Study reveals dramatic drop in river's oyster stocks

An expert study of the Noosa River system’s past fish stocks suggests declines in several important species.

The study by University of Queensland’s Dr Ruth Thurstan confirms the oyster industry collapsed around 80 years ago, and prawn catches have shown significant fluctuations over the past 50 years. 

The findings paint a sobering picture of deterioration of the river’s aquatic biodiversity, and reinforce the value of an oyster reef rehabilitation trial.

Dr Thurstan searched more than 10,000 media reports, in addition to government sources, from 1871 to 2014 to build an historical picture of the river’s fish stocks in a study co-funded by Council and the philanthropic Thomas Foundation.

"Many of us fail to appreciate environmental change occurring in our lifetimes. This is a problem for resource management as we're less likely to recognise continuing environmental degradation, or try to halt further adverse effects. It's known as the shifting baseline syndrome," Dr Thurstan says. 

“For a time, around three million oysters a year were dredged from the lakes and river and transplanted to oyster farms in Moreton Bay and the Broadwater for fattening before sale in Brisbane.  By the 1960s the oyster fishery no longer existed, at least in part due to this exploitation,” she says.

Oyster reefs are an important part of a healthy aquatic ecosystem, providing habitat and a rich food source for fish and other marine life.

“Prior to European settlement, the Kabi Kabi tribe regularly feasted on the oysters and other shell-fish, leaving enormous shell middens in Tewantin, Noosaville, on Hayes Island and at other locations – a practice that ended more than a century ago.”

The study found that prawns too were abundant, although catches varied from year to year. In 1961, 400,000kg were netted in Lake Cootharaba when an observer recalled 25 small boats operating there. The next year the catch declined to one-tenth that amount, an indication of variability. By 1969 an observer reported 40 boats working there, yet the Department of Primary Industries reported the take at just over 200,000kg. In 2014 the catch was 35,300kg.

The Thomas Foundation, Noosa Parks Association and the University of the Sunshine Coast, working with The Nature Conservancy Australia, are seeking Noosa Biosphere Reserve Foundation funding to establish pilot oyster reefs in Lake Weyba and Noosa River.

Council supports the trial, having recently part-funded a Nature Conservancy Australia study that confirmed the river is still spawning sufficient juvenile oysters to re-establish oyster beds.

Dr Thurstan’s findings highlight how Noosa's once-rich commercial fishing ground and recreational angling paradise is in decline.

This study will help determine baseline targets to improve aquatic biodiversity and fish abundance.

“Time series data for recreational fishing suggests a gradual decline in catch rates for the most popular estuarine species over the 20th century," Dr Thurstan reported.

While it did not examine the cause of the decline in catch rates, the study noted that it occurred before the introduction of bag limits or the emergence of changed attitudes to conserving fish stocks, including catch-and-release practices. 

The study outlined phenomenal fishing catches of the past, including a 250kg rock cod caught in 1872, a 206kg groper caught in 1934, a 90kg groper in 1935, and a 120kg sawfish in 1954.

In 1901 two anglers caught 60 to 70 whiting a day and more than 100 on some days. In 1915 six anglers landed 483 snapper, the biggest 47kg, in a day's fishing outside the river; while in 1923 a family of four – a father and three children – landed 440 flathead in two days.

The Nature Conservancy Australia organised an expert workshop in October last year to discuss ways of rehabilitating the Noosa River system, including oyster and prawn stock rejuvenation, which offers positive impacts on fish stocks.

The oyster and prawn projects are part of a focused long-term effort, involving the Noosa community, to restore Noosa's rich marine biodiversity. 

Efforts to restore Noosa’s shellfish reefs and fisheries have the support of Kabi Kabi elders, according to Dr Thurstan’s report.

With The Nature Conservancy Australia, The Thomas Foundation in 2012 initiated the Great Southern Seascapes program to restore bays and estuaries in southern Australia.  Pilot oyster reefs have been built in Port Phillip Bay, a program to restore sea grass meadows and reefs has begun near Adelaide, and other similar projects are being developed in Western Australia and Tasmania.

The Noosa pilot draws on this experience, and The Nature Conservancy Australia’s experience with reef restoration in the USA. 

23 November 2015