It’s the question that has eluded human-kind for decades and one of the last greatest mysteries of the natural world… just where on earth do sharks go to give birth? 

Sharks have inhabited the world’s ocean for over 400 million years, surviving mass extinctions and shaping marine ecosystems. Yet, even today, little remains known about their reproductive strategies. But the team at the Marine Biological Association wants to change all of that.

And to do so, they’ve set their sights on one of the most elusive and mysterious of all the shark species – the largest fish on Earth: the whale shark. 

The whale shark (Rhinocodon typus) is most certainly an ocean giant, growing to as much as 18 metres in length and is a recognised keystone species of biodiversity, meaning they play a significant role in shaping and defining their ecosystem. 

But despite their size and ecological importance, the reproductive behaviours of this species are poorly understood, including where females go to give birth. What’s more, sightings of neonates – baby whale sharks less than 1.5 metres in length – are extremely rare, with only approximately 35 recorded sightings worldwide between 1970 and 2020.

However, in a new study published in the journal Ecology and Evolution this week, researchers from the Ocean Predator Movement Ecology and Conservation research lab at the Marine Biological Association have started to map neonate whale shark sightings against oceanographic data in an effort to remove the shroud of mystery from the whole process.

“Just imagine these tiny, spotty baby sharks swimming around in the open ocean like tadpoles,” said Dr Freya Womersley, lead author and Post Doctoral Researcher at the Marine Biological Association. “They need to stay safe from predators and find as much food as they can, so certain areas of the ocean may suit them more than others, and that’s why we wanted to map their locations as a first step.” 

And what’s surprised scientists the most, so far, is the connection emerging between the distribution of baby whale sharks and areas of the ocean known as Oxygen Minimum Zones – regions in which levels of dissolved oxygen are extremely low.

Researchers have since proposed to main hypotheses to explain the link between neonates and these Oxygen Minimum Zones (OMZs).

The first suggests that female whale sharks may deliberately give birth near OMZs, where low-oxygen waters offer protection from predators and high surface productivity ensures plentiful food for neonates. The second theorises that neonates may be born randomly in the open ocean, but OMZs force them to remain closer to the surface due to their inability to tolerate low-oxygen waters, making them more likely to be observed by humans.

Professor David Sims, Senior Research Fellow at the Marine Biological Society, said: “Our analysis suggests that OMZs could play an important role in the early life of whale sharks, and perhaps even other young sharks, by acting as natural refuges, offering protection from predators and abundant food sources.”

Whale sharks are classified as endangered, with their populations under threat from ship collisions, habitat degradation, fishing bycatch, and climate change. Understanding the role that these OMZs habitats play in whale shark reproduction could help prioritise areas for conservation. 

Of course, with areas of OMZs predicted to increase with climate change, the survival of neonates could become greatly affected, raising questions about how they will adapt to warming seas and changes in the location of ‘safer zones’.

This research highlights areas at the boundaries of productive upwelling systems – regions where neonates are most likely to occur – as key targets for future research and protection efforts. 

One such approach, it has been suggested, could use biologging devices that measure the oxygen concentrations in certain areas to explore how whale sharks interact with OMZs across different life stages. 

These tools could also shed light on how neonates respond to climate-driven changes in ocean chemistry.

“By focusing on areas which share similar characteristics to where these baby sharks have already been spotted, we can better understand where to look for neonates and how to protect them,” said Dr Womersley. “This is a critical step toward ensuring the survival of this iconic species.”

The study, Do Whale Sharks Select for Specific Environments to Give Birth? has now been published in the scientific journal, Ecology and Evolution.

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