Tracking technologies help to identify key marine areas for basking sharks

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A range of satellite tag technologies has helped track movement behavior of this huge iconic fish species.

Tracking technologies help to identify key marine areas for basking sharks
  • We briefly compare the satellite tracking systems and tags deployed by the Basking Shark Satellite Tagging Project in the UK to track basking shark movements and understand their ranging behaviors.
  • These tags and tracking systems are used on other marine species, as well as turtles, penguins, and other birds.
  • A three-year tracking project of basking sharks highlights the importance and potential efficacy of a marine protected area in the Sea of Hebrides.

Recorded at up to 8,000 pounds, the basking shark (Cetorhinus maximus) is the world’s second largest fish species, smaller only than the whale shark (Rhincodon typus). For decades, humans hunted these sharks for their liver oil and until recently, aside from the historical human uses, relatively little was known about this species.

Researchers from Scottish Natural Heritage (SNH) and the University of Exeter (UoE) have begun to investigate the sharks’ distribution and movement behaviors through the Basking Shark Satellite Tagging Project. Dr. Suzanne Henderson from SNH, the project manager, explained to WildTech the impetus for this project:

We don’t fully understand basking shark distribution, abundance, or the influence they have on marine ecosystems. They are one of a few sharks feeding directly on zooplankton and as such may influence mesopredators’ numbers. Having been exploited heavily in the past, this iconic species, with its many secrets, deserved further investigation. Limited research has occurred so far, possibly in part due to the shark’s size, behavior (at surface in large numbers only in certain locations) and costly technology. Recent advances in satellite tagging technology (battery power and size), alongside a new focus on Marine Protected Areas in Scotland after The Marine (Scotland) Act came into force, have all been timely for this research idea.

Basking shark exhibiting impressive feeding behavior_Flickr
Basking shark exhibiting its impressive feeding behavior, which includes swimming open-mouthed with dark, bristle-like gill rakers upright and rigid to trap plankton in the water column. Photo credit: Flickr user: jidanchaomian

The researchers conducted the three-year project between 2012 and 2014, tracking 61 basking sharks in the waters between Isle of Skye and the Isle of Mull to assess whether these waters should be part of Scotland’s Nature Conservation Marine Protected Areas (MPAs) Programme.

General Study Area of Sharks Map_Google Maps
General study area of the basking sharks in the Sea of Hebrides in northwestern Scotland. Image credit: Katie Taylor/Google Maps

Throughout the project (click here for the project report) the research team deployed four types of tracking tags on the sharks. They followed the sharks’ movements using three tracking systems to address a variety of research questions, including how basking sharks use the waters off the west coast of Scotland, how long they inhabit these waters, and what migration patterns the sharks follow once their leave the area.

basking shark fitted with transmitter SPOT tag L.Campbell & SNH
Basking shark fitted with a SPOT tag (seen on the surface) on dorsal fin. Photo credit: L. Campbell/Scottish Natural Heritage

The tables below highlight general information and limitations of the different satellite tracking systems and tags used in the Basking Shark Satellite Tracking Project. Email correspondence with Dr. Henderson and the final report document of the project provided information on each system.

Satellite Tracking System  General Information  Limitations
Argos System Used to track land and marine animals

Uses the Doppler Effect and can be integrated with GPS tracking systems (see Fastloc GPS)

Best used for compact, low-power transmitters & can be used in difficult radio environments (e.g. dense rainforest, water)

Accuracy: up to 150 m using only the Doppler Effect

Tagged marine animals must be on the surface for the system to locate the tag accurately

Fastloc GPS Specially designed for use in marine environments and species that only surface briefly

GPS ‘snapshot’ processing receivers capture locations in tens of milliseconds

Radio signals from GPS satellites are processed on the platform and compressed into the ‘snapshot’, which contains the satellite ID numbers, respective pseudo ranges, and a timestamp

Accuracy: up to 10 GPS satellites can be processed, for accuracy ranging from 20 to 75 m

Tags can be deployed for several months or longer only if data acquisitions occur, at most, once daily

Tagged marine animals must be on the surface of the water (at least for a short period of time) for the receiver to transmit data to the satellites accurately

Light-based geolocation Low-resolution tracking method; used for animals that move long distances

Records essential light level information which can be processed to give location latitude and longitude; used for animals that spend little to no time at surface

Uses light levels and times of dawn and dusk to estimate location

Recent improvement in analysis capacity allows for more accurate depictions of large-scale movements

Accuracy: only to several tens of kilometers of the tagged individual

o  Animal depth in water can hinder accuracy (deeper waters have less light penetration)
o  “Latitude accuracy” decreases near the equator due to the rate of changing day length
o  Most accurate near the poles

Typically, two positions per day can be acquired

 

Scientists laptop showing basking shark logs 6223 L.Campbell & SNH
Scientists investigating basking shark movements on a tracking system. Photo credit: L. Campbell/SNH

The accuracies of tags are relative and determined more by the tracking system than the type of tag: the accuracy of tags using GPS are 20-75m, the accuracy of those using Argos is up to 150m, and for tags using light geolocation, accuracy is tens of kilometers.

Pop-up tags allow researchers to easily study marine mammals that do not reach the surface frequently or are not re-captured. At a pre-set time and date, a charge passes through a pin on the tag, making it soluble in salt water. This detaches the tag from the animal; the tag then floats to the surface, where it transmits a signal to an over-passing satellite. Once at the surface, the tag can transmit the recorded data to the satellite without needing to be recovered by the researchers. However, if they recover the tag, researchers can download and access higher-resolution data. While more expensive, pop-up tags can save time and resources in other areas of the study, i.e. re-capturing the study animal to remove the tag and extract the data.

Splash tag on shark
SPLASH tag with a tether deployed on shark. Photo credit: Dr. Matthew Witt/University of Exeter

 

Tags General Information and Key Features
SPOT (Smart Position or Temperature)
  • Communicates with Argos system
  • Tracks horizontal movements of marine animals
  • Near-real-time tracking of individuals online; public can follow the movements online
  • Remained attached to the sharks for considerable periods of time, despite having to be extremely buoyant
  • Wet-dry sensors permit tags to transmit data to satellites when tag is dry (and determine animal surfacing)
  • Weight: varying configurations; smallest approximately 30 g
  • Some locations are not accompanied by an estimate of their accuracy
PAT-F (Pop-up Archival Transmitting with Fastloc)
  • Communicates with Argos system, collects Fastloc GPS location & light geolocation
  • Collects information on depth, temperature and spatial movements
  • Tags transmit stored data to over-passing satellites once the tag detaches from study animal
  • Tags can be programmed to detach after specified amount of time
  • If physical tag is retrieved, more detailed time series of data can be downloaded from the tag’s physical memory
  • Weight: mark-report mrPAT- 31 g
SPLASH-F (part of SPLASH series)
  • Communicates with Argos system; can incorporate FastLoc GPS transmission & collect light geolocation information
  • Collects depth, temperature, light levels & wet/dry periodic data
  • User-programmable transmission regimes
  • Transmits data to over-passing satellites while still on the study animal and once it is detached
  • Wet-dry sensors permit tags to transmit data to satellites when tag is dry (and determine animal surfacing)
  • If physical tag is retrieved, more detailed time series of data can be downloaded from the tag’s physical memory
  • Battery life: uses low-power microcontroller and flash memory; expected to be able to read sensors every second for 1 year
  • Weight: varying configurations; 130 – 345 g
MiniPAT (Mini Pop-up Archival Transmitting)
  • Communicates with Argos system once detached and can collect light geolocation information
  • Archival (Designed for extended deployments –several months up to 2 years)
  • User-programmable transmissions; sampling interval options range from 75 seconds to 10 minutes
  • Summarized at 24-hour intervals
  • Data transmitted to over-passing satellites once tag detached from study animal
  • Transmits archived data following user-programmed detachment
  • Wet-dry sensors permit tags to transmit data to satellites when tag is dry
  • If physical tag is retrieved, more detailed time series of data can be downloaded from the tag’s physical memory
  • Weight: 60 g
Basking shark feeding. Photo credit: Greg Skomal / NOAA Fisheries Service
Basking shark feeding. Photo credit: Greg Skomal / NOAA Fisheries Service-Wikimedia Commons

The features and requirements of each tracking device vary slightly and affect their capacity to answer specific questions. As researchers and conservationists, knowing your question is key to choosing appropriate tools. Understanding which tags are most beneficial for collecting specific types of data is necessary to conduct effective research and obtain robust results.

Dr. Henderson explained via email some additional differences in cost and types of information provided by the different tracking tags used in the project.

WildTech: What were the general costs of the satellite tracking systems and the tags?

Costs of tags vary between $1950 USD and $5500 USD (plus VAT), and there are additional satellite costs per month based on when there are transmissions made.

WildTech: Which of the four tags used in this phase of the 3-year project:  SPOT, PAT-F, SPLASH-F, MiniPAT, was best suited for this particular project?

SPOT and SPLASH-F tags provided the project with information on surface movements through the summer, a time period when they knew sharks are mainly in Scottish waters…in addition, they also showed sharks surfacing in waters much further south during the winter, and their return to Scotland the following year.

PAT-F and especially the MiniPATs provided them with more detail on longer migrations, and on depth use—so they can begin to understand shark behaviors when they disappear from the surface waters in Scotland.

SPLASH-F tags are a combination of all the tags, so they provided GPS fixes as well as information on depth and light (for light geolocation) – giving a more detailed view of individual’s use of space.

SPOT top & MiniPAT bottom_crop
SPOT tag (above) and MiniPAT tag (below). Photo credit: Scottish Natural Heritage

I think it was the combination of the tagging technologies that made this project a real success. We were lucky to get funding for the three years’ work, and managed to successfully tag a relatively large number of sharks each year, with tag types usage adapted based on what we had found in previous years and if advances had been made in tagging or tag housing in the interim months between summer field work.

The satellite tracking systems used in this project have been used in studies of other marine animals—including ocean sunfish, tuna, sea turtles, and other sharks—as well. If you have had successes or failures using any of these technologies in your work, please leave a comment and let the group know of your experience!

Please also join us on our forum page to discuss satellite tracking technologies in marine environments and improve this brief review.

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