"The fish sense something amiss". This really needs to go.

[Mwynn]

Proof? Like website that says it exists from SE or anything?

This is a Anti-Bot Measurement, stay too long on a single Spot and you get this Message.
Simply Teleporting to New-Gridania (Any Place) and just Fishing for a single second there allows you to go back to your Old Fishing Spot and continue.

It's a Feature that is completely useless by now, Bots tend to avoid this Message and just move a bit away from their Spot or go to another Fishing Hole.