beds watched?
About the pearly mussel in question--- Margaritifera margaritifera.
Here's a fairly old, but very good article. According to it, just a little over 10 years ago, in 1996, things were looking optimistic for the mussels of the South River Esk. What a difference today.
I think it must have been similar to how many of the US rivers were fished out. I don't remember reading anything so detailed as that article, but that it was often a summer's weekend activity for the family, I think before the 40's, though.
I remember reading an old cook book by M.F.K. Fisher, a famous foodie, where she writes about the extrordinary abundance of shellfish and other seafood from Chesapeake Bay, the largest estuary in the US. She could never have imagined what it is like now. A big toxic dump of an estuary with even the crab having a had time surviving. She would have been incredulous. It was not that long ago.