The Australian marine environment is home to some of the most diverse marine biota in the world (Williams et al. 2017). To date, 33,000 marine species have been recorded in Australia’s oceans. A further 17,000 have been collected but not catalogued, and many new species are still being discovered. It is estimated that there may be as many as 250,000–500,000 Australian marine species, not including microscopic plants and animals (Williams et al. 2017).
In this report (and previous reports), we focus our attention on the state and trends of 15 species groups that are caught by fisheries or protected under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). Information for some species, including marine turtles and seabirds, is largely limited to information on land-based nesting or breeding areas (see the Coasts chapter), and little information is available about their life at sea. Several species that use the marine environments of Australia (e.g. marine turtles, marine mammals, seabirds, billfish, tunas, sharks) demonstrate connectivity with regions outside the Australian exclusive economic zone (EEZ), and many of the pressures currently affecting these populations are associated with fishing activity outside the Australian EEZ (see case study: Management of shared marine biodiversity values). Within the Australian EEZ, varying pressures are exerted on species and species groups across their ranges, because many species and species groups are widely distributed throughout Australia’s marine environment.
Aboriginal and Torres Strait Islander people are intrinsically linked with Country (including sea Country). The idea of what Country encompasses extends beyond habitats – it includes all living things, including marine species (Bird Rose 1996). For many Indigenous people, there is no separation between Country and themselves, and the continued survival of coastal Indigenous people relies on the continuation of access to healthy Country, including species (Eckert et al. 2018).
Most marine species are in good condition, but it is not clear whether this condition is stable or declining. Notable exceptions are reef fishes, sea snakes, turtles and pinnipeds (fur seals and sea lions), which are in poor condition and are deteriorating (trend unclear for turtles). Traditional Owners assessed sea snakes and sea turtles as in good condition. Note that the spatial scale of Indigenous and western science assessments may be different. Related to United Nations Sustainable Development Goal targets 14.2, 14.4, 14.5, 14.6
Knowledge of state and trends is improving, but is variable between species, species groups and marine regions. The South-east and Temperate East marine regions are, on average, worse than the national state (Kyne & Simpfendorfer 2021). The Indigenous assessment locally was good, with a stable trend.
Status is calculated at regional fisheries management scales, rather than national scales. Methods used to assess status vary between species and regions (Evans et al. 2021a). The Indigenous assessment regionally was good, with an unclear trend.
Assessed grade improved from poor in 2016 to good in 2021, based on an increase in the percentage of fish stock classified as sustainable (Koopman 2021).
Deepwater demersal and benthopelagic fish species in Australian waters are considered to be in good and stable condition, but the long-term consequences of ocean warming are uncertain (Tuck et al. 2021).
Assessments are restricted to the Temperate East, South-east and South-west marine regions. Fishing pressure has lessened, and biomass information is improving (Molony et al. 2021).
Increases in proxies of abundance in the South-east Marine Region suggest that mesopelagic fishes may be increasing (Kloser & Kunnath 2021).
Variable trends at local scales, but some declines in abundance evident at larger scales. Changes in the local fish composition are almost ubiquitous (Stuart-Smith & Edgar 2021b).
State and trends are likely to be regionally variable, with timeseries lacking for most of Australia. Some localised improvements are likely, but some areas have worsened as a result of extreme climate events and climate change–induced range expansions (Tanner & Pitcher 2021b).
The Indigenous assessment regionally was poor, with a deteriorating trend.
There are limited temporal data and historical baselines to determine state and trends. Trawling effort pressures are decreasing in the long term, but pressures associated with climate change are increasing (Tanner & Pitcher 2021a).
Seabird population trends are highly variable, with numerous threats identified and increasing pressures on Australian seabird populations (Woehler 2021). The Indigenous assessment locally was good, with a stable trend.
The status of foraging marine turtle populations is mixed. Little quantitative data are available on habitats and impacts from threats at sea. Protection of turtle foraging habitat is lacking, particularly in the Gulf of Carpentaria (Arthur 2021). The Indigenous assessment regionally was good, with a stable trend.
State and trends are variable between species and marine regions. Current state is likely good for some species, but available evidence indicates that many populations are declining. Populations in the north-west and east are in worse condition than the national average (Udyawer 2021). The Indigenous assessment locally was good, with an unclear trend.
Population state and trends for most species are unknown but assumed stable; Australian humpback and snubfin dolphins are demonstrating decreasing trends (Evans 2021). The Indigenous assessment regionally was good, with an unclear trend.
Population state and trends for most species are unknown but assumed stable; humpback and southern right whales (Australian western population only) are demonstrating clear increasing trends (Evans & Harcourt 2021).
Populations of long-nosed fur seals are in good and improving condition, but populations of Australian sea lions and Australian fur seals are in very poor and poor condition, respectively, and both are deteriorating (McIntosh 2021).
Marine mammals
Australia’s marine environment is home to 48 species of cetaceans (whales and dolphins) and 3 species of pinnipeds (seals), along with a single sirenian species, the dugong (see the Coasts chapter).
Australia’s cetaceans and pinnipeds vary in their distributions, habitats and life histories. Some occur in Australian waters seasonally and are connected to areas outside the Australian EEZ (e.g. humpback whale – Megaptera novaeangliae). Some are distributed nationally (e.g. common dolphin – Delphinus delphis), whereas others are restricted to particular latitudes (the endemic Australian sea lion – Neophoca cinerea) or highly restricted to embayments and tributaries in particular regions (e.g. the endemic Australian snubfin dolphin – Orcaella heinsohni).
Condition of cetaceans
Information on the status of most cetacean species in Australian waters is not available, although populations are assumed to be stable (Evans 2021, Evans & Harcourt 2021). Globally, under the International Union for Conservation of Nature (IUCN) Red List, the status of a number of species has been updated since the 2016 state of the environment report (Table 2), with increasing population trends for blue (Balaenoptera musculus), fin (B. physalus), humpback and sei (B. borealis) whales; decreasing population trends for Australian humpback dolphins (Sousa sahulensis) and Australian snubfin dolphins; and unknown trends for all other marine mammal species.
Australian estimates of populations are generally positive:
- Population growth rates for northward-migrating east coast humpback whales have been estimated at 10% per year (Pirotta et al. 2020). The population has likely fully recovered from commercial whaling and may soon surpass original population levels (Noad et al. 2019). As a result, humpback whales were removed from Australia’s threatened species list in 2022, having previously been listed as Vulnerable. However, if the capacity of regions to carry these population levels is exceeded, rapid decline may result. Furthermore, increasing populations lead to increases in management needs (e.g. mitigation of bycatch and entanglement; Tulloch et al. 2020).
- Population growth rates of southern right whales (Eubalaena australis) in south-eastern Australia have been estimated at 4.7% per year, but with no significant change in the numbers of cow–calf pairs at the only recognised calving ground in the region (Stamation et al. 2020). This contrasts with a growth rate of approximately 6% per year for the population in western Australia (Smith et al. 2020a).
No nationwide population estimates are available for any species of dolphin, but Australian humpback and snubfin dolphins are declining in the North-west Marine Region (Brown et al. 2016, Brown et al. 2017, Raudino et al. 2019).
Condition of pinnipeds
The endemic Australian sea lion is listed as Endangered both in Australia under the EPBC Act and globally on the IUCN Red List (Goldsworthy 2015). Both long-nosed fur seals (Arctocephalus forsteri) and Australian fur seals (Arctocephalus pusillus doriferus) are listed marine species under the EPBC Act and as Least Concern globally on the IUCN Red List (Chilvers & Goldsworthy 2015, Hofmeyr 2015, McIntosh 2021). Australia long-nosed fur seals are considered to be in good and improving condition, but populations of Australian sea lions and Australian fur seals are in very poor and poor condition, respectively, and both are deteriorating (McIntosh et al. 2018, Goldsworthy 2020, Goldsworthy et al. 2020, McIntosh 2021).
Pressures and management for marine mammals
The main pressures on marine mammals within Australian waters include bycatch in commercial fishing operations, interactions with vessels (tourism operations and recreational), ship strike, entanglement in debris and fishing gear, coastal habitat loss from development, temporary disturbance and habitat avoidance caused by vessel disturbance and noise, and changes to breeding and feeding habitats and marine food webs associated with climate change (Speakman et al. 2020). Interaction with aquaculture operations is also an important pressure for pinnipeds (Cummings et al. 2019, McIntosh 2021), and disease and toxicants are emerging threats (Taylor et al. 2018, Taylor et al. 2021).
However, given the lack of knowledge for many species, it is difficult to identify how current pressures may be affecting marine mammal populations in Australian waters, and therefore the outlook or the potential resilience of species populations to environmental change (although see Tulloch et al. 2019).
Marine mammals in Australia are listed under the EPBC Act, the only changes in status since the 2016 report has been the removal of humpback whales from listing in 2022, and uplisting of the Australian sea lion from Vulnerable to Endangered in December 2020. Individual states and territories also list marine mammals under relevant legislation, with their status sometimes varying between jurisdictions. The Australian Government and most state and territory governments have agreed to deliver a common assessment method for determining the conservation status of Australian threatened species, based on IUCN criteria (DAWE 2021d). This has meant that some states and territories have reassessed species and produced provisional assessments of status (e.g. DELWP 2022).
As part of this process, some states and territories have reassessed several whale species. Victoria has released provisional assessments of blue (Critically Endangered), southern right (Endangered) and humpback (Critically Endangered; this status is still under review) whales (DELWP 2021). In Western Australia, the humpback whale has been downlisted from Vulnerable to Specially Protected – Conservation Dependent under the Wildlife Conservation (Specially Protected Fauna) Notice 2018.
The conservation management plan for blue whale (Australian Government Department of the Environment 2015) was updated in 2015 and is in place until 2025. The conservation management plan for southern right whale (SEWPAC 2012) is in place until 2023. Conservation advice for humpback, fin and sei whales has not been updated since the 2016 state of the environment report.
Sharks, rays, and chimaeras
Chondrichthyans (sharks, rays and chimaeras) are widely varied in size, habitat and ecological roles, encompassing species that feed on plankton to those at the top of marine food chains. A total of 329 species of sharks, rays and chimaeras have been recorded in Australian waters from the coastal zone to the abyssal plain, and 42% of these are endemic – that is, they are found nowhere else (Kyne & Simpfendorfer 2021). Sharks and rays have substantial cultural significance, and some species are the focus of tourism operations (e.g. whale shark – Rhincodon typus, white shark – Carcharodon carcharias), or valuable fisheries (e.g. school shark – Galeorhinus galeus, gummy shark – Mustelus antarcticus). However, some species may also be the focus of localised fears around shark attack in coastal areas (Ryan et al. 2019b) (see case study: Shark–human interactions on Australian coasts, in the Changes in tourism section in the Coasts chapter).
The action plan for Australian sharks and rays 2021 (Kyne et al. 2021) reported 12% of chondrichthyan species as threatened (Critically Endangered, Endangered or Vulnerable), 10% as Near Threatened, 70% as Least Concern and 8% as Data Deficient (Figure 11). Based on this assessment, the overall state for chondrichthyans can be considered as good, although there have been significant population declines in some species and species groups (e.g. sawfishes, some demersal species). The recent trend is also considered to be improving, given the ongoing recovery of some protected or previously overexploited species.
The main pressure on chondrichthyans is commercial fishing (Simpfendorfer et al. 2019, Kyne & Simpfendorfer 2021). Fewer than 20 species are directly targeted in Australian commercial fisheries (Simpfendorfer et al. 2019, Kyne et al. 2021). Most other species are caught incidentally by commercial fishing operations (bycatch), and they may be retained or discarded. In the South-east Marine Region, bycatch in fishing operations has had a significant effect on bottom-dwelling populations of the upper continental slope. Chondrichthyans that are slow-growing, long-lived species with limited reproductive output are more vulnerable to overexploitation than fast-growing species.
To a lesser extent, a variety of species are also taken by shark control programs, and recreational and Indigenous fisheries. Some species, particularly those occurring in shallow coastal waters, are affected by habitat loss. The impacts of climate change on the group are currently poorly understood (Heupel et al. 2018).
Management for sharks, rays and chimaeras
Commonwealth, state and territory fisheries management frameworks aim for ecological sustainability. This approach is generally expected to be beneficial for chondrichthyans, but benefits may vary between jurisdictions, and between target and bycatch species.
Of those species that have been historically harvested or are currently harvested, several are fished sustainably in Australian waters (e.g. gummy shark), whereas others have been overfished (e.g. school shark; Simpfendorfer et al. 2019, FRDC 2021a). The recovery of some previously overfished species demonstrates the resilience of these species under effective management. Many species are likely to benefit from existing protected areas, including the Australian marine park network; however, the effectiveness of this network is yet to be established (see Marine protected areas).
Challenges remain in the recovery of threatened species, particularly those not currently protected under the EPBC Act. For those that are listed under the EPBC Act, recovery and management plans need to be effectively implemented, funded and reviewed. For globally ranging pelagic species such as many sharks, international engagement to ensure the effective implementation of existing international instruments is required (see case study: Management of shared biodiversity values). Similarly, cross-jurisdictional management within Australia is required for highly mobile species moving between different states and territories.
Shark control programs currently operate in both New South Wales and Queensland (and have been in operation for several decades) with the goal of reducing the risk of shark attacks for beach users. Both programs include lethal methods; substantial declines in catch rates, which are considered likely to reflect corresponding population declines, have been observed over the period of operation of the control programs in both states (Roff et al. 2018, Gibbs et al. 2020). The use of lethal methods for shark control has been the subject of public debate (Simmons & Mehmet 2018), and both Queensland and New South Wales shark control strategies include investment in research into nonlethal control methods (see DAF 2021, NSW DPI 2021).
Tuna and billfish
Nine species of tuna and billfish are found in Australian waters, and support highly valuable commercial and recreational fisheries (Mobsby et al. 2020). All species are wide-ranging, with populations that extend well beyond the Australian EEZ in the Pacific and Indian oceans.
The main pressure on populations is harvesting of wild stocks. All species of tuna, broadbill swordfish (Xiphias gladius) and striped marlin (Kajikia audax) are caught both commercially and recreationally, while catches of blue (Makaira nigricans) and black marlin (Makaira indica) in Australian waters are only caught recreationally.
The status of populations subject to commercial fishing is assessed within regional fisheries management organisation (RFMO) frameworks (Table 3). Species assessments carried out by RFMOs inform domestic management within Australian waters. Overall, the status of tunas and billfishes in Australian waters can be considered good and stable (Evans et al. 2021a). However, the general outlook for tuna and billfish species in Australian waters varies regionally within species and depends on management measures (Table 4).
Management of tuna and billfish commercial fisheries
A key issue for the management of commercial fisheries under the Commonwealth Fisheries Harvest Strategy Policy framework is how to accommodate both national and international management processes and requirements, and how to reconcile potential mismatches between them.
Currently, catches of bigeye tuna (Thunnus obesus), skipjack tuna (Katsuwonus pelamis), yellowfin tuna (Thunnus albacares), striped marlin and broadbill swordfish in the Pacific Ocean are managed with conservation measures, which include limits on spatial operations and capacity restrictions. Conservation measures set limits on overall catches of skipjack and yellowfin tunas in the Indian Ocean. There is currently no allocation of catches to member nations in either region.
Global total allowable catches (TACs) for southern bluefin tuna (Thunnus maccoyii) are determined from assessments conducted by the Commission for the Conservation of Southern Bluefin Tuna. The Australian Fisheries Management Authority (AFMA) manages Australia’s allocation of the TAC.
Domestically in the Eastern Tuna and Billfish Fishery, harvest strategy frameworks are used to set TACs for broadbill swordfish (Hillary et al. 2016). For other species, AFMA sets TACs based on various factors, including stock status, local catch indices, historical catch levels and conservation measures set by RFMOs (Larcombe et al. 2020).
Reef fishes
Coastal rocky and coral reefs around the Australian continent support many fishes, including both exploited and unexploited species.
Coastal reef fish communities in most regions have changed significantly in the past 5 years, with changes in both the composition and abundance of species (Stuart-Smith & Edgar 2021a) (see case study: Australia’s changing reefs).
In a few cases, these changes are positive. For example, fish biomass and abundance appear to be increasing in the subtropical eastern region, largely because resident tropical species are thriving with warmer-than-average temperatures. Declines in the biomass of populations of large fish (more than 20 centimetres in length) observed in the 2016 state of the environment report appear to have slowed or stabilised (Stuart-Smith et al. 2017).
However, most changes have been negative. For example, temperate species at the warm edge of their distribution have declined. Likewise, coastal fish communities have declined because of severe habitat degradation: coral bleaching and cyclones in the tropics, and losses of canopy-forming kelps in some parts of the temperate zone (Richardson et al. 2018, Stuart-Smith et al. 2018, Stuart-Smith et al. 2021c).
The national outlook is likely to be poor if management cannot be more proactive and flexible in taking an ecosystem-based approach, and accounting for changing fish compositions and habitats, particularly those associated with accelerating climate impacts. Some new opportunities for fisheries, recreation and tourism may provide positive outcomes if recognised early and properly managed.
Reef fish pressures and responses
Most changes observed in reef fish populations are the result of climate change. Changing climate has affected fish either directly, through warmer waters, or indirectly, through habitat degradation. Climate change appears likely to continue to have impacts on coastal fish communities at the national scale over the coming decade (Cheal et al. 2017).
Recreational fishing is also an important pressure. Limited data on location, composition and quantity of catches, and a constantly changing number of recreational fishers, create challenges for management (Townhill et al. 2019) (see Recreational fishing).
Populations of smaller reef fish species, which dominate coastal fish communities in terms of numbers of species and local abundance, have declined since the 2016 state of the environment report. Habitat degradation and direct impacts of warmer seas on population dynamics are likely to be responsible for many of the observed abundance changes; however, the relative importance of these 2 pressures is unknown. Important knowledge gaps still exist for macrotidal and turbid inshore reefs around northern Australia.
Reef fishes appear to be the most resilient component of our coastal ecosystems, responding rapidly to shifting ocean climates and bouncing back at a few locations where adequate protection from various forms of fishing has been applied (e.g. AIMS 2021a). However, resilience to habitat degradation is still largely unknown at the national scale.
Other fishes
Many other ecologically and economically important fish species are found in Australia’s oceans (as summarised in Kloser & Kunnath 2021, Koopman 2021, Molony et al. 2021, Tuck et al. 2021). Broadly, these can be grouped, according to the part of the water column they inhabit, as epipelagic (inhabiting surface waters), demersal/benthopelagic (living on or close to the sea floor) or mid-water/mesopelagic (see Water column).
In surface (epipelagic) waters, most fish species are relatively short-lived and early to mature, and can potentially spawn over large geographical ranges. This makes many epipelagic species inherently resilient in the long term, despite significant changes in biomass associated with fishing. Commercially important epipelagic species include sardines (Sardinops sagax), jack mackerel (Trachurus declivis and T. murphyi), blue mackerel (Scomber australasicus) and redbait (Ward et al. 2019, Ward et al. 2020, Ward & Grammer 2021). The outlook for epipelagic species in managed fisheries is positive, and exploitation levels are likely to remain below levels that would cause adverse impacts on the wider ecosystem (Molony et al. 2021). However, some populations of Australian sardine have not recovered from mass mortality events caused by pilchard herpesvirus in 2000, and may be unlikely to do so (Gaughan & Santoro 2020, Norriss & Grounds 2020).
Benthopelagic and demersal fish species tend to be longer-lived and slower growing, and are therefore often less resilient to fishing pressure than epipelagic species – for example, species such as orange roughy (Hoplostethus atlanticus) have been subject to widespread overfishing in the past. Although some stocks of demersal and benthopelagic fishes remain currently overfished, the percentage of stocks that are classified as sustainable is increasing nationally (Gaughan & Santoro 2020, Patterson et al. 2020, FRDC 2021b). Overall, demersal and benthopelagic fish species are considered to be in good and stable or improving condition (Koopman 2021, Tuck et al. 2021).
The state and trends of populations of fish species that are not harvested or occur in deeper waters are less well known because these parts of marine ecosystems are understudied as a result of observation and sampling challenges. These include mesopelagic fish species (those that reside at depths of 200–1,000 metres), which are increasingly appreciated for playing a key role in ocean carbon sequestration (Fulton et al. 2005, Lehodey et al. 2010, Boyd et al. 2019). Changes in observations of mesopelagic fish in the South-east Marine Region are thought to indicate that abundances may be increasing in this region.
Pressures and management for other fishes
The increasing and ongoing implementation of harvest strategies in fisheries has improved the management and transparency of management of fish stocks, as demonstrated by the decreasing proportion of stocks classified as overfished (or equivalent; Gaughan & Santoro 2020, Patterson et al. 2020, FRDC 2021b). However, climate change, marine heatwaves and natural environmental variability (BOM & CSIRO 2020) affect the distributions, abundance and recruitment of Australia’s fish stocks. Uncertainties regarding these impacts mean that the long-term outlook for Australia’s fish stocks is currently uncertain. Projected responses of Australia’s commercial fishery species to climate change include increased abundances and shifts in species distributions (Fulton et al. 2018, Pethybridge et al. 2020).
To account for these projected changes and associated uncertainties, conservative management settings have been established to ensure that fish stocks remain sustainably fished (see Commercial fishing). National initiatives (e.g. biennial reporting on the status of Australian fish stocks; FRDC 2021b) and recent state-based initiatives (e.g. Wise & Molony 2018) for Australian herring – Arripis georgianus) facilitate sharing of knowledge and complementary management approaches across Commonwealth and state and territory fisheries. The introduction of quota management in New South Wales for Australian sardine, blue mackerel and yellowtail scad (Trachurus novaezelandiae) has highlighted an increased need for a resource-sharing policy between the Commonwealth and state and territory fisheries that exploit shared stocks.
Climate-adaptive management reforms could minimise or eliminate negative impacts of changes in the distribution and productivity of marine fisheries caused by climate change (Free et al. 2020). Principles for climate-adaptive fisheries management include implementing best practices in fisheries management; having dynamic, flexible and forward-looking management; and building socio-economic resilience.
Demand for Australian small pelagic fishes for use as feed in the aquaculture industry is likely to continue to increase. There is also increasing global interest and effort in mesopelagic fishing, highlighting a need to prepare appropriate management guidance (Hidalgo & Browman 2019). Knowledge gaps about the effects of anthropogenic noise and marine pollution on mesopelagic fish species require focused investigation and management strategies.
Sea turtles and sea snakes
Six species of marine turtles and around 33 species of sea snakes occur in Australian waters (Arthur 2021, Udyawer 2021). The flatback turtle (Natator depressus) is Australia’s only endemic marine turtle. Thirteen endemic sea snakes are currently known. Overall, the outlook for marine turtles in Australia is mixed (DEE 2017b, GBRMPA 2019, Bell et al. 2020, Arthur 2021), whereas the outlook for sea snakes is poor (Udyawer 2021).
The state and trends of turtle populations vary among species, ranging from very good to very poor (DEE 2017b, GBRMPA 2019, Bell et al. 2020, Arthur 2021). The Recovery plan for marine turtles in Australia (DEE 2017b) recognises 22 genetic stocks: 2 are considered to be improving, 4 deteriorating, 5 stable and 11 unclear. All species of marine turtles are listed under the EPBC Act (Table 5).
Sea snake populations are in a poor state with a declining trend in Australia, and recent dramatic reductions have occurred in the spatial distributions of some endemic species (Udyawer et al. 2020). Two endemic sea snake species are listed as Critically Endangered under the EPBC Act and IUCN Red List; a further 2 endemic species are listed as Endangered and Near Threatened on the IUCN Red list, but have not been assessed under the EPBC Act (Eifes et al. 2013).
Pressures and management for sea turtles and sea snakes
The pressure of greatest concern for marine turtles and sea snakes is climate change and resultant habitat loss from coral bleaching, seagrass loss, mangrove dieback, sea level rise and extreme weather events. For turtles, an additional threat from climate change is increasing beach temperatures; eggs incubated at warmer temperatures produce more females. Recent research has found that northern Great Barrier Reef green turtle rookeries have been producing primarily females for more than 2 decades, leading to calls for immediate management interventions aimed at lowering incubation temperatures at key rookeries (Jensen et al. 2018).
Marine debris, pollution and fisheries bycatch are also identified as important pressures globally, along with light pollution, intentional take (including Indigenous harvest and harvesting outside Australian waters) and consumption of eggs by terrestrial predators, including introduced species (DEE 2017b).
The life history traits of marine turtles (e.g. late maturation, high natural maturity of juveniles, fidelity to breeding sites) increase their vulnerability to a wide range of human threats (DEE 2017b). However, their use of multiple breeding locations and highly dispersed foraging habitats may provide some population resilience to local pressures.
Marine turtles are of great cultural significance to many Indigenous communities and are the focus of tourism activities. Co-management of sea turtle stocks with Indigenous partners is an important and growing management approach (Kennett et al. 2004, Klein et al. 2017).
Limited information is available on resilience of populations of sea snakes to threatening processes. The low dispersal capacity of most species suggests limited re-establishment of sea snake populations on isolated reef systems where declines have been recorded (Lukoschek et al. 2013, Lukoschek 2018). It also means that individuals with home ranges that overlap with fisheries may be caught repeatedly as bycatch. Their physiology limits their diving and foraging ability to sea surface waters (Udyawer et al. 2016), where temperatures are increasing the fastest, potentially making them particularly vulnerable to climate change (Udyawer et al. 2018).
Seabirds
Around 60 species of seabirds are known to breed in and around Australia and its external territories, including albatrosses, boobies, cormorants, frigatebirds, gulls, noddies, pelicans, penguins, petrels, prions, shearwaters, storm petrels, terns and tropicbirds. The Coasts chapter has more information about shorebirds (see the Coasts chapter).
The overall state of seabird populations in Australia can be considered good. Some monitored populations are stable, but there have been widespread decreases in some species of petrels, shearwaters and tropicbirds (Garnett & Baker 2021, Woehler 2021; Table 6). Many species of seabirds are listed as threatened under IUCN criteria (IUCN Species Survival Commission 2012) and are listed under the EPBC Act. No species of seabird is currently assessed to be at risk of imminent extinction (within the next 5 years) within Australia (Garnett & Baker 2021).
However, the current sparsity of available data and variable population trends among different breeding sites around Australia and its external territories make it difficult to provide an overall trend, and limit confidence in the assessment. Long-lived species, including many seabirds, require decades of information on population abundance to determine trends. In contrast to seabirds, populations of shorebirds (i.e. species that breed and forage in coastal habitats) are in poor condition nationally (see the Coasts chapter).
Pressures and management for seabirds
Threats to seabirds come from both marine and land environments:
- Within the marine environment, the most significant threat is fisheries bycatch (longline, trawl and recreational gillnets). An emerging threat is decreasing marine productivity and prey availability associated with increasing sea surface temperatures (Garnett & Baker 2021).
- On land, major threats are loss and fragmentation of breeding habitat, invasive animals, and disease.
Albatrosses, petrels and shearwaters are at greatest risk, particularly from fisheries bycatch (Crawford et al. 2017, Rodríguez et al. 2019, Woehler & Baker 2020).
Varying management efforts, strategies and recovery plans have been established, adopted or implemented to reduce, remove or mitigate threats (e.g. Environment and Communications References Committee 2016, Australian Antarctic Division 2018, DEE 2018, Alderman 2019). Species-specific management objectives are detailed in the draft Wildlife conservation plan for seabirds (DEE 2019).
Climate change is expected to exacerbate current threats through additive or multiplicative interactions, such as increasing sea surface temperatures decreasing prey availability (Oliver et al. 2017, Osborne et al. 2020). Rising sea levels are also expected to further fragment coastal breeding habitats.
It is difficult to quantify the pressures that seabirds face during their foraging phases away from their colonies (Springer et al. 2018, Price et al. 2020) because of the lack of long-term data on pressures, populations and distributions.
Invertebrates
The Australian marine environment is home to a wide range of motile invertebrates (see Condition of seabed habitats for habitat-forming invertebrates such as corals, sponges and bryozoans). Large areas of soft sediments on the continental shelf are dominated by invertebrate infauna and motile epifauna (species that live in and on the surface of sediments).
Overall, these invertebrate species are considered to be in good condition (Tanner & Pitcher 2021b, Tanner & Pitcher 2021a), consistent with the 2016 state of the environment report. Areas of localised impacts occur where there is substantial bottom-trawling effort, such as in parts of the eastern regions and some embayments in the North-west Marine Region (Pitcher et al. 2018). Impacts of climate change are also becoming increasingly evident and widespread. In some cases, a few heavily fished taxa are in poor condition. Comprehensive assessments of trawl impacts and risks on the Great Barrier Reef and in Torres Strait suggest that very few invertebrate species have been seriously negatively affected at regional scales by past trawling, and none are at ongoing risk (Pitcher et al. 2007b, Pitcher et al. 2007a, Pears et al. 2012, Pitcher 2013). However, in the Great Australian Bight, the benthic protection zone was found to have different epifaunal assemblages (sessile and motile combined) from adjacent trawled areas, suggesting that trawling does have a measurable impact (Ward et al. 2006).
The status of targeted species is predominantly good, but can vary between jurisdictions. Western rock lobster (Panulirus cygnus) stocks appear to have continued to increase since 2016, due to conservative management since 2008 and strong recruitment (Caputi et al. 2015, de Lestang et al. 2015, de Lestang et al. 2020). A marine heatwave in Western Australia in 2011 resulted in elevated temperatures down to depths of 100 metres; although the ecological consequences in deeper water are largely unknown, the heatwave has been blamed for the collapse of scallop and blue swimmer crab fisheries in Shark Bay (Caputi et al. 2016). Blue swimmer crabs have been recovering since then (Chandrapavan et al. 2019), but only part of the fishery for scallops has recovered; in 2019, the northern Shark Bay part of this fishery was assessed to be below the reference level requiring consideration for a recovery strategy (Kangas et al. 2019, Kangas et al. 2020). In south-west Western Australia, recovery from the direct impacts of the marine heatwave was seen in the inner-shelf motile invertebrates; however, indirect impacts through habitat loss persist (Mulders & Wernberg 2020). East coast saucer scallops were listed as a depleted stock in the 2018 Status of Australian fish stocks report (FRDC 2021b).
With the exception of key fished species (e.g. abalone, rock lobster), and recent data from the Integrated Marine Observing System autonomous underwater vehicle facility, very little data are available with which to directly assess condition and trend of seabed invertebrate species and communities. Even in areas where good biodiversity surveys have been undertaken (e.g. Great Barrier Reef, Torres Strait, Pilbara, Kimberley), we lack a good understanding of what the system may have looked like before it was modified by human impacts, and there are very few places with repeated surveys and thus empirical information on trends. Where information is available, our understanding of state and recovery trends is based on model-based approaches, such as used in the Great Barrier Reef and some other regions (e.g. Pitcher et al. 2016).
Pressures and management for invertebrates
With most invertebrate species being closely associated with particular habitats, habitat degradation is the biggest threat and, to date, has had the biggest impact, together with the direct effect of marine heatwaves. Changes in invertebrate communities independent of coral loss have been recorded along the entire length of the Great Barrier Reef and are attributed to prolonged thermal stress (Stuart-Smith et al. 2018) (see Coral reefs and case study: Australia’s changing reefs). Threats to marine invertebrates from fishing have declined substantially, with substantial declines in trawling effort over the past several decades (more than 96% of seabed not trawled in recent years – see Pitcher et al. 2018), and increasing implementation of marine protected areas on the continental shelf. Threats from climate change, however, are increasing. These potential impacts are less well studied and poorly understood relative to impacts in other habitats such as reefs.