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Dive Dry with Dr. Bill

#368: The "Catalina Blue Perch"

I occasionally hear a local fish referred to as the Catalina perch or the Catalina blue perch. Now "perch" is probably a much over-used word since so many fish are referred to as appearing "perch like." The fish people are talking about in our waters is known as the halfmoon or Medialuna californiensis to us scientific types. The genus (first) part of the scientific name comes from the Spanish words for half moon and refers to the shape of the tail. This species actually belongs to the sea chub family, which has over 40 species world-wide. Other local representatives of this family include the opaleye or button-back "perch" and the zebra "perch." Why not button-back sea chub and zebra sea chub I wonder?

The halfmoon has a body that is slate blue or blue on the back with a silvery belly. The upper rear part of the gill cover or operculum often has a darkish spot. They may also exhibit dusky bars on the side of their body. As suggested, the tail is slightly indented. The mouth is small and located at the very front of the head. Each jaw has a single role of pointed teeth. Of course none match my magnificent canines!

These fish are found geographically from Vancouver Island, Canada, to southern Baja and the Gulf of California; but are only abundant from Pt. Conception south. They favor areas with good vertical relief such as rocky reefs, kelp forests and oil platforms. They may also be found associated with drifting kelp rafts out at sea. Halfmoon have been observed as deep as 130 feet, but they are most common in the 10 to 65 foot range.

Halfmoon reach a maximum length of about 19 inches, but are generally between 6 and 16 inches long.. They are a schooling species. Individuals are sexually mature between 7-8 inches. Halfmoon spawn between April and October. Females are oviparous, and the eggs are cast out into the water. Here they develop into pelagic or open water larvae that drift with the currents. Despite being a nearshore species, the larvae may be found as far as 300 miles offshore. Dr. Milton Love states that the heaviest concentrations of larvae are off northern Baja California. This suggests that region may have the optimal temperature and other ecological conditions for reproduction.

"Catalina blue perch" probably feed primarily during the day. When I see them at night they are solitary, so I assume the daytime schools break up. Entrees on their menu are somewhat varied. They are known to feed on algae, sponges, and small invertebrates including bryozoa, molluscs, worms and club anemones. This makes them omnivores like myself! Well, actually I skip the worms and bryozoa. Schools of halfmoon have impacted attempts to re-establish giant kelp forests because they will take frequent bites of the blades, reducing the kelp's ability to photosynthesize and grow. This may be to eat the alga itself, or the invertebrates that grow on it. Predators above them in the "mutual eating society" include California sea lions, northern fur seals, Brandt's cormorants and even bald eagles. I would imagine their bright silvery color would make them an easy target for the eagles to spot.

Halfmoon are a fairly common target of sport fishers, both from party boats and private vessels. When they are taken by commercial fishermen, they are often marketed as... what else but "perch." Although I've never eaten a halfmoon, I've heard that their flesh is excellent. Personally, I prefer they keep it on their bones rather than in my stomach... in part due to my poor cooking skills. Native Americans in our region must have been less picky or better cooks as halfmoon bones are found in their "kitchen middens" (refuse pits or garbage dumps).

I have noticed a few interesting behaviors while observing halfmoon. They seem to have a strong interest in chains and rope that move up and down with the swell and surge, such as those attached to the drop down buoys in the dive park. They swim around these, often in a figure eight fashion, and rub their bodies against them as they pass by. This seems to be a mechanism to scratch their itches. As for the parasites that might cause these itches, halfmoon frequently gather near the ocean floor where they are cleaned largely by male and female rock wrasse which pick the parasites and dead tissue off their bodies. Years ago I was told rock wrasse are not really a cleaning species like their senorita relatives in the wrasse family. Well, apparently no one told the rock wrasse that!

Another behavior involving cleaning that totally shocked me was observed while diving over the offshore pinnacle at Little Farnsworth. I had dived deep that day and was holding on to the anchor line at about 25 feet to complete my safety stop. An ocean sunfish or Mola mola swam into sight, probably to take a look at the strange bubble blower invading its realm. What truly fascinated me was that a halfmoon swam about the much larger ocean sunfish, inspecting its body and occasionally picking things off it. I had never heard of adult halfmoon serving as cleaners for other fish species. I guess they were "paying it forward" after being cleaned by the rock wrasse!

Earlier this decade when I had just begun filming underwater, I observed and filmed halfmoon behavior I'd never seen before... or since. I was finishing my dive, off-gassing in shallow water before returning to Terra firma or dry land. As I hovered and watched, two halfmoon literally locked lips and began mouth fighting! They gyrated about and swam around in tandem for about 10-15 seconds. I had two interpretations of this unusual behavior. One, they were indeed mouth fighting in a fashion similar to sheephead, but actually locking their mouths together rather than using ritualized open mouth displays like the wrasse. Since halfmoon are not territorial, this would seem highly unusual. The other possibility was that the two had simultaneously grabbed the same tidbit of food and were fighting one another for it. I thought of the day a former girlfriend and I were eating spaghetti together, and both of us tried to slurp up the same noodle! Unfortunately, when I captured the video footage to process it on my computer, the camera had somehow been set to a contrast level that made it difficult to actually see what was happening. Like the behavior itself, my camcorder has never done that since. Go figure.

© 2009 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 and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Individual halfmoon or "Catalina blue perch" and a school; halfmoon rubbing against mooring line
and cleaning an ocean sunfish.

This document maintained by Dr. Bill Bushing.
Material and images © 2009 Star Thrower Educational Multimedia