Pinnacles & Seamounts

The Pinnacles and Seamounts biome is hard structure that rises out of deep water. Where the Canyon biome is the seafloor falling away, this one is the seafloor standing up: rocky spires on the continental shelf, drowned banks that were islands during the last ice age, and volcanoes that climb thousands of feet from the abyss without ever breaking the surface. What these features share is a shape, and the shape is what matters. A steep obstacle sitting alone in moving water gathers food, and everything else follows from that.

Pinnacles and seamounts are grouped together here because they support the same kind of community. Geologically they have almost nothing in common, and it is worth understanding why, because the difference explains where each one is found and what a diver will see on it.

What builds a pinnacle

Along the West Coast, a pinnacle is a piece of the continent that the ocean has not managed to bury. These are hard rock outcrops on the continental shelf and the offshore banks, exposed because the softer sediment around them washed away. The US Geological Survey describes them plainly in its seafloor mapping as isolated parts of outcrop protruding through sediment cover, formed where erosion of softer layers leaves the resistant rock standing. The sediment cover itself comes and goes. Storms bury reefs and then dig them out again, so a pinnacle is a rock that keeps winning that argument.

The rock is continental, not oceanic. Cortes Bank, the most dramatic feature of the group, is a good demonstration. In 1975 an exploratory well was drilled 10,920 feet into its northeast flank, and what came up was marine sandstone and shale running from the middle Miocene down through the Eocene into Late Cretaceous rock about 95 million years old. The well report describes the bank as sitting on a southeasterly plunging anticline, a fold in the rock, whose subsurface shape closely resembles the bathymetry of the bank above it. Cortes Bank is not a volcano. It is a wrinkle in the continental margin that happens to reach the surface, with some old basalt flows buried in the middle of it.

Farther north the rock changes entirely. Cordell Bank, 22 miles west of Point Reyes, is granitic basement roughly 100 million years old, part of the Salinian terranes. The same granite forms the Farallon Islands and the Point Reyes Peninsula, and all of it rode north along the San Andreas Fault from much farther south. NOAA describes the bank as a piece of the southern Sierra Nevada; the USGS notes only that the Salinian rocks came from far to the south, possibly from the area of present-day Los Angeles, and resemble Sierra Nevada granite. Where exactly it started is still argued. That it is a mountain range that ended up underwater in the wrong place is not.

Sea level is the other builder. At the last glacial maximum, roughly 20,000 years ago, the ocean stood more than 400 feet lower than it does now, according to the USGS, which puts the old shoreline somewhere around 120 meters down. Much of what is now a bank was dry land or beach then. Researchers mapping Santa Catalina Island found 9 submerged wave-cut platforms preserved between 32 and 362 meters depth, with the ice-age shoreline sitting at 131 meters. Cordell Bank was a true island between about 20,000 and 15,000 years ago. The pinnacles divers visit today are, in a real sense, drowned sea cliffs.

There are exceptions, and Begg Rock is the good one. This spire 8 miles northwest of San Nicolas Island stands 15 feet above the waterline in otherwise deep water, and the USGS lists it as rhyolite of uncertain but probably Miocene age, bounded by seafloor outcrops of sedimentary rock. It is a genuine volcanic plug in a province where nearly everything else is folded sediment.

What builds a seamount

A seamount is a volcano. The formal definition, set by the International Hydrographic Organization, requires an elevation rising more than 1,000 meters (3,300 feet) above the surrounding seafloor, and NOAA notes that most seamounts are the remnants of extinct volcanoes. They sit on oceanic crust, well out past the shelf, and they were built upward by lava rather than left behind by erosion. NOAA states the contrast directly: banks are of continental origin, while seamounts are mainly volcanic.

California's seamounts have an unusual origin story. Guide, Gumdrop, Pioneer, Davidson, San Juan, San Marcos, Little Joe, Rodriguez, and Northeast Bank all appear to trace back to the abandonment and partial subduction of spreading center segments, which happened when the plate boundary here changed from a subduction zone into the transform margin that produced the San Andreas. Davidson and Guide were built directly on top of fossil spreading centers, according to magnetic anomaly data.

Davidson Seamount is the flagship. It rises about 2,300 meters (7,500 feet) from the seafloor 80 miles southwest of Monterey, holds roughly 320 cubic kilometers of rock, and its lavas date from 9.8 to 14.8 million years ago on crust that was already 20 million years old when the eruptions began. Its summit is still 1,250 meters (about 4,100 feet) below the surface. Volcanism at San Juan Seamount, off Point Conception, ran for around 8 million years and ended about 2.8 million years ago. Rodriguez Seamount is a guyot, a seamount with a flat top, and its summit carries beach deposits showing it stood above water as an island before it subsided.

That is the whole difference. A pinnacle is old continental rock the sea rose over. A seamount is ocean-floor rock built up from below. One is a survivor, the other is a construction project.

Why the two share one ecology

Life does not care which process made the mountain. What it responds to is hard surface, steep relief, and accelerated current, and both features deliver all 3.

When a current meets an isolated obstacle it speeds up and deflects around and over it. A classic 1986 study in Nature found that black corals and gorgonians on a multi-peaked seamount were most abundant near the peaks, and on narrow peaks they clustered on the crest itself while on wide peaks they gathered at the crest edge. The corals, in other words, map the flow. They grow densest wherever the water moves fastest, because that is where the food arrives.

Food is the constraint. Of the organic matter produced at the sunlit surface, only about 10 percent sinks past the surface layer, and only 1 percent of that reaches the bottom, according to the Woods Hole Oceanographic Institution. Roughly a tenth of a percent of what grows up top makes it all the way down. An animal that cannot move has to sit where the most water passes it per hour. Steep structure also drives mixing. Diapycnal diffusivity, the rate at which water masses stir into each other, has been measured near seamounts at more than 100 times its value in the surrounding open ocean, which lifts deep nutrients toward the light. Currents sweeping past a rise also carry larvae, and NOAA notes that seamounts give those drifting larvae a place to settle, which is why animals that would not normally live together often end up side by side on one.

The size of that effect depends heavily on depth, and this is where a lot of popular writing overstates things. Ten years of satellite data across the Pacific found long-term chlorophyll enhancement around only 17 percent of seamounts, but around 45 percent of those with summits shallower than 100 meters, and where it occurred it raised chlorophyll by as much as 56 percent. Seamounts that showed the effect produced twice the fisheries catch of those that did not. The productivity story is really a shallow-summit story, which is exactly why the shallow banks off southern California fish the way they do and a mile-deep seamount does not.

What lives there

The shallow end of the biome is what most divers and spearfishers will ever see, and it looks like a rocky reef with the volume turned up. Yellowtail (Seriola dorsalis), white seabass, and bluefin and yellowfin tuna move through the water above the structure. Rockfish (Sebastes species), including cowcod, hold on the rock alongside lingcod. A survey of Farnsworth Bank off Catalina counted 10,404 fish across at least 43 species in a single set of transects, with rockfishes accounting for 25 of those species. Giant sea bass (Stereolepis gigas) gather at these features in summer, and acoustic tagging at Catalina found them aggregating from June through October and most likely spawning at a pinnacle promontory between July and September.

The invertebrate life is the reason several of these places are protected. Farnsworth Bank carries California purple hydrocoral (Stylaster californicus), which grows between 31 and 66 meters depth, only on rock, in colonies that reach 120 centimeters (about 4 feet) across. White abalone (Haliotis sorenseni), the deepest-living abalone species, was historically most abundant on Tanner and Cortes Banks and at the offshore islands.

The deep end is another world. Below the reach of light, seamounts are covered in slow-growing corals and sponges that filter the passing current. These animals are astonishingly old. Radiocarbon dating of deep-water gold coral and black coral found radial growth rates of 4 to 35 micrometers per year, roughly the width of a human hair, and produced individual colonies aged at 2,742 and 4,265 years. Davidson Seamount holds at least 237 species and at least 18 that were new to science when it was surveyed, including 8 sponges and 4 corals. A warm-water seep on its flank hosts a brooding site where researchers counted more than 1,000 octopus in a single 45-minute pass, in water that is normally 35 degrees Fahrenheit but reaches 50 degrees where it leaks from the rock. Less than 1 percent of the seamount has been studied.

Why the biome matters

Isolated structure concentrates life far out of proportion to the space it occupies. At 3 shallow seamounts in the tropical Atlantic, researchers measured biomass enrichment relative to the surrounding open ocean ranging from 2-fold for zooplankton up to 41-fold for sharks, and tagged Galapagos sharks there showed near-continuous residence across 595 days. Analysis of more than 23,000 longline sets across the Pacific found higher species richness within 30 to 40 kilometers of a seamount summit, with mako sharks, silky sharks, blue marlin, swordfish, and yellowfin tuna all showing significant seamount association.

That concentration is why the banks matter to fishing and why they are fragile in the same breath. Seabirds respond to it too. During upwelling season Cordell Bank carries some of the highest seabird biomass in central California waters, alongside Monterey Bay and the Farallon ridge, with 68 species recorded feeding there. They include black-footed albatross that breed in the Northwestern Hawaiian Islands and sooty shearwaters that come from New Zealand and Chile.

For a spearfisher, all of this reduces to one practical fact. Fish gather at these features reliably, at predictable times of year, in numbers they do not reach anywhere else nearby. That reliability is the value and it is also the danger.

Where the biome occurs

Seamounts occur worldwide on deep ocean floor, and pinnacles occur wherever hard rock stands above the surrounding seabed. Off California the shallow pinnacles and banks include Cortes and Tanner Banks about 100 nautical miles west of San Diego, where Bishop Rock comes to within 15 feet of the surface while the seabed drops more than 5,000 feet off the flanks; Farnsworth Bank south of Catalina, where bedrock pinnacles peak near 50 feet; Begg Rock off San Nicolas Island; Osborn Bank, Nine Mile Bank, Cherry Bank, and Potato Bank; and Cordell Bank off Point Reyes, whose upper pinnacles reach within about 115 feet of the surface.

The true seamounts lie farther out and deeper. Davidson sits 80 miles southwest of Monterey, San Juan and Rodriguez off the southern and central coast, and Pioneer and Gumdrop offshore of San Francisco. None of them is divable. Their summits are thousands of feet down.

Threats and conservation

The deep parts of this biome are damaged easily and heal on a timescale no fishery operates on. Work on New Zealand seamounts found that as few as 10 bottom trawls can take average coral cover from 15 to 20 percent down to no visible cover at all, and 15 years after one seamount was closed to trawling its community still resembled that of a seamount being actively trawled. Some studies do find recovery, but at 30 to 40 years, not 5 or 10. The safest reading of the evidence is that these communities can come back on a multi-decadal to century scale, and that everything measured at the 5-to-15-year mark looks bleak.

The West Coast has made real progress here. Amendment 28 to the Pacific Coast Groundfish Fishery Management Plan took effect on January 1, 2020 and created 45,136 square kilometers of new Essential Fish Habitat conservation areas closed to bottom trawling. The largest, the Southern California Bight conservation area, covers 41,915 square kilometers from Point Conception to the Mexican border and newly protected Cortes Bank, Tanner Bank, Nine Mile Bank, and the waters off San Clemente and west Catalina. Davidson Seamount joined the Monterey Bay National Marine Sanctuary in 2008 and has its own conservation area, and Cordell Bank is its own national marine sanctuary.

Closer to shore the damage is more ordinary. At Farnsworth Bank, roughly 200 fishing and diving boats a year drop anchors directly onto hydrocoral reef, and a survey there documented large amounts of fishing line and net debris across the site. The California Ocean Protection Council has agreed to fund mooring buoys, which would solve the problem for a few thousand dollars, but the installation requires permits from 4 separate agencies and an outside organization willing to manage it, so none have gone in.

Some losses are already close to permanent. White abalone declined about 99 percent in southern California from its 1970s levels, and repeat surveys at Tanner Bank tracked the population falling from an estimated 15,323 individuals in 2002 to 3,375 in 2010. It has been listed under the Endangered Species Act since 2001.

How divers and spearfishers can help

The core problem on a pinnacle is that fish gather there predictably, which makes them easy to find and easy to erase.

The clearest cautionary tale is the giant sea bass. Boats fishing spawning aggregations off Baja California in the mid-20th century consistently brought back 70 to 100 fish per trip, and one trip produced 255 in 3 days. Once those aggregations were exploited, the fishery disappeared with the fish. That is not a metaphor. Young giant sea bass appear to learn aggregation sites from older fish, so when a site is wiped out the knowledge of it goes too, and it is not known whether a new one ever forms. California closed the fishery in 1981, and current regulations name giant sea bass explicitly in the spearfishing section as a species that may not be taken. A recent mark-recapture study estimated about 1,221 adults in southern California between 2015 and 2022 with the population growing at about 8 percent a year, so the closure is working. Leave them alone and photograph them instead; the spot patterns are individually identifiable and researchers use diver photos as data.

The same trap catches aggregating fish generally, and it hides itself while it happens. A study of barred sand bass and kelp bass in southern California found biomass fell roughly 90 percent from 1980 levels while catch rates stayed high, because anglers were fishing the aggregation sites. The authors called it the illusion of plenty. If your catch rate on a bank stays good while the fish get smaller and fewer everywhere else, that is the pattern, not an exception to it.

For anyone taking bottom fish from a boat, California has required a descending device aboard and available for immediate use since 2024 whenever federal groundfish are being taken. The rule is vessel-based, so it does not reach a beach-entry diver, but the physics do. The device works: a study across 11 California sites measured 91.4 percent initial survival for rockfish returned to depth with one, and the average release took under 3 minutes. Cowcod, yelloweye, quillback, and bronzespotted rockfish are zero-limit species year-round regardless of depth. Cowcod is worth understanding as a case study: its spawning output fell to 9 percent of unfished levels by 1989, the stock was declared rebuilt in 2019 at 57 percent, and it is still illegal to keep. Rebuilt does not mean retainable. These fish live to at least 55 years, and a large old female produces up to 2 million eggs against 200,000 for a small one, so the biggest fish on the reef is doing most of the reproductive work.

Divers using spearfishing gear are exempt from the seasonal and depth restrictions that bind rod-and-reel anglers, but not from the zero-limit species list, and several offshore banks including Cherry Bank, Potato Bank, and 43-Fathom Spot are groundfish exclusion areas closed year-round. Those rules change most years, so check the current CDFW regulations before running out to a bank.

Last, anchor on sand and stay off the coral. California purple hydrocoral grows less than a quarter of an inch a year, according to CDFW, and it snaps easily. A colony broken by a fin or an anchor chain represents decades of growth, and at Farnsworth it has been protected since 1972 precisely because it does not come back on any timescale that helps you.