Project Title: Population status of Western Australian humpback whales, 2008
Chief Investigator: Mr John Bannister
Following aerial surveys in 1999 and 2005, a further survey was planned over the two months June-August 2008, off Shark Bay, Western Australia. As previously the aim was to cover the major part of the northern migration of the Southern Hemisphere Group IV population (Breeding Stock D) past the Bay. As in 2005 there was to be a combined aerial survey and land-based operation, the latter to ground-truth the aerial survey results, in particular to estimate the proportion of animals sighted by the aircraft, g(0). There were two main objectives: i) to obtain an estimate of current (2008) absolute abundance for northbound migrating humpback whales, and ii) to compare the 2008 results with those from 1999 and 2005. As an improvement over 2005, when for logistical reasons the land-based component took place from Quobba, to the north of the original survey area, land-based operations in 2008 were from Cape Inscription at the northern end of Dirk Hartog Island. The landbased team comprised eight observers operating over two weeks for nine hours per day recording sightings by theodolite linked to a computer operating the program Cyclopes through which accurate plots of distances and bearings from the observer station were obtained, and undertaking double counts over the first week. On the aerial survey, 26 flights were completed out of 30 planned, giving much improved coverage of the migration. The flight path was somewhat reduced from the area surveyed previously to allow for two longer legs to investigate the distribution of animals beyond the area surveyed previously. Combined aerial and land-based operations, particularly to estimate the number of pods missed by the aircraft and determine accurate pod composition of pods seen from both platforms, took place on seven days. Pods were found to be migrating further from the land than expected, with many passing beyond visible range of the observers, which was restricted by the relatively low height (25.5m) of the available observation point. Analysis of the results is reported on separately.
Single platform aerial line transect and land-based surveys of Southern Hemisphere Group IV (Breeding Stock D) humpback whales Megaptera novaeangliae were undertaken to provide absolute abundance estimates of animals migrating northward along the western Australian coast. The aerial survey flew a total of 28 flights, of which 26 were completed successfully, from 24th June-19th August 2008. The land-based survey was undertaken from Cape Inscription, Dirk Hartog Island, during the expected peak of the whales northward migration, from 8th-20th July. During the first week of the land-based survey, some double count effort was undertaken to provide information on the numbers of pods missed from the land station. The assumed period of northward migration was 2nd June-7th September. Estimated abundance of northward-migrating whales during that time is 21,750 (95% CI: (17,550-43,000)). This estimate is based on an estimate of relative abundance of surface-available whales of 11,850 (9,550-23,450), and an estimated g(0) of 0.54 (±0.21). Comparison of the 2008 result with those from 2005 and 1999 in the same area is most appropriate between relative rather than absolute abundance figures and gives an increase rate of ca 9.8% per annum between 1999 and 2005, but of 14.7% between 1999 and 2008, which seems implausible; the question requires further investigation.
