I had the same issue. Once your coconuts get down to miniscule levels, they'll stop climbing the trees and harvest the crops. I was pulling my hair out til then. You'll still get some with each crop, but it's not much. I found if I let them get the coconuts, as soon as it was to the "reserve" level, they went back to the crops.

If you want them to fish, you'll have to boost your population a bit & then make 'em fish til the algae arrives. (Of course, if you already have the algae, forget this.) Fortunately, when I finally got the algae, I wasn't far from the necessary level of tech to clear it up. Now, all they want to do is fish