Human activities affect all trophic levels of marine food webs and threaten numerous species. The status of such populations can be assessed via monitoring of their size. However, in long-lived species such as most marine top predators, environmental stress on a population only affects its size after several years. It has therefore been suggested that behavioural proxies such as foraging effort could be used to assess the short-term effects of environmental conditions on the wellbeing of a predator population. We demonstrate the potential of this approach in northern gannets Morus bassanus from the Sept-Iles Archipelago, France. This colony has been expanding rapidly for the past 60 yr, and is still growing faster than predicted from its size. However, using miniaturised GPS & time-depth recorders attached to 21 breeding adults, we show that their foraging effort (assessed during the 2005 breeding season in terms of foraging trip duration and diving frequency) is higher than that of their British relatives. Further parameters such as foraging path length, path sinuosity, maximum foraging range, flight speed, flight time, dive depth, dive duration, and diet were also recorded. Using these field data and energetics modelling we predict that northern gannets breeding at the Sept-Iles will be highly sensitive to alterations of their foraging conditions. GPS-tracking indicates that birds from this colony forage exclusively within the western English Channel, with a strong preference for the tidal front between eastern and western Channel waters, northwest of Guernsey. This frontal zone attracts a wide range of marine organisms. It is also heavily fished, and a major oil spill hotspot. We conclude that the conservation of this marine area is critically important for northern gannets breeding at the Sept-Iles Archipelago.