Why California Kelp Is Disappearing: Urchins, Pollution, and the Truth About Diver Impact

California's kelp forests are in trouble. Northern California has lost over 90 percent of its bull kelp canopy since 2014. Southern California's giant kelp forests - including the ecologically important beds at Imperial Beach and Point Loma - face their own pressures from pollution and urchin overgrazing. Public conversation sometimes blames recreational spearfishing and diving for the decline. The science does not support that narrative.

The actual causes are well documented: explosive sea urchin populations driven by the collapse of urchin predators, marine heatwave conditions that stress kelp directly, and concentrated pollution sources - particularly the Tijuana River sewage plume that flows north into U.S. waters every year. Recreational divers are not the problem. In many cases, divers are part of the solution.

This guide covers what the research shows about California kelp decline, why urchins and pollution are the real drivers, and what would actually fix the problem - including the role of California sheephead recovery and cleaner-water legislation.

The Real Cause #1: Sea Urchin Barrens

The single biggest driver of California kelp loss is sea urchin overgrazing. When urchin populations explode, they mow down kelp holdfasts faster than the kelp can regrow. The result is what marine biologists call an 'urchin barren' - a rocky bottom covered in starving purple urchins with no kelp, no fish habitat, and no path back to a healthy ecosystem without intervention.

The mechanism is straightforward. Urchins eat kelp continuously, especially at the holdfast where the kelp attaches to the rock. In a healthy ecosystem, predators keep urchin populations in check - sea stars, sea otters, California sheephead, and other predator species eat enough urchins to prevent runaway grazing. When the predators disappear, the urchins explode.

What happened in California:

• 2013-2014: sea star wasting disease wiped out the sunflower sea star (Pycnopodia helianthoides) and other urchin-eating sea stars along the West Coast. Sunflower sea star populations dropped over 90 percent in many areas

• 2014-2016: 'The Blob' marine heatwave warmed coastal water 2-5 degrees above normal, stressing kelp directly and favoring urchin reproduction

• Purple sea urchin (Strongylocentrotus purpuratus) populations exploded - in some Northern California sites, urchin density increased 60-fold

• Result: bull kelp forests collapsed across hundreds of miles of Northern California coast. Southern California impacts have been more variable but still significant in specific zones

The Real Cause #2: Pollution, Especially the Tijuana River

Sea urchins are the headline cause in Northern California. Pollution is the headline cause in Southern California. The single biggest pollution source affecting California kelp is the Tijuana River, which carries untreated sewage and storm runoff from Tijuana, Mexico, across the U.S. border and into Pacific Ocean water that flows north along the California coast on the prevailing current.

• The Tijuana River regularly discharges tens of millions of gallons of untreated wastewater per day across the U.S.-Mexico border

• The plume flows north on the coastal current, fouling Imperial Beach first and reaching the Point Loma kelp beds within days

• Beach closures at Imperial Beach happen routinely; in heavy flow years, the entire south end of San Diego County is closed to water contact for weeks at a time

• The same pollution that closes beaches stresses kelp - the water at the kelp beds is loaded with sewage bacteria, sediment, and chemical contaminants

• Stressed kelp grows slower, becomes more vulnerable to urchin grazing, and is less able to recover from storm damage or warm-water events

• The Point Loma kelp bed - one of the most important kelp ecosystems in California - sits directly downstream of the Tijuana plume on the prevailing current

Other pollution sources matter too: agricultural runoff, urban storm drains, sewage outfalls along the coast, and atmospheric deposition all contribute. But the Tijuana River is the single largest persistent point source affecting Southern California kelp, and addressing it would have outsized impact on the recovery of kelp at Imperial Beach, Point Loma, and the entire south end of the county.

The Real Cause #3: Marine Heatwave

Beyond urchins and pollution, the broader ocean climate trend is hostile to California kelp. The marine heatwave that started in 2014 has not fully released its grip, and the developing El Niño pattern is adding to the warm-water signal. Warmer water:

• Stresses kelp directly - giant kelp and bull kelp both prefer cooler water and grow slower in warm conditions

• Favors urchin reproduction and survival, accelerating the urchin barren problem

• Drives away cold-water species that historically helped balance the ecosystem

• Increases storm intensity and storm-driven kelp removal

Marine heatwave is not something individual divers or conservation groups can fix directly, but it is essential context. The other stressors compound when the water is warm.

Why Spearfishing and Diving Are NOT the Cause

The narrative that recreational diving and spearfishing damage California kelp is not supported by the science:

• Recreational take of kelp-canopy species (calicos, sheephead, halibut) is heavily regulated by size limits, bag limits, and Marine Protected Areas

• California's MPA network protects 16 percent of state waters as no-take reserves - the kelp recovery research from inside MPAs vs outside shows mixed results, suggesting fishing pressure is not the dominant factor

• Spearfishing-specific impact is minimal - the species recreational divers target are not the species that maintain kelp ecosystem function

• By contrast, the Northern California kelp collapse happened in waters that had been fully protected from any fishing for years - the urchin barrens formed inside MPAs as well as outside

• Divers are not removing urchin predators in numbers that affect populations - sheephead size limits and bag limits keep recreational take well below the level that would drive predator loss

If the cause were recreational diving, MPAs would have recovered. They have not. The real causes operate at scales recreational diving cannot match: ocean-wide marine heatwave, multi-state sea star wasting disease, and watershed-scale pollution sources.

Solution #1: Restore California Sheephead Populations

California sheephead (Semicossyphus pulcher) are one of the most important urchin predators on Southern California reefs. Large sheephead - especially the big terminal-phase males - have specialized jaws that crush urchin tests. A healthy sheephead population is one of the natural controls on urchin overgrazing.

What works:

• Maintain current size limits (12-inch minimum) and bag limits that allow sheephead to reach urchin-eating size before being harvested

• Protect spawning aggregations and large breeding fish - the biggest sheephead have outsized ecological value

• Habitat-based protection at key spawning aggregations and nursery zones where measurable benefit exists - though MPAs alone do not fix kelp loss, as decline has occurred inside fully-protected reserves too. See the companion post on California MPAs for the full discussion

• Reduce commercial take where commercial fishing pressure exists on sheephead

• Restore the predator base that has been depleted by historic over-harvest - the goal is more big sheephead per acre of kelp

Spearfishing divers who target sheephead can support this by following size and bag limits strictly, releasing the biggest fish even when legal, and prioritizing other species for table use when possible. The big sheephead has more value alive eating urchins than dead on your stringer.

Solution #2: Direct Urchin Culling

Several organizations and research projects now actively remove urchins from designated kelp recovery zones. The Bay Foundation, the Reef Check California program, and various academic groups run urchin culling and kelp restoration projects up and down the coast.

• Volunteer urchin culling days organized by conservation groups - divers participate directly in kelp restoration

• Funded urchin removal contracts in priority kelp recovery zones

• Commercial purple urchin harvest for the (limited) food market - some divers have made this a small business while contributing to kelp restoration

• Sea otter restoration in Northern California, where otters are functionally absent - extremely controversial but a major potential ecological lever

Divers can participate directly. Reef Check California, the Bay Foundation, and other groups run urchin culling and kelp restoration events that recreational divers can volunteer for. This is the rare conservation effort where being a diver is the qualification, not a problem.

Solution #3: Fix the Tijuana River Sewage Problem

The Tijuana River sewage crisis has been a binational issue for decades. Real fixes require U.S.-Mexico cooperation, federal funding, and infrastructure construction on both sides of the border. The good news is that progress has been made, even if it has been slow.

• The South Bay International Wastewater Treatment Plant (SBIWTP) was built in the 1990s to treat Tijuana River wastewater on the U.S. side. It is currently undersized and overwhelmed by Tijuana population growth

• Federal funding has been allocated for expansion - the SBIWTP expansion project is funded through USMCA and IBWC (International Boundary and Water Commission)

• Mexico has committed to upgrading its own sewage infrastructure on the Tijuana side, though progress has been slow

• Binational projects with EPA, IBWC, and Mexican CONAGUA address watershed-scale problems, not just border outflow

• California state legislation (SB 1066, AB 32 related funding) has directed state funds toward the cleanup

What divers and California residents can do:

• Contact state and federal representatives to fund SBIWTP expansion fully - it has been a frequent bipartisan project but funding is never permanent

• Support nonprofit organizations working on the Tijuana River issue (Surfrider Foundation, San Diego Coastkeeper, WILDCOAST)

• Document and report sewage impacts on local kelp ecosystems - data drives funding

• Show up to public meetings on watershed and treatment funding - decisions get made when people show up

• Vote on bond measures and ballot initiatives that fund water quality infrastructure

Solution #4: Broader Water Quality Legislation

Beyond Tijuana specifically, California water quality affecting kelp depends on a wider set of policies:

• Clean Water Act enforcement at coastal outfalls

• Storm drain controls to reduce urban runoff loading - 'first flush' regulations after rainfall

• Agricultural runoff controls in coastal watersheds

• Coastal Commission permit oversight for any project that could affect water quality near kelp ecosystems

• MPA monitoring and adaptive management - scientific data should drive policy adjustments, not assumptions

• Climate policy that addresses the underlying marine heatwave problem at the source

What Divers Can Do Today

• Follow size and bag limits strictly, especially on urchin-predator species (sheephead, large lobster, lingcod)

• Don't shoot

  the biggest fish even when legal - they have outsized ecological value

• Participate in urchin culling and kelp restoration events through Reef Check California, the Bay Foundation, or local conservation groups

• Report illegal take - poaching of protected species or violations of MPA boundaries weakens the ecosystem

• Document kelp conditions at your home dive spots - photos over time become valuable scientific data

• Support water quality legislation and the Tijuana River cleanup effort financially and politically

• Educate other divers and non-divers about the actual causes of kelp decline - the diver-blame narrative is wrong and replacing it with accurate science helps the recovery effort

The Long-Term Outlook

California kelp will recover. The same ecosystem that crashed has historically rebounded from major disturbances given time and conditions. The path back requires addressing the actual drivers - urchin overgrazing, pollution, and warming water - rather than blaming user groups that have minimal impact. Spearfishing divers can be allies in the recovery effort. The first step is getting the science right. The next step is taking the actions that actually move the needle.

Final Thought

Sea urchins ate California's kelp. Pollution finished what the urchins started, especially in Southern California where the Tijuana River carries Mexican sewage into U.S. waters that flow over Imperial Beach and the Point Loma kelp beds. Recreational divers are not the cause. Real solutions exist: rebuild sheephead populations, run direct urchin culling programs, fix the Tijuana River sewage problem with binational funding, pass and enforce cleaner-water legislation, and support the scientists and conservation groups doing the actual work. The divers who learn the real story can become some of the most effective advocates for kelp recovery.