Document ID: chunk:federal_register_of_legislation:F2023L00138:front:0:p6
Version: federal_register_of_legislation:F2023L00138
Segment Type: other
Provision Reference: 
Character Range: 14469–17485

near Cape Arid (Jaensch et al. 1988; DBCA 2018).

   FIGURE 1 Indicative distribution of the Australasian Bittern

                   1.5            Population trends

             The estimated number of mature individuals is <2,000 globally with approximately 1,300 (range 750–1,800) in Australia (Herring et al. 2019a). This estimate includes combining mid-range figures for the Riverina rice field population estimate (500–1000, 750; Herring et al. 2019a) and the remaining Australian population estimate (247–796, 522; Garnett et al. 2011), with New Zealand (580–725, 626; Heather and Robertson 2000) and New Caledonia (0–50, 25; Martínez- Vilalta et al. 2019a).

             Herring et al. (2019a) demonstrated that the rice fields of the Riverina support the largest known population of Australasian Bittern. Occupancy modelling from 2013–2017 in the Riverina region of New South Wales produced estimates of 368–409 for "early permanent water" rice crops in the Murrumbidgee and Coleambally Irrigation Areas (Herring et al. 2019a). Conservative estimates for "delayed permanent water" crops and for the unsurveyed Murray Irrigation Area suggest the Riverina's rice fields support 500–1000 mature Australasian Bitterns in most years, representing about 59 per cent of the national population (Herring et al. 2019a).

             Previous estimates in The Action Plan for Australian Birds 2010 (Garnett et al. 2011), which did not account for the Riverina population, suggested there were less than 1,000 mature Australasian Bitterns within the Australian population, and that the population was likely still decreasing. Based on two years of surveys at all wetlands where Australasian Bitterns had been reported between 1998 and 2008, BirdLife Australia estimated the number of adult birds in 2009–2010 to be 3–16 in Queensland, 82–162 in New South Wales, 86–248 in Victoria, 12–100 in Tasmania, 26–116 in South Australia and 38–154 in Western Australia – a national total of 247–796 (Birds Australia 2010; Garnett et al. 2011). The Western Australian population is now considered to comprise fewer than 100 individuals (Pickering 2012; DBCA 2018), while Tasmania is now thought to support 20–80 birds (E. Znidersic, unpublished).

             Based on the published data, decreases in this species have been occurring for several decades. The reporting rate in national Atlas surveys has decreased from being recorded in 260 10-minute grid squares in 1977–1981, to 142 squares in 1998–2003, and 61 in 2003–2008 (Birds Australia 2010; Garnett et al. 2011). The reporting rate declined by >90 per cent in Tasmania and Western Australia, and by 63 per cent in the Riverina. The long-term rate of decline is estimated to be 20–30 per cent over two generations (11 years).

             The area of occupancy of the Australasian Bittern in Australia is thought to have declined by 70 per cent between 1977 and 2008. These declines are considered to have led to a comparable decline in the size of the adult