Dive Dry with Dr. Bill

045: Scythe Butterflyfish

Conditions were, shall we say, less than optimal at the East End Quarry when I dived there recently on the Catalina Divers Supply dive boat. A fairly strong surge had kicked up, the water was still unseasonably cold, and visibility was good... for the mainland (but not by Catalina standards). I entered the water with my camera before the rest of the passengers and headed off to the shore.

Within 20 minutes I saw a flash of yellow and black that had me thinking I'd been transported to the tropics... until I felt the cold water entering through the many large holes in my aging wetsuit! What had caught my eye was exactly what I had been seeking... the locally rare scythe or scythe-marked butterflyfish. I had never seen or videotaped this species before. Actually CDS dive master Ron Moore had suggested I look for it there at depths from 36 to 42 feet. Right on cue it briefly appeared in just that range from deep within a crevice giving me my first brief glimpse. I waited, bouncing in the surge, until it emerged from the hole for less than 10 seconds (just enough to allow me to catch it on tape).

Like the finescale triggerfish and guadalupe cardinalfish, this southern species has been known from Catalina waters for some time. It may have been introduced during a previous El Nino event. Although butterflyfish in general are a warm water group, this particular species is primarily a deeper and cooler water resident (120 to 500 ft) in southern areas like Cabo San Lucas. It ranges northward to Guadalupe and Catalina Islands and south to the Galapagos. Recently this species has been seen in the colder waters of the northern Channel Islands, presumably due to the 1997-1998 El Nino. They prefer habitats with rocky or boulder bottoms, something the area around our East End Quarry easily provides.

I mentioned in an earlier column that the biogeography, or geographic distribution, of species can be very interesting. Often more northerly forms seek deeper water if they move into areas of warmer water further south. This makes sense since they would find cooler water temperatures at greater depths there. The scythe butterflyfish may be an example of the reverse- it is found in shallower water off Catalina than in the warmer areas of Mexico and central America. Because of the cooler water temperatures here, it need not go so deep to be comfortable!

The scythe butterflyfish is usually 3-4" long but may reach lengths of six inches. They are yellow with a distinct scythe shaped black mark on each side. Its butterflyfish relative, the barberfish, is known to change color at night, losing its dark band. I don't know if our species does. Most butterflyfish I have seen are yellow and black, but like their namesakes they do come in a wider variety of colors in tropical seas. The dorsal fin of the scythe butterflyfish extends to the top of the head, making this fish look like it has a spike hairdo (even before it became "stylish")!

Butterflyfish have fairly pointed snouts which serve them well in gathering food. Part of their diet consists of algae and small invertebrates like snails and crustaceans picked from the substrate. However, many are also cleaner species and the pointed snout is very helpful in picking parasites off other fish. Note the long snout on the long-nosed butterfly fish in the photo above.

Our local waters do not seem to be warming up as early as they usually do. Seeing this reminder of my winter trips into subtropical and tropical waters reminds me how nice it would be to be diving there now (especially with the holes in my wetsuit). Perhaps the little scythe butterfly fish will escort me back to its more southerly waters, although I'm not sure I'm up to the swim!

© 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: The scythe butterflyfish at the East End Quarry, Catalina;
the related barberfish (schooling) and longnose butterflyfish
from the Gulf of California, Mexico

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