Evidence of localized and cumulative impacts of bottom-trawling upon bycatch abundance and diversity was found during the 8-year period the Chilean orange roughy (Hoplostethus atlanticus) fishery was open. We analyzed acoustic, orange roughy catch and fish bycatch data available from a voluntary monitoring program implemented by the fishing-rights holders. Acoustic data, recorded from the third year of the fishery (2002) until its closure in 2006, permitted us to extract detailed bathymetry and precise information about trawl tows. From these data we reconstructed the yearly and cumulative extent of the bottom trawling footprint and compared biodiversity indices, bycatch abundance and assemblage structure between nominally pristine areas and others that exhibited different levels of previous trawling impacts (as measured by within- and across-years impact indices). Although the footprint area expanded as the cumulated number of tows increased, degrees of overlapping between tows were found in all seamounts. The trawled footprint was 22 km2 for the 2002–2006 period, equivalent to 2.7% of the study area. An extrapolation to include unassigned tows and years without data (1999–2001) increased this estimate to 81 km2 (9.8% of the study area) for the entire duration of the fishery. The relative abundance of bycatch species decreased at more heavily impacted areas, suggesting a local depletion response. Evidence of bycatch species richness decline and significant changes in diversity and species assemblage composition were also found at these areas. Seamount management and conservation efforts must consider the cumulative and localized nature of bottom trawling effects we found to occur upon seamount fish communities.