California Diving News - Catalina Island Scuba Diving: Dive Spot Isthmus Reef

A diver pauses to examine a red gorgonian.

Isthmus Reef
Although few and far between, there are those dives that always seem to be a “crowd-pleaser.” Let’s face it, there are not many dive sites that a charter boat laden with two to three dozen divers can drop anchor on and please everybody, even if the hoard is comprised of a beginner class, rusty/dusty divers, and experienced salts. Isthmus Reef is just such a place. And the reef is large enough that you can actually drop three dozen divers on top and never see another buddy team your whole dive.

Fish life and gorgonians are abundant along the walls and rocks.

Isthmus Reef is a large plateau in the center of the large Isthmus Cove on the frontside (mainland side) of Catalina Island. The top of the reef is a comfortable 10 to 35 feet deep and dotted with healthy growths of kelp. There are pockets of sand, ridges, crevices and small overhangs. Perfect for a fun day of exploring if you are beginner or somebody that just wants something more low-key.

The top of the reef is not lacking in marine life excitement. Bat rays, some quite large, are common. No need to go far. Often they are found right under the boat in the boat’s shadow. In the crevices you will find small lobster, octopus, and an occasional moray eel. Tiny but brightly colorful blue-banded gobies dot the rocks. Schools of jack mackerel, anchovies, and sardines frequent the kelp passages. Garibaldi are common and very friendly.

Tube anemone

Much of the reef top is quite shallow. Several sections are less than 20 feet and some less than 10. At the east end the reef actually breaks the surface. A number of boats have inadvertently discovered this shallow portion, damaging their hulls in the process, so a few years ago a permanent marker was placed over the hazard.

Around the plateau edges is where it gets interesting for the more experienced diver. On nearly all sides the reef drops off nearly vertical from 20-35 feet to 70-100 feet, depending on location. The walls are rack with deep crevices, overhangs and holes. In some spots, caves penetrate deep into the rock. These holes are large enough for divers to peer into, only to take a frustrating look at dozens of lobster, some good sized, just out of reach (did I hear that bug laugh at me?).

Along the walls you will find big horn sharks, fat yellow and black striped treefish, and aggregating coral under the ledges. In spots you will find gorgonian sea fans, but not as prolific as other reefs on Catalina. This could be because Isthmus Reef, backed up into the cove, is not as open to currents as these other reefs. Rarely do you have to deal with currents here. The downside is sometimes the water clarity is not as good as nearby Bird Rock, although being far from shore, it is rarely bad.

Kelp growth varies here from season to season, year to year. I have seen the reef completely gone during El Niño years and thick as spinach dip at other times. The top of the reef is interspersed with patches of sand and gravel, creating openings for entry, exit and settling in on the bottom. Even so, brush up on your kelp diving techniques before heading off into the thickest parts of the kelp on Isthmus Reef.

The charter boats will usually anchor over the shallows but near the drop-off to give divers the option of diving a variety of depths and locations. On my most recent trip, we stayed at the site for our two morning dives. One excursion we headed over the wall and the other cruised the shallows, chasing bat rays. Although I’d dived this site many times before, I felt I could have made three and four dives here and been just as happy exploring the four points of the compass on this vast underwater terrain.

Dive Spot At-A-Glance
Location: At the center of Isthmus Cove, usually easy to find by the prolific kelp growth. The pole at the east end of the reef marks its shallowest point. Stay clear. Reef covers several acres.
Access: Boat only.
Depths: 15 to 35 feet across the top; 70 to 100 down the walls
Skill Level: All on top, intermediate or better down the wall.
Visibility: Good, averaging 40 feet.
Snorkeling: Good in shallow sections.
Hunting: Good for lobster in season along base of wall at night. Spearfishing poor.
Photography: Excellent wide-angle from base of wall looking up at noon-day sun through the kelp for silhouette shots. Macro good, mostly for fish photography.
Suggested Boats Serving the Area:
Encore - (310) 541-1025
King Neptune - (800) 262-3483
Sundiver - (800) 555-9446
Truth/Conception/Vision - (805) 962-1127
Horizon/Ocean Odyssey - (858) 277-7823
Mr. C - (310) 521-9737
Garibaldi - (310) 510-2800
Pacific Star - (310) 921-9210
Island Diver - (800) 642-0785
Express Divers - (866) 488-3483
Scuba Cat - (800) 353-0330
or see the California Scuba Calendar section of this issue for trips to Catalina Island