Earlier this month Chanel 10 aired a story describing the killing of juvenile Port Jackson sharks off South Australia’s Metropolitan coast and the subsequent community-driven push for stricter laws protecting this species.
What are Port Jackson sharks?
The Port Jackson shark, or Heterodontus portusjacksoni, is a species of small shark found in rocky reefs along Australia’s southern coast, they are light grey-brown in colour with black bands and grow up to around 1.5m in length. They are harmless bottom feeders, eating echinoderms (sea stars and urchins), crustaceans and molluscs on the ocean floor.
They are a nocturnal species that forage for food at night and hide during the day under rocky ledges or in caves. They are known to migrate seasonally between foraging refuges and ovipositional refuges where they deposit their eggs between August and November. In this time females can lay a pair of eggs every ten to fourteen days, depositing up to 16 eggs per season. The embryo develops for 11 months before hatching into what is known as a neonate, 7 to 8 inches long. This lengthy incubation time means eggs are prone to predation, an estimated 89% are predated prior to hatching. Female sharks reach sexual maturity at 70-90cm between 11 and 14 years of age, males reach sexual maturity at 55-70cm at the age of 8-10 years old (Thaler 2018).
"Port Jackson Shark" by PacificKlaus is marked with CC BY-NC 2.0.
Threats to Port Jackson sharks
As well as being prone to predation in the egg and early life stages, this species is threatened by overfishing, climate change and habitat loss.
Though Port Jackson’s are not commonly targeted directly by recreational or commercial fishers they are a common bycatch. Gill nets used by commercial fisheries are not effective in excluding non-targeted species such as Port Jackson sharks and other marine vertebrates. Although studies have found that post-release these sharks are resilient to capture stress and likely to have a high survival rate, mortalities do occur either through prolonged entanglement or when killed by fishermen (Fisheries Research and Development Corporation 2019).
This species is particularly affected by the loss of individuals due to their reproductive strategy in which few eggs are produced, embryonic mortality is high and sexual maturity is reached later in life (Thaler 2018).
An article published in the Australian Geographic in 2014 explored this issue, summarising that “all shark species are susceptible to overfishing due to their slow growth, late maturity and few offspring. And sharks have a top-down influence on ecosystems, so overfishing can affect other species’ survival.” (Gilligan 2014)
Furthermore, as climate change intensifies in coming years, Port Jackson sharks will be under increasing threat from rising ocean temperatures and habitat loss.
Biochronology studies conducted across 15 years have shown that Port Jackson shark growth is sensitive to temperature change. The research showed slower growth rates correlated with higher temperatures (Izzo & Gillanders 2020).
An article in the Conversation further explored this issue, stating “with a rise in water temperature of just 3℃, the energy required to survive is more than twice that of current day temperatures for the Port Jackson sharks in Adelaide. The massive shift in energy demand we observed in the Adelaide sharks means they have to prioritise survival (coping mechanisms) over other processes, such as growth and reproduction.” (Brown & Gervais 2020)
As ocean temperatures rise it is also expected that tropical species will migrate into the temperate waters surrounding South Australia’s coast, as their habitat range is extended. Tropicalisation of the temperate zone is predicted to have a dynamic effect on the current communities of our temperate reefs and will see species being displaced by competition from subtropical immigrants. It is also noted that ecosystems in the rocky reef systems in shallow waters will be more vulnerable to the impacts of climate change than the deeper layers of the ocean.
Day & Walker (2021) state “Sharks and their relatives are an ancient group that have endured extinction events in Earth's history involving high levels of carbon dioxide. But the expected rates of temperature change are much faster now than in these previous events—particularly in south-eastern Australian waters.”
Why do we care?
Although the conservation status of Port Jackson sharks is currently described as “least concern” on the IUCN list, populations are currently in decline in the local area and growing pressures on this species from fishing, climate change and habitat loss mean greater measures are needed to protect them.
Sharks are keystone species in marine environments, meaning they have top-down control over other species in the community through predator-prey interactions. For example, sharks eat urchins, and urchins feed on macroalgae such as kelp forests which provide habitat structure for many other temperate reef species. If Port Jackson numbers were to decline numbers of urchins would increase, leading to the overgrazing of kelp and reduction of species that rely on kelp for food and habitat (Brown & Gervais 2020).
The rocky reef habitats along Adelaide metropolitan coast including Christies Beach, Port Noarlunga and Aldinga, are particularly important to the conservation of this species as they are known breeding grounds where Port Jackson sharks aggregate in large numbers.
In November 2018, conservation groups banded together to stage the first “Sharkfest”, an event that focused on raising awareness about the importance of the Port Jacksons along the Adelaide metropolitan coastline. The event included guided dives, education sessions and pamphlets targeted at educating fishermen, who have been known to kill sharks, viewing them as a pest (Spence 2018).
The aggregation of Port Jackson sharks along our coastlines attracts tourists and local divers alike who describe them as “puppies of the sea”. In this regard, they are also recognised as a tourism attraction and hold economic value for dive operators. Furthermore, they play an important role in regulating the reef community and promoting steady populations of fish that are exploited through recreational and commercial fishing.
What can we do?
The best way we can care for these animals in our waters is to be educated on the matter and encourage the local community to understand the ecological value of sharks. For fisherman information can be found on the PIRSA website that outlines guidelines for the safe handling and release of sharks and rays. See the link below.
Unfortunately, not all people follow these guidelines and there are many reports of fishermen killing or mutilating Port Jackson sharks. There are currently no laws against the killing of these sharks as the only shark species protected by law in this region is the Great White Shark. In an effort to push for stricter laws to protect Port Jackson sharks a petition has been launched. The petition aims to ban the killing or deliberate harm of Port Jackson sharks and rays in Port Noarlunga and surrounding areas with penalties applied, post signage around boat ramps and jetties, educate anglers on current regulations, and increase monitoring and research of this species. This petition will be submitted to the Minister for Environment, PIRSA, Fisheries and all key leaders state government.
The petition currently has almost 15 000 signatures. These are some of the reasons people gave for signing.
“I'm signing because I was with my dive buddy on Saturday at Port Noarlunga. We discovered three juvenile Port Jackson sharks that had been stabbed in the head. They were paralysed but still alive. It was heart breaking. This senseless cruelty needs to stop.”
“Many species are needlessly harmed by a minority of fishers. I have seen species like ornate cow fish, leatherjackets and rough rock crabs left to die on jetties by anglers. A law requiring all by catch to be returned carefully, immediately after capture would be a good way to reduce the incidence of this behaviour by a minority of fishers”
“these lives are worth more alive in our oceans than what a fisherman thinks, just too avoid re catch and not wanting them to take their baits.”
“I love Port Jackson sharks and I was horrified seeing a baby just left on Semaphore Jetty dead, it made me so sad, they are so gorgeous, people don’t protect them”
If you would like to add your voice to this petition, please sign using the link below.
Article written by Mary Gordon