*Gyojin-tō* (魚人島) = literal Japanese 'Fish-Man Island' — 魚 *gyo/sakana* (fish) + 人 *jin/hito* (person) + 島 *tō* (island). The name describes the inhabitants directly. Located underwater beneath the Red Line at the gateway to the New World; the descriptive name matches the fish-man race's matter-of-fact biological/anatomical naming (cf. *Merman / Mermaid* in English). Frequently parsed as evoking Atlantis-style submerged-civilization myths.
First mentioned: Ch. 69 page 7 (Luffy: “Fish-man Island in the Grand Line is famous, isn’t it?” / Sanji: “I’ve heard there are beautiful mermaids there.”).
Famous (Luffy’s qualifier — even in East Blue it’s known by reputation).
Inhabited by fish-men (Arlong’s, Jinbei’s, Hatchan’s homeland).
Mermaids live there (Sanji’s “beautiful mermaids” line — first canonization).
First contemporary on-page arrival (Ch. 608)
On-page events
Oden-era flashback — the Roger Pirates visit (Ch. 967)
The Roger Pirates visited ~24 years ago; young King Neptune (“Neptune the Knight”) read the Sea-Forest poneglyph (Joy Boy’s apology) and 3-year-old Sharley delivered the Poseidon / Mermaid-Princess prophecy (pp. 6–10). The seditious mermaid Queen Otohime is named as alive and active (p. 8). See Ancient Weapons, Joy Boy Identity.
The island faces destruction as Decken re-aims the colossal ark Noah to crash onto it; the chapter ends “Fish-Man Island… is in terrible danger!!!” (p.19).
Reverie prelude — the Neptune family sets sail (Ch. 903)
Fish-Man Island is the post-time-skip first-arc location (Ch. 612-653) — an underwater bubble-domed kingdom at the midpoint of the Red Line. Houses the Ryugu Royal Family, the Ancient Weapon Poseidon (Shirahoshi), and the Sun Pirates / Hody Jones political history.
SBS reveals
Mermaid-biology install (Ch. 424)
Course locked onto Fish-Man Island (Ch. 435)
Reached but unreachable — the descent problem (Ch. 490)
The seafloor route + geography fix (Ch. 496)
Mermaid princess + coating requirement (Ch. 497)
The coated crews set sail (Ch. 504)
The Straw Hats’ next destination (Ch. 507)
Jinbei’s two-year appointment (Ch. 597)
The Straw Hats begin the dive (Ch. 602)
Reference + heading only.
The descent begins (Ch. 603)
The downwelling-current descent begins (Ch. 604)
The deep-sea approach detailed (Ch. 605)
The deep-sea approach route (Ch. 606)
First present-day depiction (Ch. 607)
The Kingdom of Ryugu — arrival on the island proper (Ch. 609)
Coral Hill, Big Mom’s protectorate, the Vander Decken legend (Ch. 610)
The arc-proper continues — Pappag’s store, the king, the supremacist threat (Ch. 611)
The Kingdom of Ryugu seat — the Sun Tree Eve revealed (Ch. 612)
The New Fish-Man Pirates’ invasion begins (Ch. 615)