Bottom Trawling on Seamounts: The Damage Nobody Sees

Seamounts are underwater mountains that rise at least 1,000 meters from the seafloor without breaking the surface, and they punch far above their weight biologically. Topographic upwelling pulls cold, nutrient-rich water up their slopes, fueling plankton blooms that draw fish, sharks, seabirds, and marine mammals, while their rocky surfaces give slow-growing corals and sponges a rare hard place to root in an otherwise soft-sediment deep sea. Roughly one in five seamount species are endemic, found nowhere else on Earth, which is why scientists call them oases in the deep sea.

That's exactly what made a pair of Australian studies so consequential. In 1997, researchers led by Tony Koslow and Karen Gowlett-Holmes of CSIRO Marine Research surveyed seamounts south of Tasmania to test whether bottom trawling for orange roughy was damaging habitat that had taken centuries to form, and whether a proposed marine reserve would actually protect it. What they found on heavily fished seamounts, compared to lightly fished or untouched ones, was stark: stripped reef, collapsed biomass, and far fewer species per sample. A 2009 follow-up study by Althaus and colleagues went further, analyzing tens of thousands of video frames and photos across 25 seamounts, and found coral cover reduced by roughly two orders of magnitude on trawled sites, with no sign of recovery even five years after trawling stopped.

Those findings are part of what pushed Matthew Gianni, a former deep-sea fisherman, to help found the Deep Sea Conservation Coalition and push for international protections against high seas bottom trawling. This episode walks through what seamounts are, what the research actually found, and why damage this far down might be effectively permanent on any timescale that matters to us.

Takeaways:

• Seamounts are volcanic underwater mountains that drive upwelling, concentrate biodiversity, and host high rates of endemic species found nowhere else.
• Bottom trawling for species like orange roughy drags heavy gear across seamount slopes, destroying coral and sponge habitat that can take centuries to build.
• A 1997 CSIRO-led survey south of Tasmania found dramatically lower biomass and species richness on heavily trawled seamounts compared to lightly fished or untouched ones.
• A 2009 follow-up study found roughly a hundredfold decline in coral cover on trawled seamounts, with no clear sign of recovery five years after trawling stopped.
• Because this damage happens far below the surface and outside any sunlight or recreational diving range, it's often invisible until a dedicated scientific survey documents it.
• This research helped inspire the founding of the Deep Sea Conservation Coalition and international efforts to restrict high seas bottom trawling.

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