We have similar issues and it is a pain. The new version of BRC requires you to investigate for lost knives so IMO it's best not to issue one to every person, otherwise, how do you know when it's been lost?
I think I might have been the person who said sometimes you just have to get to the point of saying "JFDI". Seriously though, here are some suggestions and it will take hard work and support from your operational collegues.
A magnetic or "peg" board so the knives have a location where they should be stored.
A sign in and out sheet for knives located with the board which includes a check on the blades for damage at the start and end of shift (it doesn't matter that there's no-one to sign them out from).
Training against this procedure including the reasons why (metal hazards are a real risk, as a PP said, it's amazing how far foreign bodies can travel, I would not be assured by the crabs having shells!) also include the fact that non conformities on audit = lower grade = less business = less work. Ensure the training is documented.
Regularly audit for compliance. Allow two strikes, i.e. find anyone not following the procedure, rebrief all staff and remind them off the rules, sign them off against this brief. Find the second person, rebrief all staff, ensure they sign off against the brief stating if you find anyone failing to follow the rules again there will be disciplinary action. Third time, take that disciplinary action. Ensure you have operational and HR support for this before you start, there is nothing worse than threatening to be tough and failing to follow through. It will be easy to identify who hasn't done it, as you audit, find people with knives and see if they've signed them out. Simples. You will also then have three separate occasions where the person has signed to say they will follow the rules so it will be very cut and dried in disciplinary terms. Tough? Yes but it will only take 4 instances in total I think and you will have an operations team who are (grudgingly) complying.
I agree it's a pain but as you said, technical isn't about being popular.