Found this after passing the thermos around twice in a row - got a message about how there's already three or so dockhands fishing there. You feel bad for fishing there since it already so crowded but they were right - the fish are really biting. [paraphrased] Not sure if that is the normal message, or if passing around twice is necessary.
You acquire: Lakebottom rock, 3 fresh mollusks
Equip: Old comp. rod, high test, "good" fishing skill.
EDIT: I got a couple more later (and another rock) just at a regular fishing spot on the docks.