When Zhuangzi sees the minnows that "swim about so freely, following the openings wherever they take them", he concludes, "Such is the happiness of fish." (17; Ziporyn) Assuming that we have established that he has reason enough to make this observation, still we might ask if he is not anthropomorphizing a bit to ascribe to anything fish do as an expression of their happiness. The problem may be, however, that we tend to anthropomorphize everything humans (anthropos) do, which is to say, we take the happiness of humans as the standard by which every happiness is to be judged.
It is true, of course, that human happiness is quite different than any other that we imagine in as much as it is self-aware and thereby experientially intensified, but then perhaps might it not be better to understand human happiness as only one expression of a happiness that pertains to all things? In this case, though human happiness might be thought the most intense, and therefore 'highest' happiness, we would have to say it is also the most tenuous and unreliable. No doubt fish have their bad days, but if doing what fish do is their happiness, then we would have to admit that they are a great deal happier than most humans. The consequence of this capability for intensified happiness is also a capability for intensified unhappiness.
Are rocks happy? If being what one is is happiness, then I think we can say they are not only happy, but consistently a great deal happier than humans. But they don't know they are happy, so it is a very 'low' form of happiness. Yes, but they do not need to know they are happy, and this is the reason for their happiness.
When Shen Dao said: "Just become like an inanimate object. There is no need for worthies and sages. Indeed, a clump of earth never strays from the Dao" (33), this was in part what he had in mind. The human experience has greatly complicated the essential happiness of all things in simply being what they are. We require to be something other than what we are, a perfect recipe for unhappiness. Even the pursuit of happiness makes us unhappy, just as unhappiness is the reason for the pursuit. Perhaps the Daoist metaphor of returning to "the unhewn block" speaks to this possibility of just being happy being, rather than in being strung out in becoming.
All this would seem to require a trustful release into Mystery so that every eventuality is affirmed and accepted.
You can check out Scott's writings on Zhuangzi here.