Animals are well known for their ability to migrate (Gauthreaux, 1980). It is a behavioural strategy by which animals enhance resource acquisition while reducing predation risk (Middleton et al., 2013). Migration is defined as “a seasonal movement of animals from a region to another” (Altizer et al., 2011). The factors initiating migration mainly involves environmental changes. Many migrations are annual events, thus their timing is in relation to the annual cycle. A possible cue is the cyclical change in day length that occurs in non-tropical regions (Aidley, 1981). In equatorial regions, however, day-length changes cannot act as a cue for migrants wintering (Baker, 1978).
- Movements of migrations
In any particular population of migrants, a factor to consider is its origin and destination (Aidley, 1981). Clues such as the changes in the population density of a species help determine the migratory journeys (Baker, 1978). For example, the occurrence of fieldfares (Turdus pilaris) in Britain in winter suggests that they have migrated there from their breeding grounds in northern Europe (Lever, 1987). In another example, the movement of commercial fish has been estimated from the seasonal variations in the catch at different sites (Harden Jones, 1968).
Another clue towards understanding the migratory journey can be inferred from sequential changes in the specific characters of the members of a species sampled at a single place (Aidley, 1981). For example, different wader populations have been distinguished on the basis of bill length and body colour (Moreau, 1972).
1.2 Migratory patterns
The diversity of migratory behavior in animals overwhelms attempts to neatly summarize its character and function. In general, animals migrate to escape unfavorable conditions or to exploit favorable ones such as habitat suitable for reproduction (Aidley, 1981; Baker, 1978). Polar and cold temperate habitats tend to have more migrant species than tropical ones. This is due to the fact that they vary more in productivity and habitability. This tendency also differs widely amongst the major taxonomic groups (Rubenstein & Hack, 2013). For example, at most southern and northern latitudes, only 135 species of birds breed in the arctic zone but all species found there will migrate south to spend most of time there (Gwinner, 1990). A majority of the species found in the temperate region, however, will migrate to more tropical region after breeding (Karr, 1980).
Migration distance and duration similarly vary to a great degree, even within groups of species that migrate for the same reason. This variability takes into account the differences in physiology of the animals. Larger body sizes can store relatively more energy to fuel longer trips. Different forms of locomotion such as flying or swimming are more efficient than others.
In addition to physiology, physical forces such as winds and currents act with habitat topography to further shape the migratory route and schedule (Rubenstein & Hack, 2013).
1.3 Social structure
1.3 Whale migration
Whales, dolphins and porpoises, the order Cetacea, form a group of highly specialized mammals living an entirely aquatic existence. The Cetacea, like land mammals, have well defined patterns of feeding, breeding and ecological behaviour. Migration is an important factor of some species (Aidley, 1981; Perrin et al., 2009).
There are two suborders of living whales, the Odontoceti (toothed whales) and the Mysticeti (baleen whales). The baleen whales are notable migrants. Most species undertake highly seasonal return migrations over long distances (Aidley, 1981; Perrin et al., 2009). Examples of baleen whales that migrate include the humpback whale (Megaptera novaengliae), the gray whale (Eschrichtius robustus) and the blue whale (Balaenoptera musculus). The toothed whales also include some migratory species such as the sperm whale (Physeter macrocephalus) and the northern bottlenose whale (Hyperoodon ampullatus), which undertake seasonal migration (Perrin et al., 2009).
1.3.1 General pattern of migration in mysticetes
The migratory species of the baleen whales include the blue, fin (Balaenoptera physalus), humpback, minke (Balaenoptera bonaerensis, Balaenoptera acutorostrata), sei (Balaenoptera borealis), gray, bowhead (Balaena mysticetus) and the right whales (Eubalaena glacialis, Eubalaena australis, Eubalaena japonica). Migrations are seasonal where the many species migrate between tropical to temperate waters for breeding in winter and high latitude or polar waters for intensive feeding in summer (Corkeron & Connor, 1999; Dawbin, 1966; Perrin et al., 2009). Similar seasonal links with breeding and feeding takes place in both hemispheres. The migration routes of the two species, the humpback whales and the gray whales, are the best studied for the mysticetes (Dawbin, 1966; Gilmore, 1960; Laake et al., 2012; Reed et al., 1988; Rice & Wolman, 1971). The migration routes of these species pass close to land therefore making observation and investigation easier over the years (Aidley, 1981; Kerry & Hempel, 1990; Paterson et al., 1994,2001,2004).
1.3.2 Factors influencing migration in mysticetes
There are several factors influencing migration in mysticetes.
- Food availability
- Climate and physical environment
Food, feeding and fattening are the major factors influencing mysticete migration. The major food of baleen whales in the Southern Hemisphere is Antarctic krill (Euphausia superba), a swarming planktonic crustacean found in the Antarctic (Marr, 1962). Antarctic krill is circumpolar in distribution and extends from the Antarctic shelf waters up to the Antarctic convergence (Deacon, 1937; Mackintosh, 1972; Marr, 1962). In the Northern Hemisphere, however, shoaling fish are taken more readily e.g. herring (Clupea harengus) and Alaska pollock (Gadus chalcogrammus) (Nemoto, 1959). Food availability outside the main polar and subpolar summer feeding grounds is less and feeding is opportunistic (Lockyer, 1976). Due to this seasonal limitation on feeding, all migratory mysticetes must store energy during the main summer feeding period. This is achieved by storing fat and oil in blubber (Lockyer, 1976; Rice & Wolman, 1971).
The gestation period of all baleen whales is approximately a year (Chittleborough, 1958). Conception takes place in winter for the mysticetes (Chittleborough, 1958). Both sexes come into breeding condition independently of sexual stimulation. This seasonal gonadal activity only occurs when environmental conditions are appropriate. The foetal growth curve of rorquals (whales of the family Baleanopteridae) has an exponential spurt approximately five months after conception (Laws, 1959). This coincides with the migration to the summer feeding grounds. The intense growth phase of the foetus ensures that it reaches term by the time the female returns to winter grounds the following year (Chittleborough, 1965; Laws, 1959). The females gives birth at winter grounds where she suckles her calf for about 7 months and weans the offspring on the first summer feeding migration. This is with the exception of the minke whale and the humpback whale. The minke whale and humpback whale suckle their young for about 4 to 11 months. Outside the main feeding grounds, the growth of the calf is dependant on food supply other than milk (Dawbin, 1998; Kasamatsu et al., 1995).
While climate and physical environment have been mentioned as a factor influencing migration in mysticetes, there are no evidences to confirm the hypothesis. In general, factors inducing migration is greatly determined by reproductive needs as well as availability of food as mentioned earlier. Though not all individuals will respond the same way, these factors may trigger the start of migration (Norris & Sciences, 1966)
1.3.3 Social organization of migration in mysticetes
The social organization and succession of different social classes in migration have already been outlined for humpback and gray whales. The migration occurs as a procession, in a structured manner, rather than a mass movement of whales at one go (Aidley, 1981). This procession mentioned by Aidley (1981) refers to a structure in the whale migration in terms of sexual categories. Dawbin (1966) and Rice and Wolman (1971) have described the temporal segregation during migration in humpback whales and gray whales. Laws (1961) described the temporal segregation of fin whales during the north and south migration. The sequences however, differ from humpback whales. During their southward migration to feeding grounds, older and pregnant female humpback whales are in advance of other groups. The sexually immature humpback whales arrive later. This differs during the northward migration to breeding grounds where the pregnant humpback whales return after the other categories. Immature humpback whales are the second group in both the south and north migration, followed by mature males alongside resting females. Lactating females travel south last and return north ahead of other humpback whales (Dawbin, 1998).
1.4 Introduction to humpback whales
The humpback whales are rorquals (Balaenopteridae), a family that includes the blue, fin, Bryde’s (Balaenoptera brydei), sei and minke whales. Adult humpback whales range in length from 12-16m and weigh about 22000-36000kg. Known for their long pectoral fins, its length is equivalent to approximately one third that of its body (True, 1904). Humpback whales have no teeth. Instead, they carry about 270-400 baleen plates on either side of their upper jaw (Clapham & Mead, 1999; Matthews, 1937; Ognev & Tomilin, 1967)
The humpback whales have a varied diet. Whales in the Southern Hemisphere prey on euphausiids (mostly Euphausia superba). Elsewhere, the species feed on other euphausiids of several genera (such as Thysanoessa and Meganyctiphanes) as well as several species of schooling fish such as herring (Clupea), Atlantic mackerel (Scomber scombrus) and capelin (Mallotus villosus) (Clapham & Mead, 1999). Humpback whales are ‘lunge feeders’. They swim rapidly into a school of prey to gulp in large quantities of water and prey. This method of feeding is a combination of a large mouth, a widely opening lower jaw and numerous extensible ventral grooves (Bannister, 2008).
Humpback whales are found in all oceans. The species is commonly found in coastal or shelf waters throughout its range except during migration when whales cross deep ocean. Its distribution changes with the seasons. In spring, summer and autumn, the whales spend their time on feeding grounds in temperate or high-latitude waters. The whales then migrate in winter to mating and calving grounds in tropical or subtropical waters (Dawbin, 1966). This is with the exception of the ‘Group X’ population that remains in the Arabian Sea year-round (Mikhalev, 1997).
Humpback whale populations are geographically separated into these locations: North Atlantic, North Pacific and Southern Hemisphere. Populations that belong in the Northern and Southern Hemisphere remain largely discrete. There are however genetic (Baker et al., 1993) and photographic evidence (Rasmussen et al., 2007) suggesting a small level of interchange that may occur. Humpback whales migrate from Antarctica to breeding grounds in Costa Rica where they spend the austral winter (Rasmussen et al., 2007). The area off Costa Rica is also used as a breeding ground by Northern Hemisphere humpback whales during the boreal winter (Calambokidis et al., 2000). This suggests the spatial as well as temporal overlap of Northern and Southern Hemisphere humpback whale breeding grounds in some regions (Rasmussen et al., 2007). Evidence of trans-equatorial genetic exchange by (Baker et al., 1993) suggests that there may be some level of temporal overlap between early-arriving whales from one hemisphere and whales that depart late from the other.
There are six different feeding aggregations in the Northern Hemisphere. These can be found in the North Atlantic in the Gulf of Maine, Iceland, North Norway, West Greenland, Labrador and Gulf of St Lawrence (Stevick et al., 2003). The main breeding grounds for these whales are in the West Indies. Smaller aggregations go around the Cape Verde Islands for breeding (Reiner et al., 1996).
In the eastern, central and western North Pacific, there are at least three main subpopulations that exist (Reilly et al., 2008). The major breeding grounds for the whales that feed in the central and northern latitude areas are in Hawaii, while those that feed in the west coast of USA from California to Washington breed mainly off mainland Mexico (Clapham & Mead, 1999).
There are now 7 major breeding stocks A through G, which are further divided into substocks (Reilly et al., 2008). The 7 breeding stocks in the Southern Hemisphere are:
- A: Southwest Atlantic coast of Brazil
- B: Southwest Atlantic coast of West Africa from the Gulf of Guinea down to South Africa
- C: Southwestern Indian Ocean coasts of eastern South Africa, Mozambique, Madagascar (southern, western and eastern coasts), Mayotte, the Comoros and other western Indian Ocean island groups;
- D: Southeastern Indian Ocean coasts of northwestern Australia
- E: Northeastern Australia, New Caledonia, Tonga and Fiji.
- F: Cook Islands and French Polynesia
- G: Ecuador, Galápagos, Colombia, Panama and Costa Rica.
1.4.2 East Australian population including its migration
Humpback whales were exploited from five locations in east Australia in the 1900s. Reports from east Australia indicated large numbers of humpback whales at 23-27oS during the months of July, August and September. Specific site reports included Cape Capricorn, Lady Elliot Island, Double Island Point and Stradbroke Island (Paterson et al., 1994). Extensive biological data including the timing of northern and southern migrations were obtained during the final phase of humpback whale exploitation (Chittleborough, 1965).
1.4.3 Temporal segregation during migration in Southern Hemisphere waters
The temporal segregation of different age and reproductive classes of humpback whales on their migration in the Southern Hemisphere has been described by (Dawbin, 1998). The peak of lactating females and yearlings are the first to leave the Antarctic feeding ground. Immature whales of both sexes follow this peaking about 12 days later, alongside mature males and resting females peaking about 20 and 23 days after. Lastly, the peaking of pregnant females migrate to the breeding grounds 31 days after the start of the migratory ‘procession’.
On the return journey, newly pregnant and resting females (along with immature whales) leave tropical waters first. Mature males follow peaking about 10 days later as well as mother with calves about 16 days after the newly pregnant females (Dawbin, 1998).
The duration of passage of the major part of the migrating population in the Southern Hemisphere is approximately three months. During this time, there is a peak density in the middle for both its northward and southward migration (Dawbin, 1966). Southern Hemisphere whales begin to leave Antarctic water in mid to late April to undertake its northward journey (Dawbin, 1998).
1.4.4 Migratory timing
Photoperiod and the movement of prey and ice appear to be the major factors to trigger migration from the feeding grounds in humpback whales (Baker, 1978; Chittleborough, 1965; Craig et al., 2003; Dawbin, 1966). There are variations however, suggesting that there are other factors to trigger migration. For example, the peak of the migration past Cook Strait, NZ varied up to 38 days (Dawbin, 1956). Smaller variations in peak migration of 3-3.5 weeks have also been recorded for whales in Hawaiian waters and whales passing Albany (Baker & Herman, 1981; Chittleborough, 1965).
Tidal cycles and water temperature have also been associated with the variation in timing of greatest abundance of humpback whales off the Ryuku Islands in Japan (Nishiwaki, 1960). Brodie (1975) hypothesized that migration is delayed when finding and exploiting a rich food source offsets the energetic costs of remaining in cold water.
Thus, a combination of factors such as photoperiod, food availability, water temperature, tidal cycles, body condition and hormonal state may contribute to when an individual humpback whale undertakes migration (Craig et al., 2003)
Humpback whales have been studied on the migratory corridor off the eastern coast of Australia through the collection of data during the whaling period (Dawbin, 1966). More recently, land based counts (Kerry & Hempel, 1990; Paterson et al., 1994,2001,2004) and studies on acoustics were done (Cato et al., 2002; Dunlop et al., 2007; Frankel et al., 1995). Little information however, has been published regarding the processions of whales and its structure.
A recent study by Bruce et al. (2014) investigated the spatio-temporal distributions of humpback whales in Jervis Bay provided insight on the role of the shallow coastal embayment for mother-calf pods during its southern migration. Evidence of clusters of mother-calf pairs were observed in this study displayed a significant preference for protected shallow waters inside Jervis Bay during the southern migration period.
As mentioned earlier, the social organization of the mysticetes have been outlined. The migration that was described to occur in processions (Aidley, 1981) however has yet to be defined or studied. Aside from the temporal segregation described by (Dawbin, 1998), no other studies were done to analyze the structure of migrating humpback whales within the Southern Hemisphere stocks.
As such, this paper aims to analyse the movements of the east Australian humpback whale population that passes Point Lookout, the primary land-based survey site for the last 40 years. The hypothesis of this study is that the whale movements are occurring at ‘random’. The alternative hypothesis of this study would be that the whale movements occur in clusters, which provides more information regarding the processions described earlier.
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