A comparative study of the behaviour and dynamics of humpback whale (Megaptera novaeangliae) mother and calf pairs during their residence in nursery waters
This study provides quantitative data on the variation in behaviour of humpback
whale (Megaptera novaeangliae) mother and calf pairs during their residence in
two known nursery regions; the westerly, leeward regions of the islands of Maui
and the Big Island, Hawaii. Data were collected over a five year period from 1999
to 2003 and exclusively within waters of less than 100 fathoms.
The initial investigation aimed to determine any age-related changes in calf
behaviour. The exact age of calves seen in nursery waters is unknown, however,
calves with distinctly furled dorsal fins are seen and flaccid or furled fins are a
commonly reported feature for very young neonates of other cetacean species.
Such furled fins reportedly unfurl over time and results in this study indicated a
significant relationship between the relative unfurling of the calf's dorsal fin and
increases in the calf's maximum dive times, a known physiological attribute in
neonate cetaceans that increases with age. The degree of furl of the dorsal fin was
therefore applied as a morphological indicator of the relative age of the calf.
Adequate photographic documentation of the dorsal fin and extended behavioural
data, obtained during extended focal follows, were compiled for 69 calves and
these were classified into two discrete relative age classes, separated by a midrange
Two distinct age-related behavioural regimes were identified; in younger calves
the predominant behaviour was persistent travel, accounting for 85 (62-97)% of
the calf's time budget, within the older age group time spent travelling dropped to
47 (26-68)% of the time budget and median time spent stalled underwater
increased by 31%. Frequency of some specific surface behaviours, such as tail slaps and tail swishes, changed slightly with the calf's relative age, whereas other
behaviours were consistently seen in calves in both relative age classes, e.g.
twirling. The calf's breathing regime also varied between age classes: mean
extended dives lengthened with age, increasing from 120 (s.d. 49) seconds for
young calves, to 175 (s.d.55) seconds for older calves and the frequency of
intermittent blows, between 30 and 60 seconds, fell from 22 (s.d. 13) to 12 (s.d.
8) per hour as the calf matured. A range of interactions between the behaviour of
the calf and the calf's breathing regime were also documented.
Variation in mother and calf behaviour between different nursery regions was
limited to changes in the orientation of the mother and calf pair; this may indicate
an adaptive response to increased levels of vessel traffic in favoured nursery
areas. Within specific nursery regions interplay between habitat and behaviour was
evident; in the four island area mother and calf pairs with older calves that were
travelling were found closer to shore than those with older, resting calves and on
the Big Island stratification by water depth, according to group composition was
A summary of incidents of calf injuries and mortalities during this period was
compiled and these provide valuable insight into factors that may impact calf
behaviour and patterns of habitat utilisation in this region. Finally a series of
management recommendations are provided that take into account the changes in
behaviour of calves during the nursery period and across the nursery region.