Each year scientists from the Marine Conservation Program at Department of Natural Resources and Environment Tasmania (NRE Tas) and colleagues undertake surveys of short-tailed shearwaters (also known as muttonbirds) in the Furneaux Island Group and Fort Direction in Tasmania's south.
Surveys are undertaken in December, immediately after the birds have laid their egg, to determine the number of burrows containing a bird (burrow occupancy), which closely reflects the number of eggs laid.
Department of Natural Resources and Environment Tasmania (NRE Tas) uses this data to assess the annual shearwater breeding productivity which helps inform decision-making in relation to the sustainable level of annual muttonbird harvest and how many pre-fledging chicks can be 'taken' under permit/licence for commercial, recreational and cultural purposes.
Surveys of Short-tailed shearwater (muttonbird) in December 2020 showed above-average burrow occupancy across regions surveyed. Breeding birds also exhibited above-average weights, suggesting good breeding condition.
Breeding for the 2020/21 season looks promising at this stage, the number of chicks available to harvest in any given year depends on several factors, especially rainfall levels at breeding colonies. Eastern Australia is currently experiencing a La Niña weather pattern leading to a predicted rainfall above average during the Tasmanian summer.
Follow-up surveys will be undertaken by the Department in early March 2021 to help guide management of the 2021 short-tailed shearwater harvest.
Four islands in the Furneaux Island Group were surveyed in early December 2020 to determine the level of burrow occupancy and the mass of breeding birds. Islands were surveyed using 100m transects, with all burrows within one meter each side of the survey line inspected to determine the presence or absence of a bird, assumed to be on an egg.
Islands surveyed (figure 1) include:
- Little Dog (6 transects)
- Little Green (6 transects)
- Big Green (10 transects)
- East Kangaroo (10 transects)
Total transect length was 3.2 km (6400m2). Where possible five birds were removed from burrows on each transect and weighed as an indication of body condition.
Further surveys will be undertaken in March 2021 to assess chick numbers before the harvest season. These surveys will be used to guide the 2021 harvest.
Figure 1. Furneaux Islands surveyed in December 2020
Survey results December 2020
The western two surveyed islands showed the highest levels of burrow occupancy: 70.97% (± 3.5%) and 69.74% (±2.53%) of burrows were occupied on East Kangaroo Island and Big Green Island respectively.
The eastern islands (Little Dog and Little Green) showed occupancy of 65.38% (± 6.99) and 62.92% (± 5.49) respectively. The mean occupancy across survey sites of 67.25% (±4.63%) is above the long-term average (figure 2).
Figure 2: Long-term burrow occupancy of short-tailed shearwaters in the Furneaux Islands during early egg incubation
The mass of breeding birds provides a useful indication of their general condition. A total of 151 birds were weighed across the four surveyed islands. The average mass of birds was 594g (figure 3), greater than the long-term average.
Figure 3: Mass of breeding short-tailed shearwaters, Furneaux island surveys 2010 to 2020 (average shown in yellow).
Fort Direction, located on the southern tip of South Arm in Tasmania’s south-east, is subject to a small cultural harvest. Fort Direction was surveyed on 3 December 2020 using the same methodology as outlined for the Furneaux Group on 4 x 100m transects (area surveyed (800m2).
Survey Results December 2020
Burrow occupancy across the four transects at Fort Direction was 65.89% (± 3.22%) (figure 4).
Figure 4: Long-term burrow occupancy of short-tailed shearwaters at Fort Direction during early egg incubation
The occupancy observed at Fort Direction was the highest burrow occupancy found at that site in the 11 years surveyed over a 16-year period.
The Friends of Maatsuyker Island group have undertaken surveys of short-tailed shearwaters on Maatsuyker Island for eight consecutive years. This season’s survey in December 2020 found above-average burrow occupancy of breeding birds.
Likely breeding success
Breeding success of shearwaters following egg laying can be highly variable and dependent on a number of factors including condition of breeding adults following migration and food availability during chick provisioning. One of the most important environmental factors appears to be rainfall: where heavy rainfall occurs, burrows can flood leading to adults abandoning eggs or chicks drowning or suffering from hypothermia, sometimes leading to death.
The Australian Bureau of Meteorology has forecast a La Niña weather pattern to continue during the 2020-21 shearwater breeding season, which typically increases the likelihood of above-average rainfall in eastern Australia during the summer months.
The 2021 harvest
The results of NRE Tas' December 2020 monitoring showed short-tailed shearwater (muttonbird) abundance at surveyed colonies to be above average.
Further monitoring will be conducted in March 2021 to check how many chicks successfully hatched.
Some rookeries on the west coast are small in size and damage from fires and storm events makes them susceptible to overharvest. Consequently, all rookeries on the west coast will remain closed to harvesting in 2021 until periodic monitoring shows the populations have increased sufficiently enough to sustain annual harvest.