I noticed this issue myself, that my farmers wanted to harvest the coconuts before anything else. By the time another child came of age to farm, the coconuts were already being hogged by my first two farmers. My new farmer was perfectly happy to fish though, and leave the coconuts to the first two. Even with the farm and algae, the farmers went for coconuts as soon as they had grown. Now that I'm post-algae, the farmers seem to actually want to fish again, but the coconut-before-farm logic seems a little skewed.