Impacts of bottom trawling on deep-coral ecosystems of seamounts are long-lasting

Complex biogenic habitats formed by corals are important components of the megabenthos of seamounts, but their fragility makes them susceptible to damage by bottom trawling. Here we examine changes to stony corals and associated megabenthic assemblages on seamounts off Tasmania (Australia) with different histories of bottom-contact trawling by analysing 64 504 video frames (25 seamounts) and 704 high-resolution images (7 seamounts). Trawling had a dramatic impact on the seamount benthos: (1) bottom cover of the matrix-forming stony coral Solenosmilia variabilis was reduced by 2 orders of magnitude; (2) loss of coral habitat translated into 3-fold declines in richness, diversity and density of other megabenthos; and (3) megabenthos assemblage structures diverged widely between trawled and untrawled seamounts. On seamounts where trawling had been reduced to <5% a decade ago and ceased completely 5 yr ago, there was no clear signal of recovery of the megabenthos; communities remained impoverished comprising fewer species at reduced densities. Differences in community structure in the trawled (as compared to the untrawled) seamounts were attributed to resistant species that survived initial impacts, others protected in natural refugia and early colonisers. Long-term persistence of trawling impacts on deep-water corals is consistent with their biological traits (e.g. slow growth rates, fragility) that make them particularly vulnerable. Because recovery on seamounts will be slow, the benefits from fishery closures may not be immediately recognisable or measureable. Spatial closures are crucial conservation instruments, but will require long-term commitments and expectations of performance whose time frames match the biological tempo in the deep sea.