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Diet of the Australasian gannet (Morus serrator) at Farewell Spit, New Zealand

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The diet of the Australasian gannet (Morus serrator) at Farewell Spit, New Zealand, was studied by the analysis of 70 regurgitations collected from the 1995 to 2001 breeding seasons. Surface schooling pilchard (Sardinops neopilchardus) was the main prey, followed by anchovy (Engraulis australis). The composition of the diet was similar in most seasons examined except in 1996 in which anchovy was the main prey item. Such a change in diet could be linked with a pilchard mass mortality in New Zealand in August 1995. The estimated annual prey consumption by birds at the Farewell Spit gannetry was 852 tonnes. Although annual catches of pilchard and anchovy by commercial fsheries in the area are still relatively small, an increase may interfere with prey availability, and in turn, increase competition between marine predators and infuence the breeding success. Our analyses of diet are consistent with previous studies showing that Australasian gannets as fexible foragers and they highlight their importance as bioindicators of fsh stocks in New Zealand.
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INTRODUCTION
Breeding success and population growth in a
number of species of seabirds has been demonstrated
to be related to food availability (Lack 1968; Croxall
1987). However, commercial shing can also
inuence seabird populations through a reduction of
food resources (Burger et al. 1984; Croxall 1987; Tasker
et al. 2000) and an increase in mortality from by-
catch (Robertson et al. 2003). Information on the diet,
foraging strategies, foraging sites and breeding cycles
of seabirds is needed to increase our understanding
of the potential for conict with sheries (Croxall
1987). Furthermore, long-term monitoring of the diet
and foraging strategies of marine apex predators,
including seabirds, provide valuable information on
the health of the environment and in some sh stocks
(Einoder 2009). The Australasian gannet (Morus
Notornis, 2012, Vol. 59: 66-70
0029-4470 © The Ornithological Society of New Zealand, Inc.
Received 28 Feb 2012; accepted 13 May 2012
*Correspondence: rschckrd@xtra.co.nz
Diet of the Australasian gannet (Morus serrator) at Farewell Spit,
New Zealand
ROB SCHUCKARD*
PO BOX 98, Rai Valley 7145, New Zealand
DAVID S. MELVILLE
1261 Dovedale Road. Dovedale, RD 2 Wakeeld, Nelson New Zealand
WILLIE COOK
23 Edens Road RD 1 Richmond, 7081, New Zealand
GABRIEL E. MACHOVSKY CAPUSKA
Nutritional Ecology Research Group & Coastal-Marine Research Group, Institute of Natural Sciences, Massey
University, Private Bag 102 904 North Shore MSC, Auckland, New Zealand
Abstract The diet of the Australasian gannet (Morus serrator) at Farewell Spit, New Zealand, was studied by the analysis of
70 regurgitations collected from the 1995 to 2001 breeding seasons. Surface schooling pilchard (Sardinops neopilchardus) was
the main prey, followed by anchovy (Engraulis australis). The composition of the diet was similar in most seasons examined
except in 1996 in which anchovy was the main prey item. Such a change in diet could be linked with a pilchard mass
mortality in New Zealand in August 1995. The estimated annual prey consumption by birds at the Farewell Spit gannetry
was 852 tonnes. Although annual catches of pilchard and anchovy by commercial sheries in the area are still relatively
small, an increase may interfere with prey availability, and in turn, increase competition between marine predators and
inuence the breeding success. Our analyses of diet are consistent with previous studies showing that Australasian gannets
as exible foragers and they highlight their importance as bioindicators of sh stocks in New Zealand.
Schuckard, R.; Melville, D.; Cook, W.; Machovsky Capuska, G.E. 2012. Diet of the Australasian gannet (Morus serrator) at
Farewell Spit, New Zealand. Notornis 59 (1&2): 66-70.
Keywords gannets; bioindicators; foraging strategies; diet; Farewell Spit; sh stocks
67
serrator) (hereafter gannet) is a marine apex predator
endemic to Australia and New Zealand (Nelson
1978). The New Zealand population is estimated to
be around 55,000 breeding pairs, distributed in 29
colonies and is considered to be increasing annually
by 2.3 % (Robertson 1992; Nelson 2005; Stephenson
2005). Farewell Spit gannetry in Golden Bay, 1 of
4 breeding sites in the South I, was established in
1983 with c.75 breeding pairs (Hawkins 1988). Since
then, the population has increased by an average
of 11.5% per annum, and in 2011 was estimated at
3,900 pairs (R. Schuckard, unpubl. data).
Several studies have previously described the
diet of gannets in other breeding colonies around
New Zealand, reporting that they feed mainly on
pilchard (Sardinops spp.), anchovy (Engraulis spp.),
saury (Scomberesox spp.), jack mackerel (Trachurus
spp.) and squid (Nototodarus spp.; Oliver 1955;
Wingham 1985; Robertson 1992; Machovsky
Capuska et al. 2011a, b). It has been suggested that
the increase in inshore commercial shing in New
Zealand has positively inuenced gannets by the
increase in surface-schooling sh that are normally
preyed on by commercial species (Robertson 1992).
However, no data are available for the Farewell Spit
gannetry to determine whether commercial sheries
have had any inuence on this breeding population.
Here we examine the diet of breeding adult
Australasian gannets obtained from regurgitations
collected at the Farewell Spit gannetry during 5
breeding seasons, to improve our knowledge of
the foraging behaviour of this marine predator and
to highlight possible overlap with sheries in the
region.
METHODS
A total of 70 gannet regurgitations from dierent
breeding adults were collected at Farewell Spit
gannetry, which is located at the northern end of
the South I of New Zealand (40°33’S 173°02’E). All
samples were collected between Oct and Jan during
the chick-rearing period of the 1995/1996 (n = 18),
1996/1997 (n =7), 1997/1998 (n = 17), 1999/2000 (n = 26),
and 2001/2002 (n = 2) breeding seasons. Considering
that gannets are sexually monomorphic in size and
weight and behaviour is not a reliable method of
sex-assignment (Nelson 1978), no sex dierences
were established for our samples.
Samples were collected in separate polythene
bags from gannets in active nests that voluntarily
regurgitated during handling. We then identied
the species of prey in the eld using published
guides (Paulin et al. 1989). For each prey species,
body length was measured as described in Meynier
et al. (2008). Following Robertson (1992) samples
were not weighed because birds from which they
were taken had not always recently returned
from feeding and thus partial digestion may have
occurred. Data were analysed as the percentage
of prey items of one type out of all prey items
(Numerical Abundance percentage, N%) and as
the presence or absence of prey on each individual
regurgitation (Frequency of occurrence, F%) (Duy
& Jackson 1986). Frequencies of prey occurrence
were compared using χ2 tests. Data were initially
tested using Levene tests for homoscedasticity and
Shapiro-Wilk for normality and Kruskal Wallis
tests for subsequent non-parametric comparisons.
For statistical comparisons, data were analysed
using PAWS Statistics version 19. We report data as
medians and range.
To estimate Annual Food Consumption (AFC)
by birds at the Farewell Spit gannetry, we assumed a
daily food intake of 259 g per bird (Wingham 1985),
and multiplied this by the number of breeding birds
at the colony and the diet composition from Table
1.
RESULTS
A total of 7 species of sh and 1 species of squid
were identied in gannet regurgitations. Surface-
schooling pilchard (Sardinops neopilchardus) was the
most frequent prey, followed by anchovy (Engraulis
australis), garsh (Hyporhamphus ihi), squid
(Nototodarus spp.), yellow-eye mullet (Aldrichea
forsteri), barracouta (Thrysites atun) and saury
(Scomberesox saurus). However, E. australis was the
most numerous prey followed by S. neopilchardus
(Table 1).
Prey occurrence varied signicantly across the
5 seasons studied (χ2 = 41.7; df = 4 ; P < 0.001). From
70 regurgitations from 1995 to 2001, 50% contained
5 prey species, 37% contained 6 prey species, 10%
contained 3 prey species and 3% contained 4 prey
species (Table 2). S. neopilchardus (in 1995, 1997,
1999 and 2001) and E. australis (1996) were the most
frequent prey. E. australis was the most numerous
Table 1. Overall composition of the diet of the Australasian
gannet breeding at Farewell Spit, New Zealand, 1995-2001.
Diet is described by percentage frequencies of occurrence
(F%) and number (N%).
F% N%
Pilchard 62.8 37.6
Anchovy 44.3 50.2
Garsh 10.0 4.3
Arrow squid 4.3 1.8
Yellow-eye mullet 4.3 4.7
Barracouta 1.4 0.2
Saury 1.4 0.4
Diet of Australasian gannets
68
prey in 1995, 1996 and 2001, whereas S. neopilchardus
was most numerous in 1997 and 1999 (Table 2).
The overall length of the prey species consumed
ranged in size from 9-33 cm and varied signicantly
between years (Kruskal Wallis: h = 227.6, df = 4, P <
0.0001, Table 3). Thus, signicant length variation
was also observed in pilchard and anchovy between
years (Kruskal Wallis: h = 219.3, df = 4, P < 0.0001,
Table 3).
The total annual prey consumption by the
Farewell Spit gannetry was also estimated (Table
4). Our analysis showed that more than 80 % of the
biomass was represented by pilchard and anchovy,
highlighting the importance of these species on the
diet of gannets.
DISCUSSION
Our analysis provides the 1st report on the diet
of breeding gannets at the Farewell Spit colony.
Pilchard and anchovy were the main prey species,
this being consistent with previous studies at other
gannetries around New Zealand (Wingham 1985;
Robertson 1992), including Waimaru colony during
the summer of 1981-1982, in which pilchard and
anchovy (90% and 10%, respectively) were the most
important prey items (Robertson 1992). Additional
diet studies from Australia revealed that Australasian
gannets based their diet on high caloric value
prey such as pilchards and to a lesser minor extent
jack mackerel (Trachurus novaezelandiae) and saury
(Bunce 2001).
The overall composition of the diet was similar
in most of the seasons examined except for the
samples collected in 1996 in which anchovy was the
main prey item. Such a change in the diet could be
linked with the pilchard mass mortality registered
in New Zealand in Aug 1995 which is thought to
have aected around 50% of the pilchard biomass
(Whiington et al. 1997) and was suggested to have
caused the biggest wreck of Australasian gannets
ever recorded in New Zealand including at the
Farewell Spit colony where hundreds of birds were
found dead (Taylor 1997; R. Schuckard, unpubl
data). This reduction in pilchard consumption and
increase in anchovy was previously reported in
Table 2. The composition of the diet of the Australasian gannet as reected by the analysis of 70 regurgitations collected
in 5 breeding seasons at Farewell Spit, New Zealand. Diet is described by percentage frequencies of occurrence (F%) and
number (N%). Sample size in parentheses.
1995 (n = 18) 1996 (n = 7) 1997 (n = 17) 1999 (n = 26) 2001 (n = 2)
F% N% F% N% F% N% F% N% F% N%
Pilchard 55.5 32.3 14.3 3.4 58.8 52.2 80.8 53.2 100.0 30.0
Anchovy 50.0 53.0 85.7 95.4 35.3 44.1 34.6 23.8 50.0 35.0
Arrow squid 14.3 1.2 14.3 1.2 5.9 0.9 - - 50.0 30.0
Garsh - - - - 11.8 1.8 15.4 12.7 50.0 5.0
Yellow-eye mullet 11.1 10.8 - - - - 3.8 7.9 - -
Barracouta - - - - - - 3.8 0.8 - -
Saury 5.5 2.0 - - - - - - - -
Table 3. Median total length (cm, and range) of prey in the diet of the Australasian gannet as reected by the analysis of
70 regurgitations collected in 5 breeding seasons at Farewell Spit, New Zealand. The number of prey considered each
year is presented in parentheses.
1995 (n = 102) 1996 (n = 88) 1997 ( n = 111) 1999 (n = 126) 2001 (n = 20)
Median Range Median Range Median Range Median Range Median Range
Pilchard 12.0 11.0-19.0 15.0 11.0-19.0 14.0 10.0-21.0 15.5 10.0-19.0 15.0 13.0-17.0
Anchovy 12.0 10.0-12.0 10.0 9.0-10.0 11.0 10.0-12.0 11.0 10.0-12.0 - -
Arrow squid - - - 11.0 - 21.0 - - - -
Garsh - - - - 22.5 22.0-23.0 17.0 15.0-20.0 - -
Yellow-eye
mullet 13.5 10.0-22.0 - - - - 12.0 12.0-14.0 - -
Barracouta - - - - - - - 20.0 - -
Saury - 33.0 - - - - - - - -
Schuckard et al.
69
Australia, supporting the view that Australasian
gannets are exible foragers (Nelson 1978; Bunce
& Norman 2000). Thus, our results conrmed
that gannets increased the number of anchovies
consumed when their diet was based mainly on
this lower caloric prey species to satisfy their
daily energetic requirements as suggested by Bunce
(2001).
Our analysis also showed that gannets at
Farewell Spit colony feed typically on sh of similar
length to prey consumed by the same species in
Australia (Brothers et al. 1993; Bunce 2001) and other
colonies in New Zealand (Wingham 1985; Robertson
1992; Machovsky Capuska et al. 2011 a). Prey size
was also similar in the related Cape gannet (Morus
capensis) in South Africa (Berruti et al. 1993). It has
been suggested that the size of these prey species
are related to the size that adults deliver to young
chicks during the breeding season (Waghorn 1982;
Bunce 2001).
Australasian gannets are specialised hunters
that forage by plunge-diving into the water with a
high prey capture rate (72%, Machovsky Capuska et
al. 2011b). It has been suggested that gannets have
an average feeding range of 268 km in the Hauraki
Gulf (Wingham 1985), however banded adults
were found during the breeding season up to 400
km away from Farewell Spit colony (Huler 2009),
although it is unknown if these were breeding
birds. An aerial and boat survey for seabirds and
marine mammals covering the whole of the Tasman
and Golden Bay areas in Dec 2010 and Jan 2011
recorded gannets as the most widespread species in
the study area (Handley et al. 2011a, b). This wide
distribution could be related to pilchard availability.
Pilchards are reported in the Marlborough Sounds
and Tasman Bay throughout the year and from
Oct to Apr are often visible as surface schools
when diatoms bloom in nutrient-rich upwelling,
coinciding with the gannet breeding season (Baker
1972, Paul et al. 2001).
The foraging range of gannets from Farewell
Spit falls within Ministry of Fisheries Area 7 which
encompasses the Marlborough Sounds, Golden and
Tasman Bays, and the West coast of South I (Annala
1992). Although current estimates of biomass for
pilchard and anchovy in New Zealand are not
available, annual catches of 115 tonnes for anchovy
and 165 tonnes for pilchard have been established
(Paul et al. 2001; Chaerton 2002). Considering
our estimation of prey annually consumed by
gannets at Farewell Spit, and that other marine
predators such as dusky dolphin (Lagenorhynchus
obscurus), shearwaters (Punus spp.), terns (Sterna
spp.), gulls (Larus spp.), shags (Phalacrocorax spp.),
fur seals (Arctocephalus forsteri), spiny dogsh
(Squalus acanthias) thresher shark (Alopias vulpinus),
kahawai (Arripis trua) and barracouta have also
been reported to feed on pilchard and anchovy
(Graham 1956; Baker 1972; Vaughn et al. 2007),
an increase in commercial shing may interfere
with prey availability and result in increased
competition between marine predators. This in turn
could inuence the breeding success of birds at the
Farewell Spit gannetry.
Our study supports previous observations of
gannets as exible predators and we recommend
the use of Australasian gannets as bioindicators
of food availability and sheries stocks in New
Zealand. Decline of pilchard and anchovy can have
serious eects on the overall food web, aecting not
only the seabird populations dependent on these
food sources but also important recreational and
commercial shing stocks. Further studies including
the use of bird-aached data loggers are necessary
to gain a beer understanding of the foraging areas
and site delity of these marine apex predators.
ACKNOWLEDGEMENTS
We thank members of the OSNZ Nelson Region for eld
assistance and the Department of Conservation, Golden
Bay for providing the long term logistical support to this
project. Landcare New Zealand supported this project.
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Schuckard et al.
... The Australasian gannet (Morus serrator) is a large pelagic seabird breeding on coastal locations and offshore islands along narrow continental shelves in south-eastern Australia and New Zealand. Its diet typically consists of small schooling prey such as pilchards (Sardinops sagax), anchovy (Engraulis australis), garfish (Hyporhamphus melanochir) and, to a lesser extent, larger species such as mackerel (Trachurus declivis), barracouta (Thyrsites atun), mullet (Upeneichthys lineatus) and squid species (Nototodarus gouldi and N. sloanii) (Bunce, 2001;Schuckard et al., 2012). Throughout its range it is an important top marine predator, with individuals from a single small colony in south-eastern Australia alone consuming an estimated 230 tonnes of fish and cephalopods during the breeding season (Bunce, 2001). ...
... The diet of Australasian gannets has previously been well documented (Robertson, 1992;Schuckard et al., 2012;Tait et al., 2014), particularly at the Pope's Eye colony (Brothers et al., 1993;Norman and Menkhorst, 1995;Bunce and Norman, 2000;Bunce, 2001;T.M. Pyk, PhD thesis, Deakin University, Australia, 2012), with the majority of regurgitate samples comprised of schooling fish, barracouta and red mullet. These studies, however, have not explicitly linked diet with foraging location, therefore limiting the understanding of preferred prey. ...
... In New Zealand, Australasian gannets display different dive behaviours and consume different prey species depending on colony location (Robertson, 1992;Schuckard et al., 2012;Machovsky-Capuska et al., 2013a,b). Similarly, northern gannets differ in foraging strategies and prey consumption in contrasting oceanographic environments, with individuals from an inshore colony feeding primarily on large prey species whereas birds from an offshore colony feed on small schooling prey (Garthe et al., 2007). ...
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Knowledge of top predator foraging adaptability is imperative for predicting their biological response to environmental variability. While seabirds have developed highly specialised techniques to locate prey, little is known about intraspecific variation in foraging strategies with many studies deriving information from uniform oceanic environments. Australasian gannets (Morus serrator) typically forage in continental shelf regions on small schooling prey. The present study used GPS and video data loggers to compare habitat-specific foraging strategies at two sites of contrasting oceanographic regimes (deep water near the continental shelf edge, n=23; shallow inshore embayment, n=26), in south-eastern Australia. Individuals from the continental shelf site exhibited pelagic foraging behaviours typical of gannet species, using local enhancement to locate and feed on small schooling fish; in contrast only 50% of the individuals from the inshore site foraged offshore, displaying the typical pelagic foraging strategy. The remainder adopted a strategy of searching sand banks in shallow inshore waters in the absence of conspecifics and other predators for large, single prey items. Furthermore, of the individuals foraging inshore, 93% were male, indicating that the inshore strategy may be sex-specific. Large inter-colony differences in Australasian gannets suggest strong plasticity in foraging behaviours, essential for adapting to environmental change.
... Fieldwork was conducted on Farewell Spit (FS, New Zealand, 40°33′S, 173°01′E), during the 2-to 5-week-old chick-rearing period in December and January 2011, 2013-2014, 2014-2016. FS is a beach colony located at sea level with a population of gannets estimated at 3,900 breeding pairs (Schuckard, Melville, Cook, & Machovsky-Capuska, 2012). ...
... This is because the niche concept refers to the resources required to maintain the population, including all stages of the life cycle (Pulliam, 2000). Indeed, sampling We are confident that the sampling regime in our study has provided a reliable and unique representation of the prey composition (Schuckard et al., 2012). Second, the range of macronutrients that we recorded in prey and diets consumed by gannets comprises most of the spectrum of marine fish wet mass concentrations of lipid (0.2%-25.0%) and protein (17.0%-25.0%; ...
... The likely explanation is that the gannets' main prey are small anchovy, pilchard and garfish that feed mostly on plankton (Schuckard et al., 2012) and respond to regional plankton blooms under nutrient-rich conditions (Hunt et al., 1998;Paul, Taylor, & Parkinson, 2001). However, warmer waters are often more stratified and characterized by nutrient limitation and reduced plankton productivity (Behrenfeld et al., 2006;Richardson & Schoeman, 2004). ...
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1.Our understanding of the niche concept will remain limited while the quantity and range of different food types eaten remains a dominant proxy for niche breadth, as this does not account for the broad ecological context that governs diet. Linking nutrition, physiology and behaviour are critical to predict the extent to which a species adjusts its nutritional niche breadth at the levels of prey (“prey composition niche”, defined as the range of prey compositions eaten), and diet (“realized nutritional niche” is the range of diets composed through feeding on the prey). 2.Here we studied adult‐chick rearing Australasian gannets (Morus serrator) to propose an integrative approach using sea surface temperature anomalies (SSTa), geographic location and bathymetry over different years, to explore their relationship with the nutritional composition of prey and diets (i.e., prey composition and nutritional niche breadth), habitat use and foraging behavior. 3.We found that gannets feed on prey that varied widely in their nutritional composition (have a broad prey composition niche), and composed diets from these prey that likewise varied in composition (have a broad realized nutritional niche), suggesting generalism at two levels of macronutrient selection. 4.Across seasons, we established “nutritional landscapes” (hereafter nutriscapes), linking the nutritional content of prey (wet mass protein to‐lipid ratio ‐P:L‐) to the most likely geographic area of capture and bathymetry. Nutriscapes varied in their P:L from 6.06 to 15.28, over time, space and bathymetry (0 to 150 m). 5.During warm water events (strong positive SSTa), gannets expanded their foraging habitat, increased their foraging trip duration and consumed prey and diets with low macronutrient content (wet mass proportions of P and L). They were also constrained to the smallest prey composition and realized nutritional niche breadths. 6.Our findings are consistent with previous suggestions that dietary generalism evolves in heterogeneous environments, and provide a framework for understanding the nutritional goals in wild marine predators and how these goals drive ecological interactions and are, in turn, ultimately shaped by environmental fluctuations.
... The study was conducted during the chick-rearing period in December and January 2011-2015 at FS, which is located at the northern end of the South Island, New Zealand (40°33′S, 173°01′E). The FS gannetry in Golden Bay was established in 1983, and since then, the population has increased by an average of 11.5 % per annum, to around 3900 pairs in 2011 (Schuckard et al. 2012). ...
... Weight of the regurgitation was calculated as the sum of the individual prey items' mass, and each prey species was assigned a mass percentage (M %), calculated as the percentage contribution of each prey species to the total weight of the regurgitation following Duffy and Jackson (1986). In addition, a numerical abundance percentage (N %) was calculated as the percentage of the total number of prey items contributed by individuals of a particular species, and a frequency of occurrence percentage (F %) calculated as the percentage of birds that had a particular species in their diet (Schuckard et al. 2012). ...
... Presumably such temporal fluctuations are not solely limited to the years in which we have sampled, but have also happened regularly in the past. Despite such temporal variation, the gannet colony at FS has shown continuous growth (Schuckard et al. 2012). Our findings may thus be seen as further evidence of this species' ability to successfully adjust to nutritional fluctuations of prey (Grémillet et al. 2008;Tait et al. 2014). ...
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The foraging challenge for predators is to find and capture food with adequate levels of energy and nutrients. Marine predators require particularly sophisticated foraging strategies that enable them to balance self- and offspring-feeding, and also in many circumstances simultaneously consider the nutritional constraints of their partners. Here we combined the use of dietary analysis, proximate composition and nutritional geometry (right-angled mixture triangle nutritional models) to examine the macronutrient preferences of Australasian gannets (Morus serrator) at Farewell Spit gannetry in New Zealand. Our results showed intra- and inter-specific variation in the protein, lipid and water composition of prey captured by our sample of 111 Australasian gannets. In addition, we observed significant differences in the Australasian gannets’ nutritional niche between seasons. We provide evidence of sex-specific macronutrient foraging strategies in a successful marine predator in the wild. We have shown that in spite of fluctuations in the nutritional composition of foods available to Australasian gannets, males consistently capture prey with higher protein-to-lipid ratios and lower lipid-to-water ratios than females. These results aid to better understand the evolutionary relationship between macronutrient selection and sex-specific traits in wild animals. They also suggest an incentive for these predators to combine individually imbalanced but nutritionally complementary foods to achieve dietary balance, further highlighting the likelihood that prey selection is guided by the balance of macronutrients, rather than energy alone.
... This region is a breeding spot for one of the most successful seabirds in New Zealand, the Australasian gannet (Morus serrator; hereafter gannets). Gannets feed mainly on pelagic fish and squid (Robertson 1992;Machovsky-Capuska et al. 2011a;Schuckard et al. 2012;Tait et al. 2014). These highly specialized marine predators have been reported to travel for food as far as 388.5 km (Machovsky-Capuska et al. 2013a, 2014 with the ability to assess prey density to increase foraging success (Machovsky-Capuska et al. 2013b). ...
... i.e. pilchard (Sardinops sagax) (Bunce et al. 2002). The effect of expanding inshore commercial fishing activity in New Zealand leading to a greater presence of surface-schooling fish, normally preyed on by commercial species, may also impact gannet foraging behavior (Robertson 1992;Schuckard et al. 2012). However, such overlap with commercial fisheries also raises the risk of gannet mortality via gear entanglements (Norman 2000). ...
... Our analysis indicates that inter-annual variability in gannet sightings was significantly and negatively correlated with SST anomaly in HG. Lack of direct correlation with other variables tested could be due to, a) our inability to detect a relationship due to data limitations, b) as flexible foragers (Schuckard et al. 2012), gannets ...
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Few studies have investigated regional and natural climate variability on seabird populations using ocean reanalysis datasets (e.g. Simple Ocean Data Assimilation (SODA)) that integrate atmospheric information to supplement ocean observations and provide improved estimates of ocean conditions. Herein we use a non-systematic dataset on Australasian gannets (Morus serrator) from 2001–2009 to identify potential connections between Gannet Sightings Per Unit Effort (GSPUE) and climate and oceanographic variability in a region of known importance for breeding seabirds, the Hauraki Gulf (HG), New Zealand. While no statistically significant relationships between GSPUE and global climate indices were determined, there was significant correlation between GSPUE and regional SST anomaly for HG. Also, there appears to be strong link between global climate indices and regional climate in the HG. Further, based on cross-correlation function coefficients and lagged multiple regression models, we identified potential leading and lagging climate variables, and climate variables but with limited predictive capacity in forecasting future GSPUE. Despite significant inter-annual variability and marginally cooler SSTs since 2001, gannet sightings appear to be increasing. We hypothesize that at present underlying physical changes in the marine ecosystem may be insufficient to affect supply of preferred gannet main prey (pilchard Sardinops spp.), which tolerate a wide thermal range. Our study showcases the potential scientific value of lengthy non-systematic data streams and when designed properly (i.e., contain abundance, flock size, and spatial data), can yield useful information in climate impact studies on seabirds and other marine fauna. Such information can be invaluable for enhancing conservation measures for protected species in fiscally constrained research environments.
... The species is also found in the coastal waters of North and South Island in New Zealand excluding southeast coast of the South Island (Froese & Pauly, 2019). Individuals of this species form compact schools and are prey to larger fi shes, marine mammals and birds (Majluf & Reyes, 1989;Bunce, 2001;Schuckard et al., 2012). ...
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Relationships between fish length and otolith length, width and mass were examined in the Australian anchovy Engraulis australis (White, 1790) recovered from the food of Gannet examined from colonies at islands of Horuhoru Rock and Mahuki Islands in the Hauraki Gulf, New Zealand. The relationships between otolith length- fish total length (TL), otolith-weight-TL, and otolith-width-TL were investigated by means of non-linear regression models (TL = 0.54 OL 16.86, TL = 4.39 OW 7.61 and TL = 26.19 OWe 2.2). This study characterizes the first reference available on the relationship of fish size and otolith size and weight for E. australis obtained from bird’s food in the Pacific Ocean region
... Like other members of the Sulidae family, the Australasian gannet is considered a generalist forager and has been shown to be adaptable in its feeding habits [40,46,47]. It displays reverse sexual dimorphism (females larger than males) and recent studies suggest individuals exhibit sex-related differences in habitat use [48][49][50] and prey selection [47]. ...
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Foraging is a behaviour that can be influenced by multiple factors and is highly plastic. Recent studies have shown consistency in individual foraging behaviour has serious ecological and evolutionary implications within species and populations. Such information is crucial to understand how species select habitats, and how such selection might allow them to adapt to the environmental changes they face. Five foraging metrics (maximum distance from the colony, bearing from the colony to the most distal point, tortuosity index, total number of dives and mean vectorial dynamic body acceleration were obtained using GPS tracking and accelerometry data in adult Australasian gannets (Morus serrator) from two colonies in southeastern Australia. Individuals were instrumented over two breeding seasons to obtain data to assess factors influencing foraging behaviour and behavioural consistency over multiple timescales (consecutive trips, breeding stages and years) and habitats (pelagic, mixed pelagic and inshore, and inshore). Colony, breeding stage and year were the factors which had the greatest influence on foraging behaviour, followed by sex. Behavioural consistency, measured as the contribution of the individual to the observed variance, was low to moderate for all foraging metrics (0.0-27.05%), with the higher values occurring over shorter timescales. In addition, behavioural consistency was driven by spatio-temporal factors rather than intrinsic characteristics. Behavioural consistency was higher in individuals foraging in inshore than pelagic habitats or mixed pelagic/inshore strategy, supporting suggestions that consistency is favoured in stable environments.
... The prey identified in the present study are consistent with the previous reports of the Australasian gannet diet in the region (Bunce, 2001;Rodríguez Malagón, 2018;Schuckard et al., 2012). For a high proportion of dives, the prey type was unknown and as most of these dives were short dives, the data suggests that small schools or single prey were targeted in these cases. ...
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Knowledge of the factors influencing foraging efficiency in top predators can provide insights into the effects of environmental variability on their populations. Seabirds are important marine predators foraging in a highly temporally and spatially variable environment. While numerous studies have focussed on search time and its effects on foraging energetics in seabirds, relatively little is known of the factors influencing capture success and prey profitability in these predators. In the present study, animal-borne cameras were used to investigate the chase durations, capture success, handling durations and profitability of prey consumed by Australasian gannets (Morus serrator) (n=95) from two breeding colonies in south-eastern Australia exposed to different oceanographic conditions. Capture success was generally lower when individuals foraged alone. However, foraging in multi-species groups and in high prey densities increased chase time, while larger prey elicited longer handling times. While prey type influenced profitability, high prey density and foraging in multispecies groups was found to lower prey profitability due to increased time expenditure. While previous studies have found group foraging reduces search time, the increased profitability explains why some animals may favour solitary foraging. Therefore future studies should combine search time and the currently found factors.
... For each prey species the percentage contribution of the individual to the total weight was calculated as a mass percentage (M%, Duffy and Jackson 1986). The total number of prey items contributed by an individual of a particular species was calculated as a percentage termed numerical abundance (N%), and the frequency of occurrence (F%) was calculated as the percentage of birds that had one particular species present in the regurgitation (Schuckard et al. 2012). The above metrics were summarised using the index of relative importance (IRI), calculated for each prey species as IRI = F% × (M% + N%) and expressed as a percentage (IRI%) by dividing by the sum of IRI values from all prey species (Cortés 1997). ...
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Sexual segregation in the behaviour, morphology or physiology of breeding seabirds can be related to divergent parental roles, foraging niche partitioning or sex-specific nutritional requirements. Here, we combine GPS tracking, dietary and nutritional analysis to investigate sex-specific foraging of Brown Boobies breeding on Raine Island, Great Barrier Reef, Australia. We observed sex-specific segregation in: (1) foraging location: females undertook longer trips, foraging at more distant locations than males; (2) foraging time: male activity and foraging occurred throughout the day, while female activity and foraging increased from midday to an afternoon peak; and (3) prey type, females mostly consumed flying fish, whereas males consumed equal proportions of flying fish and squid. Brown Booby diets contained five tropical prey species that significantly differed in their nutritional composition (Protein, Lipid and Water, wet mass). Despite this variation we found no differences in the overall nutritional content of prey caught by each sex. The observed sex-specific differences in prey type, location and time of capture are likely driven by a combination of a division of labour, risk partitioning and competition. However, Brown Boobies breeding on Raine Island, and other populations, might flexibly partition foraging niches by sex in response to varying competitive and environmental pressures. In light of such potential foraging dynamism, our inconclusive exploration of nutritional segregation between sexes warrants further investigation in the species.
... Australasian gannets predominantly forage on small schooling prey such as pilchards Sardinops sagax and anchovy Engraulis australis (Bunce 2001, Schuckard et al. 2012. These prey are found in neritic waters on the continental shelf (Fletcher & Tregon ning 1992) with a greater abundance associated with the enriched primary productivity produced by the Bonney Upwelling (Nieblas et al. 2009). ...
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The predictability of prey due to oceanographic features can result in large aggregations of apex predators. Central place foragers, such as seabirds, are limited in their foraging duration and range during the breeding period, which can restrict their ability to reach such locations. Segregation by colony and sex can further restrict foraging range and may have direct implications for the foraging ecology of a species. In parts of its range, the Australasian gannet Morus serrator breeds in colonies of relatively close proximity and has recently been found to display sexual dimorphism. Two neighbouring breeding colonies in Bass Strait that experience divergent environmental conditions were investigated to determine whether these conditions or the sex of the individual are important variables influencing foraging behaviour in this species. GPS tracking and accelerometry were paired with stable isotope analysis to compare differences in foraging effort, habitat use and diet. Birds from Point Danger, a large colony located near a seasonally strong upwelling, travelled considerably further (77%) than birds from Pope's Eye, a small colony in a nutrient-poor embayment. However, within colonies no sexual differences in foraging effort were found. While the colonies did not overlap in foraging areas, a degree of sexual segregation was apparent within both colonies (Point Danger 46.3% overlap and Pope's Eye 73.7% overlap in home range). Furthermore, stable isotope analysis indicated birds from each colony fed at different trophic niches. This study reveals differences in habitat use and, consequently, dietary niches between and within neighbouring colonies.
... the complex sets of interacting variables and the logistical challenges of collecting reliable data typically constrain the possibilities for nutritional studies. For example, in the field, it is challenging to measure the daily intake of nutrients by an animal, but an estimate of the proportional composition of the diet can be obtained using gut contents analysis (Hyslop 1980;Kamler and Pope 2001;Petry et al. 2007;Machovsky-Capuska et al. 2011;Tait et al. 2014), regurgitations (Schuckard et al. 2012;Tait et al. 2014), faecal analysis (Klare et al. 2011;Giri et al. 2011;Panthi et al. 2012), bite rates analysis (Shrader et al. 2006;Paddack et al. 2006) and related methods. Second, field-based questions often relate directly to proportions rather than absolute amounts, as is the case where the nutritional compositions of different foods or of foods versus non-foods are compared. ...
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Nutritional geometry has shown the benefits of viewing nutrition in a multidimensional context, in which foraging is viewed as a process of balancing the intake and use of multiple nutrients. New insights into nutrient regulation have been generated in studies performed in a laboratory context, where accurate measures of amounts (e.g. eaten, converted to body mass, excreted) can be made and analysed using amounts-based nutritional geometry. In most field situations, however, proportional compositions (e.g. of foods, diets, faeces) are the only measures readily available, and in some cases are more relevant to the problem at hand. For this reason, a complementary geometric method was recently introduced for analysing multi-dimensional data on proportional compositions in nutritional studies, called the right-angled mixture triangle (RMT). We use literature data from field studies of primates to demonstrate how the RMT can provide insight into a variety of important concepts in nutritional ecology. We first compare the compositions of foods, using as an example primate milks collected in both the wild and the laboratory. We next compare the diets of different species of primates from the same habitat and of the same species (mountain gorillas) from two distinct forests. Subsequently, we model the relationships between the composition of gorilla diets in these two habitats and the foods that comprise these diets, showing how such analyses can provide evidence for active nutrient-specific regulation in a field context. We provide a framework to relate concepts developed in laboratory studies with field-based studies of nutrition.
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The species composition of the prey of the Cape Gannet (Morus capensis) was monitored monthly at colonies at Lambert's Bay and Malgas Island in the southern Benguela system off western South Africa from 1977-78 to 1989. A modified form of the biplot procedure gave a parsimonious, descriptive summary of this time-series. The diet at Malgas Island showed much greater seasonality than at Lambert's Bay. Seasonal variations in the anchovy (Engraulis japonicus) and sardine (Sardinops sagax) components of gannet diet conformed largely to the known distribution of these species. However, the gannet diets demonstrated a previously undocumented presence of adult anchovy off the western Cape, particularly in summer. The contribution of sardines to the diet increased from late 1984 at Malgas Island, and from late 1985 at Lambert's Bay, with corresponding decreases in anchovy and hake (Merluccius spp.) at Malgas Island and anchovy at Lambert's Bay. The occurrence of hake in gannet diets, available as trawler offal throughout the year in deeper waters, seems to be associated with a lack of shoaling fish inshore.
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New Zealand is the breeding ground for the greatest variety of albatross and petrel species anywhere. However, large parts of the lives of these New Zealand breeding species can be spent in other national and international waters far from their breeding locations. During their foraging and migrations away from the colonies these New Zealand breeding populations can interact with a wide variety of fishing activities (demersal and pelagic longline, trawl, set net, etc.). Using the FAO fishery regions of the world as a distribution base, we identified 50 seabird species that breed wholly or partly in New Zealand waters as having been recorded in fisheries incidental bycatch throughout the world (with 37 of these caught in New Zealand fisheries). These species are tabulated (according to fishing method and area) within a listing of 175 species of bird currently recorded as having interacted with the world's fisheries. There are few fisheries and locations with reliable data about which species interact with fishing. Therefore to enable a comparative assessment of areas and fisheries where there is potential risk for fishery/bird interaction worldwide, comparative tabulations of fish catch by nation and fishery target species are provided for the FAO regions. Seabirds which have been subject to capture in New Zealand fisheries have a probability of capture in similar fishery types during the parts of their life-cycles beyond New Zealand waters. Individual summary assessments and mapped distributions are given for 25 of the principal New Zealand breeding (mainly endemic) seabirds deemed most at risk in waters outside New Zealand jurisdiction. These selected species ranged widely, with four found using the South Atlantic and four the Indian Ocean, 22 within Australian waters and the Tasman Sea, 15 crossing the South Pacific Ocean to Chile and Peru, and six crossing the equator and using the North Pacific Ocean as far north as the Bering Sea. These birds are using the national waters of at least 18 countries. For most seabirds vulnerable to incidental capture in fisheries there is little reliable information on the global distribution of pre-breeding and non-breeding birds within species populations.
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Methods of collecting, analysing and presenting data on the diets of seabirds are reviewed, with consideration of methods employed in diet studies of other organisms. Killing of birds continues to be the primary source of dietary information from birds at sea but is no longer necessary for studies on land. Stomach pumps obtain complete stomach evacuation with low mortality. Stomach samples should be examined as soon as possible after collection, to avoid biases caused by preservation. Presentation of data is best done by rank-order to facilitate comparison between studies, but as many data should be provided as possible. Future work on diets will be strengthened through knowledge of digestion rates and nutritional values of food.
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Regurgitations analysed over three seasons (1978-80) indicated that Pilchards Sardinops neopilchardus, both numerically and by weight, were the most common prey species of the Australasian Gannet at Motukaramarama. The next most common species were: numerically, Anchovies Engraulis australis and by weight, Jack Mackerel Trachurus novaeselandiae. It is thought that relative abundance of prey analysed reflects their abundance. Prey between 11-20 cm comprised 77% of the Gannet's diet. A further two prey species: Sprat Sprattus antipodum and Kahawai Arripis trutta were recorded, bringing the known food of the Gannet to 15 species. The average adult Gannet regurgitated food equal to 11% of its body weight (or 259 g and 2,000 Kj), whereas the average daily energy requirements were estimated at 2,844 Kj/day, or 353 g of food. An average feeding range of 268 km (range = 86-450 km) was estimated from the time adults spent away from the colony, resighting of marked birds and from recoveries of banded, breeding adults.
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The diet of Australasian gannets (Morus serrator) breeding in Port Phillip Bay was investigated before and after a mass mortality of pilchards (Sardinops sagax) in southern Australian waters in 1998. Gannets usually feed on inshore pelagic schooling fish, such as pilchards and barracouta (Thyrsites atun), and to a lesser extent on other species. Pilchards represented ~60% of the gannet diet examined before the spread of pilchard deaths into Victorian waters, but this component declined to 5% following the mortality event. This reduction was compensated by a substantial increase in the amount of barracouta taken, supporting the view that the gannet is a flexible forager. However, the 1998 mortality of pilchards is likely to have wider implications since pilchards are an important prey for other piscivorous fish, seabirds and marine mammals.