Wait, what? That doesn't make sense. Why would the plantation mimigas only eat fish?
I think that, just like we don't see any non-teleporter entrances to the village even though there logically has to be some, we are supposed to assume that the reservoir is in fact a larger body of water than the one we can see, and it's only the 2D nature of our viewpoint that makes it look so small. After all, the thing has to support enough fish that at least one of the mimigas can make a living fishing. Chinfish is just the only sufficiently large fish for Quote to notice that's in the section we can see.
This is even more apparent in the "Original Sin" mod, in which a guy is fishing what looks like a puddle compared to the reservoir with no chinfish, yet after you come back from saving whatshisname he claims that he caught a big fish while you were gone. It's just a limited perspective, that's all.