Each year scientists from the Marine Conservation Program at DPIPWE 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.
DPIPWE 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.
2019-2020 monitoring surveys
In 2019 there were unusual reports of hundreds of thousands of short-tailed shearwaters in the Bering Sea well after their usual arrival timeframe in Australia.
Initial surveys of key colonies in Tasmania in December have indicated that burrow occupancy is slightly below the long-term average but within the normal range.
Surveys will be repeated in early March to determine how many of the eggs have hatched and successfully produced a chick (breeding success). This information will directly inform decisions in relation to the 2020 harvesting season and the long-term sustainability of the species.
Migration and breeding
Short-tailed shearwaters are incredibly regular in their timing at their migration and breeding activities.
They usually return to their breeding colonies in southern Australia within a couple of days of 22 September each year. Upon arrival, pairs re-unite, scratch out or repair old burrows where they mate before flying south to the Antarctic convergence zone to feed, returning to their colonies to lay a single egg around 28 November.
This year there were unusual reports of hundreds of thousands of short-tailed shearwaters still in the Bering Sea in October, well after their usual arrival in Australia. This range of reports led to concerns regarding their health and if they would be in condition to breed this season.
DPIPWE surveys on four islands in the Furneaux Island Group in the State’s north, and Fort Direction at the southern point of South Arm in Tasmania’s south. This season’s surveys of
Big Green Island, East Kangaroo Island, Little Dog Island and Little Green Island
were undertaken 5—8 December 2019, the week following the peak laying date. Fort Direction was surveyed on the 17 December 2019.
Other institutions and community groups also survey short-tailed shearwater colonies at sites around Tasmania and DPIPWE consults with them prior to making any harvest decisions. This year, the University of Tasmania has conducted surveys on Bruny Island and the Friends of Maatsuyker Island (Wildcare Inc.) has surveyed on Maatsuyker Island.
Figure 1. All shearwater monitoring locations around Tasmania.
Survey results December 2019
In December 2019, burrow occupancy in the Furneaux Islands was found to be slightly below the long-term average.
Figure 2. Long-term burrow occupancy of short-tailed shearwaters in the Furneaux Islands during early egg incubation. Note that error bars are presented from 2009/10 onwards due to a change in survey methodology from overall counts to a network of transect counts per island.
- Burrow occupancy in the Furneaux Islands was 57%, slightly below the long term average of 62%.
Fort Direction showed above average numbers of breeding birds with burrow occupancy at 62%, compared to an average of 53%.
Although there was concern about the late arrival of birds at breeding colonies, based on a check of a proportion of burrows using a specialised “burrow” camera (i.e. “burrowscope”), there was no indication that egg laying was later than normal.
The University of Tasmania has also undertaken counts at Bruny Island ‘Neck’ and the Friends of Maatsuyker Island have surveyed Maatsuyker Island. Both found shearwater burrow occupancy in early December 2019 to be within normal parameters, although at the lower end of the burrow occupancy range recorded in December.
Birds weighed this year were found to be within normal weights but slightly below average compared to previous years.
The mass of shearwaters provides an indication of their health and annual monitoring of adult weight at the start of the season provides an index of body condition leading into the breeding season. During surveys, DPIPWE scientists weigh a sample of birds as an index of general health.
Figure 3. Mass (gms) of short-tailed shearwater adults on Furneaux Islands in December.
The 2020 harvest
The results of DPIPWE’s December 2019 survey showed short-tailed shearwater (muttonbird) abundance to be within the normal range.
Further monitoring in March 2020 to check how many chicks had hatched successfully also showed numbers within the normal range.
While numbers are at the lower end of the normal range, harvesting in 2020 is not expected to affect long-term sustainability of the species.
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 be closed to harvesting in 2020 until periodic monitoring shows the populations have increased sufficiently enough to sustain annual harvest.