Australia’s oceans are highly dynamic—they vary daily, monthly and annually, driven by winds and tides, the seasons, the influence of the world’s major ocean currents and the global climate. Near the shore, marine ecosystems are heavily influenced by land-based factors such as river run-off, non-point sources of pollution and the effects of human activities. Most of these impacts have historically been focused in the coastal lands (such as mangrove wetlands, shallow reefs and beaches) and the shallow inshore waters down to a depth of approximately 100 metres, which are usually found close to shore and are readily accessible by small boat.
However, technological advances have enabled our marine activities to become more intensive in nearshore waters and progressively expand into deeper waters. For example, in the past three decades, high-quality position-finding and underwater acoustic systems have become affordable and widely available. As a result, oil and gas exploration and fishing have now moved into waters more than 1 kilometre in depth. This has increased the potential for impacts in the oceans in remote places and at greater depths. The exploitation of places that were once beyond the reach of fishing, or could not be repeatedly targeted, has contributed to the problems of overexploitation in many fisheries—the refuges that once existed for many species in places remote from the coast, or where the seabed was formerly too rugged to be fished, have been reduced or removed. Equally, onshore development is reducing the size of coastal wetlands that are valued as breeding and nursery areas for marine species.
The primary broad drivers of environmental change in Australia’s marine ecosystems are outlined in Chapter 2: Drivers. These drivers are expressed in various places and times as specific pressures on the marine environment, many of which cause obvious and measurable ecosystem impacts. However, not all the impacts are measurable, because of their type, extent or complexity; and many ecosystem changes result from the cumulative effect of two or more pressures. As a result, it is rarely possible to identify a single cause for changes that may be considered detrimental.
The pressures and their impacts primarily affect the east, south-east and south-west regions; many parts of the north and north-west regions remain in near-pristine condition, although development pressures there are rapidly increasing, particularly from mining. Pressures in parts of the temperate regions are very high; they include the impacts of climate change, urban areas, ports, catchment run-off, fishing, aquaculture, tourism and mining. This pattern reflects both the existing distribution of our population and the distribution of the industries and activities that rely on coastal resources.
Australia is also following the global pattern for coastal zone areas, which are under much greater pressure than the offshore areas. Despite our investments in management systems, many of the same impacts that occur overseas are apparent in areas of Australia’s oceans and coasts. For example, the impacts of fishing—such as the large and broadscale reductions in biomass that persist even when fishing ceases—have been observed in many large species that are fished, across all the global oceans39 and in Australian waters. It is possible for biodiversity to recover when pressures are reduced, as has been observed in the case of humpback whales in Australia’s waters. However, the recovery is usually much slower than the rate of decline and often more uncertain.
This section summarises the known and likely extent of impacts from the drivers and pressures on the environment, considering the interactions between the highly complex and natural dynamics of the ocean ecosystems and the effects from human sources.