Morus Serrator Research Articles

Dive strategies and foraging effort in the Australasian gannet Morus serrator revealed by underwater videography

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 442:255-261 (2011) - DOI: https://doi.org/10.3354/meps09458 Dive strategies and foraging effort in the Australasian gannet Morus serrator revealed by underwater videography Gabriel E. Machovsky Capuska1,2,* , Robin L. Vaughn3, Bernd Würsig3, Gadi Katzir4,5, David Raubenheimer1,2 1Nutritional Ecology Research Group, Institute of Natural Sciences, Massey University, Private Bag 102 904 North Shore MSC, Auckland, New Zealand 2Coastal-Marine Research Group, Institute of Natural Sciences, Massey University, Private Bag 102 904 North Shore MSC, Auckland, New Zealand 3Department of Marine Biology, Texas A&M University at Galveston, 200 Seawolf Parkway, Galveston, Texas 77553, USA 4Department of Marine Biology, University of Haifa, Mount Carmel, Haifa 31905, Israel 5Department of Evolutionary and Environmental Biology, University of Haifa, Mount Carmel, Haifa 31905, Israel *Email: g.machovsky@massey.ac.nz ABSTRACT: Gannets are specialist plunge divers that perform short and shallow V-shaped dives and long and deep U-shaped dives in pursuit of pelagic fish and squid. We used underwater videography to examine the patterns of behaviour and relative success rates of V- and U-shaped dives in Australasian gannets. A significantly greater proportion of U-shaped dives were associated with successful prey capture than V-shaped dives (95% vs. 43%, respectively). The maximum number of prey captured per dive by the gannets was higher than previously reported, reaching up to 5 fish in a single U-shaped dive. However, V-shaped dives were more efficient in terms of grams of prey captured per time spent underwater in successful dives. In contrast, a population-level comparison of the mass of fish captured per total time spent underwater (i.e. including unsuccessful dives) suggested that the 2 dive profiles were equally efficient. We also found that gannets adjusted their dive shape in relation to the depth of their prey rather than prey type, as previously hypothesized. Further studies are needed to understand decisions made by gannets while plunge diving in complex marine environments. KEY WORDS: Gannets · Morus serrator · Dive shape · Prey capture success · Decision making · Multispecies feeding associations Full text in pdf format PreviousNextCite this article as: Machovsky Capuska GE, Vaughn RL, Würsig B, Katzir G, Raubenheimer D (2011) Dive strategies and foraging effort in the Australasian gannet Morus serrator revealed by underwater videography. Mar Ecol Prog Ser 442:255-261. https://doi.org/10.3354/meps09458 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 442. Online publication date: December 05, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Evidence for fatal collisions and kleptoparasitism while plunge-diving in Gannets

Plunge-diving is a highly successful strategy for dealing with the challenges confronting birds feeding on pelagic prey. We tested for evidence of fatal injuries due to collision between conspecifics in plunge-diving Australasian Gannets Morus serrator and Cape Gannets Morus capensis, respectively, by performing post-mortem examinations of carcasses recovered from New Zealand waters and analysing video footage of Cape Gannet foraging events from South Africa. We found evidence of accidental collisions between Gannets and also observed a case of attempted kleptoparasitism, in which a diving Cape Gannet targeted a previously captured fish in the beak of a conspecific.

Geolocation Tracking of the Annual Migration of Adult Australasian Gannets (Morus serrator) Breeding in New Zealand

The long breeding period and high reproductive investment of seabirds make use of resource-rich foraging areas pivotal both during and between breeding seasons. We tracked adult Australasian Gannets (Morus serrator) from their New Zealand breeding colony at Cape Kidnappers to Australia during the non-breeding period to assess wintering behavior and migratory routes for this species. Data from three recovered geolocation sensor (GLS) tags showed that both a male and a female M. serrator, and a hybrid M. capensis × M. serrator migrated across the Tasman Sea to winter in Australian and Tasmanian coastal waters. Tracked birds covered distances of up to 13,000 km on their migration. These movements were consistent with historical records of band recoveries.

Global population status of shy albatross and an assessment of colony-specific trends and drivers

Context Monitoring the status of albatross populations and identifying the factors driving observed trends remain international conservation and management priorities. The shy albatross is endemic to Australia and breeds only on three Tasmanian islands. Aims To provide a reliable total population estimate for shy albatross, including an assessment of demographic trends for each of the three populations where possible. We consider also key drivers of population trends for each population, particularly the potential role of fisheries by-catch, with an overall aim of determining the status of the species. Methods Aerial photography and ground surveys were used to estimate the number of annual breeding pairs and trends in adult and juvenile survival rates were calculated using mark–recapture methods. At-sea distribution data was used to identify population specific trends in the overlap of shy albatross and fisheries to evaluate the potential influence of fisheries by-catch on the populations. Key Results The Albatross Island population increased post-harvesting but has recently stabilised at around 5200 breeding pairs, less than half its estimated historic size. This trajectory change appears driven by a decrease in juvenile survival. The small (170 breeding pairs) Pedra Branca population has recently declined, probably due to reduced breeding success associated with the increasing population of Australasian gannets (Morus serrator) on the island. The largest population (on Mewstone) comprises at least 9500 breeding pairs. Trends for this population are unknown. However, this paper demonstrates that these birds have greater overlap with trawl and longline fishing effort and are consequently at higher risk of fishing-related mortality. Conclusions Given the extent of fisheries overlap, survival rates for Mewstone individuals are likely to be lower than the Albatross Island population. Combined with recent trends on Pedra Branca and Albatross Island, we suggest that the current status of the shy albatross is likely to be stable at best and quite possibly decreasing. Implications The concerns raised about the conservation status of shy albatross reinforce the importance of continued population monitoring focussed particularly on establishing the trend of Mewstone. A thorough assessment of interactions with trawl fishing operations also is a management priority for this species.

A virgin flight across the Tasman Sea? Satellite tracking of post-fledging movement in the Australasian Gannet Morus serrator

New technologies enable tracking of the route, duration, and destination of previously unassessed long-distance movements. Fledgling Australasian Gannets Morus serrator from breeding populations in New Zealand had been reported to fly across the Tasman Sea to Australia, with this historic knowledge derived from the recovery of banded carcasses and from observations of initial flight direction. We deployed Argos satellite devices on ten M. serrator fledglings at Cape Kidnappers Gannetry, North Island, New Zealand, across 2 years. Birds that were tracked leaving the colony initially appeared to have landed on the sea. A male bird and two female birds were tracked moving along the east coast to the south tip of New Zealand. The two females then crossed the Tasman Sea to eastern Australian coastal waters in 4 and 5 days, respectively. We suggest that, contrary to historic reports, the route via Stewart Island constitutes a realized migration path for fledglings from Cape Kidnappers, which might minimize the distance traveled across the open sea to southeastern Australia or Tasmania. Our results further imply that initial direction of flight needs not be indicative of the subsequent migration route taken by M. serrator. This highlights the importance of direct tracking technology for adequate assessment of dispersal and migration in seabirds and other highly mobile species.

Mate replacement entails a fitness cost for a socially monogamous seabird

Studies of the selective advantages of divorce in socially monogamous bird species have unravelled extensive variation among different lineages with diverse ecologies. We quantified the reproductive correlates of mate retention, mate loss and divorce in a highly philopatric, colonially breeding biparental seabird, the Australasian gannet Morus serrator. Estimates of annual divorce rates varied between 40-43% for M. serrator and were high in comparison with both the closely related Morus bassanus and the range of divorce rates reported across monogamous avian breeding systems. Mate retention across seasons was related to consistently higher reproductive success compared with mate replacement, while divorce per se contributed significantly to lower reproductive output only in one of two breeding seasons. Prior reproductive success was not predictive of mate replacement overall or divorce in particular. These patterns are in accordance with the musical chairs hypothesis of adaptive divorce theory, which operates in systems characterised by asynchronous territorial establishment.

Energetic consequences of plunge diving in gannets

Seabirds that forage by plunge diving dive less frequently than those that dive from the water surface, and spend less time in flight than more generalist foragers. We hypothesised that this is due to foraging by plunge diving entailing a high energetic cost, which in turn is due to high energetic costs of take-off and flight. Using heart rate as a proxy for metabolic rate, we evaluated the energetic costs of foraging by plunge diving in the Australasian gannet Morus serrator. As expected, flight entailed a high energetic cost, and energy expenditure during foraging was equivalent to that during flight and significantly higher than that when animals were resting during foraging trips or were inactive on land. These values represent the highest costs of foraging yet recorded in a seabird, and the low frequency of plunge diving can be attributed to these high costs. On average, Australasian gannets perform 2.6 dives h�1when foraging, with a mean dive duration of 3.4 s. As a result, they spend <0.25% of the duration of each foraging trip submerged. We combined this infor- mation with previously obtained data on diet to calculate an estimated rate of prey capture of ~287 g (min submerged)�1. This rate is at least 7 times greater than rates recorded in other diving birds. For plunge divers, therefore, the high costs of foraging are offset by high rates of energy gain.

Behavior of free-ranging common dolphins (Delphinussp.) in the Hauraki Gulf, New Zealand

Here we present the first data describing the behavior of common dolphins (Delphinus sp.) in the Hauraki Gulf, New Zealand. Activity budgets are used to assess the effects of diel, season, depth, sea surface temperature, group size, and composition on dolphin behavior. Additionally, the presence/absence of Bryde's whale (Balaenoptera brydei) and Australasian gannet (Morus serrator) is examined in relation to dolphin behavior. Behavioral data were collected from 686 independent dolphin groups during boat-based surveys conducted between February 2002 and January 2005. Foraging (46.7%) and social (7.2%) were the most and least frequently observed behaviors, respectively. Travel (28.9%), mill (9.5%), and rest (7.7%) accounted for the remainder of the activity budget. Behavior varied seasonally, with the highest proportion of foraging and resting groups observed during the spring and autumn, respectively. Behavior also varied with water depth, with foraging animals observed in the deepest and resting groups recorded in the shallowest regions of the Hauraki Gulf. A correlation between group size and behavior was evident, although behavior did not vary with the composition of dolphin groups. Resting, milling, and socializing animals were more frequently observed in smaller group sizes. Foraging behavior was prevalent in both small and large group sizes, suggesting foraging plasticity exists within this population. Behavior differed between single- and multispecies groups, with foraging more frequent in multispecies groups. Resting, milling, or socializing was rarely observed in the presence of any associated species, indicating the primary mechanism for association is likely prey related.

Dusky Dolphins Influence Prey Accessibility for Seabirds in Admiralty Bay, New Zealand

Although seabirds frequently aggregate with feeding delphinids, the benefits to seabirds of feeding with dolphins have been rarely reported. We examined how dusky dolphins (Lagenorhynchus obscurus) influenced prey accessibility for seabirds in Admiralty Bay, New Zealand. Interactions of dusky dolphins and seabirds were characterized during 335 feeding bouts of dusky dolphins (52 video-recorded underwater). Dolphins increased prey accessibility for seabirds because they swam under the bottom half of prey balls for 59% of passes that were within 2 m of the prey ball. During feeding bouts by dolphins, 51% of prey balls ascended, whereas only 13% descended. Dolphins also influenced prey mobility; only 24% of stationary feeding bouts became mobile after dolphins began feeding, and 17% subsequently became stationary again. Significantly more Australasian gannets (Morus serrator) were near mobile than stationary prey balls after feeding, but not during feeding bouts. This suggests that feeding gannets increase mobility of prey balls, but that feeding dolphins counteract this effect. Seabirds also used dusky dolphins to locate prey. Numbers of gannets, shearwaters (Puffinus), and gulls (Larus) increased during the first 2 min of dolphin feeding, even when other seabirds were not present. Gannets fed with dolphins for 40% of gannet feeding observations and shearwaters fed with dolphins for 24% of shearwater feeding observations.

Factors Affecting the Occurrence and Demographics of Common Dolphins (&lt;I&gt;Delphinus&lt;/I&gt; sp.) in the Hauraki Gulf, New Zealand

The common dolphin (Delphinus sp.) is the most frequently observed cetacean species in the Hauraki Gulf, a large shallow body of water on the northeastern coastline of North Island, New Zealand. Herein, we present the first data relating to the occurrence and distribution of common dolphins in this region and assess the possible effects of abiotic parameters on the demographics of this population. The presence of associated marine species is quantified, and differences in the occurrence and demographics of single and multi-species groups are examined. Sightings data were collected between February 2002 and January 2005 during boat-based surveys. We recorded 719 independent encounters with common dolphins, involving 1 to > 300 animals. Dolphin presence was significantly affected by month, latitude and depth. Group size varied significantly by month, season, depth, sea surface temperature (SST) and latitude, and was highly skewed towards smaller groups made up of < 50 animals. Larger aggregations were most frequent during the austral winter when nutrient upwelling typically leads to increased prey availability within the region. Over 70% of groups encountered contained immature animals and 25% of groups included neonates. Calves were observed throughout the year but were most prevalent in the austral summer months of December and January. Month, season, depth, and SST significantly affected group composition. Common dolphins were observed in association with four cetacean and eight avian species, most frequently with the Australasian gannet (Morus serrator) and the Bryde’s whale (Balaenoptera brydei). The distribution of dolphin-only groups differed significantly from that of dolphin-whale groups, with mono-specific groups found on average in waters that were 3.6 m shallower and 3.1oC warmer. The year-round occurrence and social organisation of common dolphins in Hauraki Gulf waters suggest this region is important both as a calving and nursery ground.

Seaweed provisioning behaviour confers thermal benefit for nesting Australasian gannets (Morus serrator)

Despite prolonged and obligate biparental care for a single offspring in the Australasian gannet Morus serrator , several reproductive behaviours are presumed to be sex specific and might indicate sexual dimorphism in mating and parental effort in this broadly monomorphic seabird. For instance, the delivery of seaweed as a nesting material has been typically considered a male specific trait. We assessed this assumption and determined whether the potential role of this behaviour is to serve as a nuptial trait preceeding copulation or to impart a thermal benefit for incubation. First, as predicted, all arriving individuals at the colony that mated following seaweed delivery assumed the top copulatory position, which is consistent with male behaviour in this species. In comparison, the likelihood of birds without seaweed copulating in top position upon arrival at the nest site was approx. 50%, indicating an even mix of the sexes. However, the sex of those individuals in our sample that did not copulate during our observations remains unresolved. Second, seaweed delivery was not related to copulation following arrival, as individuals arriving with seaweed in our sample had a lower probability of mating than did individuals arriving without seaweed. Third, to determine if seaweed provides thermoregulatory benefits to alleviate the physiological costs of incubation, the foot temperatures of incubating and non-incubating individuals and temperatures of nests with or without seaweed were recorded. Temperatures of the foot-webbings during incubation were significantly higher above ambient temperatures than those of non-incubating gannets at the colony. Nests that contained seaweed were significantly warmer at sunrise than those without seaweed. There was no consistent difference between the temperatures of nesting material in the evenings alone, with a large variance of evening nest temperatures. These correlative data are consistent with male specificity and thermoregulatory benefits associated with seaweed delivery in M. serrator , implying that further experimental work on known-sexed birds should focus on physiological benefits and reproductive consequences of seaweed delivery in this species.

Dusky dolphin (Lagenorhynchus obscurus) feeding tactics and multi‐species associations

In this study, monthly changes in dusky dolphin (Lagenorhynchus obscurus) feeding tactics and multi‐species associations in Admiralty Bay and Current Basin, New Zealand were described from August to November 2005 and from May through August 2006. Concurrent changes in dusky dolphin abundance, prey characteristics, and locations of feeding bouts were examined. Data were collected during systematic surveys and focal follows of dolphin groups. During 335 dolphin feeding bouts (52 observed underwater), data were recorded on number of associated fur seal (Arctocephalus forsteri), seabirds, and sharks; dolphin behaviour; prey ball parameters; and locations. Dolphin feeding tactics, multi‐species associations, and locations of feeding bouts were different from May through July than from August to November. From May through July, dolphins fed on mobile prey at depth; from August to November, they herded small schools of fish (including pilchard Sardinops neopilchardus) to the surface. Primarily shearwaters (Puffinus spp.) fed with dolphins from May through July; shearwaters, gannet (Morus serrator), gulls (Larus spp.), and fur seal frequently fed with dolphins from August to November. These intra‐annual differences suggest that changes in prey species or behaviour influence dolphin feeding tactics, and influence which species benefit from feeding with dolphins.

The influence of age on reproductive success and diet in Australasian gannets (Morus serrator) breeding at Pope's Eye, Port Phillip Bay, Victoria

The influence of age on reproductive success and diet was examined in ‘old’ (experienced; 12 years and older) and ‘young’ (5–8 years of age) Australasian gannets (Morus serrator) breeding at Pope’s Eye, Port Phillip Bay, Victoria during the 2002–2003 breeding period. Although food availability, as indicated by commercial fish catches, throughout this breeding period was low, there were no significant differences in breeding success or chick growth between groups. Nevertheless, old birds tended to have higher reproductive success, replacing more lost eggs and fledging chicks of a greater mass. However, old birds also laid more eggs that failed to hatch. Five fish species, including jack mackerel (Trachurus declivis), barracouta (Thyrsites atun), redbait (Emmelichthys nitidus), anchovy (Engraulis australis) and red mullet (Upeneichthys vlamingii), were important in the gannet diet during this breeding period. There were no significant differences in dietary parameters, including range of species and size of prey, between old and young gannets, nor were there any differences between those of the chicks and their parents, suggesting that adults do not forage selectively for their chicks. This study showed that even during a period of presumed low food availability, when experienced (older) birds might be expected to have enhanced success, the differences between these and less experienced (younger) birds may not be apparent.

Evaluating molecular and behavioural sexing methods for the Australasian gannet (Morus serrator)

The availability of molecular methods for avian sex identification has revolutionised the study of sexual differences in behaviour, morphology, life-history traits and conservation management. We implemented the recommendations of a recent review of DNA-based sex-identification by (1) verifying the sex-specificity and (2) estimating the accuracy of different sex-assignment methods in an apparently monomorphic seabird, the Australasian gannet (Morus serrator). The polymerase chain reaction (PCR) method based on the amplification of the sex-linked chromodomain-helicase-DNA binding gene (CHD) repeatedly assigned the same sex in 96% (n = 27 replicates) and correctly sexed all individuals with known gonadal anatomy (n = 6). PCR and sex-specific restriction fragment length polymorphism (RFLPs) showed agreement for 99.5% of individuals (n = 201). DNA-sexed pairs known to be social mates consisted of a male and a female in 96% of pairs sexed by PCR (n = 77) and 98% of pairs sexed by RFLP (n = 65). DNA-sexed females were in the bottom and males in the top copulatory position in 86% of observed copulations (n = 43 individuals). These results validate assumptions that both membership in social pairs and different copulatory positions can serve as reliable behavioural proxies for field-based sex identification in this colonial and obligately biparental seabird.

Preliminary investigation of the costs of incubation in the Australasian Gannet (Morus serrator) breeding in Port Phillip Bay, Victoria

To optimise lifetime reproductive success, individuals must balance current reproductive effort against future reproductive prospects. In birds, incubation and chick-rearing must involve costs, and manipulation of the length of incubation offers an insight into some costs affecting adults. An experiment was conducted at a colony of Australasian Gannets in Port Phillip Bay, Victoria, in which length of incubation was manipulated so that some adults experienced short (10–20 days duration), long (70–80 days) or normal (~45 days) incubation periods. Adults with a manipulated incubation period did not show significant differences in weight change (taken here to reflect cost) during incubation or chick-rearing compared with controls. Manipulation of length of incubation did not significantly affect the hatching success or the growth rate of chicks involved and is not, therefore considered to impose an increased reproductive cost. This suggests that the Australasian Gannet has the capacity to maintain body condition and successfully rear young despite modified duration of incubation.

Are age‐related variations in breeding performance greatest when food availability is limited?

Age-related improvements in reproductive performance in seabirds have been well documented, and may be explained by improvements in foraging efficiency or increased experience and reproductive effort with age. The interactive effects of parental age and food supply on reproductive performance, however, remain poorly understood. A widespread mass mortality of pilchards Sardinops sagax in southern Australian waters in 1998 provided a unique opportunity to investigate the effects of a sudden reduction in the availability of amajor prey species on Australasian gannets Morus serrator, an important local marine predator. Age-related differences in the breeding performance of gannets were evident in 1 year of reduced pilchard availability; when food was not limited, both young and experienced parents were equally capable of rearing chicks and had similar levels of breeding success. These data clearly demonstrate the interactive effects of parental age and food supply on breeding performance and suggest that such differences only become apparent when conditions become more stressful.

Do dietary changes of Australasian gannets (Morus serrator) reflect variability in pelagic fish stocks?

The relationship between changes in the diet of Australasian gannets (Morus serrator) and commercial fisheries landings was investigated. The contribution of pilchard (Sardinops sagax) to the diet of gannets was significantly related to the commercial catch of that species, suggesting that changes in the relative proportion of pilchard in the diet of gannets may be a useful index of pilchard abundance and availability. Equivalent relationships, however, were not found for other common prey species in the gannet diet. It is considered that the abundance of important prey items in the gannet diet, such as pilchards, and commercial landings are proportional to stock abundance. The implications of this relationship for fisheries management are examined.

Distribution and abundance of seabirds in Western Port, Victoria

The distribution, abundance and biomass of seabirds in Western Port, Victoria, were surveyed between April 1991 and August 1994. Individuals were counted along an 81-km series of transects from a boat at approximately monthly intervals. A total of 25 seabird taxa were recorded, of which 18 and 15 were common to those recorded by an earlier study in Port Phillip Bay and waters south of Phillip Island, respectively. The most numerous 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 within Western Port was not uniform, with pursuit divers such as cormorant and grebe species being recorded mostly in the 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) species predominated in the deeper Western Arm of Western Port. These species were also seasonally abundant, with peak numbers for most occurring in late summer-early autumn, which coincides with the reported influx of juvenile clupeoid fish into Western Port. Average biomass (686 ± 395 kg) comprised mostly Short-tailed Shearwaters, Little Penguins and Pied Cormorants (Phalacrocorax varius). Biomass density (8.5 kg km 2) was similar to that reported for Port Phillip Bay (8.1 kg km−2) but lower than off the southern coast of Phillip Island (9.9 kg km−2).

Long-term trends in the Australasian gannet ( Morus serrator ) population in Australia: the effect of climate change and commercial fisheries

The Australasian gannet (Morus serrator) population has increased considerably over the past century, both in New Zealand and Australia. Since 1980, the population in Australian waters has increased threefold, from 6,600 breeding pairs to approximately 20,000 pairs in 1999-2000, a rate of 6% per year. Reasons for the increase in the Australasian gannet population are poorly understood; here we consider the possible effects of recent fluctuations in climatic and oceanographic conditions, and changes in major local commercial fisheries. A significant trend towards more frequent, and stronger, El Nino Southern Oscillation events, warmer summer sea surface temperatures in Bass Strait, increased annual catches and catch per unit effort in the Victorian pilchard (Sardinops sagax) fishery and potential increased discarding of fisheries bycatch may account for at least some of the observed increase in the Australasian gannet population. The potential interactive effects of these factors on prey distribution and abundance and consequently on gannet numbers are discussed.

Prey consumption of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, southeast Australia, and potential overlap with commercial fisheries

Prey consumption of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria, was investigated between 1997 and 2000 and the potential overlap between gannets and commercial fisheries in this region was assessed. Approximately 1000 gannets now breed in Port Phillip Bay annually and typically feed on inshore pelagic schooling fish species, such as pilchards (Sardinops sagax), barracouta (Thyrsites atun) and, to a lesser extent, anchovy (Engraulis australis), garfish (Hyporhamphus melanochir), mackerel (Scomber australasicus) and other species. Nutritional analyses of the major prey items in the gannet diet suggests that gannets may target prey species of higher energy density; however, variations in their representation in the gannet diet between breeding periods probably reflect changes in local availability. The daily energy requirement of individual gannets breeding in Port Phillip Bay, determined from activity patterns and activity-specific metabolic rates, was calculated as 4561 kJ d 1 . Using a bioenergetics model it was estimated that gannets breeding in Port Phillip Bay consume in total some 228.2 tonnes of prey during the breeding period, including 37.8 t of pilchard, 80.5 t of barracouta, 26.5 t of anchovy and 27.5 t of garfish. The potential foraging ranges of gannets during the breeding period were estimated at approximately 100 km. Within this zone all of the major prey items in the gannet diet are taken to some extent commercially, although only pilchards are specifically targeted as a fishery in large quantities. There is considerable spatial and temporal overlap in the harvesting of pilchards by both gannets and the commercial pilchard fishery, but, limited information on the abun- dance or availability of prey stocks inhibits quantification of this level of competition. Nevertheless, it is likely that the fisheries has the potential to negatively affect gannets. 2001 International Council for the Exploration of the Sea