However, if not sustainably managed, fishing can damage fish habitats, reduce biodiversity and impair the functioning of ecosystems with negative repercussions for sustainable social and economic development. To achieve a healthy balance, fish stocks must be maintained within biologically sustainable limits—at or above the abundance level that can produce the maximum sustainable yield. Based on an analysis of assessed stocks, the proportion of world marine fish stocks within biologically sustainable levels declined from 90 per cent in 1974 to 69 per cent in 2013 and appears to have stabilized over the last few years.