煮頃鮒

Originally a compound of 似 (ni, “resembles”) +‎ 五郎鮒 (gorōbuna), short for 源五郎鮒 (gengorōbuna).

Variously spelled in kanji as 煮頃鮒 or 似五郎鮒. 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 源五郎鮒 (gengorōbuna), a closely related species also endemic to the lake.

• (Tokyo) にごろぶ​な [nìgóróbúꜜnà] (Nakadaka – [4])[1]
• IPA^(key): [ɲ̟iɡo̞ɾo̞bɯ̟ᵝna̠]

煮(に)頃(ごろ)鮒(ぶな) or 煮頃鮒(ニゴロブナ) • (nigorobuna) 

1. nigorobuna (Carassius auratus grandoculis), a type of 鮒 (funa, “crucian carp”), an edible freshwater ray-finned fish related to carp and goldfish, endemic to Lake Biwa in Japan

As with many terms that name organisms, this term is often spelled in katakana, especially in biological contexts (where katakana is customary), as ニゴロブナ. When written in kanji, the 煮頃鮒 spelling is more common.

• 1981, Hitsudai Hitomi, d. 1701, edited by Hatsuko Yoshii and Shimada Isao, 本朝食鑑 [Honchō Shokkan]‎, volume 5 (snippet), Heibonsha, ASIN B000J80JPO, page 267:

  又此魚ノ大ナルモノ、一尺二三寸位ナルモノハ源五郎鮒二似タリテ、故ニ似五郎ト呼ブト云フ説ヲ優レリトス

  (please add an English translation of this quotation)