Distribution, abundance and biology of Group V humpback whales ...

5 Migration and site fi delity
The migratory potential of humpback whales is almost
unlimited and individuals may migrate up to 10,000 km
each year between summer feeding and winter grounds
(Baker et al.1990). The humpback migration could be driven
by a breeding population exhibiting strong site fi delity to
natal wintering grounds and which then disperses to access
prey that is only adequately abundant in Antarctic waters
(Gaskin 1982).
It has been hypothesized that not all Group V humpbacks
migrate each year either from the breeding or feeding
grounds. Occasional sightings of humpbacks in northern
GBR waters (above 16°S Cairns) in summer supports
previous claims of a sub-stock resident year round in
northern Australian tropical waters (Chaloupka and Osmond
1999; Simmons and Marsh 1985). Biopsy studies (Brown et
al.1995) and whaling data (Chittleborough 1965) indicate
that some females may remain on the feeding grounds
during Austral summers.
The trigger to commence migration has been attributed to
reduced light conditions (Dawbin 1966), prey availability
(Clapham 1996), breeding condition and water temperature.
Corkeron and Connor (1999) discuss several hypotheses
to explain the migration to winter breeding grounds. They
conclude that the northward migration is undertaken by
pregnant females to reduce the risk of killer whale Orcinus
orca predation on newborn calves in low-latitude waters.
In the absence of experimental evidence, it is impossible
to establish which factor or factors is/are responsible for
migration.
The trigger to leave tropical wintering grounds and return
to feeding grounds may be related to reproductive status or
hunger, although Paterson (1994) suggested that breeding
behaviour was a stronger urge than feeding. Dawbin (1997)
stated that the fl uctuation in water temperature was too
small for it to act as a trigger for migration.
From the observations of a single humpback offshore
of North Stradbroke Island, Paterson (1994) concluded
that humpbacks migrated at a similar rate during the day
and night. This is one of the assumptions used to model
population estimates by two separate groups based on
North Stradbroke Island (Bryden et al.1996; Paterson et
al.1994). Other assumptions are detailed in Brown (1997).
5.1 Group V migratory route
The wintering grounds of humpbacks are typically located in
tropical, shallow lagoonal waters such as Hawaii, Bahamas,
West Indies, the South Pacifi c islands (Dawbin 1959;
Garrigue et al.2000) and the Great Barrier Reef (Simmons
and Marsh 1985; Paterson and Paterson 1984; Osmond and
Chaloupka 1999).
Northward Southward
Lactating females accompanied by weaning yearlings.
Immature male and female.
Mature males together with resting females.
Pregnant females.
Figure 6. Summary of Group IV population herd structure (Dawbin 1997)
The east Australian Group V population migrates northward
along the continental shelf hugging the coast until it reaches
southern Queensland and then disperses into the lagoonal
waters of the GBR (Paterson et al.1994). Bryden and Griffi th
(1980) conducted aerial surveys to determine the width of
the migratory corridor off North Stradbroke Island, southern
Queensland. The surveys were conducted to the continental
shelf edge but no humpbacks were sighted. However, the
migratory corridor was determined to be no more than
10km wide.
Due to research being focused at the ends of migratory
routes (Corkeron et al.1994), we can only speculate on
the navigation tools used by migrating humpbacks. The
humpback, like the closely related gray whale, may follow
bottom topography parallel to the coast during migration
and “spy-hop” to orient themselves when they reach deep
trenches (Gaskin 1982).
Other unsurveyed migratory routes are possible. A chain of
sea mountains lie parallel to the east Australian continental
shelf, the closest approximately 200 nautical miles off
south-east Queensland. This is defi ned northward by the
Fraser Sea Mountain and south by the Taupo Sea Mountain.
These sea mountains lead into the Coral Sea platform that
is approximately 250 nautical miles from the Queensland
coast at 17°S.
The second chain of sea mountains is further offshore and
lies approximately 560 nautical miles off the south-east
Queensland coast. This chain is defi ned northward by the
Chesterfi eld Reefs and southward by Lord Howe Island.
Reports of sightings at the Chesterfi eld Reefs indicate that
the Group V population at least occasionally visits the area
(Gill et al.1995).
The migratory stream of Group V is described as spreading
out as individuals move northward from Antarctica, and
perhaps divide into two sub-populations, one travelling up
the East Australian coast and another travelling into the
eastern Coral Sea around New Caledonia (Chittleborough
1965; Dawbin 1966; Chaloupka and Osmond 1999). Craig
and Herman (2000) suggest that undersea topography has
some infl uence on habitat choice for breeding females in
Hawaii. It may be that the Group V population separates
into three or more substocks and each group uses these
topographical features to migrate north and south.
5.2 Herd structure and behaviour
There is a distinct temporal separation of the migrating
herd dependent upon maturity and reproductive status.
Chittleborough (1965) and Dawbin (1997) describe the
age class of migrating individuals from Group IV and V
respectively. The herd structure described is similar for
both the northward and southward journeys, although the
individuals differ because of the change in reproductive
status of females (Dawbin 1997).
Mixed females (including those in early pregnancy)
and immature males and females.
Mature males.
Females in early lactation.
9 • Distribution, abundance and biology of Group V humpback whales Megaptera novaeangliae: A review • August 2002