http://news.bbc.co.uk/2/hi/science/nature/4581428.stm

You can see what happens if you walk on the same stretch of lawn overtime you wear a path and everything is dead. I would imagine it is the same down there.

Depends on the type of bottom. Sand bottoms recover quickly, gravel and rocks sustain a lot more damage.

Mike makes good points:
Sand in shallow water recovers quickly.
Sand with strong tidal currents recovers quickly
Thats why clam dredges look mean as far as impacts to the bottom, but most of it occurs in under 60ft of water and the bottom is 'resurfaced' fairly quickly by wave action, especially during storm events
Sand in deeper water; i.e. on the shelf that is deeper than wave and tidal currents, I would be less apt to say that it recovers quickly.

Softer (muddy) bottoms will show the evidence of trawling (door scars) and some evidence that there is effect from the rollers as well. Some of the effects can be fairly long lasting. This is more of a 'digging' effect. In some areas fairly deep and visible scars persist from the doors of the net dragging along the bottom. These are visible to the naked eye, i.e. diver or video and are VERY visible in some places on Side sonar.

There is less digging and more scraping on the gravel and cobble bottom (i.g. Georges) so that tends to get hit hard in terms of macro-algae and critters living right on the bottom.