Dolphins, fish and shellfish under pressure on South Australia’s coastline

Marine scientists worried that key fish species, and “near threatened” dolphin and shellfish around South Australia’s coastline are under pressure are calling for stepped up conservation efforts to protect the ecosystems.

Challenges of the South Australian marine environment have been highlighted in 3 recent scientific reports from Flinders University.

Mike Bossley, who has led a team tracking the local Indo-Pacific bottlenose dolphins (Tursiops aduncus) of the Adelaide Dolphin Sanctuary for 34 years, found a population decline between 2012 and 2020. He says recent observations show the population has stabilised in 2021–24.

Bossley is a field researcher at the charity organisation Whale & Dolphin Conservation. Scientists say the sanctuary needs better protection.

The marine area was established in 2005 to protect the dolphins and the habitat that sustains them. It is in Port Adelaide, less than 15km north of the city centre.

Indo-Pacific bottlenose dolphins are listed as “near threatened” in the IUCN Red List of Threatened Species, despite deficient data. The global population is unknown.

These dolphins can be found as far afield as the coasts of East Africa, South Asia, the Arabian Peninsula and around Australia. 

Bossley’s team collaborated with researchers at Adelaide’s Flinders University Cetacean Ecology, Behaviour and Evolution Lab (CEBEL) to produce a report on the dolphins’ long-term population trends.

The Adelaide population lives in a highly urbanised but shallow estuary. It also faces pressures from local and global effects of climate change.

The new study published in the Ecology and Evolution journal shows the dolphins have shown remarkable resilience.

“It’s important to focus conservation strategies on improving the Adelaide Dolphin Sanctuary ecosystem and promoting connectivity to the surrounding waters to help secure the future of these dolphins,” says first author Kennadie Haigh, a PhD candidate at Flinders. 

Another Flinders University study published in the journal Ocean & Coastal Management highlights the historical exploitation of South Australia’s shellfish reefs.

The researchers say urgent action is needed to restore native marine species populations and return the ecosystem to a healthy state.

“Human and environmental stresses, as well as overfishing and dredge harvesting, have combined to significantly diminish our local multi-species shellfish reefs, which once covered more than 2,600 square kilometres of the state’s coastline,” says first author and PhD candidate Brad Martin. 

“Based on historical records, we documented 140 potential shellfish reef locations, and we estimate that over 43 million flat oysters were commercially harvested statewide between 1849 and 1915, prior to their functional extinction by the 1940s.

“Shellfish reef decline was also influenced by environmental factors including drought and salinity issues, disease, heavy predation by marine species and sediment deposition from storms.”

A third study published in the journal Environmental DNA sought to develop methods to track communities of fish in marine ecosystems including remote parts of the Great Australian Bight off the far west coast of South Australia.

“Fish communities are critical indicators of ecosystem health, and comprehensive monitoring strategies are vital to effective management of marine fishes,” says senior author Michael Doane from Flinders University.

The study found 2 survey methods effective and complementary in detecting different fish species.

One method used environmental DNA (eDNA) – genetic material left behind by organisms as they swim through the water – to estimate fish dispersal. The other used Baited Remote Underwater Video Systems (BRUVS) to assess the fish communities at locations off South Australia’s coast.

“By combining both methods, we gain a much fuller picture of fish communities,” says first author Ewan Burns. “eDNA excelled at detecting large pelagic species like white sharks (Carcharodon carcharias) and southern bluefin tuna (Thunnus maccoyii), while BRUVS revealed more bottom-dwelling fish.”