Sunlight and Ship Fuel: The Unexpected Assassin of San Francisco's Herring Embryos
How a 2007 oil spill revealed a devastating phototoxicity mechanism that killed fish embryos through an unexpected combination of bunker fuel and sunlight
The Spill That Threatened an Ecosystem
When the container ship Cosco Busan struck the San Francisco-Oakland Bay Bridge on a foggy November morning in 2007, it released approximately 54,000 gallons of bunker fuel into the bay's waters 3 . This wasn't just an environmental disaster; it was a crisis waiting to happen for a silent, underwater resident—the Pacific herring. The spill occurred just one month before the herring's spawning season, contaminating the very habitats where these ecologically and commercially vital fish would lay their eggs 3 .
Key Fact
The spill timing was particularly devastating—occurring just one month before the herring spawning season, directly contaminating critical spawning grounds.
What scientists discovered in the aftermath surprised them. They expected to find the kind of heart defects documented in previous oil spills, but instead, they uncovered a far more devastating phenomenon: herring embryos in shallow waters were literally disintegrating . This is the story of how a combination of bunker oil and sunlight revealed a previously unknown lethal mechanism, changing our understanding of oil spill impacts forever.
A Tale of Two Toxicities: The Dual Assault on Fish Embryos
Following the spill, scientists initiated a comprehensive study to assess the damage to herring embryos. They placed caged embryos containing artificially fertilized eggs at various depths in both oiled and unoiled "reference" sites throughout the bay 1 .
What they found in subtidal zones (deeper than 1 meter) was the anticipated cardiotoxicity. Embryos at oiled sites showed pronounced bradycardia (abnormally slow heart rates), around 90-95 beats per minute compared to a healthy 116 beats per minute at reference sites 1 . A small but significant percentage (0.9%-2.5%) of hatched larvae also developed pericardial edema—a fluid-filled sac around the heart that impairs function 1 . This was the classic oil toxicity syndrome scientists had expected.
The real shock came when researchers examined naturally spawned embryos from the intertidal zone—the shallow waters where sunlight penetrates easily. These embryos showed catastrophic damage that went far beyond heart defects. They were "literally falling apart" with high rates of tissue necrosis and mortality 3 . Scientists described them as "liquefying before our eyes" .
The damage was so severe that in 2008, almost no live larvae hatched from natural spawn collected from oiled sites 3 . This unexpected finding prompted a deeper investigation into what was causing such dramatic effects in shallow waters but not in deeper ones.
Comparison of Embryo Impacts in Different Zones
| Location | Cardiac Effects | Tissue Necrosis | Hatching Success |
|---|---|---|---|
| Intertidal Zone (Shallow) | Moderate sublethal cardiotoxicity | Unexpectedly high rates, embryos "disintegrating" | Nearly complete failure in 2008 |
| Subtidal Zone (Deeper) | Significant bradycardia, pericardial edema | No elevated necrosis | High, though with some abnormalities |
| Unoiled Reference Sites | No cardiotoxicity | No necrosis | Normal |
Data compiled from field studies following the Cosco Busan oil spill 1 3
Scientific Detective Work: Uncovering the Phototoxicity Connection
Field Assessment (2008-2010)
Between 2008 and 2010, scientists collected naturally spawned embryos from multiple oiled and unoiled sites, while also deploying caged embryos in subtidal locations 1 . This allowed them to compare different exposure scenarios and track changes over time.
Laboratory Analysis
Embryos were transported to laboratories where researchers used digital photo- and videomicroscopy to document developmental abnormalities, heart function, and hatching success 1 . This detailed analysis revealed the extent of tissue damage in embryos from shallow waters.
Chemical Tracking
Scientists measured concentrations of polycyclic aromatic compounds (PACs)—toxic components of oil—in both the embryos and the water using passive sampling devices 1 . Surprisingly, PAC levels were lower than expected to cause such severe lethality.
The Phototoxicity Breakthrough
The breakthrough came when researchers realized the damage pattern perfectly matched the reach of sunlight. Embryos disintegrated only in shallow waters where sunlight could penetrate, regardless of other pollution sources . Laboratory tests confirmed that when zebra fish embryos were exposed to bunker oil and then sunlight, tissue breakdown began within minutes to an hour .
Key Findings from the Cosco Busan Herring Embryo Studies
| Research Aspect | Finding | Significance |
|---|---|---|
| Timeline | Severe impacts 3 months after spill; resolved after 2 years | Demonstrated acute but relatively short-term effects |
| Tissue Concentrations | PAC levels lower than expected to cause lethality | Suggested unknown toxic mechanisms beyond conventional analysis |
| Spill Specificity | Bunker oil caused phototoxicity; crude oil did not in tests | Revealed unique dangers of bunker fuels compared to crude oils |
| Geographic Scope | Impacts affected up to 25% of 2008 herring spawn | Quantified population-level consequences |
The Scientist's Toolkit: Research Reagent Solutions
Understanding this environmental disaster required specialized tools and methods. Here are the key components that enabled this groundbreaking research:
Caged Embryos
Artificially fertilized eggs deployed in specific locations to control exposure conditions and test site-specific toxicity 1
PEMDs
Passive water sampling devices that measured concentrations of oil-derived polycyclic aromatic compounds 1
Digital Microscopy
Enabled detailed documentation of cardiac toxicity, developmental abnormalities, and tissue necrosis 1
Cardiac Analysis
Quantitative measurement of heart rates and rhythm abnormalities to assess sublethal oil impacts 1
Chemical Fractionation
Laboratory technique to separate different components of bunker oil to identify phototoxic compounds
Phototoxicity Tests
Laboratory experiments exposing oil-treated embryos to specific light wavelengths
Rethinking Oil Spills: A Scientific Paradigm Shift
Beyond Crude Oil Assumptions
Prior to this research, most oil toxicity knowledge came from crude oil studies. Bunker oil contains the chemically uncharacterized remains of crude oil refinement, with unidentified chemicals interacting with sunlight to kill herring embryos 1 .
The Urban Spill Challenge
The Cosco Busan spill occurred in San Francisco Bay, an already urbanized environment with multiple background pollution sources. Despite this complicated backdrop, researchers successfully pinpointed the spill's specific effects 1 .
A New Kind of Toxicity
This research demonstrated that phototoxicity—the enhancement of chemical toxicity by light—wasn't just a laboratory curiosity but a real-world phenomenon with devastating consequences .
"Based on our previous understanding of the effects of oil on embryonic fish, we didn't think there was enough oil from the Cosco Busan spill to cause this much damage," and they didn't expect ultraviolet light would dramatically increase toxicity in the actual environment 3 .
- Gary Cherr, Director of the UC Davis Bodega Marine Laboratory
Ecological Ripple Effects: Why Herring Matter
Pacific herring are far more than just another fish species. They serve as a keystone species in the pelagic food web, forming a critical nutritional link between plankton and larger predators 1 .
Herring schools provide essential food for humpback whales, seabirds, seals, and salmon 3 . The San Francisco Bay population represents the largest coastal population of Pacific herring along the continental United States and supports the last commercial finfish fishery in the bay 1 3 .
Expert Insight
"Herring is one of the last urban fisheries, and herring is an indicator for the health of the Bay" - Gary Cherr, director of the UC Davis Bodega Marine Laboratory 3 .
The near-complete reproductive failure of herring in oiled shallow habitats during 2008 therefore threatened not just the herring population itself, but the entire coastal ecosystem that depends on them. The impacts extended through the food web, affecting predators that rely on herring as a primary food source.
Conclusion: Lessons from a Disaster
The story of the Cosco Busan oil spill and its impact on herring embryos represents more than a local environmental tragedy. It reveals critical gaps in our understanding of how pollutants affect marine life and underscores the complex interplay between human activities and ecosystem health.
The research forced scientists to reconsider what they thought they knew about oil toxicity. These findings have directly influenced how scientists assess the impacts of subsequent oil spills, including the 2010 Deepwater Horizon disaster in the Gulf of Mexico 7 . They've also led to a more sophisticated understanding of the delayed consequences of oil exposure—how seemingly minor defects in embryonic development can translate into reduced survival and reproductive success later in life 6 .
Perhaps most importantly, the legacy of this research lies in demonstrating that what we can't easily measure might be just as important as what we can. The very fact that unidentified chemicals in bunker oil were responsible for the devastating phototoxicity reminds us that nature often holds complexities beyond our current scientific reach.
This humble acknowledgement, born from the unexpected mortality of herring embryos, continues to drive more nuanced, comprehensive approaches to environmental protection today. The Cosco Busan spill taught us that we must consider not just the chemicals we know, but also their interactions with environmental factors like sunlight—lessons that continue to shape oil spill response and environmental toxicology.