From fear to fascination: Decoding the depths and distances travelled by the world’s largest fish

By  Colby Bignell 18 June 2025 6 min read

I became acquainted with sharks at a very early age. 

We spent every other Christmas holiday at my family’s old beach shack at an island in the Dampier Archipelago, 2000 km north of my home in Perth. My earliest memory of an encounter with a reef shark was while snorkelling there when I was five or six. 

As a child, I had developed an intense fear of the “monsters of the deep” depicted in books about great white sharks in the school library. Once I interacted with sharks this fear quickly transformed from fear into awe and fascination.

Ultimately, these early encounters led me to study Marine Science at Murdoch University. I started around the time that my Mum and grandparents moved to Exmouth, the focal point of Ningaloo Reef, Australia’s largest fringing coral reef that reaches across 300 kilometres of coastline. And this was the first time I swam with the world’s largest fish - the whale shark, which aggregate at Ningaloo Reef annually.

My first experience of research was during my Honours degree at the University of Western Australia. I researched behaviour and sensory biology including vision, behaviour and electroreception, where sharks detect electro-magnetic fields with special sensory pores. 

The ocean has influenced every aspect of my life, both in my studies and as an avid diver, fisher and underwater photographer. It has taken me across six continents to photograph, experience and learn everything I can about sharks. I have also been lucky to work for the South African Shark Conservancy, with the Newport Beach Lifeguards in California, USA and with the Queensland Department of Environment & Science.

Circling back to Ningaloo

These experiences working abroad were pivotal in shaping my career goal to continue with research. This led me back to the whale sharks of Ningaloo Reef in 2022, when I started a PhD with CSIRO and the University of Tasmania in the Ningaloo Outlook program. 

My research aims to better understand the movement ecology of sharks at Ningaloo Reef, using satellite and acoustic tagging data – which tracks not only where whale sharks go but how deep they dive. Movement ecology is concerned with understanding the patterns and drivers of movement behaviours, which vary amongst marine species depending on their physiology, age, sex and the environment. There are still many gaps in this knowledge for whale sharks.

The Ningaloo Outlook research team – including my PhD supervisor, Richard Pillans, and many others – have tagged more than 150 reef sharks of various species and 40 whale sharks over the past decade. 

I have been lucky to help collect this data on several field trips; swimming up close to more than 100 whale sharks in just a few days. My field work also includes maintaining a large array of acoustic listening stations throughout the lagoon and reef slope, diving in a wide range of habitats and conditions to collect the receivers and download the shark detections. 

A unique data set for a unique fish

Some of the satellite tracks in our study are among the longest ever recorded for whale sharks at Ningaloo Reef. We collected data for almost an entire year from some individuals as they travelled to Indonesia, Timor-Leste and the Gulf of Carpentaria in Queensland – 3,500 km away from Ningaloo Reef. 

The tags used were capable of transmitting both satellite locations and depth data, which allowed us to collect detailed information on depth use during the whale sharks’ migrations. We observed many deep dives up to an astonishing 1,906 m as they moved offshore. Whale sharks may even dive deeper than this, but tags can only withstand depths up to 2,000 m before they are crushed by pressure.
While the data we collected is remarkable, this type of research presents challenges as satellite tags cannot transmit data when underwater. Whale sharks do not need to surface to breathe and spend a lot of time under the surface, which means much of the data contains large gaps or errors. Resolving this requires the use of complex statistical modelling and analysis to predict the most likely locations of sharks between gaps and link them environmental variables to better understand their movements. 

Over countless hours spent writing code in the past three years to clean, analyse and predict patterns in the data, my PhD research has led to some fascinating discoveries about whale sharks. I’m excited to finally share these findings in my first lead-author publication in the scientific journal Marine Biology. 

What is unique about this study is that it explores the horizontal and vertical movements of whale sharks using a dataset that includes an equal number of males and females. Aggregations of whale sharks are dominated by immature males so obtaining comparable data from males and females is important. 

Where the satellite tracking took us

The results provide important insights into the movement ecology of male and female whale sharks. 

• Migration timing and habitat use varied between sexes. Females migrated away from Ningaloo Reef earlier (June–July) and stayed offshore for longer whereas males spent more time in shallower coastal waters in June-July and during the summer months.  
• Temperature preferences differed. There was evidence that in late spring, females occupied areas with lower Sea Surface Temperature - SST (<24 °C) than males (>28 °C). Additional data are required to determine if this result represents a preference for cooler temperatures in females, which differs from the widely accepted temperature preferences of whale sharks (26.5–30 °C).
• Habitat use was influenced by temperature. Core area size increased with increasing SST in both sexes, most likely due to animals moving away from warmer tropical areas as water temperature increases during the summer months.
• Depth use was influenced by temperature, but differently by sex. Females dived to greater depths than males in SST between 26-28 °C, possibly to avoid warmer surface waters. Deep diving in males was more frequent in SST above 28 °C).

There is still much to learn, and more research is required to understand what drives the movements of these large migratory animals, research that is essential for their conservation. I hope to continue contributing to this field during the remainder of my PhD and beyond as a post-doctoral scientist. As the first person in my family to complete a university degree — let alone undertake a PhD — I’m incredibly proud of this work and excited to see what more I can discover. 

Read more in the open access paper:

Bignell, C.J., Patterson, T.A., Donovan, A. et al. Satellite tracking reveals sex-specific differences in the geographical and vertical habitat use of Whale sharks, Rhincodon typus, in the Eastern Indian ocean. Mar Biol 172, 105 (2025). https://doi.org/10.1007/s00227-025-04616-5

Colby Bignell is a PhD Candidate at the University of Tasmania and CSIRO Ningaloo Outlook Program.

This research is funded by Ningaloo Outlook (a strategic marine research partnership between CSIRO and Woodside Energy to better understand Ningaloo Reef and its important ecological values).