Cruising along the base of the west wall of Isthmus Reef I stumbled across a most remarkable find — a four-foot long horn shark! While horn sharks are by no means uncommon, especially at this reef, seeing one this large was highly unusual.

The horn shark gets its name from the spiny “horn” protruding from the top of its dorsal. When the shark is small, the sharp point serves as a deterrent to predators. Later in life the spine serves to help the shark stay wedged in deep crevices (there are a lot of them here along these walls). Over time, the sharp spine is worn blunt.
Another experience I remember here was on opening night of lobster season a few years past. While this site is frequently visited by both private and dive charter boats, I had one of my most successful lobster dives here ever. Along the north wall I came across a deep undercut in the reef full of lobster. I had my limit of lobster in a matter of minutes.
Isthmus Reef is fun on so many different levels, both in experience and activities. The shallow plateau-like structures with walls on nearly all sides make many dive profile options possible.
The peak of the plateau nearly breaks the surface of the water at low tide. It is here that a permanent reef marker has been placed so that boats will not accidentally run aground. But most of this shallow plateau is around 20 to 30 feet deep, and features a few ledges and crevices here and there. Depending on water temperatures the kelp growth can be quite thick. Because of the recent warm water in this 2014-15 season, kelp has become very thin if not non-existent. We all love kelp but with the kelp gone it gives the reef a different perspective, revealing much of its profile.
The top of the plateau is a good spot for Open Water Diver certification classes to complete their training. The area offers plenty to delight new divers — every diver should meet a bright orange garibaldi on one of their first outings, right?
Experienced divers will likely want to head down the walls. Along the walls are deep crevices that not only hide horn sharks and lobster but also an abundance of blue-banded gobies, an occasional moray eel as well as representatives from the rockfish family such as kelp rockfish and the yellow and black striped treefish.
The walls are tremendous, dropping vertically as much as 60 feet. Depending on location the walls bottom out into jumbled boulders and sand at 70 to 90 feet. This deep sand environment holds halibut, bat rays and angel sharks.
For those visiting the deeper sections of the south wall, the profile is ideal for finishing up your dive. Ascend slowly along the wall to the top of the reef and then work your way up to the shallowest point at 15 feet for your safety stop enjoying the reef all the way through the entire dive.
Conditions here are generally very good. Seated deep inside Isthmus Cove, the location is well protected from most wind, waves and swell. Currents are also rare. Water clarity here, while still very good, is never as superb as nearby Ship Rock and Bird Rock which benefit from frequent water cleansing oceanic currents.
If you have the opportunity to dive here multiple times, do it. The large reef cannot be truly appreciated in just one dive. The huge rock faces present many different personalities often only 100 yards apart.
Location: Large reef area at the center of Isthmus Cove. Shallowest point on the east end is marked by a permanent pole and light. Marked on some charts as Harbor Reef. GPS N33°26.873′, W118°29.421′
Skill level: All
Depths: 10 to 90 feet.
Visibility: Very good averaging 40 feet. Best on the east end.
Snorkeling: Very good in the shallows
Photography: Excellent both macro and wide angle.
Hunting: Good for lobster in season.
Hazards: Boat traffic