An enduring, 20-year, multidisciplinary seal-borne ocean sensor research collaboration in the Southern Ocean Open Access

The Southern Ocean is central to regulating the Earth’s climate by controlling heat and carbon exchange between the atmosphere and the ocean (SO-CHIC consortium et al., 2023; Bennetts et al., 2024). Yet the climate is changing rapidly in response to anthropogenic activities. The vast amounts of carbon dioxide (CO₂) being released into the atmosphere intensifies the greenhouse effect, inducing global warming and major changes in the water cycle. The oceans, and especially the Southern Ocean, store most of the added heat causing water mass and stratification changes in the water column (Abraham et al., 2013; Frölicher et al., 2015; Storto and Yang, 2024). Moreover, as levels of CO₂ rise, the amount dissolved in the seas rise, acidifying the world’s oceans (Doney et al., 2009; Nissen et al., 2024). Apart from changing the physics of the ocean, acidification potentially reduces species and trait diversity and ecosystem structure and function (Teixido et al., 2024).

Recognising the central role that the oceans play in Earth’s climate (Clem et al., 2024), the Global Ocean Observing System (GOOS) was formed in 1991 to coordinate observations of essential ocean variables (EOVs), to contribute to and coordinate observations delivered to the operational meteorological services, and to provide information on marine ecosystem health (Moltmann et al., 2019). GOOS leads and supports a community of international, regional and national ocean observing programs, governments, United Nations agencies, research organisations and individual scientists and aims to provide a complete observational framework for all the world’s oceans through a suite of complementary ocean observing networks (Hermes et al., 2022). However, despite this international effort to observe the world’s oceans, some ocean basins remain poorly observed, most notably the Southern Ocean (Newman et al., 2019).

Autonomous sampling platforms are essential for improving ocean observing capacity (Harcourt et al., 2019). Diving animals, primarily seals, carrying miniaturised satellite-relayed data loggers (SRDLs), have provided a wealth of physical and biological Southern Ocean measurements to help fill this important ocean observing gap (Treasure et al., 2017; McMahon et al., 2021), particularly from the high Antarctic south of 60°S which is otherwise poorly sampled (Roquet et al., 2014). In 2004, we established a collaborative French-Australian program attaching miniaturised SRDL conductivity-temperature-depth instruments (CTD-SRDLs; Photopoulou et al., 2015a) to both southern elephant (Mirounga leonina) and Weddell (Leptonychotes weddellii) seals. Tagging occurred at Iles Kerguelen and along the Antarctic coastline to provide EOVs from across the region, including a combination of temperature, conductivity, pressure and sometimes light and/or fluorometry measurements. These two species provide ocean observations that serve complementary functions: Weddell seals provide localised, repeat observations on the Antarctic shelf (acting as biological moorings), while the more wide-ranging southern elephant seals provide excellent basin-wide water column observations from the Antarctic coast to the sub-Antarctic.

The continuous observations from this 20-year French-Australian collaboration form a major component of the Southern Ocean Observing System (SOOS) and are a primary contribution to the GOOS Animal Borne Sensors (AniBOS) emerging network (McMahon et al., 2021). This collaboration has been supported primarily by two national observational programs; Systeme National d’Observation Mammifere Echantillonneurs du Milieu Oceanique (SNO-MEMO) in France and the Integrated Marine Observing System (IMOS) in Australia, with the French polar institute (IPEV) and the Australian Antarctic Division providing logistical support for fieldwork. Here, following a symposium in France (April 10–12, 2024) entitled 'Elephant and Weddell Seal Symposium: 20 years of observations of the Southern Ocean and ecological studies', we present and review the advances in understanding of the physics and biology of the Southern Ocean and East Antarctica made over the first 20 years of our collaborative program attaching CTD-SRDLs to southern elephant and Weddell seals.

As early as the 1970s, Evans and Leatherwood (1972) attached an instrumented telemetry package first to a yearling grey whale (Eschrichtius robustus) and then to a Pacific common dolphin (Delphinus delphis) that recorded ocean temperature. In 1978, the Argos satellite-data transmission system was created as a tool for collecting and relaying meteorological and oceanographic data (Argos, 2016). By the mid-1980s, Argos opened new opportunities to use the system for diving marine mammals, which must return to the surface to breathe.

The earliest tracking of a marine species (grey seals, Halichoerus grypus) using system Argos in 1985 provided location-only data (McConnell, 1986), but questions about animal behaviour and their underwater environment remained. Argos allows for one-way transmission of very limited amounts of data, with further restrictions on transmissions imposed because the animals typically surface only briefly to breathe (e.g., <2 min for southern elephant seals). However, data compression techniques developed by the Sea Mammal Research Unit (SMRU) (Fedak et al., 2000) made possible the gathering of additional information about the animals’ behaviours and their immediate physical environment (Fedak et al., 2001). Adding a pressure sensor enabled access to the dive profiles of marine animals, providing for the first time a 3D view on their movements.

These satellite-relayed data logger (SRDL) tags were developed by SMRU and first deployed (in 1990) on southern elephant seals at South Georgia Island. They showed how the animals travelled long distances, traversing a variety of oceanographic regimes from the open ocean to shelf areas along the Antarctic Peninsula (McConnell et al., 1992), and for the first time related detailed dive data to oceanographic features. McConnell et al. (1992) wrote what was simultaneously naïve and prophetic: ‘The development of the data logger and transmitter package has provided a methodology which yields both high quality location and behavioural data. This has permitted, for the first time, visualization of the movements of a marine mammal as it moves freely through the most remote reaches of ocean. It can provide information almost in real time and allow interpretation of the pelagic behaviour in terms of the immediate oceanic environment. The continued successful use of this technique provides a unique opportunity to open a new window through which to view the behaviour of marine mammals and will impose new demands on the oceanographic data needed to provide overlays for its interpretation'.

The development of early temperature-depth recorders is discussed by Boehlert et al. (2001). Earnest interaction with the oceanographic community began in the late 1990s when researchers at the Norwegian Polar Institute approached SMRU Instrumentation to integrate a CTD profiler into their SRDL (Lydersen et al., 2002), resulting in the SMRU CTD-SRDL that could be attached to seals and send longitudinally accurate and stable data on the animals’ behaviours and the seawater properties (McMahon et al., 2005a). A new CTD sensor was developed by Valeport Ltd (UK). With funding via a small grant from the Natural Environment Research Council and more substantial support from the Office of Naval Research National Oceanographic Partnership Program via the Sloan Foundation Census of Marine Life, Valeport’s CTD sensor was incorporated into a combined package with the SMRU Argos package to form the CTD-SRDL (Boehme et al., 2009), which became the basic workhorse units used in our 20-year collaboration.

Starting in 2004, CTD-SRDL tags have been attached to seals that collect CTD profiles along with the animal movements, depth profiles and other behavioural information. Since then, other environmental sensors have been incorporated in the tags to gather vertical profiles of fluorescence (in 2007; Blain et al., 2013; Guinet et al., 2013), light levels (in 2008; Jaud et al., 2012) and oxygen (in 2009; Bailleul et al., 2015). These data serve two broad purposes, allowing us to characterise the immediate environment of the animals at a scale relevant to their behaviours and to gather oceanographic information at times and from places where it adds to general oceanographic observing and research objectives.

Details of the sensors and data characteristics, descriptions of summarisation strategies to optimise size, energy and bandwidth constraints, as well as descriptions of how the data are organised and transmitted are readily available elsewhere (Fedak et al., 2002; Photopoulou et al., 2015b). Our intention here is to provide a brief history of how biological questions and technical approaches co-evolved: a description of the circumstances driving tag development to combine the collection and relay of ecological information linked with that of their immediate environment. Current tags provide a wide range of behavioural detail, beyond simple dive patterns. Data stored onboard and/or transmitted can include details of orientation, activity level, prey capture attempts, nutritional state, and so on, while simultaneously providing contextual high-resolution oceanographic data from times and places that are otherwise challenging to sample. An important feature of recent tags is that they store all the data onboard, which can be accessed if the tags are recovered. They provide a far richer source of information than the summarised profiles recovered via ARGOS.

In 2004, a serendipitous set of conditions involving field study opportunities and national funding sources (see below) coalesced to make possible the first coordinated large-scale deployment of oceanographic tags on southern elephant seals across the Southern Ocean. This effort involved scientists from Australia, France, the United Kingdom and the United States of America, resulting in a large-scale coordinated program to link oceanographic data (Charrassin et al., 2008) to the behaviour and foraging success of southern elephant seals (Biuw et al., 2007). This work was united under the Southern Elephant Seals as Oceanographic Samplers (SEaOS) program. The SMRU telemetry team helped develop hardware and analytical tools to support this program.

The success of the SEaoS program led to the development of a new international consortium: Marine Mammals Exploring the Oceans Pole to Pole (MEOP; https://www.meop.net/). MEOP started as an International Polar Year project in 2008 and formed the precursor to the AniBOS network (Treasure et al., 2017; McMahon et al., 2021). The core mission of the MEOP consortium was to produce a unified, quality-controlled, database of oceanographic profiles. The accuracy of the post-processed measurements for the loggers is ±5 dbar in pressure, ±0.03°C in temperature and ±0.05 g kg⁻¹ or better in salinity (Roquet et al., 2011; Siegelman et al., 2019b). Comparisons with Argo floats and ship-based CTD profilers have recently shown that temperature tends to be slightly too cold, albeit within the uncertainty range, with a mean bias below 200 m of −0.025 ± 0.07°C (Gouretski et al., 2024). The most recent version of the database MEOP-CTD (version 2024-03-08; Roquet et al., 2024) includes over 850,000 post-processed temperature and salinity profiles from 2085 CTD-SRDL tags deployed since 2004. Profiles are gathered across many polar regions, including the North Atlantic and North Pacific oceans and most sectors of the Southern Ocean.

The SNO-MEMO in France and IMOS in Australia, with additional funding from the Australian Research Council, have together provided the long-term research infrastructure to support seal tagging for the study of the Southern Ocean’s role in regulating climate, and since 2004, they have provided continuous support for collecting CTD profiles within the Indian sector of the Southern Ocean. Importantly, this French-Australian collaboration has contributed half of all the MEOP-CTD profiles collected globally, making the Indian Sector the only region of the Southern Ocean continuously sampled by instrumented seals. Through major efforts by field personnel, 200 of these tags (mostly from Kerguelen Island deployments) have been recovered, giving access to the detailed vertical resolution data (sampled every 4 s) archived onboard. Also, a growing number of tags have been deployed that include a light sensor and/or a fluorometer, which can be used to estimate the chlorophyll-a content (Blain et al., 2013; Bayle et al., 2015; Le Ster et al., 2023).

Over the last 20 years, the remote Southern Ocean was mostly sampled through a combination of autonomous Argo profiling floats and animal-borne sensors (Figure 1). More than 80% of animal-borne observations are south of 45°S, and they are the dominant platform south of 60°S (Figure 1). To date, over 80% of all oceanographic profiles available south of 60°, and more than 95% of those associated with the pack ice zone, have been collected by seals. However, the data are not distributed evenly across longitudes, with the Southern Indian sector and the western Antarctic Peninsula the most sampled areas (Figure 2).

Data from tags on marine animals relayed by ARGOS operated by the French company CLS are uploaded in near real-time to the World Meteorological Organization for distribution via the Global Telecommunications System from which the data can be used immediately for operational forecasting. The data are also ingested annually by the MEOP Data portal where they are post-processed and subsequently made freely available in a standardised quality-controlled format (Roquet et al., 2014; Jonsen et al., 2024). MEOP data are widely used by the international community and routinely assimilated into ocean model products such as CODC-v1 (Zhang et al., 2024), Bran 2020, Mercator (Chamberlain et al., 2021a; Chamberlain et al., 2021b), and the Southern Ocean State Estimate (Fenty et al., 2015).

Central to the ongoing success of our 20-year program is maintaining best animal handling practices that minimise any negative effects on animal welfare. We do this through shared standard operating procedures (SOPs) detailing the capture, handling, anaesthesia, instrument attachment, and care of animals. We maintain and update these SOPs to improve methods (McMahon et al., 2000; Field et al., 2012; McMahon et al., 2021). Importantly, we have quantified the effects of instrumentation on animal behaviour, performance, and vital rates (McMahon et al., 2008; McMahon et al., 2012). Biologging effects on animals fall into two main categories: (i) procedural effects, such as the capture and restraint of the animal and attachment of the biologging device; and (ii) ongoing effects on the animal from carrying the device. In both cases, appropriate procedures have been developed that minimise capture injuries and handling time and expedite the release of the animal after the tagging procedure; we are confident that seals are not affected negatively by the capture and handling procedures or the carrying of the tags (McMahon et al., 2008). Determining the optimal number of animals needed for deployment is crucial for filling existing data gaps (Sequeira et al., 2019). The ideal sample size balances appropriate spatio-temporal coverage, sufficient statistical power (Hindell et al., 2003; Hindell et al., 2022) and animal welfare following the principles of Replacement, Reduction and Refinement (the 3Rs).

The Southern Ocean remains the least well-observed ocean (Figure 1), despite its global importance for climate, biogeochemical cycles and sea level variability (Bennetts et al., 2024). The use of Argo profiling floats improved the global sampling coverage starting in 2005, both spatially and temporally (Roemmich et al., 2019). However, the region south of the Polar Front remains less well sampled by Argo floats, due to the presence of seasonal sea ice and the isolating effect of zonal currents. Additionally, regions of importance such as shelf areas remain poorly or not at all sampled by Argo floats. Southern elephant seals instrumented on sub-Antarctic Islands have filled in many gaps in the open ocean and have been particularly successful in sampling the Antarctic continental shelf (Figures 1 and 2).

The ability of seals to collect oceanographic data in the open Southern Ocean was first demonstrated by Charrassin et al. (2008), who combined data from seals tagged at four sub-Antarctic islands to build a unique synoptic map of upper ocean properties (Figure 3). Profiles of temperature and salinity were obtained in sea-ice-covered areas for the first time, also enabling detailed mapping of the position of the Antarctic Circumpolar Current (ACC) fronts in the Atlantic (Boehme et al., 2008) and Indian (Roquet et al., 2009) sectors.

Continuous annual deployments of loggers on seals have provided a substantive database of oceanographic data making a global contribution to the ocean observing system (Roquet et al., 2013). The data serve to improve the accuracy of the Southern Ocean state estimate (Mazloff et al., 2010) and of global state estimates such as the Estimating the Circulation and Climate of the Oceans (ECCO) product (Wunsch et al., 2009; Forget et al., 2015) or the GLORYS12 global reanalysis product (Lellouche et al., 2021). For the first time, budgets of mixed layer properties within seasonally sea-ice-covered areas are available, demonstrating the dominant role of freshwater in controlling the upper stratification (Pellichero et al., 2016) and sea level variability (Kolbe et al., 2021) of these regions. Because several vertical ocean profiles per day are retrieved from seal-borne sensors, sub-mesoscale processes could also be investigated for the first time in these southern regions, demonstrating the substantial impact of sea ice on sub-mesoscale variability (Biddle and Swart, 2020) and the presence of deep-reaching sub-mesoscale fronts in the ACC region (Siegelman et al., 2019a).

The region around the Kerguelen Islands has benefitted greatly from the deployment of animal-borne sensors. These islands lie on the submarine Kerguelen Plateau, directly across the path of ACC, playing a key role in topographically steering this predominantly zonal oceanic current. Seal-borne observations obtained in 2004 provided the first-ever picture of the oceanic flow across the Kerguelen Plateau, demonstrating how the deep Fawn Trough which cuts the plateau in half, channels all the ACC water masses south of the Polar Front within a permanent jet 50 km wide (Roquet et al., 2009).

With 20 years of data now available in this region, studying the seasonal and inter-annual variability of ocean properties such as the mixed layer depth has become possible (Figure 4). The ACC Polar Front experiences large seasonal north-south migrations west of the Kerguelen Plateau highlighting the complexity of water mass pathways across the plateau (Pauthenet et al., 2018). The recent occurrence of intense marine heatwaves in this region has become a source of concern for the Kerguelen marine ecosystem, and the availability of seal-borne oceanographic data also helps to study the impact of such extreme events, which are predicted to intensify in the future (Azarian et al., 2024).

Our bilateral collaboration has contributed essential observations on the Antarctic continental shelf and slope, where ocean-ice-atmosphere interactions drive globally significant water mass transformations. Early on, deployment of sensors on crabeater seals revealed ocean properties and identified warm flows intruding onto the continental shelf along the western Antarctic peninsula (Costa et al., 2008). Data obtained from instrumented Weddell seals and southern elephant seals have since provided important data complementary to traditional ship, mooring and float-based observations to provide information about the frontal positions and water mass properties around Antarctica. These circumpolar regions include (from west to east): the Weddell Sea (Nicholls et al., 2008; Nøst et al., 2011; Arthun et al., 2012; Labrousse et al., 2021; Darelius et al., 2024), Cape Darnley (Ohshima et al., 2013), Prydz Bay (Williams et al., 2016), Vincennes Bay (Kitade et al., 2014), Adelie Land (Williams et al., 2011), the Ross Sea (Piñones et al., 2019), Amundsen Sea (Mallet et al., 2018) and the Bellingshausen Sea (Zhang et al., 2016).

Instrumented seals assisted the discovery of a new fourth major source of Antarctic Bottom Water (AABW) at Cape Darnley, west of Prydz Bay, contributing 6–13% of the circumpolar AABW total (Ohshima et al., 2013). Moorings deployed in canyons along the slope observed very saline (>34.8) dense shelf water sinking to depths, while the animal-borne sensors documented this Dense Shelf Water (DSW) production to occur within the coastal Cape Darnley polynya, driven primarily by the salt flux from the intense sea-ice formation rates here. Being able to incorporate Cape Darnley Bottom Water into global climate assessments can improve long-term climate change predictions. Further prominent studies of Prydz Bay explained the complex contributions (from polynyas and ice shelves) into the gyre circulation within this embayment; particularly the freshening impact of glacial meltwater from the Amery Ice Shelf in conditioning the water properties to ultimately export a fresher DSW (approximately 34.67) through the Prydz Channel (Herraiz-Borreguero et al., 2015; Williams et al., 2016). These studies highlighted the vulnerability of this system to warming, with further ice shelf melting likely to suppress DSW and ultimately inhibit AABW formation.

As observational datasets expand, we are learning how each polynya region varies dynamically from the others, highlighting the complexities around DSW formation and AABW production. We have a new understanding of how physical factors, sea-ice formation, source water masses, glacial meltwater and water column preconditioning interact within coastal polynyas along the Antarctic shelf to control rates of DSW formation (Portela et al., 2021; Ribeiro et al., 2021; Portela et al., 2022). It exposes a vulnerability of processes essential for AABW formation to increased ocean heat and export of glacial meltwater in a warming climate (Silvano et al., 2018).

The long-held view of the East Antarctic margin being isolated from intrusions of warm offshore waters has been updated fundamentally by the new observational capacity. Most recently, seal-borne observations in conjunction with historical ship-based observations have documented long-term ocean warming and freshening at the Shackleton Ice Shelf (Ribeiro et al., 2023). With their study spanning 60 years of data, Ribeiro et al. (2023) found that post-2010 warm (≥1°C) modified Circumpolar Deep Water (CDW) intrusions were widespread west of the ice shelf, causing basal melting and reducing DSW formation.

Supporting evidence from seal-borne instruments has shown signals of long-term change across the whole East Antarctic caused by the poleward shift of warm CDW (Herraiz-Borreguero and Naveira Garabato, 2022). The unique animal-borne observations over the Antarctic continental shelf are invaluable in mapping on-shelf DSW and CDW properties in the Indian sector of the Southern Ocean and assessing changes in on-shelf water masses. These assessments are central to quantifying mass loss from the Antarctic Ice Sheet, freshwater input to the ocean, and changes in ocean heat content.

Our collaboration has tagged 875 southern elephant seals, 785 of which were from three sites in the Indian Ocean: Iles Kerguelen (n = 695), Davis station (n = 64) and Casey Station (n = 26). Over the 20 years of study, an average of 37 seals were tagged per year (minimum = 9 in 2008, maximum = 81 in 2021) across sites. This effort constitutes the largest and longest running tagging program in the Southern Ocean and has enabled many important insights into the biology, behaviour and ecological role of this species within the marine ecosystem. The deployments have focussed on adult females (n = 323) and subadult males (aged 2 to 6 years; n = 353), as they provide broad geographic coverage for ocean sampling and are both easily accessible during the summer months for tag deployments. This focus means that the youngest age classes and adult males (which attain massive size) remain under sampled. The large-scale nature of the study has nonetheless delivered a solid understanding of how the seals use the Southern Indian Ocean. Here we highlight a few key findings but refer the reader to the substantial body of literature (e.g., Bailleul et al., 2007; Bailleul et al., 2010; Dragon et al., 2010; Bestley et al., 2013; Bestley et al., 2015; Cotté et al., 2015; Labrousse et al., 2017).

Southern elephant seals range widely throughout the south Indian sector, latitudinally from 36.6°S to 70.0°S and longitudinally from 10°E to 149.0°W, equating to 5452 km of Antarctic Coastline (at 65°S) and 44% of the Southern Ocean, with some individual seals travelling up to 5700 km from their deployment site. However, the seals do not use this space uniformly, with at-sea locations concentrated to the east of Kerguelen Plateau and many seals using the winter sea-ice zone (Figure 5; modified from Hindell et al., 2021). How the seals use this space varies with both sex and time of year. Adult females are predominantly oceanic foragers, spending on average 80% of their time in the open ocean and only 20% in shelf waters (Hindell et al., 2021). In contrast, subadult males spend more time in shelf waters; up to 80% in autumn, with some focusing on the Kerguelen Plateau (30% of total time) and others the Antarctic continental shelf (50% of total time). By spring, however, their use of the Antarctic shelf drops to 10% and the Kerguelen Plateau increases to 56%, likely in response to the increasing density and extent of the sea ice combined with reproductively active females returning to their natal islands to breed. The difference in habitat use between the sexes is attributed to the highly polygynous nature of their breeding system driving extreme sexual dimorphism, with few large males defending large groups of much smaller females (called harems) during the breeding season (Beltran et al., 2022). This breeding system imposes an imperative on rapid growth in the male seals, leading to their use of highly productive shelf waters where they may experience higher risk of predation (Henderson et al., 2020) from killer whales. In contrast, female seals, which maximise their lifetime reproductive output by investing in a single pup each year and spread their reproductive effort throughout their lives (approximately 12 reproductive years), can use open ocean habitats, hunting mesopelagic fish and squid.

The importance of the Kerguelen Plateau as a habitat for southern elephant seals exposes them to potential risks from interacting with commercial fisheries (Hindell et al., 2022). Longline fishing catches a few young males every year (van den Hoff et al., 2017). While unlikely to present a threat to the population, there is still the possibility of indirect ecological interactions where the seals and fishery may compete for resources. The large tagging sample size means that the seals’ use of the Kerguelen Plateau has been described in unprecedented detail, to the extent that the occupied area recorded represents 91% of the potential theoretical plateau habitat for (a completely sampled population of) subadult males (Hindell et al., 2022). This level of coverage provided a high degree of confidence in describing the seals’ spatial usage, allowing comparison with known fishery areas. Seals spent 30% of their time on the plateau within the commonly used fishing grounds, indicating the potential for competition for resources there. However, males were also more likely than females to feed on the benthos, where the fishery is most active. Simple energetic models suggested that male seals consumed 7.8–19.1% of the fishery’s total annual catch and females, 3.6–15.1%. Lack of data on the importance of toothfish in the seal diet meant that fully establishing the degree of interaction was not possible. Altogether though, the highly polygynous nature of this species and the lack of a measurable effect on population growth rates or the fishery catch metrics suggest that any interactions are of insufficient magnitude to affect either the seal population or the fishery.

Weddell seals were tagged at Davis station in 2006–2008 and 2010 and more continuously at Dumont d’Urville (DDU) since 2006, providing key insights into their ecology (Figure 6). Post-moult studies from 2007 to 2009 indicated that seals foraged predominantly within a 200 km range east and west of the station (Heerah et al., 2013; Heerah et al., 2016). Foraging and hunting dives were concentrated within the dense sea-ice zone along the Antarctic continental shelf (Heerah et al., 2016). Combined with research conducted in McMurdo Sound (Testa, 1994; Burns et al., 1999; Stewart et al., 2000; Harcourt et al., 2021), DDU tagging shows that Weddell seals adapt to different local sea-ice conditions. Seals tagged at DDU typically travelled less far and foraged over a limited spatial extent due to stable sea-ice conditions, in comparison with seals at Davis which roamed further (Harcourt et al., 2021). Weddell seals typically show a much more residential movement behaviour than southern elephant seals (Bestley et al., 2015), although their high propensity to haul out onto sea ice has implications for how we interpret their activity budgets and at-sea spatial usage (Bestley et al., 2016). Their haul out behaviour follows a diurnal cycle and changes with environmental conditions, tending to haul out more in lower winds and at higher temperatures (Andrews-Goff et al., 2010). Weddell seal diving and movement behaviour appears influenced by icescape features like open water areas and ice variability, rather than just sea-ice concentration (Heerah et al., 2016; Harcourt et al., 2021).

Female Weddell seal diets consist of high trophic-level prey including Pleuragramma antarcticum, Dissostichus mawsoni and squid (Heerah et al., 2016), as well as benthic prey Trematomus spp. and Channichthyidae spp. Seals primarily make pelagic dives to depths around 115 m for mid-water prey. Weddell seals at DDU dived deeper and longer during the day, with pelagic dives mostly at night. These seals demonstrate opportunistic feeding behaviour, adjusting foraging in response to possible intra-specific competition and ice conditions. There are also seasonal changes in habitat use, with increased area-restricted search behaviour as winter advances and shallower and longer winter dives near DDU and in McMurdo Sound without increasing foraging effort (Heerah et al., 2013; Heerah et al., 2016; Yong et al., 2024). Sea-ice thickening increases the risk of moving between breathing holes, leading to longer and shallower dives to maximise horizontal travel and orienting time at each breathing hole.

During winter, Weddell seals forage primarily in association with modified CDW (mCDW; Heerah et al., 2013), which is also important to southern elephant seals (Biuw et al., 2007; Hindell et al., 2016). Intrusions of nutrient-rich water masses onto the continental shelf, such as mCDW, are known to stimulate primary productivity (Nicol, 2006), supporting the population growth of mid- (Prézelin et al., 2000) and upper-trophic level biota (La Mesa et al., 2010), including top predators that feed on these prey. Weddell seals also forage in modified Warm Deep Water on the Filchner-Ronne Ice Shelf in the southwestern Atlantic (Labrousse et al., 2021).

Seal dive depths recorded in conjunction with in situ oceanographic data have provided unique insights into how ocean conditions influence seal behaviour and therefore allow inferences about the distribution and abundance of their mesopelagic prey. Combining a relative measure of dive effort with information on the vertical position of a key Southern Ocean water mass (CDW) showed that mesopelagic prey distribution (inferred from seal dive depth) was influenced by the physical hydrographic processes that structure their habitat (McMahon et al., 2019). Specifically, mesopelagic prey had a more restricted vertical migration and higher relative abundance closer to the surface where CDW rises to shallower depths. Combining these observations with a future projection of Southern Ocean conditions suggests that changes in the coupling of surface and deep waters will potentially redistribute mesopelagic prey (McMahon et al., 2019). While these changes are likely to be small in absolute terms, they show important spatial variability: prey will increase in relative abundance to the east of the Kerguelen Plateau and decrease to the west. The consequences for deep-diving specialists such as southern elephant seals are likely to be minor, but have implications for other predators that forage within the mesopelagic zone.

Green et al. (2020) reported that pelagically foraging southern elephant seals aggregate in regions of high mesoscale activity, where eddies concentrate prey. In these regions seals dived deeper and spent less time hunting, presumably targeting more profitable (higher quality) prey patches. Seals generally avoided areas of low eddy activity where prey may be more dispersed due to the lack of aggregating oceanographic features. Most seals foraged south of the sub-Antarctic Front, despite the region north of the front exhibiting consistently high simulated prey field biomasses (Lehodey, 2004; Green et al., 2020; Green et al., 2021). These findings demonstrate the value of coupling mechanistic representations of prey biomass with predator observations to provide insight into how biophysical processes combine to shape species distributions. This approach will be increasingly important for the robust prediction of species’ responses to rapid system change.

As detailed above, tagged southern elephant seals have provided novel insights into globally significant physical processes occurring within East Antarctic coastal polynyas. However, these areas are also ecologically important as ‘biological oases’ in the high Antarctic (Labrousse et al., 2018). Based on 7 years of data from Isles Kerguelen to the Antarctic continental shelf, a total of nine different coastal polynyas were used across East Antarctica (Labrousse et al., 2018). From a larger sample of 119 seals foraging over the Antarctic shelf, Arce et al. (2022) demonstrated that 96% used polynya areas, spending a significant amount of their time (on average 62%) inside polynyas.

The French-Australian collaboration has identified Antarctic coastal polynyas as key foraging habitats, not only as open water areas during the period of extensive sea ice but also as ‘post-polynyas’ because they harbour higher biological resources year-round. Tagging initially revealed that juvenile male southern elephant seals remained resident within four Prydz Bay polynyas during summer–autumn and into winter, and that their residency and behaviour related to the biophysical characteristics of these environments (Malpress et al., 2017). Bathymetry, chlorophyll-a, surface net heat flux (representing polynya location) and bottom temperature were identified as significant biophysical predictors of their spatio-temporal habitat usage. Diving behaviours of seals also differ between those polynyas with high versus low sea-ice production (Labrousse et al., 2018). In low sea-ice production polynyas, seals dive pelagically to the bottom of the deepening winter mixed layer (in Antarctic Surface Water), likely benefiting from enhanced secondary production associated with the remixing of entraining nutrients from below. Strong sea-ice production polynyas have a homogeneous water column with no pycnocline, leading seals to forage on the seafloor, within DSW. These two studies suggest that as the water column becomes fully convected during autumn–winter, this promotes pelagic-benthic linkages; these polynyas are associated with rich benthic communities and prey due to vertical carbon flux, important for benthic foraging.

When foraging inside polynyas, the seals gain more energy, as indicated by increased buoyancy from greater fat stores (Arce et al., 2019; Arce et al., 2022). This higher-quality foraging exists even when ice was not present in the study area, indicating that these polynya areas are important and predictable foraging grounds year-round. This study is one of very few examples able to directly demonstrate the foraging benefit of energy consumed within polynyas by upper-trophic levels. Taken altogether this body of work provides otherwise unobtainable knowledge of the critical biophysical processes occurring within Antarctic coastal polynyas and highlights their global significance in both supporting ecological function and driving ocean circulation.

While many areas of the Southern Ocean are categorised as high-nutrient low-chlorophyll zones, the Kerguelen region is naturally enriched in iron coming off the island or from resuspended sediments over the submarine plateau; this enrichment favours the development of large phytoplankton blooms over and downstream from Kerguelen Islands (Schallenberg et al., 2018). The high primary production sustains one of the largest marine ecosystems in the Southern Ocean. The development of miniaturised fluorometers embedded in the CTD unit (Guinet et al., 2014) has improved estimates of the chlorophyll concentration in this region (Blain et al., 2013) and highlighted the difficulty of retrieving phytoplankton information from satellites, both because the chlorophyll peak is often observed in the sub-surface and because satellite measurements suffer from large biases when they are not correctly adjusted using in situ observations (Johnson et al., 2013). Combining data types provides the possibility to upscale information: Le Ster et al. (2023) showed that the combined use of light profiles and satellite ocean colour matchups enabled the effective calibration of tag-based fluorescence data and the filling of spatial gaps in sampling.

A high-latitude study by Bourreau et al. (2023) shed light on chlorophyll fluorescence within Antarctic polynyas from late summer to winter. This study analysed 698 profiles collected between 2011 and 2019–2021 in Cape Darnley and Shackleton polynyas. The animal-borne sensors revealed the first in situ chlorophyll fluorescence signal, observing a substantial signal in both polynyas until April. The study showed clear differences in biological and environmental conditions between the two polynyas. A unique signal at 130 m depth in Shackleton polynya in April was associated with fresher and warmer waters, potentially associated with ice-shelf melting. This research emphasises the importance of polynyas as active regions within the sea-ice zone throughout the year.

Strong interest has developed in the long-term conservation of the Southern Ocean, but authorities face the considerable challenge of identifying regions that should be considered for protection, for reasons such as their high biodiversity, biological productivity or particular importance for certain life-history stages of species. The distribution and demography of marine predators provides a viable basis for this consideration because other integrated ecosystem measures are difficult to obtain at management-relevant ocean-basin scales. Spatial aggregations of predators at sea identify not only areas that are important to the predator species themselves, which depend on lower trophic levels, but also areas of ecological significance (AES), defined as regions of elevated productivity, biodiversity and biomass (Hindell et al., 2020). There is a growing recognition of the value of tracking data for making decisions about conservation (McGowan et al., 2017; Hays et al., 2019). The southern elephant and Weddell seal data from the French-Australian collaboration were part of a synthesis of more than 4000 tracks from 17 bird and mammal species that identified AES around sub-Antarctic islands in the Atlantic and Indian oceans and over the Antarctic continental shelf (Hindell et al., 2020). Climate change over the next century is predicted to impose pressure on these areas, particularly around the Antarctic continent. At present, only 7.1% of the ocean south of 40°S is under formal protection, encompassing 29% of the total AES.

Behavioural and physical data like those collected by southern elephant seals play an important conservation role in establishing marine protected areas (Becker et al., 2024). Seal tracking and dive data collected from the more than 800 southern elephant seals as part of our collaboration has been central to assessing how the seals use the Kerguelen Shelf and quantifying the overlap with fisheries. The Kerguelen Shelf is a critical foraging habitat for the male component of this population along the Kerguelen Shelf and shelf break within 100–300 km of the island to the south of the Polar Front. These seal observations were critical in designating, in 2019, the French Southern Lands and Seas marine protected zone, a unique World heritage-listed reserve network encompassing 672,969 km² in the Southern Indian Ocean (http://taaf.fr).

All techniques used to obtain information observing the physical and biological environment come with some impact to the environment, including a carbon footprint, and animal-borne systems are no exception. As a dedicated environmental observing program, it is important that we consider the environmental impact, quantify the carbon contribution of the program and as far as reasonable limit the carbon emissions from the project. These considerations are especially pertinent given that the foci of our program are quantifying how the Southern Ocean is changing and evaluating the consequences of its changes on the physical, biogeochemical and biological components of the integrated system.

To this end we assessed the carbon footprint of the program by calculating the carbon footprint of each temperature-salinity (T-S) profile. Our estimate includes the carbon costs of transporting instruments from the Sea Mammal Research Unit in Scotland to the deployment sites, transport costs of deploying field personnel and the associated costs of the consumables needed for the deployments and data infrastructure and digital storage space. For the Iles Kerguelen deployments (the vast majority of profiles), we estimated that on average the program emitted 0.41 kg of CO₂ per sampled T-S profile. This estimate is low compared to other hydrographic profile sampling systems (e.g., oceanographic ships, Argo profilers, underwater gliders, etc.). Indeed, a typical dedicated research vessel may emit up to 75,000 kg day⁻¹ of CO₂ (Jun et al., 1999). A standard CTD cast typically requires 2–5 hours (i.e., 0.08–0.21 of a day), and the cost of each profile may thus vary between 6250 kg and 15,625 kg of CO₂ per CTD profile. We note that emissions from continental Antarctic deployments (e.g., Davis, Casey, Dumont d’Urville stations) are likely to be higher, albeit they represent a small component of the total profiles, and are unlikely to incur a change of four orders of magnitude. While 0.41 kg of CO₂ emissions per sampled T-S profile is not negligible, it remains low compared to the alternative costs of ocean sampling in the remote Southern Ocean.

Recovering instruments can further reduce the carbon footprint of the program, which not only reduces waste into the environment but also extends the operational life of the tags. Each year some of the instruments are recovered (up to 65%) when the tags are moulted off or removed when the seals return to their natal islands. This recovery reduces the ecological costs of lost equipment and, because recovered instruments can be reconditioned (typically twice), the lifespan of a tag increases from 1 year to 3 years.

Despite the low carbon cost of animal-borne ocean observations (compared to other observing networks) and their importance to understanding Earth’s climate, animal-borne ocean observations like all the observing communities have a responsibility to further reduce the carbon costs of ocean observing: greenhouse gas emissions are driving increases in global temperature. Options for reducing the carbon cost include technological developments, such as increasing the use of renewable materials, and behavioural changes, for example, reducing manufacturing and transport costs and ensuring that quality-controlled data are available to the broader ocean observing community to reduce observational duplication.

The main objectives of our collaboration are to quantify the long-term oceanographic conditions and seal (elephant and Weddell seal) ecology by evaluating seasonal temperature, salinity, water mass transformations, circulation patterns, seal foraging areas, diet changes, primary production processes and to address data gaps under glacier platforms. We also investigate the frequency of extreme events, for example, the calving of the Mertz glacier tongue in 2010 (Kusahara et al., 2011; Tamura et al., 2012), and their impacts on seal ecology, and the effects on marine predators and ecosystems (Heerah et al., 2013; Nachtsheim et al., 2017; Photopoulou et al., 2020; Harcourt et al., 2021[). Our animal-borne observations have over the last 21 years provided exactly those priority observations (detailed below) identified by the SOOS community (Meredith et al., 2013; Newman et al., 2022) and remain a focus of our study in the Southern Indian Ocean (Newman et al., 2019; Newman et al., 2022).

Of the five community-agreed science themes within which a number of key science challenges are identified (Newman et al., 2022), our French-Australian collaboration provides input to these three themes and key science challenges:

1. Theme 1: Understanding and quantifying the state and variability of the Southern Ocean cryosphere, which includes

   • the dynamical processes in the Southern Ocean and their likely changes in the future, and

   • how climate change will alter surface fluxes and freshwater input from the cryosphere, and the impact of these changes on water mass properties.

2. Theme 2: Understanding and quantifying the state and variability of the Southern Ocean, which includes

   • circulation formation and circulation, and implications for heat and carbon,

   • dynamical processes in the Southern Ocean and their likely changes in the future, and

   • how climate change will alter surface fluxes and freshwater input from the cryosphere, and the impact of these changes on water mass properties, formation and circulation, and implications for heat and carbon.

3. Theme 4: Understanding and quantifying the state and variability of Southern Ocean ecosystems and biodiversity, which includes

   • the drivers of environmental change and their effects on Southern Ocean ecosystems, and

   • the biodiversity of Southern Ocean benthic and pelagic ecosystems, and the distribution of species in relation to the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR), Marine Protected Area (MPAs) and climate change.

Climate change is a globally pressing issue, and data-driven models and decision support techniques can help understand its complexity and inform mitigation and adaptation policies. The Southern Ocean, which plays an important role in the world’s climate and weather system, has the capacity to store and release more heat than any of the other ocean basins. However, the lack of observations in this region limits our ability to detect critical changes and develop improved Earth system models. Animal-borne oceanographic sensors and the long time series of ocean observations they have provided have helped to address this gap, allowing for more accurate climate predictions. Maintaining and building on these observational times series are therefore a key priority.

There is a growing recognition that emerging chemical sensors that measure dissolved gases, for example, oxygen (Bailleul et al., 2015), and optical sensors that measure ocean colour and light levels (Blain et al., 2013; Guinet et al., 2013; Guinet et al., 2014; Biermann et al., 2015; Vacquie-Garcia et al., 2017; Bourreau et al., 2023; Le Ster et al., 2023) would be invaluable additions to the existing CTD and FCTD tags. The optical sensors provide information on the concentration, composition and physiological state of the phytoplankton. There is also a suite of sensors that can record prey encounters (Adachi et al., 2023), with estimates of prey sizes and density (Goulet et al., 2019; Antoine et al., 2023).

While existing observations and technologies have provided physical and biological information that have improved our understanding, there remains much to be learnt about ocean chemistry (e.g., pH) and animal foraging ecology (e.g., identification of prey species). Obtaining more information requires investment into and integration of new miniaturised sensors and better data-transmitting abilities, for example, real-time satellite-linked communication systems and improved, smaller and longer-lived power sources. Keeping instruments small is important for maintaining high animal welfare standards. Further miniaturisation of sensors and instruments will also provide opportunities to expand the range of animals on which tags can be deployed and the systems that can be observed, for example, small instruments deployed on Ross seals would allow us to better observe physical, biological and behavioural information within the marginal ice zone. The advances we have made in sensor miniaturisation, integration of new sensors and better telecommunications have been possible due to the close relationships we have fostered with sensor and instrument developers and manufactures, and this collaboration between the researchers and manufactures remains an important priority.

One of the cornerstones of the success of the 20-year France-Australia collaboration has been the delivery of open access data, both in near real-time and delayed quality-controlled mode, through the international MEOP and Australia’s IMOS. A key step towards building a comprehensive and sustained circumpolar network is developing a formal optimal sampling plan, for example, an Observing System Simulation Experiment. Such a plan will allow us to set quantitative targets for the number and frequency of deployments, their geographic scope and the temporal range of observations needed to achieve complete coverage of the Southern Ocean. It will also allow a formal framework for assessing and measuring success.

Despite the importance of bathymetry much of the ocean including the Southern Ocean remains unmapped (Mayer et al., 2018; Dorschel et al., 2022). Bathymetry profoundly affects water circulation pathways across and along the Antarctic shelf and influences how ocean heat is transported towards cavities beneath floating ice shelves. Understanding the detail of these processes is important to resolving the underlying mechanisms missing from global ocean and climate models. Animal observations can improve seafloor mapping and identify target areas for detailed ship-based surveys (McMahon et al., 2023), thereby making an important contribution to continued building of a more complete and comprehensive view of Earth’s bathymetry.

Sustained long-term observations in the Southern Ocean remain sparse, and building a diverse, integrated and complementary observing system is desirable for addressing remaining key observing gaps. However, despite the extensive geographic coverage achieved from deployments in the southern Indian sector on southern elephant seals and the more regional coverage provided by deployments on Weddell seals, there are many areas around Antarctica that are infrequently or in some cases never sampled. Expanding the observing capacity into regions where sampling frequency is low will depend on effective integration and coordination of animal-borne deployments, across polar programs (including the Alfred Wegner Institute, Antarctica New Zealand, Brazilian Antarctic Program, British Antarctic Survey, Institut Polaire Français Paul-Émile Victor, Korea Polar Research Institute, National Institute for Polar Research, Norwegian Polar Institute, South African National Antarctic Program, and United States Antarctic Program) under the broader framework provided by AniBOS (McMahon et al., 2021).

The number of studies using animal-borne instruments continues to increase significantly (McMahon et al., 2011; McMahon et al., 2021). Unfortunately, the number of research projects aiming to identify and minimise tag effects has not kept up (Vandenabeele et al., 2012). In addition to the technological advances and the incorporation of new sensors, quantifying and ultimately reducing the drag or resistance of tags needs to be considered. Kyte et al. (2018) and Kay et al. (2019) provide an approach for estimating tag effect, which can guide the design and deployment of animal-borne tags. Our awareness of animal welfare offers significant opportunities to improve the capture and restraint of animals (e.g., Field et al., 2002; McMahon et al., 2005b; Wheatley et al., 2006; Harcourt et al., 2010) and the attachment of devices (Field et al., 2012; Horning et al., 2019). Such continual refinement of our protocols to minimise stress, reduce the effects of instruments on animals and reduce restraint times is an important component of ensuring we maintain our social licence to provide the EOVs and ecologically essential variables to the broader community.

Clearly, the long-term interdisciplinary collaboration we show here has advanced our understanding of the physical and biological marine environment and how it is responding to changes in climate and increased resource exploitation. Our time series is especially valuable because it enables the study of the inter-seasonal and inter-annual variability of oceanographic conditions as well as the detection of decadal-scale changes. The observations are ingested as real-time temperature profiles through the global telecommunications system to improve marine weather forecasting, while the delayed mode data have been assimilated into a number of ocean-climate reanalyses, for example, BRAN2020 (Chamberlain et al., 2021a; Chamberlain et al., 2021b), GLORYS12 (Lellouche et al., 2021) and quality-controlled data sets such as EN4 quality-controlled ocean data: EN.4.2.2 (Gouretski and Cheng, 2020).

The rapid development of new technology has opened new windows for observing the physical sub-surface structure of our remote oceans and the life history of seals, providing insights into how seals gather the resources to survive and reproduce within their dynamic ecosystem and an ability to monitor their success in prey capture and quantify at-sea seal body condition. Making these life history and physical sub-surface observations helps to improve larger scale oceanographic models and make predictions on how the marine ecosystem will respond to future exploitation and climatic shifts. A major achievement of our project has been convincing an at-times sceptical—stemming from concerns of data and location accuracy and the non-random seal movement behaviours—oceanographic community of the value of the animal-borne observations. The large-scale uptake and use of these data are testament to their importance as a legitimate, reliable and essential source of observations that is used increasingly by the oceanographic community. The uptake of the in situ physical information improves model predictive capacity that enhances greatly our ability to monitor how the ocean and climate change as human activities continue to warm Earth’s surface and its ocean.

The quality-controlled data collected from 2004 to 2024 are all available freely through the Marine Mammals Exploring the Oceans Pole to Pole (MEOP) data portal: https://www.meop.net/database/meop-databases/index.html. Currently the data are made available as three databases:

• the MEOP-CTD database: quality-controlled CTD profiles

• the MEOP-SMS database: sub-mesoscale-resolving high density CTD data

• the MEOP-TDR database: high spatial density temperature/light data

All tagging procedures were approved and executed under University of Tasmania Animal Ethics Committee guidelines (A12141, A14523), the Comité d’éthique Anses/ENVA/UPEC (no. APAFiS: 21375) and by Macquarie University Ethics Committee ARA 2014_057. We thank Adélie Antoine for help and expertise in preparing Figure 6.

This research was supported by the Integrated Marine Observing System (IMOS), and as part of the IPEV programs no. 109 (PI H. Weimerskirch) and no. 1201 (PI C. Gilbert). Research at Terre Adelie was supported by Terre-Océan-Surface Continentale-Atmosphère from the Centre National d’Etudes Spatiales (TOSCA CNES, France) along with IMOS. IMOS is enabled by the National Collaborative Research Infrastructure Strategy (NCRIS). It is operated by a consortium of institutions as an unincorporated joint venture, with the University of Tasmania as Lead Agent. Fieldwork at Macquarie Island and at the Australian Antarctic stations was supported by the Australian Antarctic Division (AAS 2265, AAS 2794, AAS 4329 & AAS 4630). The Australian Research Council supported this work through Discovery Projects DP0345010, DP0770910, DP180101667 and DP23010136 and the ARC Special Research Initiative SR200100008.

The authors declare no competing interests.

CRM, CG and MAH conceived the manuscript, CRM led the writing to which all authors contributed. All authors reviewed the manuscript. All authors have secured funding and contributed to data collection, data curation and data quality control. All authors approved the submission of the manuscript.