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