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).

AFMA-managed fisheries are using the newly developed harvest strategy (see Section 1.5.2),²³ to move towards a more secure and sustainable level of production for the various species within the Commonwealth jurisdiction. The need for this strategy is illustrated by the history of the Western Deepwater Trawl Fishery (Box 6.3).

Both offshore and coastal fisheries have suffered substantial declines over the past century. A recent study of the coastal fish of Tasmanian waters⁴⁷ suggests that both climate change and fishing have had severe impacts on approximately 20% of the island’s coastal fish species, beginning with the arrival of Europeans and their fishing practices in the early 1800s, and made worse more recently by accelerating climate change. The reduction in a number of popular fishing species has been offset by the appearance of several alternative species that are expanding their range southwards from the mainland. These substantial changes in species composition demonstrate that the drivers of long-term shifts in coastal diversity may have a variety of sources, and their ecological impacts may extend beyond a reduction in fishing resources to include direct impacts on coastal ecosystems by affecting interactions within the food chain.⁴⁷

In other states, the coastal fisheries are also suffering declines—many species are considered to be fully fished, while others are recognised to be depleted and suffering population declines.^18,48 In coastal waters and the continental shelf, the species that can be fished are mostly fished to their limits and, for some, overfishing has resulted in population collapse.⁴⁸ So, while modern-day fishing practices are generally much improved over practices used as recently as 30 years ago, the legacy effects from the intense fishdown phase of virgin stocks (such as in the South-east Tiger Flathead Fishery—see Box 6.4) are a dominant feature of the population structure of most fishable species. The relative risks from other impacts are now increasing, requiring intense vigilance from fishery managers to avoid catastrophic and long-term impacts on populations of these (mostly large) marine species that were once considered to be abundant and widespread in our oceans and estuaries.

There is a high risk that, after heavy fishdown or other forms of overfishing, depleted stocks may not be resilient or recover quickly (such as the eastern gemfish example in Figure 6.13). While they are in such poor condition, they may be subject to other environmental pressures, including climate change impacts. The flow-on effects on the ecological functions of the oceans are largely unknown. It is likely, however, that fishdowns of most of the fished species have left Australia’s oceans much less resilient by reducing diversity, modularity and feedback within ecosystems (see Section 5 of this chapter). This probably has important consequences for the capacity of marine ecosystems to adapt to the combined effects of the present-day pressures of climate change, habitat loss and fishing pressure.⁴⁵

Declining stocks may lead fishers to fish harder to catch the remaining fish. In fish catch data, this pattern can be detected as a progressive reduction in the size of catches, reduction in the size of fish being caught, or a change in the type of fish being caught. It can also bring about a shift in the trophic level of the fish in the catch. Where this occurs, fish catches shift from species higher in the food chain (at a high trophic level), including large carnivores such as sharks, to successively lower trophic levels of smaller and less valuable fish. This is measured by the marine trophic index (MTI)—an international marine indicator.⁴⁹

For most Australian stocks, there are insufficient data to calculate a shift in trophic structure of the fisheries, such as that estimated by the MTI.⁵⁰ However, there is evidence in places that fishing may have altered species composition. Although the MTI is a gross index, it is the only indicator that is widely used to detect and report on gross changes in the trophic structure of fished ecosystems over time. The MTI has not been adopted in Australia, and there is no requirement for fisheries to report on such matters. Most of the impacts of fisheries in Australia are now historical, and present-day management practices are (generally) much improved. However, most of today’s fisheries have harvest strategies that manage the stock biomass at an agreed level that is significantly lower than pristine levels (typically 40%), with management arrangements to reduce pressure if stocks drop below this level. The pressure of present-day fishing (both commercial and recreational) acts to maintain low abundances and biomass (relative to pristine levels) and probably to reduce the resilience of the populations being fished and their ocean ecosystems.

However, not all fished stocks have failed to recover from overfishing, and there are a number of documented recoveries and management success stories, most notably the South-east Tiger Flathead Fishery (Box 6.4). After extensive management intervention, this species has been found to be remarkably resilient and has shown significant population recovery.

Recovery of fish stocks is a common objective of modern fisheries management, and Australia has a number of success stories. It has long been known that the key to success is to ensure that populations are fished at rates that are below the level at which optimum yield could be taken, allowing stocks to gradually rebuild while continuing to provide for sustainable fishing.⁵² By extracting slightly less each year than the maximum sustainable yield, a fishery can gradually increase both the overall stock size and the annual yield, providing for substantial long-term gains at the cost of minor short-term losses (in terms of catch). Such approaches are now in wide use, together with the careful application of no-take marine protected areas and reserves (see Box 6.5), to begin the long process of rebuilding stocks and recovering degraded ecosystem functions. However, the effectiveness of these management interventions to achieve long-term stock rebuilding remains to be assessed in most Australian fisheries.