LEMONPEEL ANGELFISH
Centropyge flavissima (Cuvier, 1831)
   As far as we know, this common Indo-Pacific angelfish has never established itself naturally in the Hawaiian Islands. It is often imported for the aquarium trade, however, and individuals were apparently released off Oahu in the 1990s in spots such as Keehi Lagoon, Kaneohe Bay, and the Ala Wai Canal. Rare sightings continue to be made, some from as far away as the Big Island. It is possible, therefore, that released Lemonpeel Angels are now reproducing in low numbers in Hawaii. On the other hand, ichthyologist Henry W. Fowler reported the species as a stray in Hawaiian waters in 1928, long before the aquarium trade was established; perhap these rarely-seen angels drift in as larvae from time to time, sometimes surviving to adulthood. To about 5 in. Photo: Kona, Hawaii, Robyn Smith.

NAHACKY'S ANGELFISH
Centropyge nahackyi Kosaki, 1989
   In 1987, Randall Kosaki discovered this gorgeous fish at Johnston Atoll, where it lives at depths of 80 ft. or more on gently sloping outer reef areas devoid of ledges or other vertical relief. It is dark blue-brown with a bright yellow tail base, abdomen and head. The top of the head (nape) is marked with iridescent blue bars interspersed with black. In 1988, fish collector Anthony Nahacky caught a single individual of the as-yet unnamed species off Honaunau, Hawai`i, at a depth of about 115 ft. He kept it in his aquarium for some time before donating it to science. Nahacky's find was almost certainly a stray (as was another individual found at South Point, Hawaii Island, in 2013, shown above). Kosaki named the fish after Nahackyi in honor of his numerous contributions to our knowledge of Indo-Pacific fishes. The species is closely related to C. multicolor (pictured below), an angelfish common in the Marshalls and other central and south Pacific island groups. Curiously, a stray C. multicolor was also once captured along the Kona coast of the Big Island. To about 3 in. Nachakyi's Angelfish is endemic to Johnston Atoll.

EMPEROR ANGELFISH
Pomacanthus imperator (Bloch, 1787)
  Stu Ganz and Bill Stohler both sent me photos of this Emperor Angelfish, seen by each them individually on May 9, 2010, in a swim-through cave at Sharks Cove, Pupukea, O`ahu. The species is native to coral reefs from Africa to the Line Islands and French Polynesia, but not to Hawai`i. Given that it was seen at one of O`ahu's most popular dive sites, the fish is almost certainly an aquarium release. There is, of course, the more remote possibility that it got here naturally. Such things have happened. In 1948, before the advent of the commercial aquarium trade, an adult was captured in a trap off Ewa at a depth of 90 ft. But there is no evidence that a breeding population has ever existed in Hawai`i.
   If this Pupukea fish arrived naturally, it would likely have grown up in the area where it was photographed. And if a juvenile Emperor Angelfish (even more spectacular than the adult) had been present last summer, it would certainly have been noticed and photographed by dozens of divers, and the word would have certainly got out. Of course, in the unlikely event that it did grow up here unnoticed, it is questionable whether such a fish could survived the onslaught of this winter's waves.
  As a matter of interest, another Emperor Angelfish lived for some years in Honolulu Harbor, near the Falls of Clyde and could easily be seen from shore.
  Other species from outside Hawai‘i, such as the
Lemonpeel Angelfish (Centropyge flavissimus), Semicircle Angelfish (Pomacanthus semicirculatus), and Regal Angelfish (Pygoplites diacanthus), are occasionally released by irresponsible aquarists or fish importers, but are not known to be reproducing in Hawaiian waters.

UPDATE from Dave Lum, June 2014
I was browsing through some websites on marine life this evening and came across the page on your site that described an Emperor Angelfish living in a swim-through at Shark’s Cove (Pupukea). I believe that I saw this exact same fish this year (21 June 2014), while I was exploring a long lava tube at a depth of about 40ft just to the north of the mouth of Shark’s Cove. The fish was about 8” in length without a streamer on its dorsal fin. It was surprisingly tame and followed me through most of my tour of the tube, which was quite dark and occupied by many nocturnal species. After exiting the lava tube on the other side of the structure, I swam back to the entrance where again I was met by the imperator. According to your site, it was last reported in 2010, so it’s somehow managed to survive in this habitat for since the last report.