Does this happen when you've inadvertently cross-mated too many of your villagers, or is this a food related stat?

Seriously, how do I get my folks to catch the crabs? Do I have to wait until I have a zillion tech points for level 3 farming?

As your farmers improve, they will catch more crabs. That is how your food supply goes up quickly.

Since the crabs live in the sea, your farmers have to know how to fish in order to catch them.

I noticed that generally my Master Farmer catch the giant red crabs.

So, I do need Level 3 Harvesting to catch the crabs -- the ones that live in the sea, I mean

Yes. You can't fish without level 3 harvesting. It would be a nice variety if the fishers could catch lobster, or swordfish, but no such luck. I'm thinking the villagers eat a lot of sushi, because I have yet to see anyone make a fire. Spicy tuna rolls all around!

Well if that's the case, I hope they're planting a lot of soy beans (for soy sauce) and wasabi! Now that the Sims can fish, I do notice that the Masters (in anything) get it a lot faster and catch more fish than the trainees.

For fun and to see what would happen, I've dragged a few of them to the lagoon to see what kind of fish are in there. None so far...

Make sure you have level 3 harvesting before tossing them into the lagoon

I do. But I've tossed then in there before. Perhaps I'm too hasty - I have a tendency to drag and drop like the Russians were in Milwaukee (so now you know were I hale from).