2. Biodiversity

Sharks and rays (class Chondrichthyes) are fish that have a cartilaginous skeleton, rather than bone. Approximately 140 species of sharks and rays are found in the Region.³⁸⁷ These range from species that remain on a single reef for their entire life (such as epaulette sharks) to transient species that pass through reef habitats (such as cownose rays and tiger sharks).³⁸⁸

Since 2019, knowledge of the fine-scale movements, behaviour and habitat use of juvenile sharks and rays in shallow waters and coastal nursery areas has improved.^389,390 For example, in the north of the Great Barrier Reef the waters around Milman Island have been identified as a possible nursery area for both blacktip reef sharks and giant shovel‑nose rays, based on observations of social grouping behaviour in juveniles.^391,392 In the south, a repeat observation of an individual ornate eagle ray suggests that Lady Elliot Island may be an important location for this species.³⁹³

Sharks and rays play a range of important roles in the ecosystem.³⁸⁸ Some are apex predators that provide top-down control over prey populations, while others are mid‑level predators ³⁹⁴ (mesopredators), benthic invertivores and filter feeders.³⁹⁵ Most reef sharks are mesopredators.^394,396 Larger predatory sharks modify behaviours of herbivores such as turtles and dugongs,^397,398 and of mesopredator fish species, such as groupers, snappers and emperors. These fish species spend less time undertaking risk-prone behaviours when they live on a reef with sharks.^397,399,400 Recent research indicates that great hammerhead sharks play an apex predator role in shallow, inshore habitats, potentially shaping space-use and foraging behaviours of rays.⁴⁰¹ Rays may assist ecosystem processes, such as bioturbation,⁴⁰² and indirectly affect behaviour of species in their habitat.^402,403 Generally, declines in shark numbers on coral reefs have been associated with an increased dominance by rays, indicating a community-wide shift that may be associated with reduced ecological functioning.⁴⁰⁴

Globally, populations of oceanic sharks and rays have declined by 71 per cent over the past half century due to an increase in fishing pressure.⁴⁰⁵ Within Queensland, long‑term declines may have occurred in coastal populations of several shark species over the period 1962 to 2017, with corresponding declines in body size also observed.⁴⁹  Several sharks found in the Region are assessed as critically endangered, vulnerable or conservation dependent under the Environment Protection and Biodiversity Conservation Act 1999 (Cth) (EPBC Act), while many other species are listed as threatened at a global scale by the International Union for Conservation of Nature.²⁵⁸ Nevertheless, a recent global assessment of sharks and rays highlighted Australia as among the nations with greatest shark presence and potential for conserving sharks (based on sites including the Great Barrier Reef).⁴⁰⁶

Many sharks and rays are long lived, mature late and produce relatively few young, characteristics that can make them vulnerable to anthropogenic impact. Species such as great hammerhead sharks that have specialised foraging behaviour and limited demographic connectivity may be particularly vulnerable.⁴⁰¹ Overfishing is the main threat to sharks and rays worldwide, including through unintentional catch by recreational fishers ⁴⁰⁷ and in small- and large-scale fisheries.^{406,408,409,410} Other threats include climate change and habitat loss and degradation.^388,411 Understanding of the effects of climate change on sharks and rays has expanded since 2019, with fast swimming species expected to be disproportionately affected by increasing temperatures.⁴¹² Impacts on embryos and juveniles of some species as a result of warming ^413,414 and acidification ⁴¹⁵ are also likely.

Protected area management offers the greatest protection for sharks and rays,^416,417,418 particularly when complemented with other management tools (such as bycatch reduction devices).^406,419 These tools are implemented within the Region and are likely to reduce pressures on sharks and rays. Sustainable fisheries are currently being advanced in the Region to help protected threatened species and reduce bycatch incidents (Box 5.2). In 2023, all species of hammerhead sharks, guitar fish and requiem sharks were listed on Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) (Section 5.4.1). Since January 2024, taking of hammerhead sharks from the Region is no longer permitted.³⁷⁶

Fisheries-related 2020 stock status assessments for east coast stocks of the Australian blacktip shark and common blacktip shark classified them as sustainable.^420,421 The same was true for the northern Australia stock of the spot-tail shark.⁴²² However, in general, robust population data is lacking for many shark and ray species in the Region. For example, historical declines are recognised for multiple sawfishes across their broader Australian range but contemporary estimates of population size or change in the Region are not available.^{423,424,425,426}

Overfishing (including unintentional catch) and climate change-related ocean warming and acidification affect many species of sharks and rays worldwide. In the Region, multiple species are in a reduced state compared to historical population levels. Current management tools may be helping to reduce further impacts. More work is needed to understand the condition (and trend in condition) of individual species.