In the Sophist (220), the visitor from Elea and Theaetetus try to come up with a kind of definition of what an angler is. To do this they use the method of division, which is nothing more than taking a class of things and then dividing it in two by some criterion. For example, they divide hunting into hunting of land animals and hunting of swimming animals. Then they divide hunting of swimming animals into fishing and fowling. Then they divide fishing into fishing by netting and fishing by striking (with a spear or hook).
At this point in the division they make what seems to me to be a curious division. They divide fishing by striking (which includes both angling and spearing, since the spears (tridents) have hooks on them) into fishing at night and fishing during day. This division seems out of place. Why didn't it come prior to the division into fishing with hooks and fishing with nets. Can't both of them be done either at night or in the day?
Kenneth Sayre offers a suggestion about this division. "To the modern fisherman [this is] an unfamiliar form of angling, undoubtedly. A quotation from Homer at Ion 538d removes the puzzlement of why angling must be performed in daylight" (Plato's Late Ontology, 308n40).
Here's the quotation from Homer in the Ion: "And she passed to the bottom like a lead sinker which, set on a horn from an ox of the field, goes in haste to bring mischief among the ravenous fishes" (trans. Lamb). From what I can gather from this quotation, a fisherman would probably cut a small hole in the tip of an ox horn, put a line through the hole, attach a lead sinker to the end of the line, drop the horn in the water, and when a fish swam into the horn, he would pull the horn out of the water with the fish trapped in the horn. Clearly, this could be best (or only) done during the day since the fisherman would need to be able to see the fish swimming into the horn.
If this is correct, then Sayre's explanation cannot be right because, Theaetetus and the visitor having gone with the daytime division, the visitor clearly states that the kind of angling they're considering is "done with a hook" and hooks not "just any part of the fish's body but always [its] head" (220e-221a, trans. White). This description does not fit the one given in the Ion, and I remain puzzled both about what nocturnal fishing in ancient Greece was like and the placement in the Sophist of the daytime/nighttime division.
One completely unrelated point: when I was writing this I noticed that it was natural to write the phrase "fishing at night," but when I started to write the corresponding phrase "fishing at day" I noticed it didn't sound right. So I changed it to "fishing during day." I could also have said "fishing in day." Curious that modern English will allow "at night" but not "at day."