Sharon,
I don't know why this happens, but I've been plagued by this too. In my most recent village, I solved the algae puzzle and then all four of my farmers (which is a hefty portion of my population) continued to water the crops instead of fishing in the ocean. It was so frustrating because the food level was below 200 and the scientists would leave the table to go worry about food.

The only solution I have found is micro-managing and forcing the farmers to fish rather than water crops. After a little while doing this, I close the program and leave it four a couple of hours (on 2x speed). When I return, the food level has risen to over 1000.

Good luck!