 For example, radically depleted fisheries have been evaluated by researchers who used the state of the fishery at the start of their careers as the baseline, rather than the fishery in its unexploited or untouched state.Areas that swarmed with a particular species hundreds of years ago may have experienced long term decline, but it is the level a few decades previously that is used as the reference point for current populations.In this way large declines in ecosystems or species over long periods of time were, and are, masked.