Halfmoon Sea Chub
Medialuna californiensis

Physical Description

• Perch-like, football-shaped body, with a tail fin shaped like a half moon (hence their name).
• Have a rounded head with a small mouth and a blunt nose.
• Dark blue to blue-gray coloration on backs, fades to light blue/silver coloration on bellies.
• Offshore chubs are bluer, inshore more blue-gray in color.
• Dark spot just above the gill opening on sides.
• Scales are small, thick, and rough.

Range

• Vancouver Island, British Columbia to Gulf of California, Mexico.
• Most common south of Point Conception.

Habitat

• Commonly swim high in the water column.
• Found at depths of 9-130 feet.
• Live over rocks, kelp, or oil rigs.
• Very young adults live on outer fringes of kelp beds.

Reproduction

• Become mature at about 2 years old and 8 inches long.
• Eggs and sperm are broadcast spawned into the water.
• Spawning season is July through October.
• Fertilized eggs and early larvae float in the water column as plankton.

Diet

• Kelp, red algae, and green algae.
• Opportunistic carnivores that will eat sponges, anemones, and small invertebrates when they encounter them.

Predators

• California sea lions, seals, larger fish, marine birds, and bald eagles.

Interesting Facts

• Their coloration allows them to camouflage with the surrounding open water. Their silver bellies blend into the surface reflection as lower fish look up at them, and they are bluer when they are offshore in bluer water. 
• Because they feed on kelp, their presence in kelp forests helps control kelp growth, and they are important to the health of the overall forest.

Sources: Aquarium of the Pacific

Photo: Matthew Meier