This topic came up some time ago - probably under "Saltwater". It would be worth doing a search. From memory the person who was most convincing on that thread reckoned that you should boil them for only 10 minutes instead of 15 and then freeze them. Thaw them and boil them for an added 5 minutes before you want to eat them. I've forgotten the rationale, but I decided at the time that was what I would do if I caught so many that I couldn't get them eaten in a few days. This would only happen if I went crabbing (successfully) every day