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The influence of multi-species feeding associations on the foraging behaviour of Australasian gannets (Morus serrator) in the Hauraki Gulf, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Conservation Biology at Massey University, Auckland, New Zealand
In
order
to
successfully
capture
prey
in
a
challenging
and
physically
demanding
environment,
Australasian
gannets
(Morus
serrator)
have
developed
a
number
of
foraging
strategies.
Foremost
among
such
strategies
is
the
joining
in
formation
of
mixed
species
feeding
associations
(MSFA’s)
and
the
use
of
specialised
plunge
diving
behaviours.
This
study
sought
to
determine
how
gannets
vary
individual
and
group
plunge
diving
behaviours
in
order
to
maximise
the
benefits
and
mitigate
the
risks
associated
with
feeding
in
densely
packed,
highly
active
MSFA’s.
Specifically
this
study’s
aims
are
to
examine
how
variations
in
gannet
dive
heights,
angles
and
completion
rates
are
influenced
by
external
stimuli,
including
weather
parameters,
oceanographic
conditions
and
MSFA’s
species
composition
and
behaviour.
This
study
additionally
seeks
to
examine
what
mixed
and/or
con-‐specific
cues
are
associated
with
the
formation
of
synchronous
diving
bouts,
and
determine
its
role
as
a
form
of
local
enhancement
for
gannets
foraging
in
MSFA’s.
Between
March
2013
and
June
2014,
45
independent
boat
based
surveys
were
conducted
aboard
Dolphin
Explorer,
a
20m
tour
boat
based
in
New
Zealand’s
Hauraki
Gulf.
Using
a
Canon
XH
A1S
high
definition
video
camera,
approximately
11.9hrs
of
MSFA
video
footage
was
collected,
containing
5565
recorded
gannet
plunge
dives.
Variations
in
these
dives,
including
their
height,
angle
of
entry,
and
synchronous
timing
were
examined
in
context
of
environmental
variables
including
wind
speeds,
sea
surface
temperatures,
and
Beaufort
sea
states
as
well
as
against
MSFA
variables
including
the
abundance
of
other
foraging
gannets,
the
abundance
and
behaviour
of
common
dolphins
(Delphinus
sp.),
and
the
presence
of
other
species
including
shearwaters
(Puffinus
spp.),
petrels
(Fregetta
spp.),
terns
(Sterna
spp.),
and
Bryde’s
whales
(Balaenoptera
edeni).
While
weather
influences
were
found
to
have
minimal
effects
on
gannet
dive
behaviours,
higher
winds
were
associated
with
lower
altitude
dives,
and
less
frequent
synchronous
diving
bouts.
This
is
likely
due
to
the
decreased
visibility
of
deeper
prey
caused
by
increased
surface
disturbances
as
a
result
of
higher
wind
speeds,
coupled
with
the
inherent
difficulties
of
performing
highly
coordinated
aerial
manoeuvres
in
high
wind
conditions.
Increased
gannet
flock
sizes
were
found
to
correlate
with
more
frequent
high-‐
altitude
90°
dives,
and
increased
rates
of
dive
synchrony.
This
may
be
a
result
of
the
need
to
better
coordinate
dives
in
denser
assemblages
of
con-‐specifics
to
avoid
collisions,
coupled
with
the
increased
foraging
success
associated
with
larger
foraging
groups,
and
local
enhancement.
Increased
foraging
dolphin
pods
were
also
associated
with
more
frequent
high-‐
altitude
vertical
dives.
This
may
result
from
gannets
capitalising
on
the
increased
bait-‐ball
density
and
stability
associated
with
larger
foraging
pods
by
taking
deeper
dives
in
order
to
make
multiple
prey
captures
with
greater
ease.
Gannet
synchrony,
however,
was
only
found
to
decrease
in
the
smallest
dolphin
pods,
again
attributed
to
the
decreased
foraging
abilities
of
smaller
pods.
Dolphin
foraging
behaviours
most
associated
with
the
maintenance
of
dense
stationary
bait-‐balls
were
most
heavily
associated
with
high,
vertical
gannet
dives,
and
increased
gannet
synchronous
dives.
This
is
likely
the
result
of
gannets
capitalising
on
denser
prey
assemblages
by
taking
deeper
dives
in
pursuit
of
multiple
prey
capture.
The
presence
of
Bryde’s
whales
at
MSFA’s
was
found
to
decrease
the
frequency
of
both
high
vertical
dives
and
synchronous
dives,
as
lunge
feeding
on
bait-‐balls
scatters
fish
and
decreases
prey
density.
The
presence
of
hetero-‐specific
avian
species
had
various
effects
on
gannet
dive
behaviours.
Most
notably,
the
presence
of
shearwaters
and
petrels
was
found
to
be
associated
with
greater
frequencies
of
low
altitude
gannet
dives,
thought
to
be
a
result
of
an
increased
collision
risk
in
their
presence.
Terns
meanwhile
were
associated
with
decreased
dive
synchrony,
possibly
as
a
result
of
false
hetero-‐
specific
signalling
caused
by
their
white
plumage.
