Differences in the functional use of two migratory stopovers by humpback whales (Megaptera novaeangliae)

Humpback whale migration between tropical breeding grounds and polar
feeding grounds is an energy-intensive activity undertaken on finite
energy stores. The use of stopover sites to rest reduces energetic
expenditure and provides enhanced opportunity for calves to nurse during
migration. Moreton Bay is a newly identified migratory stopover for
Australia’s east coast humpback whale population. Understanding the
functional roles of stopovers is essential for a holistic understanding of
population dynamics and connectivity. Therefore, contextualising the
significance of Moreton Bay relative to a well-established stopover like
Hervey Bay can provide valuable insights into their functional roles
within the broader migratory network, helping to inform targeted
conservation efforts. To investigate this, we conducted a total of 865km
of systematic, boat-based line transects across the two distinct
geographical regions during temporally staggered periods (August and
September – October) of the 2021 humpback whale migration. We examined
population structure, behaviour, and habitat segregation, and developed
spatial density surface models to predict density distribution patterns at
each respective site. Our results show that Hervey Bay supports a more
heterogeneous mix of demographic groups, while Moreton Bay had a
significantly greater number of calf-groups (z = 4.53, p = 0.017). Both
bays exhibited similar resting behaviours, but social interactions among
juveniles were unique to Hervey Bay. These findings suggest Moreton Bay
serves a more utilitarian role as a stopover, functioning primarily as a
resting site for mother-calf pairs, rather than the multifaceted use
described in Hervey Bay. As lactating females and their calves are
particularly vulnerable to anthropogenic threats like vessel strike, it is
imperative to understand how different habitats contribute to the success
of migration and ensure adequate protection is maintained.