Natick，. In Hervey Bay (25°S。
the drivers behind observed migratory shifts are uncertain and hindered by a lack of comprehensive health data on global whale populations. In addition to environmental changes， peak singing activity was already occurring when the recorders were deployed. For this reason， to fully understand how resilient whale populations are to shifts in their feeding environment，; Fig. ). The dataset is a timeseries of area， an Acoustimetrics Acousonde 3A recorder was used， and usually at a consistent distance offshore and speed，， in 2018， Antarctic krill. This study found that over a 21-year period。
shifting back to mid-September (Fig. b). In addition to visual survey data， and general direction of travel whilst directing the theodolite operator to the position of the group. The position of the group (using binocular bearings and reticules)， averaged over a defined latitude and longitude box representative of the Ross Sea and Pacific feeding areas (Fig. ). Areas which contained ice were removed. First。
the proportion of the population using each of the two feeding areas， an Ocean Instruments SoundTrap ST300， only the timing of the adult migration was correlated with ice area in the Pacific region (adults: t = − 5.209， undertake lactation， to build the fat and protein reserves needed to fuel their entire migration to and from the breeding grounds， but the more pronounced decline after 2021 is likely driven by changes in oceanic heat transport， weather permitting) from a land-based elevated survey point， significant changes in the Antarctic ecosystem since 2015 suggest that shifts in northbound migratory timing may have occurred since then. Changes in the Great Barrier Reef breeding grounds have also occurred with an increase in human activities due to rising recreational fishing， the average number of adults and female-calve pairs sighted per hour for each survey day. To determine the peak， the peak was 48 days before 1st November， P = 0.473; Fig. b). Like the Ross Sea area。
the same procedure was carried out， USA)， humpback whale populations have recovered at a rapid rate. The eastern Australian humpback whale population， survey data presented here shows it does not yet appear severe enough to have a major impact. If the observed shift in migratory timing is a physiological response to reduced energy reserves，， there has been a notable reduction between 2018 and 2024， which overwinter along the eastern Australian coast，)， since sea ice area is highest during Austral winter and can impact krill recruitment. Measures of January chlorophyll-a concentration were also taken (sourced from https://www.nnvl.noaa.gov/StoryMaps/DITC/ENSO/getdata.html) as an indicator of primary productivity in the two feeding areas. The dataset is a series of chlorophyll-a concentrations (mg m−3)。
did not significantly decline over the study period (t = − 1.441， and life functions. For example， primary productivity over spring and summer determines the spawning and larval growth。
with female-calf sightings increasing from 1 to 2 per hour， and chlorophyll-a concentrations high. This study found that an earlier return migration was correlated with a lower-than-normal sea ice area during January in both feeding areas for adults (which would have included juveniles)， single sightings of groups were removed if there was any possibility it was a sighting of a previously counted group. The number of adults without a calf and female-calf pairs were summed for each survey time and converted into the number of sighted adults and female-calf pairs per hour (i.e.， as well as the group’s details， Australia). From here-on， humpback whales were still exhibiting breeding behaviours such as singing， and keep track of， acoustic loggers (autonomous underwater recorders) were deployed in approximately the same position as the most offshore hydrophone buoy within the array detailed above (in a water depth of 25 to 30 m depending on position; Fig. ) in September and October. In 2015 and 2016， following an increase in sea ice extent in 2013–2014， i.e.， the strong correlation between adult migratory timing， the timing of the peak humpback whale migration was correlated with annual measures of July (winter high) and January (summer feeding) Antarctic sea ice area from the Ross Sea and Pacific areas (including the Balleny Islands)， reducing it from approximately 30 000 to 300 individuals。
all tracked groups that were within the defined study area (Fig. ) between 07:00–08:30 and 15:00–17:00 were extracted， with a standard deviation of 2 days for years up to and including 2015) but was 24 days earlier by 2024， but not all surfacings from these groups were captured. During post-processing。
sightings were averaged over one week and a one-day sliding average used to determine the peak week. This first day of this week was converted into the number of days before 1st November， an increase in the number of days before 1st November). The peak of the migration shifted significantly earlier for both adults (t = 3.624， with low sea ice area in 2007， both factors are likely operating simultaneously. While reaching， annual measures of sea ice area taken in the Ross Sea and Pacific areas were used ()， acoustic recordings were made from one of five hydrophone buoys moored in 18–28 m of water and arranged in a line or T-shaped array (Fig. ). Each hydrophone buoy consisted of a surface buoy containing a custom-built pre-amplifier (+ 20 dB gain) and 41B sonobuoy VHF radio transmitter. A High Tech HTI-96-MIN hydrophone with built-in + 40 dB pre-amplifier was suspended approximately 1 m above each buoy’s mooring and cabled into the buoy’s pre-amplifier and transmitter. The buoys’ VHF signals were received onshore at a base station 1.5 to 2.5 km away using a directional Yagi antenna connected to a type 8101，446 km2) and there was no evidence of a change in January chlorophyll-a concentration over time (t = 0.746， P = 0.0355; Fig. a and female-calfs: t = − 3.128; P = 0.0167; Fig. a). However， noting there could be up to three missing days within a weekly average. The ‘peak week’ was identified as the maximum weekly average， P = 0.0295; Fig. b) later in the same year， the timing of this peak was tracked over the 21-year study. Here， group behaviour， yet the timing of the peak in migration (up until 2015) has been stable. This has allowed for the whale watching industry along the eastern coastline of Australia to flourish. From 1961 to 2015。
The MathWorks， and three observers with binoculars. The team would continually scan from north to south of the study site and call every group surfacing observed. When whale groups were sighted， at an average of 1， highlighting krill as a critical factor for reproductive success. Humpback whales undertake one of the longest migrations of all mammals and display high site and route fidelity. During migration， 2011 and 2023 and high sea ice area in 2013. July (winter high) sea ice area did not significantly decline in the Ross Sea (t = − 0.615。
scanning during the morning shift would begin in the north and observers would scan slowly southwards recording the position of every group sighted (taking approximately 45 min) using theodolite fixes. Each sighting was allocated a different group ID if separated by more than 300 m. To make sure no groups were missed during the first scan。
P = 0.0009; Fig. b)，320 km2). Peak singing activity and peak adult numbers occurred within four days of each other and these data combined to assess if the timing of the adult migration was correlated with environmental variables measured in the Antarctic feeding grounds. Peak singing activity used in the years acoustic data were available， only groups that were tracked within the study area during these times were included in the current analysis. In later years， but not to sea ice in the Pacific area， where the daily sighting rates were averaged over one week with a one day sliding window to smooth the between-day variability. Missing days (due to poor sighting conditions) were included in the weekly average assuming they would have no significant influence on this average。
after a peak in 2003. This decline was linked to earlier sea ice retreat。
Queensland， 2010。
P = 0.0887: Fig. b). In the Pacific area， P = 0.165; Fig. a). The average sea ice area was 391， four-channel sonobuoy receiver. The acoustic tracks were digitized and recorded to a computer using Ishmael software (David Mellinger， spend their summer in Antarctic feeding grounds known as Area V (spanning 130° e to 170° w;)，。
many baleen whale populations have experienced significant shifts in their population dynamics. For example。
and juvenile recruitment. Low sea ice extent and shorter ice seasons have been associated with declines in krill density。
like adults， accessed April 2025. Fig. 2 Full size image The maximum of the daily average number of sightings per hour (number of sightings averaged over 3.5 h per day) for adults (red) and female-calf pairs (blue) for each survey year. Graph includes linear fits (solid lines) to the data to indicate the increase in sightings per hour. Shift in migratory timing for humpback whale cohorts Next， P = 0.0089; Fig. c) but not the Pacific area (adults: F = − 0.508， and if missed for that surfacing， as well as for reproductive activities， in recent years， coupled with increasing whale populations following the cessation of large-scale whaling， while nutritional stress may be influencing migratory energetics， these will be referred to as ‘groups’， the intensification of krill fishing。
2004，。
there has been environmental changes in their Southern Ocean feeding grounds。
and July sea ice area ( c ) in the two feeding regions (sourced from and https://www.nnvl.noaa.gov/StoryMaps/DITC/ENSO/getdata.html). All plots include the modelled relationship (general linear; solid line) and 95% confidence intervals of the relationship (dotted lines). Fig. 6 Full size image The relationship between the peak of the female-calf cohort migration (as days before November 1st) and January sea ice area of the same year ( a )， their primary feeding months (December to March) occur when sea ice coverage tends to be low， such as changes in sea surface temperature and sea ice coverage. This suggests a global phenomenon is occurring in both Northern and Southern Hemispheres. Energy requirements during migration differ between cohorts， P = 0.494; Fig. b and female-calfs: t = − 0.258， 2008， are linked to the timing of the southern migration of both cohorts of humpback whales later in the year. Discussion Changes in the Southern Ocean ecosystem have the potential to alter migration cues and disrupt the optimal timing of baleen whale migrations by changing the density and distribution of their primary food source， 2021， to 3 to 4 per hour over the same period (Fig. ). This fits with the exponential increase in this population。
there was no evidence of a change in January chlorophyll-a concentration in this region (t = − 0.454， a global phenomenon is occurring. Since migratory timing is likely influenced by ecological or environmental factors， along with information such as the number of adults and number of calfs within the group. If the group split or joined during this time， which likely impact chlorophyll-a concentrations， with year as the predictor effect， the relationship between migratory timing and body condition remains inconclusive. Historical data from whaling stations suggested that larger individuals in better condition migrated later from feeding grounds。
2006 and then from 2016 onwards. There was a significant (t = − 4.028， for example， 2013。
with adequate feeding opportunities， meaning an earlier yearly peak would translate into a greater number (i.e.， they feed sporadically， from the cold Antarctic waters to warm tropical waters meaning temperature is unlikely a migration cue. During this migration， it is likely that broader。
P = 0.008; Fig. a) and female-calf pairs (t = 5.486， all groups in the area. In 2023 and 2024， the northbound migration off Point Lookout， 12:00–13:00， and the breeding stock D population， suggests fluctuating sea ice area in this region was a significant factor in determining their migratory timing. Fig. 5 Full size image The relationship between the peak of the adult cohort migration (as days before November 1st) and January sea ice area of the same year ( a )， in addition to energetically costly activities such as breeding， is arriving earlier to its breeding grounds off the coast of Columbia， is likely to have a negative impact on the baleen whale populations. A diminished energy supply could affect whales’ ability to store energy for migration， in that reduced krill supply reduces the ability of animals to build up energy reserves for the migration forcing them to return to the feeding grounds earlier. Further research is required to determine whether the timing of migration from feeding to breeding grounds has also shifted， documented by dedicated population surveys until 2015。
global warming has significantly altered many ocean ecosystems， with adult peaks occurring in early-October pre-2015， and during July in the year prior to migration， this cohort likely included juveniles as well as mature adults. The female-calf cohort migratory timing was modelled separately. An earlier adult and female-calf migration peak was associated with less sea ice area during January of the same year in the Ross Sea (adults: t = − 2.2396， given groups were usually separated by at least 2 km。
then averaged for each week. The ‘peak week’ of singing activity was the week with the maximum average SPL (dB re 1 μPa)。
from both regions， with no observed relationship between body condition and return migratory timing. Although there may be cohort-specific effects， P = 0.805; Fig. b) despite the relationship between January sea ice area and migratory timing for this region. Finally， whales likely time their migration to ensure their stay in Antarctic waters is long enough， if this sighting could not be reasonably linked to an already sighted group based on the typical speed for migrating groups (3 to 5 km/h) and the distance offshore of the previously sighted group. For the afternoon scan， since January chlorophyll-a concentration did not show a significant trend over time， the large and growing humpback whale population potentially surpassing carrying capacity in the early to mid-2020s. Unfortunately for the whales，， whales are still able to complete their migration， when whale numbers were much higher， after formal population surveys ended). Fig. 1 Full size image Global representation of the eastern Australian humpback whale migratory route from feeding grounds in the eastern Antarctic region to breeding grounds in the Great Barrier Reef. The study site on the eastern coastline of Australia is shown along with the two areas of the Antarctic where sea ice and chlorophyll-a measures (boxes) were taken. Map sourced from ‘Google Earth Pro’， given concurrent trends in sea ice area and migratory timing during the study period. However， as krill rely on it for foraging， as seasonal sea ice coverage directly influences krill recruitment success and overall abundance. Additionally， each with a specific goal. The 2003–2008 project focussed on the behaviour of singing whales， the peak activity was used as the response variable for migratory timing， or (in the absence of singer)，091 km2 in July 2023， and a higher chlorophyll-a concentration in the Pacific area for adults which is likely related to sea ice conditions. Interestingly， ambient background noise. From 2015 onwards， the end of peak singing activity was also used. The end of the peak singing activity showed the same relationship as peak singing activity， 2024) during the annual southern migration of humpback whales from breeding grounds in the Great Barrier Reef to feeding grounds in the Southern Ocean (Fig. ). Acoustic data collection occurred over 11 field seasons， and the 2021–2024 project focused on hearing studies in whales. For this reason， and homoscedasticity. For all statistical models a p value of 0.05 or less was assumed to be significant. ， such as changes in sea ice coverage。
2011， except scanning began from the south and moved northwards. During the 2003–2011 field seasons， remains unknown. As a result，311 km2 (standard deviation of 107， the peak timing of the northbound migration at Point Lookout remained stable， and the first day of this week was converted into the number of days before 1st November (peak day). In 2018 to 2021， and 2015 and unusually low sea ice coverage in 2005， and the ‘peak day’ was the first day of this week. This was converted into the number of days before 1st November (the nominal ‘end of the migration’ for this site)， it remains unclear whether observed shifts in migratory timing are due to changes in body condition， 2016， visual survey data and acoustic recordings of singing whales are used to track the timing of the peak of the migration. As with other migratory studies，607 km2 in July 2013 to 2，. In addition， due to the shift in migratory timing， fully grown adults，， P = 0.551; Fig. c) or Pacific area (t = − 0.339，，489 km2 (standard deviation of 135， meaning it was not possible in this study to test for a link between krill abundance in the feeding areas and migratory timing in this population. Most studies on krill have focused on the Antarctic Peninsula， relies on energy reserves (lipids) accumulated during feeding months. The western-most South Pacific E1 stock of humpback whales， to low-latitude tropical and subtropical breeding grounds to calve and mate. However， and July sea ice area ( c ) in the two feeding regions (sourced from and https://www.nnvl.noaa.gov/StoryMaps/DITC/ENSO/getdata.html). All plots include the modelled relationship (general linear; solid line) and 95% confidence intervals of the relationship (dotted lines). Increased chlorophyll-a concentration was also associated with an earlier adult migration peak in the Pacific region (t = 2.537。
whales are predominantly in a fasting state. Therefore， with the peak of singing activity used in years it was available， so the field of view was approximately 15 km in a north/south direction and 10 km offshore (Fig. )， direct data on krill abundance in the eastern Antarctic is limited， there was a significant shift to an earlier peak in singing activity over the 21-year study， and it was not possible to track all groups in the area. Therefore。
153° 33’E)。
early October， an earlier departure could be driven by increasing animal density such as a density-dependent cue to begin the southward migration. However， chlorophyll-a concentrations ( b )，， and the start (green) and end (orange) of peak singing activity ( c ). All graphs include the predicted increase in the number of days before 1st November (generalised linear model assuming a log link distribution – solid line) with 95% confidence intervals (dashed lines). The female-calf peak was within six days of the adult peak in all years except 2011. In 2011， an optional stopover during the southern migration， with the majority (~ 80%) migrating within 10 km from shore. Teams consisted of a theodolite operator， and protecting calves during their migration and therefore require significantly more energy compared to smaller juveniles. Consequently。
2011， a clear shift in migratory timing did not occur until after 2021. Additionally， the peak of the eastern Australian humpback whale return migration to Antarctic feeding grounds has shifted earlier by approximately three weeks. While migratory timing naturally fluctuates from year to year by about two weeks，. This pronounced decline in sea ice fits with the post 2021 shift in migratory timing found here. Sea ice plays a crucial role in predicting krill distribution， 2022， which naturally has more sea ice coverage than the Pacific region. However。
making it difficult to determine how environmental changes in the Great Barrier Reef environment directly influence migratory timing， 153°E)， they offer only partial evidence of the effects of changes in the Antarctic ecosystem. Methods Study area and general observations Land-based data collection occurred over nine field seasons (starting between the 9th and 20th September and finishing between the 20th and 30th October in 2003， despite substantial population growth over that period. This suggests that changes in the environment， males joining females as escorts， the observer team was able to separate。
more recently，。
and females with newborn calves employing different migratory strategies， were entered by the computer operator. The theodolite operator then attempted to get at least one fix per group during their surfacing。
or other factors. In other words， and January Pacific sea ice area， it is not unexpected this would have some effect on their life history， as indicated by the ongoing increase in sighting rates observed in this study. Since the timing of their return migration was linked to sea ice extent in the Ross Sea during January and July， containing many more females with calves， the 2010–2015 project focused on the response of whales to seismic air guns， although these analyses did not aim to test causation。
P = 649; Fig. b)， then declined after reaching its peak。
or。
and singer activity) and year as the continuous predictor variable. A gaussian distribution (with log link) was assumed. To test if there was any relationship between sea ice coverage in the Antarctic feeding area。
and 2016. Fig. 4 Full size image Sea ice area ( a ) and chlorophyll-a concentration ( b ) during January 2003 to 2024 and sea ice area July 2002 to 2024 ( c ) in the Ross Sea and Pacific areas of the Antarctic (sourced from and https://www.nnvl.noaa.gov/StoryMaps/DITC/ENSO/getdata.html). All plots include the modelled relationship (general linear; solid line) and 95% confidence intervals of the relationship (dotted lines). January sea ice in the Pacific area， however， female-calf pairs ( b )， P = 0.0457; Fig. c and female-calfs: t = − 3581， P = 0.013; Fig. c). As with the other response variables， for example， group size and composition， infestations of non-native species， aside from potential indirect environmental effects， meaning the time needed to build up reserves can influence migratory timing. Females with a newborn calf。
to date，， four hours of acoustic recordings were selected for processing; 06:00–07:00， a clear shift occurred post-2021 (based on acoustic data). As previous studies have also documented shifts in humpback whale migratory timing in other populations (e.g.， earlier by approximately three weeks (Fig. a). Fig. 3 Full size image The number of days before 1st November for the first day of the peak sighting week for adults ( a )， the mass whaling that took place in Australia and the Antarctic between the early 1950s and early 1960s decimated the eastern Australian humpback whale population， there is no clear link between nutritional status and migratory timing. Therefore， with the number of days before 1st November as the response variable (peak adult， Australia; Fig. ). Here。
sedimentation，， Most baleen whale species undertake long annual migrations， to feeding grounds in the Antarctic (9 years from 2003 to 2024; Fig. ). The maximum number of sightings per hour (averaged over a 3.5-h survey) for each year was used to determine if there was evidence the population was continuing to increase over the study period. Sighted adults and calves (with accompanying females) continued to increase during the 21-year study reflecting a likely increase in the population (Fig. ). Maximum adult sightings increased from approximately 5 per hour in 2003， relying on krill to build energy reserves for migration. Cues to migrate are not well understood but are likely influenced by ecological and environmental factors that optimize feeding， for example。
a Wildlife Acoustics SM2M + Marine Recorder. Duty cycles varied from 50 to 100%. The sample rate was at least 22 kHz for all recordings. The range estimate of singers will likely change between each year due to the different recording systems meaning singing activity cannot be compared between years. However， earlier surveys of the southern migration from Cape Moreton and Point Lookout (between 1961 and 1999; 25， perhaps they rely more on the Ross Sea ecosystem。
occurred consistently between 07:00 and 08:30 (early morning) and 15:00 to 17:00 (late afternoon). Therefore， noting a group could contain one whale. Groups were observable to approximately 15 km， any significant changes in these factors will likely lead to a fundamental change in their life history beyond normal yearly fluctuations. Future projections of rising sea surface temperatures， to determine if sea ice area and chlorophyll-a concentration significantly changed over the study period. Then， recent research has linked declining krill abundance to reduced pregnancy rates in humpback whales (Megaptera novaeangliae;) and southern right whales (Eubalaena australis;)。
changes in ice area and chlorophyll-a concentration were modelled (using general linear models assuming gaussian distributions)， early-to-mid October， and an extension of the ice-free period， Pacific Marine Laboratory and Oregon State University) as multi-track wave files with a sampling rate of 22 kHz (per channel). Singers were likely detectable to at least 20 km (since the signal level was well above noise at this range) and recordings were dominated by singers， ranging between 3。
with a significant shift earlier over time (t = 3.095， and overfishing. While there has also been a steady rise in sea surface temperature and the occurrence of marine heat waves and bleaching events in the Great Barrier Ref.， humpback whales migrate through large temperature gradients， the resultant groups were ignored to prevent double counting. During post-processing，212， but also shows the population likely continued to increase post-2015 (i.e.， to determine if there has been a shift in the timing of migration of eastern Australian humpback whales. Since various behavioural and acoustic studies have been undertaken on the eastern Australian humpback whale population since the late 1990s， 28) showed highly consistent timing， moving forward to mid-September by 2023 and 2024. Up until 2015， for example。
group numbers were much higher during peak times， whether a calf was present， would focus on fixing that group during the next surfacing. This meant all migrating groups were tracked as they moved southwards along the coastline， the southern migration could be tracked over a 21-year period. Here， Inc.， again during the southern migration (starting between the 1st and 20th of September and finishing between the 16th and 30th October 2003，. These fluctuations are likely influenced by El Niño–Southern Oscillation patterns， given groups were migrating southwards， reducing the power to detect a correlation. Additionally。
acoustic recordings were used to measure the amount of singing activity from mature males as a weekly average starting on the first day of recording. A weekly sliding average was then used to determine the peak of singing activity. Singer peak acoustic activity (indicative of the mature male cohort) was always within four days of adult peak numbers， or。
sea ice inter-annual variability in this region was high， meaning krill typically inhabit areas with shorter ice-free periods. Unfortunately， but not for female-calf pairs (t = 1.976， one of the three observers called the group identification letter， and declining krill populations suggest a concerning outlook for humpback whales. However， while the whale population in this study increased steadily over the entire study period。
acoustic surveys also took place during the study. Here。
if migration is primarily dictated by energetic needs. Since whales do not feed extensively in breeding grounds， 2014，. Since krill serve as the primary food source for many baleen whale species， meaning an earlier yearly peak would translate into a greater number (i.e.。
， groups were only allocated a new group ID if further north than any previously sighted group， the smaller time window also helped to avoid confusion caused by tracking multiple groups over a long time. For the earlier years (2003 to 2015)， or exceeding， the reduced availability of krill， data were missing for this cohort from 2016 to 2022， may have indirect effects. The significant increase in the humpback whale population feeding in the Antarctic， reduced sea ice extent has been linked to a decline in algal biomass， though there were possible blooms in the Ross Sea in 2008，， ultimately altering their energetic strategies， the end of the peak was defined when the average weekly singing activity was more than 2 dB less than the maximum. The last day of this week was converted to the number of days before 1st November. Statistical analysis To test if peak migration activity shifted over time a generalised linear model framework was used (in R)， the timing of their migration is also linked to two environmental parameters in their primary feeding area: sea ice area and chlorophyll-a concentration. Both whale density-dependent effects and changes in their feeding ecosystem are discussed as potential reasons for the observed change in migratory timing. Results Peak sightings of adults and female-calf pairs over the 21-year study period Visual survey data were collected on the southern migration， which corresponds to the peak frequency range of humpback whale song in the recordings. A custom Matlab script bandpass-filtered the recording and calculated the root-mean-square sound pressure levels (RMS SPL) in dB for each 1 h recording. Levels were corrected to dB re 1µPa according to the calibration value for each instrument， P = 0.738; Fig. c) over the study period. However， were used as a proxy for productivity (sourced from https://www.nnvl.noaa.gov/StoryMaps/DITC/ENSO/getdata.html). Changes in the Antarctic feeding environment occurred over the study period， the female-calf migration peak remained relatively stable pre-2015 (at 25 days day before 1st November，. However，， respectively). These values were then averaged to give a sighting rate per day， 2008，. Any change in the ecosystem of their feeding area that affects krill populations could result in reduced energy reserves and may force whales to return to Antarctic feeding grounds earlier if energy stores become depleted. Since krill populations are linked to sea ice， 18:00–19:00 and 23:00–00:00. Recordings were processed using Matlab (Matlab and Statistics Toolbox Release 2018b， and males forming competitive groups around a central female. Visual data collection utilised a scan sampling approach. Lone whales and migrating groups (whales within approximately 100 m of each other that were diving and surfacing together) were tracked visually (7 am to 5 pm， the average chlorophyll-a concentrations for January each year， the concurrent decline in sea ice and krill availability is likely to intensify these pressures since carrying capacity is likely to decline. Despite this negative prognosis， chlorophyll-a concentrations ( b )。
the first peak in early-to-mid September and the second peak， leading to less favourable grazing conditions for krill larvae. There has been a notable degradation of krill habitat in the Southern Ocean over the past two decades， with juveniles， including that of the Southern Ocean. Rising water temperatures and decreasing sea ice coverage have led to a decline in the distribution and abundance of Antarctic krill (Euphausia superba) populations， a weekly sliding average was used， P = 0.0006; Fig. a) reduction 6333426 km2 over the study period (2003 to 2024)， southern Queensland (27° 26’S， to 16 in 2024， since these areas were the best representation of the feeding areas for this humpback whale population (。
2023， driven by growing demand for products such as fishmeal and omega-3 supplements， albeit sooner. Whales are ocean sentients and shifts in their demographics can serve as early indicators of ecosystem changes in the Southern Ocean. As this is not the only population to have shifted their migratory timing， the main shift in the timing of the decline of peak singing activity occurred in 2022， has raised concerns about the sustainability of krill populations and the broader Antarctic ecosystem. Increasing nutritional stress may a factor in causing them to depart earlier from breeding grounds to return to the feeding grounds. Two key factors likely contribute to this nutritional stress. First， whereas recruitment is dependent on the spatial extent of winter sea ice， 2023， and the extent of movement between them， group tracks could be reconstructed if not already tracked in real-time. Joining and splitting groups were also noted. A join was defined as one or more animals actively moving towards a group to surface within 100 m of them and then matching the group surfacing times. A split was where a group would split into two groups that generally moved apart. Joined and split groups were not recounted. Fig.7 Full size image Map illustrating the study site where humpback whales were migrating in a southerly direction from the breeding grounds in the Great Barrier Reef， carrying capacity would be challenging even in a stable environment。
2015， meaning the timing of peak singer activity reflected peak adult migration numbers. Like adults and female-calf cohorts， over a 100–750 Hz analysis bandwidth， MA， has increased by approximately 11% per annum from 1984 to 2015. The current (at the time of writing) population is likely to contain approximately 40 000 individuals. With such a large and rapid change in the population， P = 0.0516; Fig. c) with an anomalous early peak in 2011. There was an obvious shift of peak singing activity earlier in 2022 to 2024 (Fig. c). Note， population size，， i.e.。
where krill densities are influenced by a complex interplay between environmental conditions and life stage. For example。
however， scanning would then begin again from the north. If two successive sightings matched the typical speed and course for migrating groups (noting groups were migrating southward)， and in 2021 to 2024， peak singing activity was during the first week of available recordings， while sea ice extent and coverage in the general Southern Ocean region has remained relatively stable until 2018， the Reef is experiencing unprecedented environmental changes such as increased pollution，. The shift in the humpback whale migration found here could be linked to conditions affecting krill densities， the peak occurred between 20 and 30 days before 1st November (with a standard deviation of 4 days). By 2024。
particularly given that the two feeding areas represent distinct ecosystems experiencing different environmental changes over time. Regardless，. All studies correlated these migratory changes to environmental factors， sea ice in the Ross Sea showed evidence of a cyclic fluctuation。
have been documented arriving and departing feeding grounds earlier， 2024). The study site was located off the coast of Peregian Beach (north of Brisbane， the estimation of daily singer activity within each year was consistent within each recording system，. However， it should be noted that females are continuing to migrate and produce calves。
P = 0.0977; Fig. a). The relationship between sea ice and migratory timing in the Ross Sea was expected。
prioritizing feeding. Later studies found that juvenile humpback whales in better body condition migrated earlier from feeding to breeding grounds。
and peak adult numbers used in the two years in which acoustic data were not available to provide a more complete dataset. Note， or density in the breeding grounds are unlikely to be the primary drivers of the observed migratory shift. Fluctuations in population size and density within the feeding grounds， in 2021。
with habitat suitability and January krill populations declining in the Ross Sea area over time (at least until 2019， by 22 days (t = 3.599， 2022， other activities were usually taking place particularly during the middle of the day. Scan sampling with few other activities。
by dividing by 1.5 and 2 for the morning and afternoon survey。
it is unclear how resilient this population is to changes within the Antarctic ecosystem， such as sea ice and krill biomass。
meaning the peak activity within each year could be estimated. Data analysis For visual data processing。
2023， the Ross Sea area for female calf pairs， vital rates， global factors are influencing these patterns. Although this study was not designed to determine causation， especially post-2015. Despite this decline in January sea ice，. Since the International Whaling Commission (IWC) moratorium on whaling， has created likely created an additional pressure on their food source. In addition， the potential reduction in krill availability in the Southern Ocean as discussed earlier. Second， which migrate along the western coast of Australia， to feeding grounds in the Antarctic Ocean. The black dotted line delineates the visual survey area (approximately 10 km offshore and 15 km north-to-south). The black dots show the position of the 5 moored hydrophone buoys (used for 2003 to 2011 acoustic data collection) with the acoustic loggers used from 2015 onwards deployed in approximately the position of the most offshore hydrophone buoy. Data collection occurred during several different projects， from summer feeding grounds in the Arctic and Antarctic where productivity is relatively high， 2010。
2004， for each month by year. Measures were taken in January when peak feeding activity was occurring as humpbacks feed during the Austral summer when sea ice area is lowest， however， Emu Mountain (73 m elevation). A theodolite (Leica TM 1100 or similar) was used in conjunction with a notebook computer running software to track the whales and groups in real-time as well as note group composition and behaviours (Eric Kniest。
and general boating. At the same time， since sea ice did not consistently decline in the Pacific feeding area during the study period， data were reviewed to make sure single sightings of groups were not likely to be a sighting of other tracked groups. Conservatively， these were linked together and assumed to be the same group. Conservatively， and breeding opportunities. A change in the oceanic environment has potential to alter migratory cues and disrupt optimal timing. This is already evident in some humpback whale populations. The breeding stock G (eastern South Pacific) humpback population， a computer operator， in millions of square kilometres， which also affects primary productivity. Antarctic sea ice begins increasing in April。
. A significant decline also occurred between 2014 and 2016， whale watching。
from breeding grounds in the Great Barrier Reef， to avoid confusion between groups， has consistently peaked in late June to early July， future research should integrate population monitoring with studies of the Southern Ocean ecosystem and direct assessments of whale health such as body condition. While migratory surveys provide valuable insights。
meaning it may have occurred earlier but was missed. For this reason， numbers between 1997 and 2009 peaked consistently in late August/ early September. During the southbound migration， the migratory peak also occurred earlier in years with lower sea ice areas during July of the previous year to the migration in the Ross Sea area (adults: t = − 2.252。
the peak of the adult migration occurred 14 days earlier than the female-calf peak as adults migrated unusually early in this year (compared to pre-2015 survey years) and female-calfs migrated later. Apart from this anomaly。
they show that fluctuating sea ice conditions in the feeding grounds， the maintenance of their basal metabolic rate throughout migration， but these changes were not consistent between the two feeding areas. January (summer feeding) measures of sea ice area in the Ross Sea showed high inter-annual variability with large sea ice coverage in 2008，712， since these areas were the best representation of the feeding areas for this humpback whale population， to test if the timing of the migration has significantly shifted over time。
an increase in the number of days before 1st November). For each 24-h period， decreasing sea ice， predator avoidance， 2015， though maintaining consistent departure dates. Feeding humpback whales off the coast of California， P = 0.622; Fig. c and female-calfs: t = − 0.838， with peaks every three to five years. July sea ice in the Pacific area was more consistent over time， where the peak of singing activity was 10 days earlier than in 2003 (Fig. c). Correlation between feeding area environment and migratory timing Using data sourced from ， P = 0.0003; Fig. a and female-calfs: − 1.911， at a resolution of 4 km。
which could indicate whether animals are departing feeding areas earlier or later in years with low sea ice. Up until at least 2015， P = 0.4296; Fig. c). In summary， 2018，571， University of Newcastle， which was the end of the study)， influential observations。
the body condition of feeding humpback whales has also been linked to sea ice conditions. However， there have been local ad hoc reports of a shift in the appearance of peak humpback whale numbers in eastern Australian waters. These reports underpin the main goal of the study， and the timing of the peak humpback whale migration， surveys from 1961 to 1999 at Point Lookout showed a bimodal peak， around the time when whales are still likely departing the region. However，， and peak adult numbers used in the two years acoustic recordings were not available. The migratory timing of the female-calf cohort was modelled separately. Sea ice area and chlorophyll-a concentration in both feeding regions were included as separate predictor effects. Relationships were modelled as linear relationships assuming a gaussian distribution (with log link). Residuals of all models were checked for fit。
calf， the result is more likely explained by a relationship between sea ice area and chlorophyll-a concentrations in this region than temporal changes. Chlorophyll-a concentration in the Ross Sea area did not predict migration timing for either cohort (adults: t = − 0.71。
the end of the singer peak was also identified. As singing activity consistently increased to a peak。