Fishing

[Legbreaker]

They're not farms, they're backyard vegetable plots!

100k seems very excessive to me, but as has been posted, it's possible I think provided the inhabitants aren't picky about what they eat. Rats, dogs, cats, pigeons, you name it would all see duty on the dinner plate.
Anywhere something can grow, somebody is almost certainly trying to grow something. Even in dank, dark cellars! (mushrooms)
Of course we can't forget parks, vacant blocks, rooftops and so on. Hydroponic systems are likely to be fairly widespread and especially useful for the winter months if set up indoors. Indoor (even outdoor) swimming pools could be converted into fish ponds (food, not gold).

So, I don't think food production is likely to be on a large scale, but just about everyone will be involved in it somehow. Yes, fishing the surrounding waters is very likely, but I believe that would represent a very small percentage of the total.