Shellfish reefs enhance marine biodiversity

Local shellfish including native razorfish and flat oysters, and wild pacific oysters, have demonstrated strong ecosystem benefits for marine species within the Port River estuary ecosystem in Adelaide—with Flinders University researchers also discovering an exotic whelk finding a foothold in the popular coastal area.

Environmental monitoring of Port Adelaide’s marine environment, with adjacent natural assets including remnant mangrove forests and the International Bird and Dolphin Sanctuaries, has seen significant changes in water quality and species composition over the years.

This leads to the Flinders University Ph.D. candidate Brad Martin, from the Ecosystem Resilience research group, to conduct fieldwork at various shellfish reefs throughout the Port River earlier this year to assess the species that use these poorly understood habitats.

Following an outbreak of disease in wild oysters, the South Australian Government introduced restrictions on the collection of shellfish including oysters, mussels and razorfish in the Port River in 2018. At the same time, local community groups also began restoration projects for native flat oysters, with native shellfish reefs that were once common on the Australian coast.

As well as finding many native fish and invertebrates, the fieldwork also found many invasive species using the wild oyster habitat, including the first sighting of the Hercules club whelk (Pyrazus ebeninus), which was “probably introduced through ballast water from the ship’s hull.”

“Currently these intertidal species are unlikely to have significant negative impacts on other species, but may spread in the future beyond the Port River,” he said.

“The establishment of species from the east coast such as the mud whelk and Sydney cockles is also an indication of warmer conditions. Until recently, these species only appeared in the fossil record in SA and died out during one of the previous ice ages.”

The Hercules club whelk is a large marine snail that grows up to 10cm in length and is common in east coast estuaries.

He says the benefits of many shellfisheries in the area include improving water quality from their filter feeding, and providing habitat and nursery grounds for other species including recreational important such as whiting, bream and blue swimmer crab.

Bradley Martin’s research from Port River “Density and death: the habitat function of intertidal razorfish (Pinna biocolor) beds” recently won the Fisheries Research Development Corporation (FRDC) Prize for Best Oral Presentation at the Australian Marine Sciences Association conference on the Gold Coast.

Also presented by his supervisor Dr. Ryan Baring’s “Sediment stabilization of the restored shellfish reefs,” from research conducted on the Glenelg shellfish reef, with Associate Professor Graziela Miot da Silva, from the Beach and Dune Systems Laboratory at Flinders University

In the two years since the establishment, Dr. Baring said oysters now grow on the restored Glenelg shellfish reef.

“The restored Glenelg shellfish reef has many oysters growing that contribute to an ecologically important nutrient sink to the seafloor, which is important for turning unhealthy nutrients from stormwater runoff into useful nitrogen in the ocean and atmosphere,” he said.

“We have also identified a reliable hydrodynamic and sediment model to assess the performance and repair of the Glenelg Shellfish Reef to improve its ability to stabilize sediments that are more challenging to manage along highly developed metropolitan coastlines under climate change.”

• Native oyster (Ostrea angasi) reefs were common in Southern Australia until the 1940s when they declined due to overfishing, disease and habitat change. In South Australia, shellfish reefs occur on coastal beaches and estuaries, covering rocky reefs and the surface of razorfish (Pinna bicolor). Today less than <1% of these reefs remain and Ostrea angasi survives in small numbers.
• In recent years several reef restoration projects have been established in Gulf St Vincent by government and community groups and The Nature Conservancy.
• Indigenous oyster harvesting was one of South Australia’s earliest fishing industries and was regulated by the “Oyster Bed” Act in 1853, the state’s first fisheries legislation.
• Port Adelaide has historically been a hub for the oyster trade with naturally occurring shellfish beds located throughout the Port River, aquaculture/farming of native oysters from the late 1850s-1910s around Jervois Bridge and regular shipments of oysters from Eyre and Spencer Gulf.
• The collapse of the native oyster industry led to the introduction of Pacific oysters (Magallana gigas) to Australia in the 1950s where aquaculture in SA took place from 1969 onwards. Pacific oysters have since established a wild population in the Port River, which in 2018 resulted in a ban on shellfish harvesting due to the explosion of POMs.
• Recognizing the Indigenous and colonial history of Port Adelaide’s shellfish, the Estuary Care Foundation conducted an initial subtidal restoration trial of native oysters at Jervois Bridge in 2018, with Ozfish as a collaborator, leading to an additional oyster basket being added in December 2022.
• Despite the poor water quality in the Inner Harbor, the area now attracts some bream, whiting, blue crab and gobies as well as the occasional snapper and salmon.