P u b l i s h i n g
Emu Volume 100, 2000 © Birds Australia 2000
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EMU Vol. 100, 312-317, 2000 © Royal Australasian Ornithologists Union 2000 0158-4197/00/04312 + 5 10.1071/MU9918 Received 19-4-1999, accepted 1-9-1999
Seabirds off the South-western Coast of Australia C.A. Surman and R.D. Wooller* Biological Sciences, Murdoch University, WA 6150 * To whom correspondence should be addressed Summary: All seabirds encountered at sea from 6–19 December 1994, between Port Lincoln, South Australia and Fremantle, Western Australia were recorded mostly over continental shelf waters. Shearwaters were the seabirds observed most often, three species replacing each other along the coastline in shelf waters adjacent to their known
breeding localities. Terns were also largely confined to shelf waters. In contrast, a great diversity of albatrosses and petrels was associated with two intrusions of cooler, lowsalinity waters. Inshore, the influence of the warm Leeuwin Current on seabird distributions was apparent on the southern, in addition to the western, coast of Western Australia.
Although our knowledge of the breeding sites of most Australian seabirds is now reasonable (Ross et al. 1996) our understanding of their distribution and abundance at sea is still poor. In some areas, visited regularly, it is now possible to discern patterns in the distributions of seabirds at sea (e.g. Blaber & Milton 1994; Blaber et al. 1998; Reid et al. 1999), but off parts of the Australian coastline seldom visited by ornithologists, such information is sketchy. Over the last 30 years, an increasing number of studies have sought relationships between seabirds and the water types over which they forage (e.g. Ribic et al. 1997; Ballance 1997). For instance, Pocklington (1979) and Dunlop et al. (1988a,b) found seabird distributions and abundances to be linked to sea surface temperature and salinity in the eastern Indian Ocean, off Western Australia, resulting in distinctive assemblages of species associated with differing water masses. We conducted a survey of seabirds off the south coast of Western Australia and part of South Australia during December 1994. At this time, most seabirds known to breed in the area were involved in breeding, which maximised their dependence on the coast. The Leeuwin Current is a body of warm, low-salinity water of tropical origins that flows southwards down the Western Australian coast before rounding Cape Leeuwin and flowing eastwards off the south coast, reaching as far east as 120°E during December (Smith et al. 1991; Cresswell & Peterson 1993). This current varies in strength and our survey was undertaken in a year in which the Leeuwin Current flowed strongly (Allan et al. 1996).
Port Lincoln, South Australia to Fremantle, Western Australia between 6 and 19 December 1994. The course of the voyage followed continental shelf waters across the Great Australian Bight, then penetrated oceanic waters for several days before returning to shelf waters for the remainder of the journey north (Fig. 1). All seabirds within a 150 m radius forward of the bridge were recorded during a series of 10 minute counts, using the protocols of Tasker et al. (1984). Three continuous ten-minute counts were conducted during each hour of daylight that the vessel was moving, usually at 12 knots (22 km/h). The observer was 10 m above sea level ensuring a view 12 km to the horizon on clear days. Species not readily identified by the observer using binoculars were confirmed either by a second observer or by sketches and notes made at the time of observation. At the start and end of each 10-minute transect, records were made of position, sea surface temperature and salinity, water depth, current speed and direction, ambient temperature, wind speed and direction, sea conditions and visibility. Seabirds observed outside the sampling occasions were also recorded and the positions of any feeding flocks noted, together with any associations with marine mammals or fish. Mean abundances were calculated for each seabird species using pooled observations for each series of three ten-minute counts from the formula: Abundance = Number of birds S × T × 300 where S is the speed of the vessel (km/h), T is the time elapsed during the count and 300 is the width of the transect surveyed (m). Mean abundances were compared with each other using Student t-tests. The cruise track was divided into 29 blocks, each 1° square, that
Methods The survey was conducted during voyage 11/94 of the CSIRO Oceanographic Research Vessel Franklin from 312
Surman & Wooller: Seabirds off the SW Coast of Australia
contained at least six 10-minute counts (Fig. 1). The mean abundance along the cruise track for each species was then calculated using all the counts contained in each block.
Results Oceanography The water bodies encountered during this voyage were similar to those described by Cresswell & Peterson (1993) for 1986–87. The Leeuwin Current influenced continental shelf waters from the western coast of Western Australia around to 34°30′S, 120°10′E. These warm, low salinity waters were characterised by sea surface temperatures of 18.5-21.3°C and surface salinities of 35.70-36.06‰. The Great Australian Bight was characterised by cooler surface waters (17-18°C) with a larger range of surface salinities (35.2-36.2‰). The third major water-body encountered was represented by two intrusions of cooler, fresher water with temperatures from 14.5 to 15.5°C and salinity from 35.05 to 35.20‰. The easternmost finger of cold water was first encountered at 38°S, 120°E and was traversed for approximately 425 km towards the shelf edge at 36°S, 118°E. The second intrusion was first encountered at about 40°S, 116°E and extended north-west for 400 km to 37°S, 115°E.
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tailed Shearwater P. tenuirostris in the easternmost part of the survey (Fig. 2a). Observations of shearwaters were not confined to waters over the continental shelf, but their abundances were far higher in these waters than elsewhere on this survey. Flesh-footed and Shorttailed Shearwaters did not differ significantly in the mean sea-surface temperatures or salinities where they were encountered, nor in their abundances (Table 1), but were observed over waters that differed significantly in depth (t2,97 = 5.34, P < 0.001). Flesh-footed Shearwaters were wider ranging and recorded at significantly higher abundances (t2,98 = 6.56, P < 0.001) than Great-winged Petrels Pterodroma macroptera, a species encountered mainly over warm, shelf waters and apparently restricted to the south-west of the area surveyed. The two species differed signifi-
Seabirds Shearwaters (Puffinus spp.) were the species most commonly encountered (Table 1). The most abundant species, the Flesh-footed Shearwater P. carneipes was replaced by the Wedge-tailed Shearwater P. pacificus over waters north of Cape Naturaliste and with sea surface temperatures exceeding 20°C, and by the Short-
Figure 1 The cruise track of the survey vessel between Port Lincoln, South Australia and Fremantle, Western Australia during 1994. The hollow squares indicate the 29 blocks, each 1° square, used to calculate seabird densities from 127 separate counts.
Figure 2 Records of shearwaters (top, 2a), petrels (centre, 2b) and storm-petrels (bottom, 2c) during a survey off southern Australia in December 1994. The size of the symbol for each species is an approximate indication of the relative numbers of that species at that position.
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cantly in the salinity (t2,85 = 3.53, P < 0.001) and depth (t2,73 = 5.56, P < 0.001) of waters over which they were observed. Highest abundances of Flesh-footed Shearwaters were recorded over waters on the continental shelf, where birds were encountered singly or in small flocks that rarely exceeded 10 individuals. Petrel diversity was greatest over cold waters well off the shelf edge (Figs 2b & 2c). Great-winged Petrels were recorded more often nearer the coast, in contrast
to the more oceanic White-chinned Procellaria aequinoctialis, White-headed Pterodroma lessonii, Gould’s P. leucoptera, Blue Halobaena caerulea and Northern Giant Petrels Macronetes halli. Gould’s and White-chinned Petrels both had strong affinities with intrusions of cold water. The most common petrels, Great-winged Petrels and White-chinned Petrels, showed no significant difference in abundance, but the waters over which they were recorded (Fig. 2) differed
Table 1 The species and numbers of seabirds recorded and the characteristics of the waters over which they were encountered. Species
Total birds counted
Number of counts
Abundance index
Wandering Albatross Diomedea exulans
9
5
0.70
Royal Albatross Diomedea epomorpha
1
1
0.29
Shy Albatross Diomedea cauta
7
7
0.31
Black-browed Albatross Diomedea melanophris
2
2
0.41
Yellow-nosed Albatross Diomedea chlororhynchos
(0.17)
Mean (± s.e.) Sea surface Salinity temperature (°C) (‰) 17.03
(0.53)
15.67 (0.01)
17.84
(0.31)
17.36
3927 4494
35.64
(0.06)
35.44
23
0.57
2
0.66
16.42
35.45
5145
Northern Giant Petrel Macronectes halli
1
1
0.39
15.35
35.01
5136
Blue Petrel Halobaena caerulea
2
1
0.81
15.08
35.00
3336
103
43
0.91
(0.09)
17.70
(0.28)
35.49
(0.04)
3158
(313)
26
9
1.17
(0.38)
15.58
(0.19)
35.17
(0.03)
4880
(88)
4
2
0.84
35.06
(0.01)
4968
White-chinned Petrel Procellaria aequinoctialis
15.02
35.64
(742)
1734
3
White-headed Petrel Pterodroma lessonii
(0.27)
1307
(889)
43
Great-winged Petrel Pterodroma macroptera
18.21
35.48 (0.09) 35.28
Sooty Albatross Phoebetria fusca
Gould’s Petrel Pterodroma leucoptera
(0.10)
Water depth (m)
(0.05)
1653
(424)
26
11
0.94
(0.26)
15.43
(0.12)
35.21
(0.04)
4924
(95)
Flesh-footed Shearwater Puffinus carneipes
1767
96
5.90
(0.75)
17.95
(0.03)
35.69
(0.13)
1106
(188)
Short-tailed Shearwater Puffinus tenuirostris
983
13
26.35 (0.10)
17.82
(0.08)
35.88
(0.08)
91
(18)
Wedge-tailed Shearwater Puffinus pacificus
84
9
3.61
20.91
(0.17)
35.82
(0.02)
1568
(484)
(1.46)
Fluttering Shearwater Puffinus gavia
2
1
0.29
17.17
35.33
Little Shearwater Puffinus assimilis
1
1
0.30
17.74
35.88
70
Wilson’s Storm-Petrel Oceanites oceanicus
3
2
0.66
15.94
35.40
5302
33
17
0.68
White-faced Storm-Petrel Pelagodroma marina Black-bellied Storm-Petrel Fregetta tropica
5
2
1.09
Australasian Gannet Morus serrator
13
8
0.50
Black-faced Cormorant Phalacrocorax fuscescens
15
2
2.28
(0.02)
18.26
(0.13)
18.34
(0.41)
35.74
(0.19)
35.88
17.03
126
(0.09)
35.30
17.11
1441
(502)
4300 (0.08)
36.05
70
(5)
14
Pomarine Jaeger Stercorarius pomarinus
10
5
0.73
(0.24)
18.07
(0.12)
35.75
(0.41)
1536
Silver Gull Larus novaehollandiae
14
3
1.46
(0.49)
17.35
(0.35)
35.87
(0.05)
86
(3)
Pacific Gull Larus pacificus
13
3
1.34
(0.43)
16.93
(0.10)
36.00
(0.03)
48
(19)
Crested Tern Sterna bergii
65
22
3.17
(0.20)
18.08
(0.20)
35.72
(0.05)
89
(12)
1
1
0.3
Bridled Tern Sterna anaethetus
43
10
1.81
Sooty Tern Sterna fuscata
13
1
9.77
3292
127
Common Tern Sterna hirundo
Total
18.08 (2.12)
19.64 21.17
36.16 (1.00)
35.80 35.79
63 (0.09)
112 3862
(762)
Surman & Wooller: Seabirds off the SW Coast of Australia
significantly in surface temperature (t2,52 = 7.32, P < 0.001), surface salinity (t2,35 = 4.27, P < 0.001) and depth (t2,49 = 5.25, P < 0.001). Among the albatrosses, species diversity was greatest over oceanic waters and along the shelf edge (Fig. 3a). Wandering Albatrosses Diomedea exulans tended to be more oceanic in their habits, whereas Shy D. cauta and Yellow-nosed Albatrosses D. chlororhynchos were more abundant over waters on the continental shelf (Fig. 3a). The abundance of Yellow-nosed Albatrosses was significantly higher than that of Shy Albatrosses (t2,23 = 2.44, P = 0.02). However, there were no significant differences in abundances or water characteristics between Wandering Albatrosses and either Shy or Yellow-nosed Albatrosses. The two tern species encountered most frequently did not differ significantly in the depths or surface salinities of the waters over which they were observed.
315
However, Bridled Terns Sterna anaethetus were observed predominantly over the warmer coastal waters of the Leeuwin Current (t2,16 = 3.93, P < 0.001) while observations of Crested Terns S. bergii were more widely distributed (Fig. 3c). The highest density of Bridled Terns was on the shelf edge 75 km south-west of their breeding site at Penguin Island, near Fremantle (Fig. 3c). Bridled Terns recorded in the Recherche Archipelago were about 1000 km east of their nearest known breeding islands, off Cape Leeuwin. Except for an isolated flock of Sooty Terns S. fuscata, 350 km south of their nearest breeding station at Pelsaert Island, Houtman Abrolhos, all terns were observed either over water on the shelf edge or over the continental shelf itself (Fig. 3c). Similar coastal distributions were recorded for gulls, skuas, gannets and cormorants (Fig. 3b).
Discussion
Figure 3 Records of albatrosses (top, 3a) gannets, skuas, gulls and cormorants (centre, 3b) and terns (bottom, 3c) during a survey off southern Australia in December 1994.
A single survey such as this has obvious limitations in its spatial and temporal resolution, so that rare species and those associated with a narrow range of water-types are likely to be under-represented. Thus, for many colder waters species, such as White-chinned Petrels, the survey only sampled a small proportion of their distribution at sea (Woehler et al. 1990). Nonetheless, the two intrusions of cooler, low-salinity waters were where most petrels and albatrosses were encountered and had a markedly greater seabird diversity. Although we have no direct data to support our view, we presume that this species richness was linked to the higher productivity and greater array of prey items common in such waters. The apparently anomalous presence in these waters of the gadfly petrel Gould’s Petrel, far from its nearest known breeding location, has been discussed separately elsewhere (Surman et al. 1997). The timing of the survey may have influenced the results obtained, in that some species were probably restricted in their distributions at sea by the need to forage near their breeding grounds. However, foraging ranges in excess of 2000 km have been demonstrated for breeding Short-tailed Shearwaters (Weimerskirch & Cherel 1998) and Sooty Shearwaters Puffinus griseus (Weimerskirch 1998), as well as for White-chinned Petrels (Weimerskirch et al. 1999) and some albatrosses (Waugh et al. 1999), so that such restrictions are by no means universal. The high numbers of Flesh-footed Shearwaters recorded is consistent with the high number of adult
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females of this species caught on longlines in the Australian Fishing Zone, especially during the summer months off south-western Australia (Gales et al. 1998). Of the two other shearwaters, Short-tailed Shearwaters were recorded mainly in the eastern part of the Bight, nearer their major breeding area around Tasmania, despite breeding colonies present on the Recherche Archipelago. Wedge-tailed Shearwaters, a tropical species that breeds as far south as Rottnest Island, near Perth (Johnstone & Storr 1998), largely replaced Fleshfooted Shearwaters north of 33°S, with overlap between the species around Cape Leeuwin. Thus, the observed distributions at sea of these three common species were such that they could have been predicted from their known breeding distributions. However, this was only partly true for Bridled Terns, a sub-tropical species that has extended its range southwards markedly in recent years, probably as a consequence of the increasing influence of the Leeuwin Current (Dunlop & Wooller 1990). Bridled Terns were recorded foraging adjacent to Penguin Island, near Perth, where they breed in high numbers (Dunlop & Jenkins 1994), and south-west of Cape Leeuwin, near islands where it has been recorded breeding (Johnstone & Storr 1998). However, a third set of foraging birds were associated with the Recherche Archipelago where the species is not known to breed. The little studied Recherche Archipelago is the most easterly group of islands regularly influenced by the Leeuwin Current and may contain islands with small breeding populations of Bridled Terns. In general, all terns recorded during the survey were largely restricted to waters over the continental shelf.
Acknowledgements We thank CSIRO for allowing us to participate in the Franklin 11/94 cruise, all members of that cruise for their assistance, particularly the captain, Neil Cheshire. Also George Cresswell for his advice and support throughout and David Milton and Eric Woehler for their comments as referees.
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2001 Serventy Medal Nominations are now called for the award of the 2001 D.L. Serventy Medal. This award, which commemorates the outstanding contribution to Australian ornithology of Dr D.L. Serventy, is made solely on the basis of a nominee’s published scientific contribution to Australasian ornithology, and is not necessarily made every year. This Award is available only to living past or present members of Birds Australia (Royal Australasian Ornithologists Union), who must be proposed and seconded by two other members. Nominations for the award must include a written statement, outlining the scientific contribution of the nominee to Australian ornithology, a curriculum vitae, list of publications and the names of three referees. Decisions regarding the award are made by the Birds Australia Council, based on the recommendations of a sub-committee comprising three senior medallists, currently Hugh Ford (Chair), Harry Recher and Ian Rowley. Nominations for the 2001 award should be sent to: Dr H.A. Ford, Zoology Department, University of New England, Armidale, NSW 2351.
