Sharks equipped with tags, sentinels of the oceans in the Gulf of Alaska

04/09/2024

9 minutes

oceans and technology

Over the last few decades, oceanography has made significant progress thanks to the integration of new technologies, in particular the use of tags attached to marine animals to collect data in real time. A recent study demonstrated the effectiveness of a new tag mounted on the dorsal fin of a shark for observing ocean conditions in the Gulf of Alaska. This device could fill existing gaps in ocean observation networks.

By Laurie Henry

Since the early 2000s, oceanography has taken a major step forward in scientific knowledge of the oceans, thanks to the international Argo initiative and the establishment of a network of autonomous underwater floats deployed throughout the world’s oceans. These Argo floats, capable of diving to depths of 2000 metres, record data in real time on pressure, temperature, salinity and ocean currents. Today, with nearly 4,000 active floats, Argo is the cornerstone of ocean surveillance.

But despite this progress, there are still major gaps in the observation of the sea, particularly in polar and coastal areas where extreme conditions and the variability of water masses make it difficult to acquire accurate, continuous data.

Meeting the challenges of ocean observation

These gaps pose a major challenge for the scientific community, especially in the context of accelerating climate change, which is having a major impact on the health of the oceans.

In particular, the polar regions are 4 times more affected than other ocean regions by climate change, but they are also the pillars of global climate regulation and balance, keystones of global ocean circulation and the carbon cycle. Yet they remain among the least sampled areas, and the data available is often insufficient to model the oceanic and atmospheric processes that take place there accurately, which limits the accuracy of long-term climate forecasts.

The idea of using marine animals as observation platforms is not new, but it has gained momentum over the last two decades. Some marine mammals, such as elephant seals, are already equipped with CTD tags, enabling ocean conductivity, temperature and depth to be measured. Attached directly to the animals, these tools take advantage of their natural behaviour to explore regions that are often inaccessible to humans, particularly the southern seas and areas submerged under ice. Since the MEOP network was set up in 2002, more than 650,000 temperature and salinity profiles have been recorded using these devices. These data have revealed information about the characteristics of water masses, the structure of oceanic fronts and small-scale ocean dynamics.

However, although marine mammals have made it possible to fill certain gaps, their capabilities remain limited to specific areas and times of year. The idea of using other large marine predators, such as sharks, makes perfect sense.

By exploiting the mobility and diversity of the habitats frequented by these animals, the researchers hope to extend the coverage of ocean observations to regions that have not yet been studied, while increasing the resolution of the data collected, both in terms of frequency and depth. This approach could provide a more complete and integrated view of the state of the oceans in a rapidly changing world. Researchers from Stanford University, in collaboration with the University of Hawaii and the University of Delaware, wanted to test this idea in the Gulf of Alaska and have just published the results of their work in Scientific Reports.

A natural laboratory for testing animals

The Gulf of Alaska remains under-sampled despite worldwide efforts. Located in the north-east of the Pacific Ocean, it is a vast maritime region bordered by the south coast of Alaska and the Aleutian Islands to the west.

This gulf is characterised by cold, nutrient-rich waters that support exceptional marine biodiversity, including numerous species of fish, marine mammals and birds. Ocean currents are powerful, particularly the Alaskan Current, which carries sub-Arctic waters southwards. The Gulf of Alaska is also known for its extreme climatic conditions, marked by frequent storms and pronounced seasonal variations.

This region plays a key role in regulating the northern hemisphere’s climate systems, making it a natural laboratory for studying the effects of climate change on marine ecosystems. A perfect study area to test the salmon shark research team’s project, with its great capacity to cover vast distances and explore varied depths.

Technological innovation serving science

To carry out this study, the researchers developed a CTD-SRDL (Satellite Relay Data Logger) specially adapted for sharks. Attached to the animal’s dorsal fin, the tag transmits data collected on water temperature, salinity and depth via satellite.

« twin » tag. © C. M. L. S. Pagniello et al., 2024

Two models of tags were tested: the ‘twin’ model and the ‘single’ model. The ‘twin’ model consists of two separate units: one for the CTD sensor, positioned at the base of the fin to ensure that it remains immersed at all times, and the other for the satellite transmitter, positioned higher up on the fin to optimise data transmission when the shark partially emerges from the water. These two units are linked by a ribbon cable, a choice which, although innovative, presents certain vulnerabilities, particularly because of its low elasticity and the lack of protection against damage in the marine environment.

The ‘single’ model, on the other hand, combines these two components into a single package, thereby reducing the risks associated with physical connections, but at the cost of increasing the overall size of the tag, which can potentially affect the shark’s natural behaviour.

In August 2015, a twin tag was fitted to a shark. Once the tag was attached and the shark released, it travelled over 1,360 kilometres in 36 days. During this journey, the CTD recorded 56 temperature and salinity profiles at depths ranging from 24 to 297 metres, including temperatures between 5.6 and 16.5°C.

Water properties and anomalies along the path of the salmon shark (A) Temperature-salinity diagram of all the profiles collected. Temperature (B) and salinity (C) profiles obtained as a function of depth, with one colour per date. © C. M. L. S. Pagniello et al., 2024

This data, collected in real time and accurately geolocated, provides a detailed view of ocean conditions along the shark’s path. In particular, it allows us to observe variations in the physical properties of water masses in response to currents, eddies and seasonal changes. The tag’s ability to transmit its data by satellite in near-real time is a considerable advantage for researchers, who can analyse current ocean conditions and adjust their hypotheses and numerical models accordingly.

A new era for oceanography

Although still in the experimental phase, this technology is paving the way for a new era in oceanographic monitoring. The data collected by the CTD tag attached to the salmon shark has revealed significant anomalies in the physical properties of the ocean.

Among the 56 temperature and salinity profiles recorded, the researchers observed predominantly positive temperature anomalies, with deviations frequently exceeding 1.5°C from seasonal averages in the top 30 metres of the water column. These anomalies are closely linked to the presence of the ‘Blob’, a vast area of abnormally warm water that emerged in the north-east Pacific between the winter of 2013 and 2014 and persisted until 2016. This stretch of warm water, sometimes 3 to 4°C warmer than normal, has had a profound effect on the marine ecosystem, disrupting the reproductive cycles of many species and modifying ocean currents.

(A) Trajectory of the salmon shark fitted with a CTD-SRDL tag in the Gulf of Alaska, between 14 August and 18 September 2015, with geographical position of profiles collected as a function of date (coloured circles) and estimates of climatological profiles from the World Ocean Database (black circles). The white triangle indicates the tagging site. (B) Mean dynamic topography, and profiles collected by the shark coloured according to whether they are in an anticyclonic eddy (red), cyclonic (blue) or out of an eddy (black). © C. M. L. S. Pagniello et al., 2024

In addition to temperature anomalies, the tag also detected significant variations in salinity, revealing an almost equal distribution between positive anomalies (saltier waters) and negative anomalies (cooler waters). These variations in salinity reveal the mixing dynamics of water masses and their impact on ocean circulation.

One of the most significant results of this study is the ability of sharks to cross and sample medium-scale eddies, dynamic structures that strongly influence the physical properties of the ocean. The shark fitted with the tag passed through seven eddies, including three anticyclonic ones, where dense, salty water is often transported to deeper layers. The data showed that these eddies modify the depth of the mixed layer, creating vertical gradients in temperature and salinity that can have large-scale impacts on the ecosystem.

Potential temporal coverage of profiles collected by sharks, compared with those from the WOD (World Ocean Databas)e climatology between 2002 and 2019 in the Gulf of Alaska: (A) by year and (B) by month. CTD-SRDL shark profiles (blue), World Ocean Database (red), and number of SPOT-tagged sharks (dotted green line and circles). © C. M. L. S. Pagniello et al., 2024

Extending this technology to a larger number of sharks and other marine species could transform our understanding of small-scale oceanic processes that are still largely unknown. The creation of a biological observation network capable of providing real-time data on variations in ocean conditions would be an innovative approach. It would not only improve the accuracy of climate models, but also offer new prospects for the sustainable management of marine resources in the face of the challenges posed by climate change.


Source : Pagniello, C.M.L.S., Castleton, M.R., Carlisle, A.B. et al. “Novel CTD tag establishes shark fins as ocean observing platforms”. Sci Rep 14, 13837 (2024). https://doi.org/10.1038/s41598-024-63543-5

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