煮頃鮒

Etymology
Originally a compound of, short for.

Variously spelled in kanji as or. The 煮頃 spelling ("perfect doneness when boiling") for nigoro is likely ateji. Meanwhile, the 似五郎鮒 spelling points to the likely derivation: this spelling parses out to "resembling (gen)gorō-buna", and as the nigorobuna grows to sizes of 30cm or longer, it begins to resemble the, a closely related species also endemic to the lake.

Noun

 * 1) nigorobuna, a type of , an edible freshwater ray-finned fish related to carp and goldfish, endemic to Lake Biwa in Japan

Usage notes
When written in kanji, the 煮頃鮒 spelling is more common.