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6 Single platform aerial line transect and land-based surveys of Southern Hemisphere Breeding Stock ‘D’ humpback whales 7 Megaptera novaeangliae were undertaken off Shark Bay, Western Australia to provide absolute abundance estimates of animals 8 migrating northward along the western Australian coast. The aerial survey flew a total of 28 flights, of which 26 were completed 9 successfully, from 24 June-19 August 2008. The land-based survey was undertaken from Cape Inscription, Dirk Hartog Island, 10 Shark Bay, during the expected peak of the whales’ northward migration, from 8-20 July. During the first week of the land11 based survey, some double count effort was undertaken to provide information on the numbers of pods missed from the land 12 station. The assumed period of northward migration was 2 June-7 September. Estimated abundance of northward-migrating 13 whales during that time is 33,850 (95% CI: (27,340-50,260)), representing an annual rate of increase of 12.7% (CV=0.19) since 14 an estimate of 11,500 in 1999. This estimate is based on an estimate of relative abundance of surface-available whales of 11,100 15 (8,960-16,480), and an estimated g(0) of 0.33 (±0.026). There were considerable practical difficulties encountered during the 16 land-based survey which reduced the effectiveness of the dual-survey approach for estimating g(0) for the aerial survey. 17 Furthermore only about 15% of whales were estimated to be within the visual range of the land-based station. Alternative 18 approaches for estimating g(0) from these data are therefore also presented, resulting in considerably higher estimates of around 19 0.6-0.7, and yielding a conservative abundance estimate of 17,500 (14,130-25,980). 20 INTRODUCTION 21 Following increasing reports of humpback whale (Megaptera novaeangliae) sightings in winter off the western 22 Australian coast in the early-mid 1970s, aerial surveys of humpback whales during their northward migration 23 were undertaken from Carnarvon, Western Australia (WA) in an area off Shark Bay where aerial spotter and 24 other data from whaling operations were available for the last year of humpback whaling, 1963. Results of those 25 surveys to 1988 (Bannister et al., 1991) demonstrated that significantly more whales were seen in the area in the 26 1980s than in 1963. Further surveys, in 1991 and 1994, demonstrated an annual increase rate of 10.15 ± 4.6% to 27 1994 (see Bannister and Hedley 2001). In comparison to the estimated population size of 568 at the end of 1963 28 (Bannister, 1964), the population size in 1994 was calculated to be some 4000-5000 animals (Bannister, 1995). 29 The 1994 survey results showed that to detect a significant difference in population in future years, at an annual 30 increase of 10%, an interval of three years would be required between surveys, leading to a proposed further 31 survey in 1997. Given funding constraints, that survey took place in 1999, its aim being to provide an estimate of 32 absolute abundance. This aim was more ambitious than for its predecessors, from which only a relative index 33 had been obtained. The survey was planned to cover as much of the northern migration period as possible, with 34 flights every other day over a two month period, mid June – mid August. Given the prevailing generally poor 35 weather conditions, only 18 of the 30 planned flights could be flown, of which only 15 were completed. 36 Nevertheless allowing for animals missed while submerged, 1999 population size was estimated as 8200-13600 37 (Bannister and Hedley, 2001). 38 Given the disappointing coverage, a further survey was planned to take place as soon as possible over the same 39 period and area, but to include an additional land-based component. That survey took place in 2005; the results 40 are reported in Paxton et al. (in press). Unfortunately, although the 2005 survey had been designed with the aim 41 of improving on earlier surveys (which were only able to apply ad hoc corrections to adjust for uncertain 42 trackline detection), last-minute logistical changes to the land-based survey in 2005 reduced its effectiveness. In 43 particular, the location of the land-based survey had to be moved northward to a location where, in the event, 44 whales often exhibited ‘milling’ behaviour rather than directional swimming more typical of migrating animals, 45 and to where the offshore distribution of whales extended far beyond the visual range of the land-based 46 observers. 47 Given rather equivocal results from the 2005 survey, improvements to the design of the 2008 survey were 48 planned as follows: 49 1 CREEM, The Observatory, Buchanan Gardens, St Andrews, Fife KY16 9LZ, UK. 2 The Western Australian Museum, Locked Bag 49, Welshpool DC, Western Australia 6986, Australia. 3 Cetacean Ecology and Acoustics Laboratory School of Veterinary Science University of Queensland St Lucia, Qld 4072, Australia. J. CETACEAN RES. MANAGE. 2 1. The aerial survey component was expanded in area to extend offshore coverage (following some 50 experimental work in 2007 to determine the most appropriate survey area). 51 2. Aerial survey data were collected using a direct data acquisition system. 52 3. The land-based component of the survey was expanded to include some double-platform 53 independent observer counts, and thus allow estimation of a correction factor for whales missed by 54 the land-based observers. 55 4. The location of the land-based platforms was at Cape Inscription, Dirk Hartog Island, Shark Bay. 56 From previous surveys, it was expected that whales passing this location would be more 57 identifiable as ‘northward-migrating’ and furthermore, that they would pass closer to the shore at 58 this latitude. 59 This report details the analysis of data from the 2008 survey, the aerial component of which took place from 24 60 June-19 August, with the land-based component from 8-20 July. 61 FIELD METHODS AND DATA 62 Aerial survey 63 In 1999, most sightings were made within about 30km of the eastern edge of the survey area – an area of 64 coastline delineated by the western coastlines of Bernier, Dorre and Dirk Hartog Islands (see Bannister and 65 Hedley, 2001), although the transects had extended out to about 56km from those islands. In 2005, the sightings 66 were spread more evenly (in relation to distance offshore) throughout the survey area (Paxton et al., in press). A 67 small set of flights in 2007 over the same area but with two legs extending 92km offshore (to 112°E) suggested 68 that humpback whales might be found out to 65km offshore (i.e. to about 112° 25’E) but with only a very few 69 further out. The 2008 flight path was therefore planned to cover an area reduced in latitudinal coverage from that 70 surveyed previously, approximately 55km x 75km immediately west of Dorre and Dirk Hartog Islands on the 71 western boundary of Shark Bay. The reduction in latitudinal effort allowed for two extended transect legs of 72 about 70km length to be flown to provide information on the possible distribution of animals further offshore. 73 These were located off the north of Dirk Hartog Island. In addition, on seven flights when the land-based survey 74 was operating, short legs of about 20km were flown at the latitude of Cape Inscription. The survey area and a 75 typical flight path are shown in Figure 1. The approximate length of the two most northerly and two most 76 southerly east-west transects was 45-50km. The survey area covered a region of approximately 6570km. 77 78 [Figure 1 about here] 79 80 As in 1999 and 2005, the 2008 survey flights were undertaken from a high-wing, twin-engine aircraft, mainly a 81 Partenavia P68B (fitted with bubble windows), under charter from TropicAir Services Pty Ltd, flying out of 82 Carnarvon, WA. On four flights, a Cessna 337 (with flat windows) was chartered from Norwest Air Work Pty 83 Ltd, based in Exmouth, WA. On all flights, a GPS and on-board computer system were available to plot 84 waypoints (as on previous surveys) and to log data (such as time, position and altitude); in addition, in 2008 85 Cyclopes software (Kniest, unpublished) was employed to map the flight path. Separately for each side of the 86 aircraft, the two observers recorded various weather covariates, including glare strength (a factor with four 87 levels); glare angle; Beaufort sea state; wind strength (in knots); wind direction; percentage cloud cover; and 88 ‘sightability’, a subjective overall assessment of the sighting conditions (a factor with four levels). Observers 89 used a clinometer (industry standard Suunto PM-5/360PC) and an angleboard to measure declination and 90 horizontal angles to sightings. For each sighting, observers made every effort to record pod size and swimming 91 direction. All sighting details were recorded on a Sony digital recorder for post-flight data entry. A total of four 92 observers participated in the survey, with their levels of participation ranging from flying 24 of the 28 flights 93 (85%) down to 7 (25%). 94 Of the 28 flights flown, 26 of were successfully completed and included in the analysis. The first three flights 95 (on 24, 26 and 29 June) were flown in a northerly direction; the remainder were flown in a southerly direction. 96 Because of glare, usually the latter is preferable for surveys in this location; historically (when transects were 97 closer together) such a strategy has also been used in order to minimise the risk of double-counting animals 98 (flying was in the opposite direction to the whales’ migration path). 99 Table 1 details the date, total transect length and number of sightings for each flight. ‘NM’ sightings are those 100 pods recorded with a northward swimming (migration) direction. NM+ sightings additionally include some pods 101 J. CETACEAN RES. MANAGE. 3 of undetermined direction, randomly allocated to be travelling north in proportion to the sightings of known 102 direction on a given day which were

the journal of cetacean research and management

Abundance estimates of Southern Hemisphere Breeding Stock ‘D’ humpback whales from aerial and land-based surveys off Shark Bay, Western Australia, 2008