I'd hardly call it a _bug_. The farmers' AI is not perfect, and it is hoped to improve it in a later release. Farmers tend to spend to much time harvesting coconuts if available, and also in watering the crops even when fish are still available. These are not life-threatening, though. The important thing is to start growing crops as soon as possible, so it you can build up a cushion of food to see you through the times when the crops are not available. It is also important to stop the birds from eating the seed and to be able to prepare a restorative stew to help your villagers if they do take starvation damage. As Terina says, make sure the village doesn't exceed about 15 villagers at first.
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To err is human; to arr is pirate.