I would simplify -and have done in previous sessions -
1. try to deduce number of boats and crew available
2. split into 1 or 2 caregories
cean /deep sea and the other coastal
3. set a base value of kg fish that they pull in pr workday and multiply with number of boats
4.roll a dice to determine if its a bad or good month and modify total catch acccordingly
5. roll a dice to gleen the number of people eating bad stuff and getting sick
6.new month /week -repeat
7.obviously fishfarms and shell fish breeding grounds will be another matter ,but for simplicity just follow the above procedure and change names ,places and capacity .
GM tip -be stricter rather than lenient to keep your players from creating a surplus that takes the fear of hunger out of them .
As for other means of feeding people ratfarms,hogfarms,rabbitfarms,mushrooms in the dung cellars,every available pot and bucket filled with soil and potatoted ( stacks of old tires can be used to this end as well )
Insects are very protein rich and I guess cockroaches could be cultivated in a sense .
(yeah..I know..but it is after the nuclear holocaust after all )
look up potato farming -it doesnt require the expertise of most agriculture and it demands little in the way of infrastructure.Good dirt,dung and water will do most of it . Ask Graebearde for number of square feet needed pr person with agricultural program.(Look up the farming thread -his stuff is good ).
give each "farm" or "fishery" a monthly or weekly value and add/subtract after the "RESULT " roll is done .
Lack of fuel or foul weather etc will be bad , but PCs managing to conquer a traincar with fertilizers from marauders will be good etc .
Just suggestions of course .