Marine ecosystems and fisheries face serious threats from over-fishing, run-off of land-based pollutants, introductions and invasions of exotic species, coastal development and habitat alteration, unintended by-catch, and climate change. Annual global fish catches fluctuate between 80 and 90 million metric tonnes (MMT) and appear to be declining. As a result of limitations on wild capture, aquaculture has emerged as a major player in seafood production and marketing worldwide. During the past decade, global output of farmed finfish and shellfish almost tripled in weight and nearly doubled in value. This paper shows how aquaculture is both a contributing factor and a possible solution to the decline in world fisheries. The threats include overfishing of small pelagic fish (low on the marine food chain) to feed farmed fish, the transmission of diseases from farms to the wild, and genetic changes caused by the escape of farmed fish from netpens. The introduction of offshore aquaculture facilities and genetically modified farmed fish present further risks to wild fish populations. The paper discusses the benefits and risks of aquaculture for marine ecosystems and wild fish supplies, and presents ideas for a more sustainable future for fish production.