The description for this fish reads:

This voracious freshwater fish, once indigenous to the island, has long since found its way into Thavnair's water sources.
But the fishing log entry says,

Once presented to the satrap as a gift, this non-native wavekin is a rare find in Thavnair. Its propagating prowess, which in other lands presents a problem, is mitigated by its seeming inability to acclimatize to Thavnairian waters.
The implication here seems to be that the Bluegill is not indigenous to Thavnair. The description seems to be a mistake.