Fishing has provided an important commercial, recreational and subsistence resource for Australians for many decades. As fishing effort has expanded, so have the environmental impacts that inevitably accompany such exploitation. These impacts include the direct effects of fishing on the species being caught (related to the intensity and extent of fishing effort); the effects on other species that may depend on the targeted species as predators or prey; the direct effects of fishing gear on habitats; and the catch of unwanted species (bycatch). Fishing in all its forms is now recognised as a major factor affecting marine ecosystems through these various impacts. Jointly, exploitation and habitat loss are considered to be the primary threats to fish stocks, with major potential impacts on the ecology of ocean ecosystems.35 Almost all the species that are large enough and abundant enough to be fished are targeted, and they comprise important ecological components of the ecosystems.45 Despite this, there is no nationally integrated analysis of the cumulative impacts of fishing or fisheries on ecosystem structure or function, and no national-level initiatives to assess and report on ecological sustainability of commercial or recreational fishing sectors. This major gap limits the extent to which the pressures on marine ecosystems can be assessed.
With increasing population and rapidly improving technology, virtually all of Australia’s marine areas that are less than 1 kilometre in depth are, or have been, fished to some extent. In the sanctuary zones of marine protected areas and other small areas protected from fishing as nursery grounds (less than 5% of our marine environment), all forms of fishing are permanently banned to protect biodiversity, and there are some areas where fishing gear is too difficult to use. These highly protected areas and topographic refuges are mainly found offshore and in deep waters; the biodiversity of these deeper regions is poorly understood, with more than half of species in some surveys previously undescribed. Some regions have areas with high levels of permanent restrictions on fishing (for example, within more than a third of the Great Barrier Reef Marine Park). Numerous smaller fishery closures have been implemented in recent years to protect sensitive habitats and species.
The historical patterns of catches over the period of post-European exploitation of Australia’s fish stocks reveal that there have been major changes in many of the stocks and probably also in their associated ocean ecosystems. In many cases, fishing has shifted from one species to another as a target species becomes difficult to catch. This is known as serial depletion—the systematic ‘fishdown’ of target species to levels that become uneconomic to exploit. In Australian waters, there are a number of examples of such depletion and, although it has not resulted in the extinction of any fished species, many stocks have been left at such low levels that they may take many years (and possibly centuries) to recover. Most stocks are managed to avoid such extremely low biomass, and a number have been restored by strong management actions after very low stock sizes were detected. Possibly the worst contemporary example of fishdown is the eastern gemfish population in south-eastern Australian waters, which has been intensively fished down over the past 50 years (Figure 6.13).