Or perhaps the drop rate is keyed to something, like the fishing skill of the character who first looks in the crate? (I mean, the ability to find fish would be of considerably more use to someone with a low fishing score as opposed to a character whose fishing is almost maxed out, wouldn't it?)
I tested this recently. My main already has 'find fish' and spent around 1hr or so fishing in feralas and wetlands. Fished out about 15 chests, 6 of which when mailed to alt actually had book.
Find Fish · Find Herbs · Find Minerals
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The best way to provide journals to others is to have one toon fish up the crates, and mail them UNOPENED to another toon who has fishing but not find fish. He opens the trunks, but does NOT LOOT them. This allows you to find the trunks with the journals in them, and mail them off or bark them in trade.
If I had my choice, I mean, I know I crank more to find fish when I'm looking for them, and most times it's a square bill. And the time of year and kind of the position the fish, that's another thing I really try to figure out as quick possible is what stage they're in. If they are in post spawn, or if they're in late post spawn, or early post spawn, if they're in summer pattern, and that tells me what size baits they're going to want, if they're going to want a big bait, or a smaller bait, a fast bait, a slow bait, or a suspended bait.