When this happens I usually have to look it up online, and discover it's in a completely different place. For example, Razor Clams. According to the Fish Guide, it's prime location is Lower La Noscea. According to the Fishing Log, it's not actually IN Lower La Noscea, it's in Western La Noscea, Skull Valley (this is were the guild leve turn-in NPC is, so that makes sense). After fishing here with different baits for like half an hour, I looked it up online and found that it was actually fished up in Crescent Cove, Western Thanlan. So I went there, and it was a very common thing to fish up with Rat's Tail bait.

I guess the point I'm trying to make is, the Fishing Log and the Fish Guide seem to contradict each other and are rarely actually helpful. Compared to the other logs in the game such as for gathering which shows you exactly where to find the item and what type of tree/bush/rock/etc.

Thanks for reading~