Southern hemisphere humpback whales (SHHW) (Megaptera novaeangliae) have evolved an extreme migratory life history that exploits the seasonal productivity of their principal prey item, Antarctic krill (Euphausia superba). Their annual migrations between the Southern Ocean and equatorial breeding grounds are among the longest migratory journeys of any mammal on Earth, covering c.a.10 000 km (Andriolo et al., 2006; Zerbini et al., 2006). The migration is associated with five to nine months of fasting, representing a period of prolonged negative energy balance, exacerbated among females that simultaneously bear the energetic demands of pregnancy, parturition, and lactation. Consequently, individuals may lose between one-third to one-half of their summer body mass during this time (Slijper, 1962; Dawbin, 1966; Lockyer and Brown, 1981; Baraff et al., 1991; Clapham et al., 1999). Understanding the species’ energy conserving behaviours and strategies is of increasing importance in a rapidly changing Antarctic sea-ice ecosystem.

Managing energy balance is of particular importance for capital breeders such as SHHWs that fund the energetic cost of reproduction via temporally constrained periods of hyperphagia (Frisch, 1984; Gittleman and Thompson, 1988; Jönsson, 1997; Stephens et al., 2009). A single sub-optimal feeding season can carry individual fitness consequences, such as exhaustion, immunosuppression, and excessive mobilisation of lipophilic environmental toxins (Jönsson, 1997; Bengtson Nash et al., 2013; Bengtson Nash, 2018; Bengtson Nash et al., 2018). For sexually mature females, single or repeated sub-optimal feeding seasons may further carry population consequences via larger inter-pregnancy intervals, sub-optimal maternal provisioning of foetus’ and dependent calves, or aborted pregnancies (Lockyer, 1986; Lockyer, 2007; Williams et al., 2013; Christiansen et al., 2014; Christiansen et al., 2016; Christiansen et al., 2018; Castrillon and Bengtson Nash, 2020; Christiansen et al., 2022).

Most SHHW populations are recovering from their post-whaling bottleneck, with some approaching pre-whaling numbers (Paterson et al., 2001; Noad et al., 2011). Concerns are, however, growing for many migratory species utilising increasingly impacted and fragmented habitats. Degradation of habitats through industrial, agricultural, and residential development, climate change, and pollution are some of the major threats that migratory species face (Robinson et al., 2005). Due to the extensive geographical range that these species occupy throughout their annual cycle (Runge et al., 2014; Schuster et al., 2019), conservation efforts are complicated by their migration across the jurisdictional limits of various nations (Ruckelshaus et al., 2008; Miller et al., 2018). The Convention on Conservation of Migratory Species of Wild Animals (CMS), also known as the Bonn Convention, was established in 1979 and seeks to conserve migratory species across their entire range. Conservation efforts primarily focus on breeding and feeding areas (Mehlman et al., 2005; Sheehy et al., 2011), with migratory corridors often neglected. Notably, many species of migratory ungulates, birds, and sea turtles (Shillinger et al., 2008; Harris et al., 2009; Murray et al., 2014; Runge et al., 2015), have dramatically declined in numbers despite well-established protected areas at both migration terminus points. For SHHWs that seasonally migrate between Antarctic feeding grounds, far removed from permanent human settlements, to urbanised coastal areas, the importance of safeguarding the entire migratory corridor of the species’ range is particularly well exemplified.

The major drivers behind the seasonal migration of SHHWs out of Antarctica remain under active theoretical debate. Aside from diminished winter food availability, avoidance of predation at high latitudes (Corkeron and Connor, 1999), calf winning mass (Norris, 1967), and more recently skin molting (Pitman et al., 2020), are all theories that have been proposed. Such uncertainty regarding the fundamental ecology of the species, translates to carry-over uncertainty when forecasting the response of populations to rapidly changing functional habitats. In extra-Antarctic waters SHHWs are known to travel in close proximity to the coastline, preferring routes less than 200 m in depth, and generally within 20 km of the coast (Peel et al., 2018). Travel from northern breeding grounds, with accompanying newborn calves, is characterised by slower travel close to shore, with frequent resting stops (Simmons and Marsh, 1986; Mattila et al., 1989; Ersts and Rosenbaum, 2003; Félix and Guzmán, 2014). A ‘stopover site’ for migratory species is a site where the migration is paused to rest, conserve energy, and refuel (Mehlman et al., 2005; Dingle and Drake, 2007). Such sites, therefore, carry a disproportionate impact on an individual’s mass gain and reproductive success, relative to time spent there (Drent et al., 2003; Smith and Moore, 2003; Schaub et al., 2008; Sheehy et al., 2011). The social and physiological drivers of humpback whale resting stopovers have previously been described (Chittleborough, 1953; Corkeron et al., 1994; Jenner et al., 2001; Braithwaite et al., 2012), and can summarised as a pause in migratory travel for mothers to engage in nursing and socialising with calves-of-the-year and older calves during the early stages of their southern migration (Franklin et al., 2011), contributing to the social development and high survival rates of calves and younger humpback whales (Franklin, 2014). Females with accompanying calves will favour lagoons with calm, shallow, and warm waters (Bruce et al., 2014; Ejrnæs and Sprogis, 2021) and utilise these areas as resting stopovers as the conditions facilitate coordination of nursing behaviour, and protect the calves from rough sea conditions, predators, and aggression from sexually active males (Craig et al., 2014; Bejder et al., 2019; Ejrnæs and Sprogis, 2021). Resting and nursing in these lagoon habitats, further plays a critical role in optimising energy balance of both mother and calf by minimising energy expenditure and maximising conversion of the mother’s lipid rich milk to calf body mass. Recently, Videsen et al. (2017) quantified calf nursing dynamics and found that mothers and young calves were engaged in resting and nursing 20% of the total time observed. Resting stopover sites are therefore of key functional significance for SHHWs and even a relatively small amount of habitat disturbance or loss could carry negative population impacts (Runge et al., 2014) through for example disrupted nursing behaviour, or increased travel distance between suitable resting stopovers.

By comparison to feeding and breeding grounds, the ecological importance of resting stopovers have received less attention (Possingham et al., 2001; Wilson et al., 2006), yet the importance of properly identifying, characterising, and effectively managing human interactions in such areas are clear. For Australia’s transient humpback whale populations, the Australian coastline offers numerous potential resting stopovers, yet only a few have been adequately characterised and formally recognized in the literature, notably Exmouth Gulf in Western Australia and Hervey Bay in Queensland (Jenner et al., 2001; Franklin et al., 2011; Franklin, 2014; Bejder et al., 2019).

Moreton Bay is located approximately 1700km south of the Great Barrier Reef, the breeding ground of the east coast of Australia migrating humpback whale population (Paterson and Paterson, 1984). It is a semi-enclosed embayment, sheltered by the large sand Islands, Moreton Island (Mulgumpin) and North Stradbroke Island (Minjerribah) to the east. The average winter sea surface temperature in the bay is 20°C with an average depth of 8m (Pantus and Dennison, 2005). The natural physical environment of Moreton Bay offers similar conditions to other well-established SHHW resting stopovers. The bay, however, it is also located adjacent to one of Australia’s fastest developing regions, and is the location of Australia’s fastest growing container port, the Port of Brisbane. Humpback whales are regularly sighted within Moreton Bay, however, little is known about the proportion of the population that utilises the bay, their distribution, or their habitat use; all necessary parameters for effective management of human-wildlife interactions. This study sought to quantify these through systematic species abundance and distribution surveys across five consecutive years (2017 – 2021). In addition, surveys covered the main migratory corridor as a reference for findings. Here we describe the migratory dynamics and behaviours observed which are of broad relevance for the management of Moreton Bay, and similar embayment along Australia’s migration corridors.

Quantification of distribution, relative abundance, behaviour, and habitat of migrating humpback whales in Moreton Bay, over a five-year period, revealed substantive evidence that Moreton Bay is not only visited by a significant proportion of the annual migratory cohort, but that it also that it plays a functional role as a resting stopover for this population.

The unique significance of Moreton Bay, particularly to mothers and calves, was demonstrated by the marked change in spatial habitat use between the two migration time-points. Regardless of the survey year, there was a substantial increase in the number of whales entering the bay between the northward and southward migrations. The most notable feature of whale utilisation of Area 1 on the southward migration was the influx of pods with accompanying calves. Pods with calves made up 76.7% of sightings in Area 1 during the southward migration, whereas pods with calves amounted to less than half of the sightings in Area 2. By comparison to Area 1, consistent whale numbers were observed in Area 2 across both time periods, supporting the well-known role of Area 2 as a main migratory corridor of this population.

The role of Moreton Bay as a resting stopover for the east coast of Australia migrating humpback whale population was further supported by the observation that of the pods with accompanying calves observed within the bay, 47.82% were resting or logging at the time of observation. These findings suggest a unique functional role of Moreton Bay, particularly for mother-calf pairs. The shallow, and sheltered, lagoon-like habitat present in Moreton Bay match those described for resting stopovers in other regions in Australia, such as Hervey Bay (Franklin, 2014), Jervis Bay (Bruce et al., 2014), and Exmouth Gulf (Jenner et al., 2001; Bejder et al., 2019). These shallow lagoon-type habitats offer protection to calves from turbulent offshore conditions, predators, or aggression from sexually active males (Whitehead and Moore, 1982; Glockner and Venus, 1983; Smultea, 1994; Ersts and Rosenbaum, 2003; Craig et al., 2014). It has been suggested that premature conspecific social encounters increase the risk of elevated energy expenditure, interruption of nursing bouts, mistaken imprinting, or nursing attempts, potential separation of calves from mums, and calf injury or death (Jones and Swartz, 1984; Thomas and Taber, 1984; Smultea, 1994; Craig et al., 2014; Ransome et al., 2022). In addition, these waters facilitate effective coordination of nursing behaviour (Whitehead and Moore, 1982; Glockner and Venus, 1983; Smultea, 1994; Ersts and Rosenbaum, 2003). Stationary nursing, is energetically advantageous for both mother and calf; as it allows the mother to provide energy to her newborn while conserving as much of her own limited energy stores, and at the same time allows the calf to allocate energy for growth rather than movement (Harrison, 1969; Herman and Tavolga, 1980). While nursing behaviour is challenging to study in cetaceans since suckling occurs below the surface and milk transfer is difficult to verify, much of the literature describes that nursing behaviour occurs at a depth of 10 to 20 m (Glockner and Venus, 1983; Cartwright, 2005). In the current study, 76% of the pods with calves observed to be resting or logging were in waters between 5 and 20m in depth at the time of observation. Although this study did not attempt to detect or quantify nursing behaviour, the prevalent observations of resting and logging whales with calves in shallow waters, commensurate with nursing activity, provide strong indication that nursing occurs within the bay. The large proportion of time that mother-calf pairs dedicate to nursing and resting (Videsen et al., 2017; Bejder et al., 2019; Ejrnæs and Sprogis, 2021), is reflective of the critical importance of energy balance in this extreme migrating species, and the implications for individual fitness. For example, bigger calf size has been observed to be associated with better breath-holding ability, reduced vulnerability to predators, reduced heat loss during migration to high latitudes, and greater resilience to environmental variability (Christiansen et al., 2018).

Given the significant functional importance of resting stopovers for humpback whale energy balance, the discovery of Moreton Bay as a previously undocumented resting stopover is of significant conservation and management importance. Moreton Bay exhibits high levels of species diversity and richness but is also a region of high levels of all-year maritime traffic (Maritime Safety Queensland, 2005). As such, whales entering the bay must increasingly navigate anthropogenic stressors and threats such as disturbance and ship strike (Mayaud et al., 2022). Vessel proximity and underwater noise may disrupt critical nursing and resting behaviours (Pirotta et al., 2019), whilst whales engaged in resting and nursing are known to be less responsive to approaching vessels, and therefore more susceptible to ship strike (Laist et al., 2001). Ship strike risk is further exacerbated among calves and young that spend a greater amount of time at the surface (Laist et al., 2001; Lammers et al., 2013). Finally, vulnerability of this demographic group is further compounded by the low detectability of resting animals, and the restriction of avoidance by both whales and ships at shallow depths. Whilst further work is needed to understand the risk of humpback whale ship-strike from a range of vessel-categories in Moreton Bay (Mayaud et al., 2022), the change in vessel traffic across Area 1 sub-areas a, b and c, likely provide clear clues as to the major risks associated with each sub-area. Sub-area a, which forms the entrance to the main Port of Brisbane shipping channel, is dominated by larger commercial vessels aggregating from deeper offshore waters to enter into the bay (Mayaud et al., 2022). The absence of the shipping channel in sub-area c results in a lack of large commercial vessels further south in the bay. By contrast, the protected inshore waters of sub-areas b and c, coincide with numerous islands, reefs, and fishing grounds making these areas particularly popular for smaller recreational vessels, as well as regular passenger ferry traffic.

Systematic abundance and distribution surveys of Moreton Bay, and the adjacent main migratory corridor, over the five-year period provided substantive evidence for the previously undocumented role of Moreton Bay as a resting stopover for the east coast of Australia migrating humpback whale population. Notably, the current study revealed a temporally constrained influx of whales into the bay during the southward migration, particularly by pods with accompanying calves. Pods with accompanying calves were further reported to be resting or logging 47.82% of the time, indicative of nursing behaviour. These findings present a local environmental management challenge for the sustainable use of this rapidly developing region. Further, they bring focus to the need for identifying similar resting stopover sites of these extreme migratory populations across their species range as continued degradation of key stopover sites will carry a disproportionate impact to species’ energetic budgets.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was reviewed and approved by Griffith University Animal Ethics Committee.

JC and SB conceived and designed the study. JC, GD, JA and RM Performed fieldwork and data collection. JC and SB analysed the data. JC took the lead in writing the manuscript with substantial support from SB. All authors contributed to the article and approved the submitted version.