distribution and abundance of seabirds in western port, victoria

© Royal Australasian Ornithologists Union 2003 10.1071/MU03017 0158-4197/03/040307

Emu, 2003, 103, 307–313

CSIRO PUBLISHING

www.publish.csiro.au/journals/emu

Distribution and abundance of seabirds in Western Port, Victoria

Peter DannA, John P. Y. ArnouldA,B,C, Ros JessopA and Marg HealyA

APhillip Island Nature Park, PO Box 97, Cowes, Vic. 3922, Australia.BDepartment of Zoology, University of Melbourne, Vic. 3010, Australia.

CTo whom correspondence should be addressed. Email: [email protected]

Abstract. The distribution, abundance and biomass of seabirds in Western Port, Victoria, were surveyed betweenApril 1991 and August 1994. Individuals were counted along an 81-km series of transects from a boat atapproximately monthly intervals. A total of 25 seabird taxa were recorded, of which 18 and 15 were common tothose recorded by an earlier study in Port Phillip Bay and waters south of Phillip Island, respectively. The mostnumerous species by far was the Short-tailed Shearwater (Puffinus tenuirostris) followed by the Silver Gull(Larus novaehollandiae), Little Penguin (Eudyptula minor) and Crested Tern (Sterna bergii). Distribution withinWestern Port was not uniform, with pursuit divers such as cormorant and grebe species being recorded mostly inthe shallow Eastern Arm. In contrast, surface-seizing (e.g. albatrosses), surface-plunging (e.g. Crested Terns),shallow-plunging (Australasian Gannet, Morus serrator) and pursuit-plunging (e.g. shearwaters) speciespredominated in the deeper Western Arm of Western Port. These species were also seasonally abundant, with peaknumbers for most occurring in late summer–early autumn, which coincides with the reported influx of juvenileclupeoid fish into Western Port. Average biomass (686 ± 395 kg) comprised mostly Short-tailed Shearwaters, LittlePenguins and Pied Cormorants (Phalacrocorax varius). Biomass density (8.5 kg km–2) was similar to that reportedfor Port Phillip Bay (8.1 kg km–2) but lower than off the southern coast of Phillip Island (9.9 kg km–2).

Introduction

Knowledge of the distribution and abundance of seabirds atsea is of importance in determining the factors that influencetheir movements and foraging areas and in understandingtheir role in nutrient and energy fluxes in the marine ecosys-tem (Adams et al. 1992; Springer et al. 1996; Fauchald et al.2000; Forbes et al. 2000; Reid and Hindell 2000). Informa-tion on the distribution of seabirds off the Victorian coastlineis limited. Species lists have been provided for sections of thecoast and Bass Strait (Learmonth 1966; Simpson 1972) andthe distribution and abundance of seabirds at breeding siteshas been surveyed (Harris and Norman 1981). Few studies,however, have investigated seasonal variations in distributionand abundance (Norman 1992a, 1992b; Dann et al. 2001).

Bays, inlets and estuaries can be important spawning andnursery areas for fish species (Harris and Cyrus 2000;Nagelkerken et al. 2002; Weerts and Cyrus 2002) and theseasonal movements of fish species to, and within, suchwater bodies have been shown to influence the distribution,diversity and abundance of seabirds (Vaneerden et al. 1993;Skov et al. 2000). Fluctuations in the number of seabirds fre-quenting such water bodies have also been shown to be asso-ciated with seasonal influxes of animals to nearby breedingcolonies (Ryan et al. 1991; Benoit and Bretagnolle 2002).Along the Victorian coastline, Port Phillip Bay and WesternPort are considered to be significant spawning and nursery

zones (Hoedt et al. 1995; Neira et al. 1999). Previous studieshave documented seasonal variations in the diversity andabundance of seabirds in Port Phillip Bay that are associatedwith seasonal increases in post-larval fish populations aswell as breeding periods and fluctuations in sea-surface tem-perature (Norman 1992a, 1992b). Similarly, peaks in LittlePenguin (Eudyptula minor) numbers in Western Port havebeen shown to coincide with influxes into the area of juvenilepilchards and anchovies (Dann et al. 2001). There is littleinformation, however, on the distribution and diversity ofother seabird species in Western Port and the factors influ-encing their seasonal abundance.

The aims of this study, therefore, were to describe thespecies composition of seabirds in Western Port and assesstheir seasonal distribution and abundance.

Methods

Western Port is a shallow bay (maximum depth 30 m) situated on thesouthern coast of Victoria. It covers 680 km2, including 270 km2 of tidalmudflats, with a coastline of 263 km, of which 107 km are lined byWhite Mangroves (Avicennia marina) (Shapiro 1975). One large island(Phillip Island) lies in the southern entrance and a larger island (FrenchIsland) occupies the centre of the bay (Fig. 1).

Thirty-six surveys were completed in the 40 months between April1991 and August 1994. Boat engine failure and inclement weathercaused four counts to be abandoned or postponed which impacted ourattempts to space the counts at four-weekly intervals. Seabirds werecounted along an 81-km series of transects (Fig. 1) from a boat

308 Emu P. Dann et al.

travelling at 20–25 knots h–1. Transects were perpendicular to the mainchannels to cover the entire range of sub-tidal water depths in eachsection of the bay. Intertidal areas were not traversed and, with theexception of those areas in the eastern part of the Bay, subtidal areasless than 2 m deep at high tide were not counted (Fig. 1).

Two observers were stationed on either side of a 7-m boat (eyeheight ~3 m above sea level) and counted birds in the right-angledsector from the bow to starboard or port. Observers recorded each birdseen up to 500 m from the boat onto a tape cassette. Each bird wasidentified to species or commonest taxon where possible. Whencounting accuracy was impaired by either choppy sea conditions orlarger numbers of birds, the boat was stopped briefly while the countswere made. Difficulties of visibility caused by weather (Tasker et al.1984) were reduced by only counting on days with wind speeds of lessthan 10 knots. The counts usually took 5 h and were carried out aroundthe middle of the day and, when possible, within a few hours of hightide. Identifications were made using 8×40 binoculars.

Body masses (averaged across sexes) were taken from Higgins andDavies (1995) and Marchant and Higgins (1990) to calculate thebiomass of individual species observed throughout the surveys. Dataare presented as means ± s.e. unless otherwise indicated.

Results

Species diversity and distribution

In total, 25 different seabird taxa were recorded during thestudy (Table 1). An additional nine species of waders andwaterbirds (Black Swan, Cygnus atratus; White-facedHeron, Ardea novaehollondiae; Oyster Catcher spp.,Haematopus spp.; Masked Lapwing, Vanellus miles; Egretspp., Egretta spp.; Spoonbill spp., Platalea spp.; CurlewSandpiper, Calidris ferruginea; Sacred Ibis, Threskiornisaethiopica; Straw-necked Ibis, T. spinicollis) were recordedat sea during the surveys. These were excluded from furtheranalyses. Of the 25 seabird taxa, 12 are known to have localbreeding colonies (within 50 km), another seven are knownto breed within Victoria and Bass Strait and six are consid-ered long-distance migrants/visitors.

The distribution of the various species within WesternPort was not uniform (Table 1). The numbers of Little

Penguin, Australasian Gannet (Morus serrator), albatrossand shearwater individuals recorded were greatest in theWestern Arm of Western Port. Silver Gulls (Larus novae-hollandiae), Crested Terns (Sterna bergii), Arctic Skua(Stercorarius parasiticus) and Black-faced Cormorants(Phalacrocorax fuscescens) were more numerous in both theWestern and Northern arms of Western Port while theremainder of species were either evenly distributed or mostnumerous in the Eastern Arm of Western Port.

Abundance, seasonal variation and biomass

Though they were observed on relatively few occasions(48% and 32% of surveys, respectively), Short-tailed Shear-waters (Puffinus tenuirostris) and Fluttering Shearwaters(P. gavia) were by far the most numerous species recorded,numbering in their thousands when present (Fig. 2). Theirpresence in Western Port was also highly seasonal, occurringmostly in January–March, and greatest in 1992 and 1993.Silver Gulls, Crested Terns and Little Penguins were the mostnumerous regularly recorded species, with peak numbers of50–240 individuals. Numbers were generally highest in latesummer–autumn and lowest in winter–spring, with theexception of a peak for all three species occurring in October1992. The remainder of species were recorded in numbers<40 with the exception of one record of 120 Fairy Terns(S. nereis) in December 1992. The presence of AustralasianGannets and Pacific Gulls (L. pacificus) was highly seasonal,with peak numbers generally occurring in February–May inboth species. Similarly, Black-browed Albatross (Diomedeamelanophrys) and Shy Albatross (Diomedea melanophrys)were recorded only in autumn and winter months whereasArctic Skuas were observed only between late spring andearly autumn. Great Crested Grebes and Hoary-headedGrebes were recorded in peak numbers in April–June of1991 and 1992 but not at all in subsequent years. The

Australia

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Bass Strait

Phillip Is.

French Is.

145°

38°30′

Fig. 1. Map of the study area showing theseries of transects (black lines) used tocensus seabirds in Western Port. The shadedareas indicate depths <2 m at high tide.

Distribution and abundance of seabirds in Western Port, Victoria Emu 309

Australian Pelican and cormorant species were regularlyobserved and their numbers showed no obvious seasonalfluctuations (Fig. 2).

The highest observed biomass (12974 kg) occurred inFebruary 1992 and was heavily influenced by the presence ofShort-tailed Shearwaters (Fig. 3). From April to December,Little Penguins contributed the greatest proportion of theobserved biomass (29.5 ± 22.7% to 68.6 ± 7.4%; Fig. 4).Their lowest contribution to biomass (<10% in January–February) coincided with when their absolute biomass was atits lowest (28–31 kg). Despite there being fewer than 22recorded in any one month, Pied Cormorants (P. varius) con-tributed as much as 21% of the average observed biomassduring winter. Similarly, the few Black-browed Albatrosses(≤5) and Australian Pelicans (Pelecanus conspicillatus) (≤20)recorded at any one time accounted for an average of 10% ofthe observed biomass in August and 11–13% in March andOctober, respectively. The biomass contribution of CrestedTerns and Little Pied Cormorants (P. sulcirostris) peaked inNovember (14.0 ± 9.3% and 11.2 ± 9.8%, respectively)whereas that of Pacific Gulls was greatest in May–October(5–6%). The biomass contribution of Australasian Gannetsand Silver Gulls fluctuated between 0.6% and 6.6% and 1.5%and 13.9%, respectively, with no apparent seasonal peaks.

Averaged across all months, mean biomass was 686 ± 395 kg(249 ± 33 kg excluding Short-tailed Shearwaters).

Discussion

The number of different taxa recorded in the present study(n = 25) is similar to that reported by Norman (1992b) in PortPhillip Bay and off the southern coast of Phillip Island(n = 25 and 28, respectively). However, only 18 and 15 of thetaxa observed in the present study were common to PortPhillip Bay and waters south of Phillip Island, respectively.The most noticeable difference between the two enclosedwater bodies was the absence of albatross species andCaspian Terns from Port Phillip Bay. These speciesaccounted for few individuals in the present study (45 intotal). In contrast, the differences between Western Port andthe waters to the south of Phillip Island (>2400 individuals)reflected a greater occurrence of pelagic procellariiformspecies (e.g. Fairy Prion, Pachyptila turtur; Yellow-nosedAlbatross, Diomedea chlororhynchos; Common DivingPetrel, Pelecanoides urinatrix) and the absence of grebespecies and Caspian Terns in the latter.

The distribution of the various taxa generally reflectedtheir different foraging modes. Species that predominantlyuse ‘pursuit diving’ feeding methods (Ashmole 1971;

Table 1. Summary of seabird species observed within Western Port (between April 1991 and August 1994) and the total number of individuals recorded during the 36 counts in each of its three main regions

Species Western Arm Northern Arm Eastern Arm

Little Penguin (Eudyptula minor)A 299 187 21Australasian Gannet (Morus serrator)A 123 14 34Black-browed Albatross (Diomedea melanophrys)C 21 0 0Shy Albatross (Diomedea melanophrys)B 8 0 0Australian Pelican (Pelecanus conspicillatus)A 0 43 32Short-tailed Shearwater (Puffinus tenuirostris)A 28089 5 15Fluttering Shearwater (P. gavia)C 2056 0 0Silver Gull (Larus novaehollandiae)A 1043 1096 322Pacific Gull (L. pacificus)A 47 94 90Dominican Gull (L. dominicanus)A 0 0 2Arctic Skua (Stercorarius parasiticus)C 19 15 11Great Cormorant (Phalacrocorax carbo)B 9 15 21Little Black Cormorant (P. sulcirostris)A 5 6 3Pied Cormorant (P. varius)A 76 141 166Little Pied Cormorant (P. melanoleucos)A 28 263 98Black-faced Cormorant (P. fuscescens)B 48 17 2Cormorant spp.A 9 8 7Crested Tern (Sterna bergii)A 567 501 275Caspian Tern (S. caspia)C 1 6 8Common Tern (S. hirundo)C 0 0 3Fairy Tern (S. nereis)B 4 0 143Arctic Tern (S. paradisae)C 1 0 0Great Crested Grebe (Podiceps cristatus)B 0 0 23Hoary-headed Grebe (Poliocephalus poliocephalus)B 0 4 18Australasian Grebe (P. novaehollandiae)B 0 1 0

ALocal breeding colonies (within 50 km). BBreeding colonies within Victoria and Bass Strait. CLong-distance migrant/visitor.


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Fig. 2. The variation in the abundance of the various seabird species observed during monthly surveys in Western Port 1991–94.


Griffiths et al. 1982) such as cormorants (with the exceptionof the Black-faced Cormorant) and grebes were recordedmostly in the shallow Eastern Arm of Western Port. Thesespecies were recorded in relatively low numbers and theiroccurrence showed no apparent seasonality. An exception tothis is the Little Penguin which, along with species that pre-dominantly use ‘surface seizing’ (e.g. albatrosses), ‘surfaceplunging’ (e.g. Crested Terns, Silver Gulls), ‘shallow plung-ing’ (Australasian Gannet) and ‘pursuit plunging’ (e.g.shearwaters) feeding methods, were recorded mostly in thedeeper Western Arm (and to a lesser extent, Northern Arm)with strong seasonality.

Dann et al. (2001) noted that the peak in numbers of LittlePenguins in late summer–early autumn coincided with theinflux of juvenile clupeoid fish species into Western Port(Hoedt et al. 1995). Crested Terns breeding on Phillip Islandfeed on small Australian Anchovy (Engraulis australis) andJack Mackerel (Trachurus declivis) (Chiaradia et al. 2002)and this species was also recorded in peak numbers duringlate summer–early autumn. While the occurrence of largenumbers of Short-tailed Shearwaters in the Western Armduring January–March 1992 and 1993 could reflect the closeproximity to the large breeding colonies on Phillip Island, theconcurrent large numbers of Fluttering Shearwaters(endemic to New Zealand) may indicate a similar reliance onjuvenile fish moving into the area. In addition, whereasSilver Gulls may obtain a significant proportion of their dietfrom terrestrial scavenging on nearby Phillip Island (Norman1992b), peak numbers for this species also occurred duringearly autumn. This is in contrast to Port Phillip Bay whereNorman (1992b) found peak numbers occurred in winter andspring in association with movement to and from breedingcolonies. Similarly, the abundance of wader species inWestern Port has also been shown to peak during summermonths, coinciding with their breeding periods (Dann et al.

1994; Loyn et al. 1994). Furthermore, numbers of PacificGulls (which breed in Victoria mostly around Wilson’sPromontory) and Australasian Gannets (which breed innearby Port Phillip Bay) showed strong seasonality, withpeaks also occurring in late summer–early autumn. Whilelocal dietary information is still lacking for numerousspecies involved, these results suggest that the late summer–autumn influx of juvenile clupeoid fish into Western Port(Hoedt et al. 1995) may be an important seasonal foodresource for numerous seabird species.

Correspondingly, peak biomass of seabirds in WesternPort occurred in late summer–early autumn. In the waters offthe southern coast of Phillip Island, biomass is highestthroughout the summer months due largely to the influx ofShort-tailed Shearwaters to extensive breeding colonies onthe island (Norman 1992b). In contrast, peaks in seabirdbiomass in Port Phillip Bay, comprising mostly non-breedingLittle Penguins and locally breeding Pied Cormorants, havebeen observed during the winter months (Norman 1992b).Norman (1992b) suggested that these were supported byincreased post-larval fish populations following blooms ofchlorophyll-a and crustacea-dominated zooplankton broughton by nutrient levels in Port Phillip Bay being raised duringwinter (Anon 1973).

Calculated mean biomass density for Western Port(8.5 kg km–2) was similar to that reported for Port PhillipBay (8.1 kg km–2) but lower than off the southern coast ofPhillip Island (9.9 kg km–2) (Norman 1992b). As noted byNorman (1992b), extrapolation of these densities to feedingrequirements is hindered by the lack of local foraging detailsfor many species and the fact that not all individualsobserved may have been foraging in the area. This is espe-cially pertinent when considering the high contribution ofShort-tailed Shearwaters to total biomass in Western Portand south of Phillip Island and that this species may forageas far away as Antarctica during the breeding season(Nicholls et al. 1998; Klomp and Schultz 2000). ExcludingShort-tailed Shearwaters greatly reduces average biomassdensity to 3.1 kg km–2 in Western Port and 2.2 kg km–2 offthe southern coast of Phillip Island (calculated from data inNorman 1992b). Similarly, Australasian Gannets may con-tribute >50% of the total biomass in Port Phillip Bay duringtheir breeding season (Norman 1992b) but are known toforage extensively in Bass Strait while raising their chicks(Bunce 2001).

In summary, the results of the present study indicateseasonality in the diversity and abundance of seabirds inWestern Port. For most species, these seasonal peaks coin-cide with the reported movements of juvenile clupeoid fishinto the area. However, more detailed information on the dietof seabirds in Western Port is needed to fully assess theimportance of such fish movements to the ecology of thesespecies and their role in local marine ecosystem.

AMJ J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J FMAMJ J A0

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Fig. 3. Total seabird biomass observed during monthly surveys inWestern Port 1991–94.


Acknowledgments

This study was funded by Esso, BHP Petroleum, BHP Com-munity Trust and the Phillip Island Nature Park. We areindebted to Darrell Bray and Ian Hunter for their capableboat handling, to Wendy D’Amore and Leanne Renwick forconsiderable field assistance, and to Marion Wood for tran-scribing the data sheets. Additional assistance was providedby Maria Belvedere, Mark Collins, Frank Hoedt andWetgens Dimmlich.

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Manuscript received 5 May 2003; accepted 26 September 2003

distribution and abundance of seabirds in western port, victoria

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