In marine and coastal areas, the main threat to biodiversity arises from the over-exploitation of marine and coastal resources and from related fishing practices, although pollution and coastal degradation also represent significant threats.
Habitat conversion and degradation, overexploitation, pollution and sedimentation, coastal erosion, eutrophication, species introductions and climate change are considered the major causes of marine biodiversity loss.
The threats to marine and particularly to coastal biodiversity are similar in many respects to those facing biodiversity on land, with most threats originating from land-based activities. Such threats include marine pollution, from domestic sewage, industrial waste, stormwater drains, and oil spills; coastal zone degradation, from rapid urbanisation, tourism, recreation, infrastructural development, and mining on parts of the coast and in the ocean; the overexploitation of marine resources, primarily by industrial fisheries, but also by recreational fishers, and in some intertidal areas, by subsistence communities; and the introduction of alien species, either inadvertently through ballast water or on ship hulls, or intentionally through activities such as mariculture.
Between 1970 and 2002 the ocean's biodiversity reduced by a third.
Many marine fisheries have been grossly over-exploited, and their recovery will be slow. Future growth in demand for fish will have to be satisfied by aquaculture - itself a practice fraught with environmental dangers.