For background, see here and here.
I missed the note in Lamb's translation of the Ion on the use of an ox horn in fishing. He says, "the nature of this device is still in dispute. Plutarch (De sollertia animal. 977) supports Aristotle's view that the horn acted as a sheath to protect the line from being bitten through by the fish."
The quotation in the Ion is from the Iliad 24.80: "Down sped she to the depths like a plummet of lead, the which, set upon the horn of an ox of the field, goeth down bearing death to the ravenous fishes" (trans. Murray). The word Murray translates as "plummet of lead" is molu/bdaina, which means a piece of lead and not, in case you were wondering (I was), something like a spear.
To review my interest in ancient fishing practices, in the Sophist, the visitor and Theaetetus agree to call the fishing done during the day by the name of "hooking" since all the fishing that is done during the day uses hooks. But that's strange, because people fish with nets during the day, and nets don't have hooks. So I am still wondering (1) what exactly fishing at night (with torches) looked like and (2) why the distinction between fishing at night and fishing during the day is where it is.