Dive Dry with Dr. Bill

007: Halfmoon or Catalina Blue Perch

A fairly common and subtly beautiful fish in our kelp beds is the halfmoon. This fish is also commonly known as the Catalina perch or Catalina blue perch, even though it is not a member of the perch family. Their color ranges from a very light to a dark, slate blue. Halfmoon get their common and scientific (Medialuna) names from the halfmoon shape of their tails.

Although found as far north as Vancouver Island, they are common from Pt. Conception south into the Gulf of California (Sea of Cortez). Their preferred habitat is over rocky reefs and kelp forests. Halfmoon are sexually mature at just over seven inches, and large ones reach 19 inches.

Based on my observations underwater, halfmoon feed on algae and a range of small invertebrates. Scientists report stomach contents are largely kelp and other seaweeds. Halfmoon are known to cause significant damage to some kelp forests by feeding on the kelp and attached forms. In turn they are eaten by sea lions, cormorants and in our waters, by the bald eagle. Party boats from the mainland also take them in significant numbers.

Halfmoon are inshore fish, living close to the coast where the kelp forests and rocky reefs provide appropriate habitat. It is interesting to speculate on how nearshore fish or invertebrates get to offshore islands like Catalina. Such species do not normally cross open oceans where protection from predators and food are limited. Would you swim "26 Miles" (or 19.7 or...) across the Channel to Catalina on a whim? Neither do they.

Deep water, open ocean stretches like the San Pedro Channel create barriers not only for land plants and animals, but also for many species of marine life. In fact, some common mainland fish and invertebrates never make it out to offshore islands like Catalina. So how do the ones that arrive in our waters make it?

Halfmoon, like many other nearshore species, cast their eggs into the water from April to October. Here the eggs develop in the plankton and hatch into larvae. It is these larvae, drifting with the currents, that allow the halfmoon to colonize offshore habitats like Catalina. These young stages can be quite abundant in the ocean, and are found as far as 300 miles from shore.

Based on my scientific research at Toyon Bay in the late 60's and 70's, another way halfmoon and similar nearshore species arrive on islands is by accompanying drifting mats of detached kelp. During that period I sampled more than 100 drifting kelp "rafts" in the open ocean and found a wide range of fish and invertebrates on them. Unlike halfmoon, a number of these species had no drifting eggs or larvae which meant "hitchhiking" on kelp might be an important means of dispersal for them.

Fishermen often report seeing nearshore fish like halfmoon drifting with these mats in the Channel (as well as yellowtail and other predators feeding on them!). Some kelp rafts I collected had drifted more than 100 miles and probably much more, and a Harvard associate of mine reported a similar journey of possibly 7,000 miles in the Southern Ocean!

© 2003 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Image caption: Group of halfmoon in Dive Park waiting to be cleaned by rock wrasse

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia