Halfmoon, Medialuna californiensis

Halfmoon, Medialuna californiensis, Juvenile. Fish caught from within the Avalon Marina, San Clemente Island, Avalon, California, December 2022. Length: 15 cm (5.9 inches). Catch, photograph and identification courtesy of  Luke Ovgard, Klamath Falls, Oregon.

Halfmoon, Medialuna californiensis. Fish caught from coastal waters off Catalina Island, California, October 2015. Length: 33 cm (13 inches). Catch, photograph and identification courtesy of Ben Cantrell, Peoria, Illinois.

Halfmoon, Medialuna californiensis. Fish caught out from coastal waters off Catalina Island, California, October 2015. Length: 35 cm (14 inches). Catch, photograph and identification courtesy of Chris Wheaton, Fullerton, California.

Halfmoon, Medialuna californiensis. Fish courtesy of Dan, F/V Sustainable Fisheries, San Diego, California, November 2014. Length: 38 cm (15 inches).

Halfmoon, Juvenile, Medialuna californiensis. Underwater photograph taken in coastal waters off San Diego, California, May 2014.  Photograph and identification courtesy of Bob Hillis, Ivins, Utah.

The Halfmoon, Medialuna californiensis, is a member of the Sea Chub or Kyphosidae Family, and is known in Mexico as chopa medialuna. Globally, there are two species in the genus Medialluna, of which one is found in Mexican waters, this species from the Pacific Ocean.

The Halfmoon has an elongated oval compressed body with a depth that is 36% to 40% of standard length. They are dark blue dorsally and transition to paler blue ventrally. Some have indistinct short bars on their sides. They have a dark blotch on the upper corner of their gill covers, a key to identification. Their anal, caudal, and dorsal fins are gray with dark gray outer margins. They have a small rounded head, a blunt nose, and a small mouth that opens at the front with one row of pointed teeth. Their anal fins have 3 elevated spines and 17 to 21 rays; their caudal fin (after which they are named) is lunate; their dorsal fin is continuous with 9 or 10 spines and 22 to 27 rays; their pectoral are short; and, their pelvic fins are short and originate behind the pectoral fins. Their body is covered with small, thick, and rough scales.

The Halfmoon is a found either demersal or mid-water in small loose schools with other fish at depths up to 44 m (145 feet). They reach a maximum of 48 cm (19 inches) in length. As of January 1, 2024, the International Game Fish Association world record stood at 0.82 kg (1 lb 13 oz) with the fish caught from coastal waters off Catalina Island, California in July 2013. They consume algae, sponges, seaweed, shrimp, mussels, squid, and small fish. In turn they are preyed upon by sea lions, seals, larger fish, and marine birds. Many live in kelp forests and are considered to be important for controlling kelp growth, however, they can also become a problem for commercial kelp farmers as they can destroy crops. Reproduction is external with the release of pelagic eggs. They have life spans of up to eight years. The Halfmoon is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, growth, longevity, movement patterns, diet, habitat use, and reproduction.

The Halfmoon is a resident of Mexican waters of the Pacific Ocean but has a limited distribution being found along the entire west coast of the Baja and along the east coast of Baja from Loreto south to Cabo San Lucas, Baja California Sur.

The Halfmoon is similar in size and shape to the Lowfin Chub, Kyphosus vaigiensis, the Cortez Chub, Kyphosus elegans, the Opaleye, Girella nigricans, the Gulf Opaleye, Girella simplicidens, and the Zebraperch, Hermosilla azurea, but none of these have the blue coloration with the dark spot over the gill cover.

From a conservation perspective the Halfmoon is currently considered to be of Least Concern with stable, widely distributed populations. They are a target of recreational fishermen commonly taken off Southern California piers adjacent to rocks, reefs, and kelp structures on fresh mussels or small crabs providing a good foe when hooked. They are considered to be a quality food fish and are caught as a by-catch of commercial fishermen and marketed fresh.