Anyone who has ever deployed a menhaden purse seine knows that it does not drag across the bottom of the sea floor, which is one of the reasons MSC certified this fishery.  The bottom of the purse seine is actually taken underneath the school of fish and brought up to the top of the water to create the 'bowl' effect that captures the fish so they cannot escape.  Crews control the depth of the bottom of the seine by keeping strain on the bottom purse line and by the depth they lower the 'tom' weight.  No fisherman would drag the net across the bottom because he would quickly snag everything on the bottom and either tear his net or prevent him from recovering it at all.  Having been associated with this business for many years, I have never seen a crab, oyster, clam, rock, or anything else associated with the sea floor captured in a menhaden purse seine net.  Additionally, the offloading equipment, as well as the menhaden processing equipment ashore is not designed for hard objects and would be destroyed if such items were included in the catch.  It is understandable that people who are not familiar with purse seining may not understand how they do not engage the sea floor, but this is a proven technique that has been deployed effectively in the Chesapeake Bay for nearly 150 years.